Support for maps in the MRI C Ruby extension.

This adds the Map container and support for parsing and serializing maps
in the protobuf wire format (as defined by the C++ implementation, with
MapEntry submessages in a repeated field). JSON map
serialization/parsing are not yet supported as these will require some
changes to upb as well.

12 files changed, 1873 insertions, 163 deletions

diff --git a/ruby/ext/google/protobuf_c/defs.c b/ruby/ext/google/protobuf_c/defs.c
index bb6f10e1..5c94a74a 100644
--- a/ruby/ext/google/protobuf_c/defs.c
+++ b/ruby/ext/google/protobuf_c/defs.c
@@ -923,6 +923,7 @@ DEFINE_CLASS(MessageBuilderContext,
 void MessageBuilderContext_mark(void* _self) {
   MessageBuilderContext* self = _self;
   rb_gc_mark(self->descriptor);
+  rb_gc_mark(self->builder);
 }
 
 void MessageBuilderContext_free(void* _self) {
@@ -935,6 +936,7 @@ VALUE MessageBuilderContext_alloc(VALUE klass) {
   VALUE ret = TypedData_Wrap_Struct(
       klass, &_MessageBuilderContext_type, self);
   self->descriptor = Qnil;
+  self->builder = Qnil;
   return ret;
 }
 
@@ -943,24 +945,29 @@ void MessageBuilderContext_register(VALUE module) {
       module, "MessageBuilderContext", rb_cObject);
   rb_define_alloc_func(klass, MessageBuilderContext_alloc);
   rb_define_method(klass, "initialize",
-                   MessageBuilderContext_initialize, 1);
+                   MessageBuilderContext_initialize, 2);
   rb_define_method(klass, "optional", MessageBuilderContext_optional, -1);
   rb_define_method(klass, "required", MessageBuilderContext_required, -1);
   rb_define_method(klass, "repeated", MessageBuilderContext_repeated, -1);
+  rb_define_method(klass, "map", MessageBuilderContext_map, -1);
   cMessageBuilderContext = klass;
   rb_gc_register_address(&cMessageBuilderContext);
 }
 
 /*
  * call-seq:
- *     MessageBuilderContext.new(desc) => context
+ *     MessageBuilderContext.new(desc, builder) => context
  *
- * Create a new builder context around the given message descriptor. This class
- * is intended to serve as a DSL context to be used with #instance_eval.
+ * Create a new message builder context around the given message descriptor and
+ * builder context. This class is intended to serve as a DSL context to be used
+ * with #instance_eval.
  */
-VALUE MessageBuilderContext_initialize(VALUE _self, VALUE msgdef) {
+VALUE MessageBuilderContext_initialize(VALUE _self,
+                                       VALUE msgdef,
+                                       VALUE builder) {
   DEFINE_SELF(MessageBuilderContext, self, _self);
   self->descriptor = msgdef;
+  self->builder = builder;
   return Qnil;
 }
 
@@ -1065,6 +1072,96 @@ VALUE MessageBuilderContext_repeated(int argc, VALUE* argv, VALUE _self) {
                           name, type, number, type_class);
 }
 
+/*
+ * call-seq:
+ *     MessageBuilderContext.map(name, key_type, value_type, number,
+ *                               value_type_class = nil)
+ *
+ * Defines a new map field on this message type with the given key and value types, tag
+ * number, and type class (for message and enum value types). The key type must
+ * be :int32/:uint32/:int64/:uint64, :bool, or :string. The value type type must
+ * be a Ruby symbol (as accepted by FieldDescriptor#type=) and the type_class
+ * must be a string, if present (as accepted by FieldDescriptor#submsg_name=).
+ */
+VALUE MessageBuilderContext_map(int argc, VALUE* argv, VALUE _self) {
+  DEFINE_SELF(MessageBuilderContext, self, _self);
+
+  if (argc < 4) {
+    rb_raise(rb_eArgError, "Expected at least 4 arguments.");
+  }
+  VALUE name = argv[0];
+  VALUE key_type = argv[1];
+  VALUE value_type = argv[2];
+  VALUE number = argv[3];
+  VALUE type_class = (argc > 4) ? argv[4] : Qnil;
+
+  // Validate the key type. We can't accept enums, messages, or floats/doubles
+  // as map keys. (We exclude these explicitly, and the field-descriptor setter
+  // below then ensures that the type is one of the remaining valid options.)
+  if (SYM2ID(key_type) == rb_intern("float") ||
+      SYM2ID(key_type) == rb_intern("double") ||
+      SYM2ID(key_type) == rb_intern("enum") ||
+      SYM2ID(key_type) == rb_intern("message")) {
+    rb_raise(rb_eArgError,
+             "Cannot add a map field with a float, double, enum, or message "
+             "type.");
+  }
+
+  // Create a new message descriptor for the map entry message, and create a
+  // repeated submessage field here with that type.
+  VALUE mapentry_desc = rb_class_new_instance(0, NULL, cDescriptor);
+  VALUE mapentry_desc_name = rb_funcall(self->descriptor, rb_intern("name"), 0);
+  mapentry_desc_name = rb_str_cat2(mapentry_desc_name, "_MapEntry_");
+  mapentry_desc_name = rb_str_cat2(mapentry_desc_name,
+                                   rb_id2name(SYM2ID(name)));
+  Descriptor_name_set(mapentry_desc, mapentry_desc_name);
+
+  // The 'mapentry' attribute has no Ruby setter because we do not want the user
+  // attempting to DIY the setup below; we want to ensure that the fields are
+  // correct. So we reach into the msgdef here to set the bit manually.
+  Descriptor* mapentry_desc_self = ruby_to_Descriptor(mapentry_desc);
+  upb_msgdef_setmapentry((upb_msgdef*)mapentry_desc_self->msgdef, true);
+
+  // optional <type> key = 1;
+  VALUE key_field = rb_class_new_instance(0, NULL, cFieldDescriptor);
+  FieldDescriptor_name_set(key_field, rb_str_new2("key"));
+  FieldDescriptor_label_set(key_field, ID2SYM(rb_intern("optional")));
+  FieldDescriptor_number_set(key_field, INT2NUM(1));
+  FieldDescriptor_type_set(key_field, key_type);
+  Descriptor_add_field(mapentry_desc, key_field);
+
+  // optional <type> value = 2;
+  VALUE value_field = rb_class_new_instance(0, NULL, cFieldDescriptor);
+  FieldDescriptor_name_set(value_field, rb_str_new2("value"));
+  FieldDescriptor_label_set(value_field, ID2SYM(rb_intern("optional")));
+  FieldDescriptor_number_set(value_field, INT2NUM(2));
+  FieldDescriptor_type_set(value_field, value_type);
+  if (type_class != Qnil) {
+    VALUE submsg_name = rb_str_new2("."); // prepend '.' to make name absolute.
+    submsg_name = rb_str_append(submsg_name, type_class);
+    FieldDescriptor_submsg_name_set(value_field, submsg_name);
+  }
+  Descriptor_add_field(mapentry_desc, value_field);
+
+  // Add the map-entry message type to the current builder, and use the type to
+  // create the map field itself.
+  Builder* builder_self = ruby_to_Builder(self->builder);
+  rb_ary_push(builder_self->pending_list, mapentry_desc);
+
+  VALUE map_field = rb_class_new_instance(0, NULL, cFieldDescriptor);
+  VALUE name_str = rb_str_new2(rb_id2name(SYM2ID(name)));
+  FieldDescriptor_name_set(map_field, name_str);
+  FieldDescriptor_number_set(map_field, number);
+  FieldDescriptor_label_set(map_field, ID2SYM(rb_intern("repeated")));
+  FieldDescriptor_type_set(map_field, ID2SYM(rb_intern("message")));
+  VALUE submsg_name = rb_str_new2("."); // prepend '.' to make name absolute.
+  submsg_name = rb_str_append(submsg_name, mapentry_desc_name);
+  FieldDescriptor_submsg_name_set(map_field, submsg_name);
+  Descriptor_add_field(self->descriptor, map_field);
+
+  return Qnil;
+}
+
 // -----------------------------------------------------------------------------
 // EnumBuilderContext.
 // -----------------------------------------------------------------------------
@@ -1190,7 +1287,8 @@ void Builder_register(VALUE module) {
 VALUE Builder_add_message(VALUE _self, VALUE name) {
   DEFINE_SELF(Builder, self, _self);
   VALUE msgdef = rb_class_new_instance(0, NULL, cDescriptor);
-  VALUE ctx = rb_class_new_instance(1, &msgdef, cMessageBuilderContext);
+  VALUE args[2] = { msgdef, _self };
+  VALUE ctx = rb_class_new_instance(2, args, cMessageBuilderContext);
   VALUE block = rb_block_proc();
   rb_funcall(msgdef, rb_intern("name="), 1, name);
   rb_funcall_with_block(ctx, rb_intern("instance_eval"), 0, NULL, block);
diff --git a/ruby/ext/google/protobuf_c/encode_decode.c b/ruby/ext/google/protobuf_c/encode_decode.c
index 8aba3c9e..6263edcc 100644
--- a/ruby/ext/google/protobuf_c/encode_decode.c
+++ b/ruby/ext/google/protobuf_c/encode_decode.c
@@ -174,9 +174,222 @@ static void *submsg_handler(void *closure, const void *hd) {
   return submsg;
 }
 
+// Handler data for startmap/endmap handlers.
+typedef struct {
+  size_t ofs;
+  const upb_fielddef* key_field;
+  const upb_fielddef* value_field;
+  VALUE value_field_typeclass;
+} map_handlerdata_t;
+
+// Temporary frame for map parsing: at the beginning of a map entry message, a
+// submsg handler allocates a frame to hold (i) a reference to the Map object
+// into which this message will be inserted and (ii) storage slots to
+// temporarily hold the key and value for this map entry until the end of the
+// submessage. When the submessage ends, another handler is called to insert the
+// value into the map.
+typedef struct {
+  VALUE map;
+  char key_storage[NATIVE_SLOT_MAX_SIZE];
+  char value_storage[NATIVE_SLOT_MAX_SIZE];
+} map_parse_frame_t;
+
+// Handler to begin a sequence of map entries: simple no-op that exists only to
+// set context for the map entry handlers.
+static void *startmap_handler(void *closure, const void *hd) {
+  return closure;
+}
+
+// Handler to begin a map entry: allocates a temporary frame. This is the
+// 'startsubmsg' handler on the msgdef that contains the map field.
+static void *startmapentry_handler(void *closure, const void *hd) {
+  MessageHeader* msg = closure;
+  const map_handlerdata_t* mapdata = hd;
+  VALUE map_rb = DEREF(Message_data(msg), mapdata->ofs, VALUE);
+
+  map_parse_frame_t* frame = ALLOC(map_parse_frame_t);
+  frame->map = map_rb;
+
+  native_slot_init(upb_fielddef_type(mapdata->key_field),
+                   &frame->key_storage);
+  native_slot_init(upb_fielddef_type(mapdata->value_field),
+                   &frame->value_storage);
+
+  return frame;
+}
+
+// Handler to end a map entry: inserts the value defined during the message into
+// the map. This is the 'endmsg' handler on the map entry msgdef.
+static bool endmap_handler(void *closure, const void *hd, upb_status* s) {
+  map_parse_frame_t* frame = closure;
+  const map_handlerdata_t* mapdata = hd;
+
+  VALUE key = native_slot_get(
+      upb_fielddef_type(mapdata->key_field), Qnil,
+      &frame->key_storage);
+  VALUE value = native_slot_get(
+      upb_fielddef_type(mapdata->value_field), mapdata->value_field_typeclass,
+      &frame->value_storage);
+
+  Map_index_set(frame->map, key, value);
+  free(frame);
+
+  return true;
+}
+
+// Allocates a new map_handlerdata_t given the map entry message definition. If
+// the offset of the field within the parent message is also given, that is
+// added to the handler data as well. Note that this is called *twice* per map
+// field: once in the parent message handler setup when setting the startsubmsg
+// handler and once in the map entry message handler setup when setting the
+// key/value and endmsg handlers. The reason is that there is no easy way to
+// pass the handlerdata down to the sub-message handler setup.
+static map_handlerdata_t* new_map_handlerdata(
+    size_t ofs,
+    const upb_msgdef* mapentry_def) {
+
+  map_handlerdata_t* hd = ALLOC(map_handlerdata_t);
+  hd->ofs = ofs;
+  hd->key_field = upb_msgdef_itof(mapentry_def, 1);
+  assert(hd->key_field != NULL);
+  hd->value_field = upb_msgdef_itof(mapentry_def, 2);
+  assert(hd->value_field != NULL);
+  hd->value_field_typeclass = field_type_class(hd->value_field);
+
+  return hd;
+}
+
+// Set up handlers for a repeated field.
+static void add_handlers_for_repeated_field(upb_handlers *h,
+                                            const upb_fielddef *f,
+                                            size_t offset) {
+  upb_handlerattr attr = UPB_HANDLERATTR_INITIALIZER;
+  upb_handlerattr_sethandlerdata(&attr, newhandlerdata(h, offset));
+  upb_handlers_setstartseq(h, f, startseq_handler, &attr);
+  upb_handlerattr_uninit(&attr);
+
+  switch (upb_fielddef_type(f)) {
+
+#define SET_HANDLER(utype, ltype)                                 \
+  case utype:                                                     \
+    upb_handlers_set##ltype(h, f, append##ltype##_handler, NULL); \
+    break;
+
+    SET_HANDLER(UPB_TYPE_BOOL,   bool);
+    SET_HANDLER(UPB_TYPE_INT32,  int32);
+    SET_HANDLER(UPB_TYPE_UINT32, uint32);
+    SET_HANDLER(UPB_TYPE_ENUM,   int32);
+    SET_HANDLER(UPB_TYPE_FLOAT,  float);
+    SET_HANDLER(UPB_TYPE_INT64,  int64);
+    SET_HANDLER(UPB_TYPE_UINT64, uint64);
+    SET_HANDLER(UPB_TYPE_DOUBLE, double);
+
+#undef SET_HANDLER
+
+    case UPB_TYPE_STRING:
+    case UPB_TYPE_BYTES: {
+      bool is_bytes = upb_fielddef_type(f) == UPB_TYPE_BYTES;
+      upb_handlers_setstartstr(h, f, is_bytes ?
+                               appendbytes_handler : appendstr_handler,
+                               NULL);
+      upb_handlers_setstring(h, f, stringdata_handler, NULL);
+    }
+    case UPB_TYPE_MESSAGE: {
+      upb_handlerattr attr = UPB_HANDLERATTR_INITIALIZER;
+      upb_handlerattr_sethandlerdata(&attr, newsubmsghandlerdata(h, 0, f));
+      upb_handlers_setstartsubmsg(h, f, appendsubmsg_handler, &attr);
+      upb_handlerattr_uninit(&attr);
+      break;
+    }
+  }
+}
+
+// Set up handlers for a singular field.
+static void add_handlers_for_singular_field(upb_handlers *h,
+                                            const upb_fielddef *f,
+                                            size_t offset) {
+  switch (upb_fielddef_type(f)) {
+    case UPB_TYPE_BOOL:
+    case UPB_TYPE_INT32:
+    case UPB_TYPE_UINT32:
+    case UPB_TYPE_ENUM:
+    case UPB_TYPE_FLOAT:
+    case UPB_TYPE_INT64:
+    case UPB_TYPE_UINT64:
+    case UPB_TYPE_DOUBLE:
+      // The shim writes directly at the given offset (instead of using
+      // DEREF()) so we need to add the msg overhead.
+      upb_shim_set(h, f, offset + sizeof(MessageHeader), -1);
+      break;
+    case UPB_TYPE_STRING:
+    case UPB_TYPE_BYTES: {
+      bool is_bytes = upb_fielddef_type(f) == UPB_TYPE_BYTES;
+      upb_handlerattr attr = UPB_HANDLERATTR_INITIALIZER;
+      upb_handlerattr_sethandlerdata(&attr, newhandlerdata(h, offset));
+      upb_handlers_setstartstr(h, f,
+                               is_bytes ? bytes_handler : str_handler,
+                               &attr);
+      upb_handlers_setstring(h, f, stringdata_handler, &attr);
+      upb_handlerattr_uninit(&attr);
+      break;
+    }
+    case UPB_TYPE_MESSAGE: {
+      upb_handlerattr attr = UPB_HANDLERATTR_INITIALIZER;
+      upb_handlerattr_sethandlerdata(&attr, newsubmsghandlerdata(h, offset, f));
+      upb_handlers_setstartsubmsg(h, f, submsg_handler, &attr);
+      upb_handlerattr_uninit(&attr);
+      break;
+    }
+  }
+}
+
+// Adds handlers to a map field.
+static void add_handlers_for_mapfield(upb_handlers* h,
+                                      const upb_fielddef* fielddef,
+                                      size_t offset) {
+  const upb_msgdef* map_msgdef = upb_fielddef_msgsubdef(fielddef);
+  map_handlerdata_t* hd = new_map_handlerdata(offset, map_msgdef);
+  upb_handlers_addcleanup(h, hd, free);
+  upb_handlerattr attr = UPB_HANDLERATTR_INITIALIZER;
+  upb_handlerattr_sethandlerdata(&attr, hd);
+  upb_handlers_setstartseq(h, fielddef, startmap_handler, &attr);
+  upb_handlers_setstartsubmsg(h, fielddef, startmapentry_handler, &attr);
+  upb_handlerattr_uninit(&attr);
+}
+
+// Adds handlers to a map-entry msgdef.
+static void add_handlers_for_mapentry(const upb_msgdef* msgdef,
+                                      upb_handlers* h) {
+  map_handlerdata_t* hd = new_map_handlerdata(0, msgdef);
+  upb_handlers_addcleanup(h, hd, free);
+  upb_handlerattr attr = UPB_HANDLERATTR_INITIALIZER;
+  upb_handlerattr_sethandlerdata(&attr, hd);
+  upb_handlers_setendmsg(h, endmap_handler, &attr);
+
+  add_handlers_for_singular_field(
+      h, hd->key_field,
+      // Convert the offset into map_parse_frame_t to an offset understood by the
+      // singular field handlers, so that we don't have to use special
+      // map-key/value-specific handlers. The ordinary singular field handlers expect
+      // a Message* and assume offset is relative to the data section at the end, so
+      // we compensate for that addition.
+      offsetof(map_parse_frame_t, key_storage) - sizeof(MessageHeader));
+  add_handlers_for_singular_field(
+      h, hd->value_field,
+      offsetof(map_parse_frame_t, value_storage) - sizeof(MessageHeader));
+}
+
 static void add_handlers_for_message(const void *closure, upb_handlers *h) {
-  Descriptor* desc = ruby_to_Descriptor(
-      get_def_obj((void*)upb_handlers_msgdef(h)));
+  const upb_msgdef* msgdef = upb_handlers_msgdef(h);
+  Descriptor* desc = ruby_to_Descriptor(get_def_obj((void*)msgdef));
+
+  // If this is a mapentry message type, set up a special set of handlers and
+  // bail out of the normal (user-defined) message type handling.
+  if (upb_msgdef_mapentry(msgdef)) {
+    add_handlers_for_mapentry(msgdef, h);
+    return;
+  }
+
   // Ensure layout exists. We may be invoked to create handlers for a given
   // message if we are included as a submsg of another message type before our
   // class is actually built, so to work around this, we just create the layout
@@ -193,80 +406,12 @@ static void add_handlers_for_message(const void *closure, upb_handlers *h) {
     const upb_fielddef *f = upb_msg_iter_field(&i);
     size_t offset = desc->layout->offsets[upb_fielddef_index(f)];
 
-    if (upb_fielddef_isseq(f)) {
-      upb_handlerattr attr = UPB_HANDLERATTR_INITIALIZER;
-      upb_handlerattr_sethandlerdata(&attr, newhandlerdata(h, offset));
-      upb_handlers_setstartseq(h, f, startseq_handler, &attr);
-      upb_handlerattr_uninit(&attr);
-
-      switch (upb_fielddef_type(f)) {
-
-#define SET_HANDLER(utype, ltype)                                 \
-  case utype:                                                     \
-    upb_handlers_set##ltype(h, f, append##ltype##_handler, NULL); \
-    break;
-
-        SET_HANDLER(UPB_TYPE_BOOL,   bool);
-        SET_HANDLER(UPB_TYPE_INT32,  int32);
-        SET_HANDLER(UPB_TYPE_UINT32, uint32);
-        SET_HANDLER(UPB_TYPE_ENUM,   int32);
-        SET_HANDLER(UPB_TYPE_FLOAT,  float);
-        SET_HANDLER(UPB_TYPE_INT64,  int64);
-        SET_HANDLER(UPB_TYPE_UINT64, uint64);
-        SET_HANDLER(UPB_TYPE_DOUBLE, double);
-
-#undef SET_HANDLER
-
-        case UPB_TYPE_STRING:
-        case UPB_TYPE_BYTES: {
-          bool is_bytes = upb_fielddef_type(f) == UPB_TYPE_BYTES;
-          upb_handlers_setstartstr(h, f, is_bytes ?
-                                   appendbytes_handler : appendstr_handler,
-                                   NULL);
-          upb_handlers_setstring(h, f, stringdata_handler, NULL);
-        }
-        case UPB_TYPE_MESSAGE: {
-          upb_handlerattr attr = UPB_HANDLERATTR_INITIALIZER;
-          upb_handlerattr_sethandlerdata(&attr, newsubmsghandlerdata(h, 0, f));
-          upb_handlers_setstartsubmsg(h, f, appendsubmsg_handler, &attr);
-          upb_handlerattr_uninit(&attr);
-          break;
-        }
-      }
-    }
-
-    switch (upb_fielddef_type(f)) {
-      case UPB_TYPE_BOOL:
-      case UPB_TYPE_INT32:
-      case UPB_TYPE_UINT32:
-      case UPB_TYPE_ENUM:
-      case UPB_TYPE_FLOAT:
-      case UPB_TYPE_INT64:
-      case UPB_TYPE_UINT64:
-      case UPB_TYPE_DOUBLE:
-        // The shim writes directly at the given offset (instead of using
-        // DEREF()) so we need to add the msg overhead.
-        upb_shim_set(h, f, offset + sizeof(MessageHeader), -1);
-        break;
-      case UPB_TYPE_STRING:
-      case UPB_TYPE_BYTES: {
-        bool is_bytes = upb_fielddef_type(f) == UPB_TYPE_BYTES;
-        upb_handlerattr attr = UPB_HANDLERATTR_INITIALIZER;
-        upb_handlerattr_sethandlerdata(&attr, newhandlerdata(h, offset));
-        upb_handlers_setstartstr(h, f,
-                                 is_bytes ? bytes_handler : str_handler,
-                                 &attr);
-        upb_handlers_setstring(h, f, stringdata_handler, &attr);
-        upb_handlerattr_uninit(&attr);
-        break;
-      }
-      case UPB_TYPE_MESSAGE: {
-        upb_handlerattr attr = UPB_HANDLERATTR_INITIALIZER;
-        upb_handlerattr_sethandlerdata(&attr, newsubmsghandlerdata(h, offset, f));
-        upb_handlers_setstartsubmsg(h, f, submsg_handler, &attr);
-        upb_handlerattr_uninit(&attr);
-        break;
-      }
+    if (is_map_field(f)) {
+      add_handlers_for_mapfield(h, f, offset);
+    } else if (upb_fielddef_isseq(f)) {
+      add_handlers_for_repeated_field(h, f, offset);
+    } else {
+      add_handlers_for_singular_field(h, f, offset);
     }
   }
 }
@@ -558,6 +703,88 @@ static void putary(VALUE ary, const upb_fielddef *f, upb_sink *sink,
   upb_sink_endseq(sink, getsel(f, UPB_HANDLER_ENDSEQ));
 }
 
+static void put_ruby_value(VALUE value,
+                           const upb_fielddef *f,
+                           VALUE type_class,
+                           int depth,
+                           upb_sink *sink) {
+  upb_selector_t sel = 0;
+  if (upb_fielddef_isprimitive(f)) {
+    sel = getsel(f, upb_handlers_getprimitivehandlertype(f));
+  }
+
+  switch (upb_fielddef_type(f)) {
+    case UPB_TYPE_INT32:
+      upb_sink_putint32(sink, sel, NUM2INT(value));
+      break;
+    case UPB_TYPE_INT64:
+      upb_sink_putint64(sink, sel, NUM2LL(value));
+      break;
+    case UPB_TYPE_UINT32:
+      upb_sink_putuint32(sink, sel, NUM2UINT(value));
+      break;
+    case UPB_TYPE_UINT64:
+      upb_sink_putuint64(sink, sel, NUM2ULL(value));
+      break;
+    case UPB_TYPE_FLOAT:
+      upb_sink_putfloat(sink, sel, NUM2DBL(value));
+      break;
+    case UPB_TYPE_DOUBLE:
+      upb_sink_putdouble(sink, sel, NUM2DBL(value));
+      break;
+    case UPB_TYPE_ENUM: {
+      if (TYPE(value) == T_SYMBOL) {
+        value = rb_funcall(type_class, rb_intern("resolve"), 1, value);
+      }
+      upb_sink_putint32(sink, sel, NUM2INT(value));
+      break;
+    }
+    case UPB_TYPE_BOOL:
+      upb_sink_putbool(sink, sel, value == Qtrue);
+      break;
+    case UPB_TYPE_STRING:
+    case UPB_TYPE_BYTES:
+      putstr(value, f, sink);
+      break;
+    case UPB_TYPE_MESSAGE:
+      putsubmsg(value, f, sink, depth);
+  }
+}
+
+static void putmap(VALUE map, const upb_fielddef *f, upb_sink *sink,
+                   int depth) {
+  if (map == Qnil) return;
+  Map* self = ruby_to_Map(map);
+
+  upb_sink subsink;
+
+  upb_sink_startseq(sink, getsel(f, UPB_HANDLER_STARTSEQ), &subsink);
+
+  assert(upb_fielddef_type(f) == UPB_TYPE_MESSAGE);
+  const upb_fielddef* key_field = map_field_key(f);
+  const upb_fielddef* value_field = map_field_value(f);
+
+  Map_iter it;
+  for (Map_begin(map, &it); !Map_done(&it); Map_next(&it)) {
+    VALUE key = Map_iter_key(&it);
+    VALUE value = Map_iter_value(&it);
+
+    upb_sink entry_sink;
+    upb_sink_startsubmsg(&subsink, getsel(f, UPB_HANDLER_STARTSUBMSG), &entry_sink);
+    upb_sink_startmsg(&entry_sink);
+
+    put_ruby_value(key, key_field, Qnil, depth + 1, &entry_sink);
+    put_ruby_value(value, value_field, self->value_type_class, depth + 1,
+                   &entry_sink);
+
+    upb_status status;
+    upb_sink_endmsg(&entry_sink, &status);
+    upb_sink_endsubmsg(&subsink, getsel(f, UPB_HANDLER_ENDSUBMSG));
+  }
+
+  upb_sink_endseq(sink, getsel(f, UPB_HANDLER_ENDSEQ));
+}
+
 static void putmsg(VALUE msg_rb, const Descriptor* desc,
                    upb_sink *sink, int depth) {
   upb_sink_startmsg(sink);
@@ -580,7 +807,12 @@ static void putmsg(VALUE msg_rb, const Descriptor* desc,
     upb_fielddef *f = upb_msg_iter_field(&i);
     uint32_t offset = desc->layout->offsets[upb_fielddef_index(f)];
 
-    if (upb_fielddef_isseq(f)) {
+    if (is_map_field(f)) {
+      VALUE map = DEREF(msg_data, offset, VALUE);
+      if (map != Qnil) {
+        putmap(map, f, sink, depth);
+      }
+    } else if (upb_fielddef_isseq(f)) {
       VALUE ary = DEREF(msg_data, offset, VALUE);
       if (ary != Qnil) {
         putary(ary, f, sink, depth);
diff --git a/ruby/ext/google/protobuf_c/extconf.rb b/ruby/ext/google/protobuf_c/extconf.rb
index 7cf7bf6a..8d60392c 100644
--- a/ruby/ext/google/protobuf_c/extconf.rb
+++ b/ruby/ext/google/protobuf_c/extconf.rb
@@ -5,6 +5,6 @@ require 'mkmf'
 $CFLAGS += " -O3 -std=c99 -Wno-unused-function -DNDEBUG "
 
 $objs = ["protobuf.o", "defs.o", "storage.o", "message.o",
-         "repeated_field.o", "encode_decode.o", "upb.o"]
+         "repeated_field.o", "map.o", "encode_decode.o", "upb.o"]
 
 create_makefile("google/protobuf_c")
diff --git a/ruby/ext/google/protobuf_c/map.c b/ruby/ext/google/protobuf_c/map.c
new file mode 100644
index 00000000..9d5de9cd
--- /dev/null
+++ b/ruby/ext/google/protobuf_c/map.c
@@ -0,0 +1,883 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2014 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include "protobuf.h"
+
+// -----------------------------------------------------------------------------
+// Basic map operations on top of upb's strtable.
+// -----------------------------------------------------------------------------
+
+// Map values are stored using the native_slot abstraction (as with repeated
+// field values), but keys are a bit special. Since we use a strtable, we need
+// to store keys as sequences of bytes such that equality of those bytes maps
+// one-to-one to equality of keys. We store strings directly (i.e., they map to
+// their own bytes) and integers as sequences of either 4 or 8 bytes in
+// host-byte-order as either a uint32_t or a uint64_t.
+
+// Forms a key to use with the underlying strtable from a Ruby key value. |buf|
+// must point to TABLE_KEY_BUF_LENGTH bytes of temporary space, used to
+// construct a key byte sequence if needed. |out_key| and |out_length| provide
+// the resulting key data/length.
+#define TABLE_KEY_BUF_LENGTH 8  // sizeof(uint64_t)
+static void table_key(Map* self, VALUE key,
+                      char* buf,
+                      const char** out_key,
+                      size_t* out_length) {
+  switch (self->key_type) {
+    case UPB_TYPE_BYTES:
+    case UPB_TYPE_STRING:
+      // Strings: use string content directly.
+      Check_Type(key, T_STRING);
+      native_slot_validate_string_encoding(self->key_type, key);
+      *out_key = RSTRING_PTR(key);
+      *out_length = RSTRING_LEN(key);
+      break;
+
+    case UPB_TYPE_BOOL:
+    case UPB_TYPE_INT32:
+    case UPB_TYPE_INT64: {
+      // Signed numeric types: use an int64 in host-native byte order.
+      int64_t key_val = 0;
+
+      // Do a range/value check.
+      switch (self->key_type) {
+        case UPB_TYPE_BOOL:
+          if (key != Qtrue && key != Qfalse) {
+            rb_raise(rb_eTypeError, "Key must be true or false");
+          }
+          key_val = (key == Qtrue) ? 1 : 0;
+          break;
+        case UPB_TYPE_INT32:
+          native_slot_check_int_range_precision(self->key_type, key);
+          key_val = NUM2INT(key);
+          break;
+        case UPB_TYPE_INT64:
+          native_slot_check_int_range_precision(self->key_type, key);
+          key_val = NUM2LL(key);
+          break;
+        default:
+          break;
+      }
+
+      int64_t* int64_key = (int64_t*)buf;
+      *int64_key = key_val;
+      *out_key = buf;
+      *out_length = sizeof(int64_t);
+      break;
+    }
+
+    case UPB_TYPE_UINT32:
+    case UPB_TYPE_UINT64: {
+      // Unsigned numeric types: use a uint64 in host-native byte order.
+      uint64_t key_val = 0;
+
+      // Do a range/value check.
+      native_slot_check_int_range_precision(self->key_type, key);
+      switch (self->key_type) {
+        case UPB_TYPE_UINT32:
+          key_val = NUM2UINT(key);
+          break;
+        case UPB_TYPE_UINT64:
+          key_val = NUM2ULL(key);
+          break;
+        default:
+          break;
+      }
+
+      uint64_t* uint64_key = (uint64_t*)buf;
+      *uint64_key = key_val;
+      *out_key = buf;
+      *out_length = sizeof(uint64_t);
+      break;
+    }
+
+    default:
+      // Map constructor should not allow a Map with another key type to be
+      // constructed.
+      assert(false);
+      break;
+  }
+}
+
+static VALUE table_key_to_ruby(Map* self, const char* buf, size_t length) {
+  switch (self->key_type) {
+    case UPB_TYPE_BYTES:
+    case UPB_TYPE_STRING: {
+      VALUE ret = rb_str_new(buf, length);
+      rb_enc_associate(ret,
+                       (self->key_type == UPB_TYPE_BYTES) ?
+                       kRubyString8bitEncoding : kRubyStringUtf8Encoding);
+      return ret;
+    }
+
+    case UPB_TYPE_BOOL:
+    case UPB_TYPE_INT32:
+    case UPB_TYPE_INT64: {
+      assert(length == sizeof(int64_t));
+      int64_t* int64_key = (int64_t*)buf;
+
+      if (self->key_type == UPB_TYPE_BOOL) {
+        return *int64_key ? Qtrue : Qfalse;
+      } else {
+        return LL2NUM(*int64_key);
+      }
+    }
+
+    case UPB_TYPE_UINT32:
+    case UPB_TYPE_UINT64: {
+      assert(length == sizeof(uint64_t));
+      uint64_t* uint64_key = (uint64_t*)buf;
+      return ULL2NUM(*uint64_key);
+    }
+
+    default:
+      assert(false);
+      return Qnil;
+  }
+}
+
+static upb_ctype_t upb_table_value_type(upb_fieldtype_t value_type) {
+  switch (value_type) {
+    case UPB_TYPE_BOOL:
+    case UPB_TYPE_INT32:
+    case UPB_TYPE_INT64:
+    case UPB_TYPE_UINT32:
+    case UPB_TYPE_UINT64:
+    case UPB_TYPE_ENUM:
+    case UPB_TYPE_FLOAT:
+    case UPB_TYPE_DOUBLE:
+    case UPB_TYPE_STRING:
+    case UPB_TYPE_BYTES:
+    case UPB_TYPE_MESSAGE:
+      return UPB_CTYPE_UINT64;
+
+    default:
+      assert(false);
+      return 0;
+  }
+}
+
+static void* value_memory(upb_value* v) {
+  return (void*)(&v->val.uint64);
+}
+
+// -----------------------------------------------------------------------------
+// Map container type.
+// -----------------------------------------------------------------------------
+
+const rb_data_type_t Map_type = {
+  "Google::Protobuf::Map",
+  { Map_mark, Map_free, NULL },
+};
+
+VALUE cMap;
+
+Map* ruby_to_Map(VALUE _self) {
+  Map* self;
+  TypedData_Get_Struct(_self, Map, &Map_type, self);
+  return self;
+}
+
+void Map_mark(void* _self) {
+  Map* self = _self;
+
+  rb_gc_mark(self->value_type_class);
+
+  if (self->value_type == UPB_TYPE_STRING ||
+      self->value_type == UPB_TYPE_BYTES ||
+      self->value_type == UPB_TYPE_MESSAGE) {
+    upb_strtable_iter it;
+    for (upb_strtable_begin(&it, &self->table);
+         !upb_strtable_done(&it);
+         upb_strtable_next(&it)) {
+      upb_value v = upb_strtable_iter_value(&it);
+      void* mem = value_memory(&v);
+      native_slot_mark(self->value_type, mem);
+    }
+  }
+}
+
+void Map_free(void* _self) {
+  Map* self = _self;
+  upb_strtable_uninit(&self->table);
+  xfree(self);
+}
+
+VALUE Map_alloc(VALUE klass) {
+  Map* self = ALLOC(Map);
+  memset(self, 0, sizeof(Map));
+  self->value_type_class = Qnil;
+  VALUE ret = TypedData_Wrap_Struct(klass, &Map_type, self);
+  return ret;
+}
+
+static bool needs_typeclass(upb_fieldtype_t type) {
+  switch (type) {
+    case UPB_TYPE_MESSAGE:
+    case UPB_TYPE_ENUM:
+      return true;
+    default:
+      return false;
+  }
+}
+
+/*
+ * call-seq:
+ *     Map.new(key_type, value_type, value_typeclass = nil, init_hashmap = {})
+ *     => new map
+ *
+ * Allocates a new Map container. This constructor may be called with 2, 3, or 4
+ * arguments. The first two arguments are always present and are symbols (taking
+ * on the same values as field-type symbols in message descriptors) that
+ * indicate the type of the map key and value fields.
+ *
+ * The supported key types are: :int32, :int64, :uint32, :uint64, :bool,
+ * :string, :bytes.
+ *
+ * The supported value types are: :int32, :int64, :uint32, :uint64, :bool,
+ * :string, :bytes, :enum, :message.
+ *
+ * The third argument, value_typeclass, must be present if value_type is :enum
+ * or :message. As in RepeatedField#new, this argument must be a message class
+ * (for :message) or enum module (for :enum).
+ *
+ * The last argument, if present, provides initial content for map. Note that
+ * this may be an ordinary Ruby hashmap or another Map instance with identical
+ * key and value types. Also note that this argument may be rpesent whether or
+ * not value_typeclass is present (and it is unambiguously separate from
+ * value_typeclass because value_typeclass's presence is strictly determined by
+ * value_type).
+ */
+VALUE Map_init(int argc, VALUE* argv, VALUE _self) {
+  Map* self = ruby_to_Map(_self);
+
+  // We take either two args (:key_type, :value_type), three args (:key_type,
+  // :value_type, "ValueMessageType"), or four args (the above plus an initial
+  // hashmap).
+  if (argc < 2 || argc > 4) {
+    rb_raise(rb_eArgError, "Map constructor expects 2, 3 or 4 arguments.");
+  }
+
+  self->key_type = ruby_to_fieldtype(argv[0]);
+  self->value_type = ruby_to_fieldtype(argv[1]);
+
+  // Check that the key type is an allowed type.
+  switch (self->key_type) {
+    case UPB_TYPE_INT32:
+    case UPB_TYPE_INT64:
+    case UPB_TYPE_UINT32:
+    case UPB_TYPE_UINT64:
+    case UPB_TYPE_BOOL:
+    case UPB_TYPE_STRING:
+    case UPB_TYPE_BYTES:
+      // These are OK.
+      break;
+    default:
+      rb_raise(rb_eArgError, "Invalid key type for map.");
+  }
+
+  int init_value_arg = 2;
+  if (needs_typeclass(self->value_type) && argc > 2) {
+    self->value_type_class = argv[2];
+    validate_type_class(self->value_type, self->value_type_class);
+    init_value_arg = 3;
+  }
+
+  if (!upb_strtable_init(&self->table, upb_table_value_type(self->value_type))) {
+    rb_raise(rb_eRuntimeError, "Could not allocate table.");
+  }
+
+  if (argc > init_value_arg) {
+    Map_merge_into_self(_self, argv[init_value_arg]);
+  }
+
+  return Qnil;
+}
+
+/*
+ * call-seq:
+ *     Map.each(&block)
+ *
+ * Invokes &block on each |key, value| pair in the map, in unspecified order.
+ * Note that Map also includes Enumerable; map thus acts like a normal Ruby
+ * sequence.
+ */
+VALUE Map_each(VALUE _self) {
+  Map* self = ruby_to_Map(_self);
+
+  upb_strtable_iter it;
+  for (upb_strtable_begin(&it, &self->table);
+       !upb_strtable_done(&it);
+       upb_strtable_next(&it)) {
+
+    VALUE key = table_key_to_ruby(
+        self, upb_strtable_iter_key(&it), upb_strtable_iter_keylength(&it));
+
+    upb_value v = upb_strtable_iter_value(&it);
+    void* mem = value_memory(&v);
+    VALUE value = native_slot_get(self->value_type,
+                                  self->value_type_class,
+                                  mem);
+
+    rb_yield_values(2, key, value);
+  }
+
+  return Qnil;
+}
+
+/*
+ * call-seq:
+ *     Map.keys => [list_of_keys]
+ *
+ * Returns the list of keys contained in the map, in unspecified order.
+ */
+VALUE Map_keys(VALUE _self) {
+  Map* self = ruby_to_Map(_self);
+
+  VALUE ret = rb_ary_new();
+  upb_strtable_iter it;
+  for (upb_strtable_begin(&it, &self->table);
+       !upb_strtable_done(&it);
+       upb_strtable_next(&it)) {
+
+    VALUE key = table_key_to_ruby(
+        self, upb_strtable_iter_key(&it), upb_strtable_iter_keylength(&it));
+
+    rb_ary_push(ret, key);
+  }
+
+  return ret;
+}
+
+/*
+ * call-seq:
+ *     Map.values => [list_of_values]
+ *
+ * Returns the list of values contained in the map, in unspecified order.
+ */
+VALUE Map_values(VALUE _self) {
+  Map* self = ruby_to_Map(_self);
+
+  VALUE ret = rb_ary_new();
+  upb_strtable_iter it;
+  for (upb_strtable_begin(&it, &self->table);
+       !upb_strtable_done(&it);
+       upb_strtable_next(&it)) {
+
+    upb_value v = upb_strtable_iter_value(&it);
+    void* mem = value_memory(&v);
+    VALUE value = native_slot_get(self->value_type,
+                                  self->value_type_class,
+                                  mem);
+
+    rb_ary_push(ret, value);
+  }
+
+  return ret;
+}
+
+/*
+ * call-seq:
+ *     Map.[](key) => value
+ *
+ * Accesses the element at the given key. Throws an exception if the key type is
+ * incorrect. Returns nil when the key is not present in the map.
+ */
+VALUE Map_index(VALUE _self, VALUE key) {
+  Map* self = ruby_to_Map(_self);
+
+  char keybuf[TABLE_KEY_BUF_LENGTH];
+  const char* keyval = NULL;
+  size_t length = 0;
+  table_key(self, key, keybuf, &keyval, &length);
+
+  upb_value v;
+  if (upb_strtable_lookup2(&self->table, keyval, length, &v)) {
+    void* mem = value_memory(&v);
+    return native_slot_get(self->value_type, self->value_type_class, mem);
+  } else {
+    return Qnil;
+  }
+}
+
+/*
+ * call-seq:
+ *     Map.[]=(key, value) => value
+ *
+ * Inserts or overwrites the value at the given key with the given new value.
+ * Throws an exception if the key type is incorrect. Returns the new value that
+ * was just inserted.
+ */
+VALUE Map_index_set(VALUE _self, VALUE key, VALUE value) {
+  Map* self = ruby_to_Map(_self);
+
+  char keybuf[TABLE_KEY_BUF_LENGTH];
+  const char* keyval = NULL;
+  size_t length = 0;
+  table_key(self, key, keybuf, &keyval, &length);
+
+  upb_value v;
+  void* mem = value_memory(&v);
+  native_slot_set(self->value_type, self->value_type_class, mem, value);
+
+  // Replace any existing value by issuing a 'remove' operation first.
+  upb_value oldv;
+  upb_strtable_remove2(&self->table, keyval, length, &oldv);
+  if (!upb_strtable_insert2(&self->table, keyval, length, v)) {
+    rb_raise(rb_eRuntimeError, "Could not insert into table");
+  }
+
+  // Ruby hashmap's :[]= method also returns the inserted value.
+  return value;
+}
+
+/*
+ * call-seq:
+ *     Map.has_key?(key) => bool
+ *
+ * Returns true if the given key is present in the map. Throws an exception if
+ * the key has the wrong type.
+ */
+VALUE Map_has_key(VALUE _self, VALUE key) {
+  Map* self = ruby_to_Map(_self);
+
+  char keybuf[TABLE_KEY_BUF_LENGTH];
+  const char* keyval = NULL;
+  size_t length = 0;
+  table_key(self, key, keybuf, &keyval, &length);
+
+  upb_value v;
+  if (upb_strtable_lookup2(&self->table, keyval, length, &v)) {
+    return Qtrue;
+  } else {
+    return Qfalse;
+  }
+}
+
+/*
+ * call-seq:
+ *     Map.delete(key) => old_value
+ *
+ * Deletes the value at the given key, if any, returning either the old value or
+ * nil if none was present. Throws an exception if the key is of the wrong type.
+ */
+VALUE Map_delete(VALUE _self, VALUE key) {
+  Map* self = ruby_to_Map(_self);
+
+  char keybuf[TABLE_KEY_BUF_LENGTH];
+  const char* keyval = NULL;
+  size_t length = 0;
+  table_key(self, key, keybuf, &keyval, &length);
+
+  upb_value v;
+  if (upb_strtable_remove2(&self->table, keyval, length, &v)) {
+    void* mem = value_memory(&v);
+    return native_slot_get(self->value_type, self->value_type_class, mem);
+  } else {
+    return Qnil;
+  }
+}
+
+/*
+ * call-seq:
+ *     Map.clear
+ *
+ * Removes all entries from the map.
+ */
+VALUE Map_clear(VALUE _self) {
+  Map* self = ruby_to_Map(_self);
+
+  // Uninit and reinit the table -- this is faster than iterating and doing a
+  // delete-lookup on each key.
+  upb_strtable_uninit(&self->table);
+  if (!upb_strtable_init(&self->table,
+                         upb_table_value_type(self->value_type))) {
+    rb_raise(rb_eRuntimeError, "Unable to re-initialize table");
+  }
+  return Qnil;
+}
+
+/*
+ * call-seq:
+ *     Map.length
+ *
+ * Returns the number of entries (key-value pairs) in the map.
+ */
+VALUE Map_length(VALUE _self) {
+  Map* self = ruby_to_Map(_self);
+  return INT2NUM(upb_strtable_count(&self->table));
+}
+
+static VALUE Map_new_this_type(VALUE _self) {
+  Map* self = ruby_to_Map(_self);
+  VALUE new_map = Qnil;
+  VALUE key_type = fieldtype_to_ruby(self->key_type);
+  VALUE value_type = fieldtype_to_ruby(self->value_type);
+  if (self->value_type_class != Qnil) {
+    new_map = rb_funcall(CLASS_OF(_self), rb_intern("new"), 3,
+                         key_type, value_type, self->value_type_class);
+  } else {
+    new_map = rb_funcall(CLASS_OF(_self), rb_intern("new"), 2,
+                         key_type, value_type);
+  }
+  return new_map;
+}
+
+/*
+ * call-seq:
+ *     Map.dup => new_map
+ *
+ * Duplicates this map with a shallow copy. References to all non-primitive
+ * element objects (e.g., submessages) are shared.
+ */
+VALUE Map_dup(VALUE _self) {
+  Map* self = ruby_to_Map(_self);
+  VALUE new_map = Map_new_this_type(_self);
+  Map* new_self = ruby_to_Map(new_map);
+
+  upb_strtable_iter it;
+  for (upb_strtable_begin(&it, &self->table);
+       !upb_strtable_done(&it);
+       upb_strtable_next(&it)) {
+
+    upb_value v = upb_strtable_iter_value(&it);
+    void* mem = value_memory(&v);
+    upb_value dup;
+    void* dup_mem = value_memory(&dup);
+    native_slot_dup(self->value_type, dup_mem, mem);
+
+    if (!upb_strtable_insert2(&new_self->table,
+                              upb_strtable_iter_key(&it),
+                              upb_strtable_iter_keylength(&it),
+                              dup)) {
+      rb_raise(rb_eRuntimeError, "Error inserting value into new table");
+    }
+  }
+
+  return new_map;
+}
+
+// Used by Google::Protobuf.deep_copy but not exposed directly.
+VALUE Map_deep_copy(VALUE _self) {
+  Map* self = ruby_to_Map(_self);
+  VALUE new_map = Map_new_this_type(_self);
+  Map* new_self = ruby_to_Map(new_map);
+
+  upb_strtable_iter it;
+  for (upb_strtable_begin(&it, &self->table);
+       !upb_strtable_done(&it);
+       upb_strtable_next(&it)) {
+
+    upb_value v = upb_strtable_iter_value(&it);
+    void* mem = value_memory(&v);
+    upb_value dup;
+    void* dup_mem = value_memory(&dup);
+    native_slot_deep_copy(self->value_type, dup_mem, mem);
+
+    if (!upb_strtable_insert2(&new_self->table,
+                              upb_strtable_iter_key(&it),
+                              upb_strtable_iter_keylength(&it),
+                              dup)) {
+      rb_raise(rb_eRuntimeError, "Error inserting value into new table");
+    }
+  }
+
+  return new_map;
+}
+
+/*
+ * call-seq:
+ *     Map.==(other) => boolean
+ *
+ * Compares this map to another. Maps are equal if they have identical key sets,
+ * and for each key, the values in both maps compare equal. Elements are
+ * compared as per normal Ruby semantics, by calling their :== methods (or
+ * performing a more efficient comparison for primitive types).
+ *
+ * Maps with dissimilar key types or value types/typeclasses are never equal,
+ * even if value comparison (for example, between integers and floats) would
+ * have otherwise indicated that every element has equal value.
+ */
+VALUE Map_eq(VALUE _self, VALUE _other) {
+  Map* self = ruby_to_Map(_self);
+
+  // Allow comparisons to Ruby hashmaps by converting to a temporary Map
+  // instance. Slow, but workable.
+  if (TYPE(_other) == T_HASH) {
+    VALUE other_map = Map_new_this_type(_self);
+    Map_merge_into_self(other_map, _other);
+    _other = other_map;
+  }
+
+  Map* other = ruby_to_Map(_other);
+
+  if (self == other) {
+    return Qtrue;
+  }
+  if (self->key_type != other->key_type ||
+      self->value_type != other->value_type ||
+      self->value_type_class != other->value_type_class) {
+    return Qfalse;
+  }
+  if (upb_strtable_count(&self->table) != upb_strtable_count(&other->table)) {
+    return Qfalse;
+  }
+
+  // For each member of self, check that an equal member exists at the same key
+  // in other.
+  upb_strtable_iter it;
+  for (upb_strtable_begin(&it, &self->table);
+       !upb_strtable_done(&it);
+       upb_strtable_next(&it)) {
+
+    upb_value v = upb_strtable_iter_value(&it);
+    void* mem = value_memory(&v);
+    upb_value other_v;
+    void* other_mem = value_memory(&other_v);
+
+    if (!upb_strtable_lookup2(&other->table,
+                              upb_strtable_iter_key(&it),
+                              upb_strtable_iter_keylength(&it),
+                              &other_v)) {
+      // Not present in other map.
+      return Qfalse;
+    }
+
+    if (!native_slot_eq(self->value_type, mem, other_mem)) {
+      // Present, but value not equal.
+      return Qfalse;
+    }
+  }
+
+  // For each member of other, check that a member exists at the same key in
+  // self. We don't need to compare values here -- if the key exists in both, we
+  // compared values above; if not, we already know that the maps are not equal.
+  for (upb_strtable_begin(&it, &other->table);
+       !upb_strtable_done(&it);
+       upb_strtable_next(&it)) {
+    upb_value v;
+    if (!upb_strtable_lookup2(&self->table,
+                              upb_strtable_iter_key(&it),
+                              upb_strtable_iter_keylength(&it),
+                              &v)) {
+      return Qfalse;
+    }
+  }
+
+  return Qtrue;
+}
+
+/*
+ * call-seq:
+ *     Map.hash => hash_value
+ *
+ * Returns a hash value based on this map's contents.
+ */
+VALUE Map_hash(VALUE _self) {
+  Map* self = ruby_to_Map(_self);
+
+  st_index_t h = rb_hash_start(0);
+  VALUE hash_sym = rb_intern("hash");
+
+  upb_strtable_iter it;
+  for (upb_strtable_begin(&it, &self->table);
+       !upb_strtable_done(&it);
+       upb_strtable_next(&it)) {
+    VALUE key = table_key_to_ruby(
+        self, upb_strtable_iter_key(&it), upb_strtable_iter_keylength(&it));
+
+    upb_value v = upb_strtable_iter_value(&it);
+    void* mem = value_memory(&v);
+    VALUE value = native_slot_get(self->value_type,
+                                  self->value_type_class,
+                                  mem);
+
+    h = rb_hash_uint(h, NUM2LONG(rb_funcall(key, hash_sym, 0)));
+    h = rb_hash_uint(h, NUM2LONG(rb_funcall(value, hash_sym, 0)));
+  }
+
+  return INT2FIX(h);
+}
+
+/*
+ * call-seq:
+ *     Map.inspect => string
+ *
+ * Returns a string representing this map's elements. It will be formatted as
+ * "{key => value, key => value, ...}", with each key and value string
+ * representation computed by its own #inspect method.
+ */
+VALUE Map_inspect(VALUE _self) {
+  Map* self = ruby_to_Map(_self);
+
+  VALUE str = rb_str_new2("{");
+
+  bool first = true;
+  VALUE inspect_sym = rb_intern("inspect");
+
+  upb_strtable_iter it;
+  for (upb_strtable_begin(&it, &self->table);
+       !upb_strtable_done(&it);
+       upb_strtable_next(&it)) {
+    VALUE key = table_key_to_ruby(
+        self, upb_strtable_iter_key(&it), upb_strtable_iter_keylength(&it));
+
+    upb_value v = upb_strtable_iter_value(&it);
+    void* mem = value_memory(&v);
+    VALUE value = native_slot_get(self->value_type,
+                                  self->value_type_class,
+                                  mem);
+
+    if (!first) {
+      str = rb_str_cat2(str, ", ");
+    } else {
+      first = false;
+    }
+    str = rb_str_append(str, rb_funcall(key, inspect_sym, 0));
+    str = rb_str_cat2(str, " => ");
+    str = rb_str_append(str, rb_funcall(value, inspect_sym, 0));
+  }
+
+  str = rb_str_cat2(str, "}");
+  return str;
+}
+
+/*
+ * call-seq:
+ *     Map.merge(other_map) => map
+ *
+ * Copies key/value pairs from other_map into a copy of this map. If a key is
+ * set in other_map and this map, the value from other_map overwrites the value
+ * in the new copy of this map. Returns the new copy of this map with merged
+ * contents.
+ */
+VALUE Map_merge(VALUE _self, VALUE hashmap) {
+  VALUE dupped = Map_dup(_self);
+  return Map_merge_into_self(dupped, hashmap);
+}
+
+static int merge_into_self_callback(VALUE key, VALUE value, VALUE self) {
+  Map_index_set(self, key, value);
+  return ST_CONTINUE;
+}
+
+// Used only internally -- shared by #merge and #initialize.
+VALUE Map_merge_into_self(VALUE _self, VALUE hashmap) {
+  if (TYPE(hashmap) == T_HASH) {
+    rb_hash_foreach(hashmap, merge_into_self_callback, _self);
+  } else if (RB_TYPE_P(hashmap, T_DATA) && RTYPEDDATA_P(hashmap) &&
+             RTYPEDDATA_TYPE(hashmap) == &Map_type) {
+
+    Map* self = ruby_to_Map(_self);
+    Map* other = ruby_to_Map(hashmap);
+
+    if (self->key_type != other->key_type ||
+        self->value_type != other->value_type ||
+        self->value_type_class != other->value_type_class) {
+      rb_raise(rb_eArgError, "Attempt to merge Map with mismatching types");
+    }
+
+    upb_strtable_iter it;
+    for (upb_strtable_begin(&it, &other->table);
+         !upb_strtable_done(&it);
+         upb_strtable_next(&it)) {
+
+      // Replace any existing value by issuing a 'remove' operation first.
+      upb_value oldv;
+      upb_strtable_remove2(&self->table,
+                           upb_strtable_iter_key(&it),
+                           upb_strtable_iter_keylength(&it),
+                           &oldv);
+
+      upb_value v = upb_strtable_iter_value(&it);
+      upb_strtable_insert2(&self->table,
+                           upb_strtable_iter_key(&it),
+                           upb_strtable_iter_keylength(&it),
+                           v);
+    }
+  } else {
+    rb_raise(rb_eArgError, "Unknown type merging into Map");
+  }
+  return _self;
+}
+
+// Internal method: map iterator initialization (used for serialization).
+void Map_begin(VALUE _self, Map_iter* iter) {
+  Map* self = ruby_to_Map(_self);
+  iter->self = self;
+  upb_strtable_begin(&iter->it, &self->table);
+}
+
+void Map_next(Map_iter* iter) {
+  upb_strtable_next(&iter->it);
+}
+
+bool Map_done(Map_iter* iter) {
+  return upb_strtable_done(&iter->it);
+}
+
+VALUE Map_iter_key(Map_iter* iter) {
+  return table_key_to_ruby(
+      iter->self,
+      upb_strtable_iter_key(&iter->it),
+      upb_strtable_iter_keylength(&iter->it));
+}
+
+VALUE Map_iter_value(Map_iter* iter) {
+  upb_value v = upb_strtable_iter_value(&iter->it);
+  void* mem = value_memory(&v);
+  return native_slot_get(iter->self->value_type,
+                         iter->self->value_type_class,
+                         mem);
+}
+
+void Map_register(VALUE module) {
+  VALUE klass = rb_define_class_under(module, "Map", rb_cObject);
+  rb_define_alloc_func(klass, Map_alloc);
+  cMap = klass;
+  rb_gc_register_address(&cMap);
+
+  rb_define_method(klass, "initialize", Map_init, -1);
+  rb_define_method(klass, "each", Map_each, 0);
+  rb_define_method(klass, "keys", Map_keys, 0);
+  rb_define_method(klass, "values", Map_values, 0);
+  rb_define_method(klass, "[]", Map_index, 1);
+  rb_define_method(klass, "[]=", Map_index_set, 2);
+  rb_define_method(klass, "has_key?", Map_has_key, 1);
+  rb_define_method(klass, "delete", Map_delete, 1);
+  rb_define_method(klass, "clear", Map_clear, 0);
+  rb_define_method(klass, "length", Map_length, 0);
+  rb_define_method(klass, "dup", Map_dup, 0);
+  rb_define_method(klass, "==", Map_eq, 1);
+  rb_define_method(klass, "hash", Map_hash, 0);
+  rb_define_method(klass, "inspect", Map_inspect, 0);
+  rb_define_method(klass, "merge", Map_merge, 1);
+  rb_include_module(klass, rb_mEnumerable);
+}
diff --git a/ruby/ext/google/protobuf_c/message.c b/ruby/ext/google/protobuf_c/message.c
index 105b7807..de38dd7b 100644
--- a/ruby/ext/google/protobuf_c/message.c
+++ b/ruby/ext/google/protobuf_c/message.c
@@ -139,7 +139,14 @@ int Message_initialize_kwarg(VALUE key, VALUE val, VALUE _self) {
              "Unknown field name in initialization map entry.");
   }
 
-  if (upb_fielddef_label(f) == UPB_LABEL_REPEATED) {
+  if (is_map_field(f)) {
+    if (TYPE(val) != T_HASH) {
+      rb_raise(rb_eArgError,
+               "Expected hashmap as initializer value for map field.");
+    }
+    VALUE map = layout_get(self->descriptor->layout, Message_data(self), f);
+    Map_merge_into_self(map, val);
+  } else if (upb_fielddef_label(f) == UPB_LABEL_REPEATED) {
     if (TYPE(val) != T_ARRAY) {
       rb_raise(rb_eArgError,
                "Expected array as initializer value for repeated field.");
@@ -450,13 +457,15 @@ VALUE build_module_from_enumdesc(EnumDescriptor* enumdesc) {
  * call-seq:
  *     Google::Protobuf.deep_copy(obj) => copy_of_obj
  *
- * Performs a deep copy of either a RepeatedField instance or a message object,
- * recursively copying its members.
+ * Performs a deep copy of a RepeatedField instance, a Map instance, or a
+ * message object, recursively copying its members.
  */
 VALUE Google_Protobuf_deep_copy(VALUE self, VALUE obj) {
   VALUE klass = CLASS_OF(obj);
   if (klass == cRepeatedField) {
     return RepeatedField_deep_copy(obj);
+  } else if (klass == cMap) {
+    return Map_deep_copy(obj);
   } else {
     return Message_deep_copy(obj);
   }
diff --git a/ruby/ext/google/protobuf_c/protobuf.c b/ruby/ext/google/protobuf_c/protobuf.c
index d5862284..30552705 100644
--- a/ruby/ext/google/protobuf_c/protobuf.c
+++ b/ruby/ext/google/protobuf_c/protobuf.c
@@ -82,6 +82,7 @@ void Init_protobuf_c() {
   EnumBuilderContext_register(internal);
   Builder_register(internal);
   RepeatedField_register(protobuf);
+  Map_register(protobuf);
 
   rb_define_singleton_method(protobuf, "encode", Google_Protobuf_encode, 1);
   rb_define_singleton_method(protobuf, "decode", Google_Protobuf_decode, 2);
diff --git a/ruby/ext/google/protobuf_c/protobuf.h b/ruby/ext/google/protobuf_c/protobuf.h
index c3a5d653..91a97a68 100644
--- a/ruby/ext/google/protobuf_c/protobuf.h
+++ b/ruby/ext/google/protobuf_c/protobuf.h
@@ -123,6 +123,7 @@ struct EnumDescriptor {
 
 struct MessageBuilderContext {
   VALUE descriptor;
+  VALUE builder;
 };
 
 struct EnumBuilderContext {
@@ -213,10 +214,13 @@ void MessageBuilderContext_free(void* _self);
 VALUE MessageBuilderContext_alloc(VALUE klass);
 void MessageBuilderContext_register(VALUE module);
 MessageBuilderContext* ruby_to_MessageBuilderContext(VALUE value);
-VALUE MessageBuilderContext_initialize(VALUE _self, VALUE descriptor);
+VALUE MessageBuilderContext_initialize(VALUE _self,
+                                       VALUE descriptor,
+                                       VALUE builder);
 VALUE MessageBuilderContext_optional(int argc, VALUE* argv, VALUE _self);
 VALUE MessageBuilderContext_required(int argc, VALUE* argv, VALUE _self);
 VALUE MessageBuilderContext_repeated(int argc, VALUE* argv, VALUE _self);
+VALUE MessageBuilderContext_map(int argc, VALUE* argv, VALUE _self);
 
 void EnumBuilderContext_mark(void* _self);
 void EnumBuilderContext_free(void* _self);
@@ -239,6 +243,8 @@ VALUE Builder_finalize_to_pool(VALUE _self, VALUE pool_rb);
 // Native slot storage abstraction.
 // -----------------------------------------------------------------------------
 
+#define NATIVE_SLOT_MAX_SIZE sizeof(void*)
+
 size_t native_slot_size(upb_fieldtype_t type);
 void native_slot_set(upb_fieldtype_t type,
                      VALUE type_class,
@@ -254,11 +260,18 @@ void native_slot_deep_copy(upb_fieldtype_t type, void* to, void* from);
 bool native_slot_eq(upb_fieldtype_t type, void* mem1, void* mem2);
 
 void native_slot_validate_string_encoding(upb_fieldtype_t type, VALUE value);
+void native_slot_check_int_range_precision(upb_fieldtype_t type, VALUE value);
 
 extern rb_encoding* kRubyStringUtf8Encoding;
 extern rb_encoding* kRubyStringASCIIEncoding;
 extern rb_encoding* kRubyString8bitEncoding;
 
+VALUE field_type_class(const upb_fielddef* field);
+
+bool is_map_field(const upb_fielddef* field);
+const upb_fielddef* map_field_key(const upb_fielddef* field);
+const upb_fielddef* map_field_value(const upb_fielddef* field);
+
 // -----------------------------------------------------------------------------
 // Repeated field container type.
 // -----------------------------------------------------------------------------
@@ -282,7 +295,6 @@ extern VALUE cRepeatedField;
 
 RepeatedField* ruby_to_RepeatedField(VALUE value);
 
-void RepeatedField_register(VALUE module);
 VALUE RepeatedField_each(VALUE _self);
 VALUE RepeatedField_index(VALUE _self, VALUE _index);
 void* RepeatedField_index_native(VALUE _self, int index);
@@ -302,6 +314,59 @@ VALUE RepeatedField_hash(VALUE _self);
 VALUE RepeatedField_inspect(VALUE _self);
 VALUE RepeatedField_plus(VALUE _self, VALUE list);
 
+// Defined in repeated_field.c; also used by Map.
+void validate_type_class(upb_fieldtype_t type, VALUE klass);
+
+// -----------------------------------------------------------------------------
+// Map container type.
+// -----------------------------------------------------------------------------
+
+typedef struct {
+  upb_fieldtype_t key_type;
+  upb_fieldtype_t value_type;
+  VALUE value_type_class;
+  upb_strtable table;
+} Map;
+
+void Map_mark(void* self);
+void Map_free(void* self);
+VALUE Map_alloc(VALUE klass);
+VALUE Map_init(int argc, VALUE* argv, VALUE self);
+void Map_register(VALUE module);
+
+extern const rb_data_type_t Map_type;
+extern VALUE cMap;
+
+Map* ruby_to_Map(VALUE value);
+
+VALUE Map_each(VALUE _self);
+VALUE Map_keys(VALUE _self);
+VALUE Map_values(VALUE _self);
+VALUE Map_index(VALUE _self, VALUE key);
+VALUE Map_index_set(VALUE _self, VALUE key, VALUE value);
+VALUE Map_has_key(VALUE _self, VALUE key);
+VALUE Map_delete(VALUE _self, VALUE key);
+VALUE Map_clear(VALUE _self);
+VALUE Map_length(VALUE _self);
+VALUE Map_dup(VALUE _self);
+VALUE Map_deep_copy(VALUE _self);
+VALUE Map_eq(VALUE _self, VALUE _other);
+VALUE Map_hash(VALUE _self);
+VALUE Map_inspect(VALUE _self);
+VALUE Map_merge(VALUE _self, VALUE hashmap);
+VALUE Map_merge_into_self(VALUE _self, VALUE hashmap);
+
+typedef struct {
+  Map* self;
+  upb_strtable_iter it;
+} Map_iter;
+
+void Map_begin(VALUE _self, Map_iter* iter);
+void Map_next(Map_iter* iter);
+bool Map_done(Map_iter* iter);
+VALUE Map_iter_key(Map_iter* iter);
+VALUE Map_iter_value(Map_iter* iter);
+
 // -----------------------------------------------------------------------------
 // Message layout / storage.
 // -----------------------------------------------------------------------------
diff --git a/ruby/ext/google/protobuf_c/repeated_field.c b/ruby/ext/google/protobuf_c/repeated_field.c
index 6bd13b07..6e3f0bc7 100644
--- a/ruby/ext/google/protobuf_c/repeated_field.c
+++ b/ruby/ext/google/protobuf_c/repeated_field.c
@@ -324,6 +324,10 @@ VALUE RepeatedField_deep_copy(VALUE _self) {
  * element types are equal, their lengths are equal, and each element is equal.
  * Elements are compared as per normal Ruby semantics, by calling their :==
  * methods (or performing a more efficient comparison for primitive types).
+ *
+ * Repeated fields with dissimilar element types are never equal, even if value
+ * comparison (for example, between integers and floats) would have otherwise
+ * indicated that every element has equal value.
  */
 VALUE RepeatedField_eq(VALUE _self, VALUE _other) {
   if (_self == _other) {
@@ -458,7 +462,7 @@ VALUE RepeatedField_plus(VALUE _self, VALUE list) {
   return dupped;
 }
 
-static void validate_type_class(upb_fieldtype_t type, VALUE klass) {
+void validate_type_class(upb_fieldtype_t type, VALUE klass) {
   if (rb_iv_get(klass, kDescriptorInstanceVar) == Qnil) {
     rb_raise(rb_eArgError,
              "Type class has no descriptor. Please pass a "
diff --git a/ruby/ext/google/protobuf_c/storage.c b/ruby/ext/google/protobuf_c/storage.c
index c4d801af..f20ddec2 100644
--- a/ruby/ext/google/protobuf_c/storage.c
+++ b/ruby/ext/google/protobuf_c/storage.c
@@ -57,7 +57,17 @@ size_t native_slot_size(upb_fieldtype_t type) {
   }
 }
 
-static void check_int_range_precision(upb_fieldtype_t type, VALUE val) {
+static bool is_ruby_num(VALUE value) {
+  return (TYPE(value) == T_FLOAT ||
+          TYPE(value) == T_FIXNUM ||
+          TYPE(value) == T_BIGNUM);
+}
+
+void native_slot_check_int_range_precision(upb_fieldtype_t type, VALUE val) {
+  if (!is_ruby_num(val)) {
+    rb_raise(rb_eTypeError, "Expected number type for integral field.");
+  }
+
   // NUM2{INT,UINT,LL,ULL} macros do the appropriate range checks on upper
   // bound; we just need to do precision checks (i.e., disallow rounding) and
   // check for < 0 on unsigned types.
@@ -76,12 +86,6 @@ static void check_int_range_precision(upb_fieldtype_t type, VALUE val) {
   }
 }
 
-static bool is_ruby_num(VALUE value) {
-  return (TYPE(value) == T_FLOAT ||
-          TYPE(value) == T_FIXNUM ||
-          TYPE(value) == T_BIGNUM);
-}
-
 void native_slot_validate_string_encoding(upb_fieldtype_t type, VALUE value) {
   bool bad_encoding = false;
   rb_encoding* string_encoding = rb_enc_from_index(ENCODING_GET(value));
@@ -156,14 +160,14 @@ void native_slot_set(upb_fieldtype_t type, VALUE type_class,
       int32_t int_val = 0;
       if (TYPE(value) == T_SYMBOL) {
         // Ensure that the given symbol exists in the enum module.
-        VALUE lookup = rb_const_get(type_class, SYM2ID(value));
+        VALUE lookup = rb_funcall(type_class, rb_intern("resolve"), 1, value);
         if (lookup == Qnil) {
           rb_raise(rb_eRangeError, "Unknown symbol value for enum field.");
         } else {
           int_val = NUM2INT(lookup);
         }
       } else {
-        check_int_range_precision(UPB_TYPE_INT32, value);
+        native_slot_check_int_range_precision(UPB_TYPE_INT32, value);
         int_val = NUM2INT(value);
       }
       DEREF(memory, int32_t) = int_val;
@@ -173,10 +177,7 @@ void native_slot_set(upb_fieldtype_t type, VALUE type_class,
     case UPB_TYPE_INT64:
     case UPB_TYPE_UINT32:
     case UPB_TYPE_UINT64:
-      if (!is_ruby_num(value)) {
-        rb_raise(rb_eTypeError, "Expected number type for integral field.");
-      }
-      check_int_range_precision(type, value);
+      native_slot_check_int_range_precision(type, value);
       switch (type) {
       case UPB_TYPE_INT32:
         DEREF(memory, int32_t) = NUM2INT(value);
@@ -246,8 +247,9 @@ void native_slot_init(upb_fieldtype_t type, void* memory) {
       break;
     case UPB_TYPE_STRING:
     case UPB_TYPE_BYTES:
-      // TODO(cfallin): set encoding appropriately
       DEREF(memory, VALUE) = rb_str_new2("");
+      rb_enc_associate(DEREF(memory, VALUE), (type == UPB_TYPE_BYTES) ?
+                       kRubyString8bitEncoding : kRubyStringUtf8Encoding);
       break;
     case UPB_TYPE_MESSAGE:
       DEREF(memory, VALUE) = Qnil;
@@ -322,6 +324,35 @@ bool native_slot_eq(upb_fieldtype_t type, void* mem1, void* mem2) {
 }
 
 // -----------------------------------------------------------------------------
+// Map field utilities.
+// -----------------------------------------------------------------------------
+
+bool is_map_field(const upb_fielddef* field) {
+  if (upb_fielddef_label(field) != UPB_LABEL_REPEATED ||
+      upb_fielddef_type(field) != UPB_TYPE_MESSAGE) {
+    return false;
+  }
+  const upb_msgdef* subdef = (const upb_msgdef*)upb_fielddef_subdef(field);
+  return upb_msgdef_mapentry(subdef);
+}
+
+const upb_fielddef* map_field_key(const upb_fielddef* field) {
+  assert(is_map_field(field));
+  const upb_msgdef* subdef = (const upb_msgdef*)upb_fielddef_subdef(field);
+  const upb_fielddef* key_field = upb_msgdef_itof(subdef, 1);
+  assert(key_field != NULL);
+  return key_field;
+}
+
+const upb_fielddef* map_field_value(const upb_fielddef* field) {
+  assert(is_map_field(field));
+  const upb_msgdef* subdef = (const upb_msgdef*)upb_fielddef_subdef(field);
+  const upb_fielddef* value_field = upb_msgdef_itof(subdef, 2);
+  assert(value_field != NULL);
+  return value_field;
+}
+
+// -----------------------------------------------------------------------------
 // Memory layout management.
 // -----------------------------------------------------------------------------
 
@@ -334,9 +365,12 @@ MessageLayout* create_layout(const upb_msgdef* msgdef) {
   size_t off = 0;
   for (upb_msg_begin(&it, msgdef); !upb_msg_done(&it); upb_msg_next(&it)) {
     const upb_fielddef* field = upb_msg_iter_field(&it);
-    size_t field_size =
-        (upb_fielddef_label(field) == UPB_LABEL_REPEATED) ?
-        sizeof(VALUE) : native_slot_size(upb_fielddef_type(field));
+    size_t field_size = 0;
+    if (upb_fielddef_label(field) == UPB_LABEL_REPEATED) {
+      field_size = sizeof(VALUE);
+    } else {
+      field_size = native_slot_size(upb_fielddef_type(field));
+    }
     // align current offset
     off = (off + field_size - 1) & ~(field_size - 1);
     layout->offsets[upb_fielddef_index(field)] = off;
@@ -357,7 +391,7 @@ void free_layout(MessageLayout* layout) {
   xfree(layout);
 }
 
-static VALUE get_type_class(const upb_fielddef* field) {
+VALUE field_type_class(const upb_fielddef* field) {
   VALUE type_class = Qnil;
   if (upb_fielddef_type(field) == UPB_TYPE_MESSAGE) {
     VALUE submsgdesc =
@@ -380,7 +414,7 @@ VALUE layout_get(MessageLayout* layout,
     return *((VALUE *)memory);
   } else {
     return native_slot_get(upb_fielddef_type(field),
-                           get_type_class(field),
+                           field_type_class(field),
                            memory);
   }
 }
@@ -398,9 +432,8 @@ static void check_repeated_field_type(VALUE val, const upb_fielddef* field) {
     rb_raise(rb_eTypeError, "Repeated field array has wrong element type");
   }
 
-  if (upb_fielddef_type(field) == UPB_TYPE_MESSAGE ||
-      upb_fielddef_type(field) == UPB_TYPE_ENUM) {
-    RepeatedField* self = ruby_to_RepeatedField(val);
+  if (self->field_type == UPB_TYPE_MESSAGE ||
+      self->field_type == UPB_TYPE_ENUM) {
     if (self->field_type_class !=
         get_def_obj(upb_fielddef_subdef(field))) {
       rb_raise(rb_eTypeError,
@@ -409,17 +442,48 @@ static void check_repeated_field_type(VALUE val, const upb_fielddef* field) {
   }
 }
 
+static void check_map_field_type(VALUE val, const upb_fielddef* field) {
+  assert(is_map_field(field));
+  const upb_fielddef* key_field = map_field_key(field);
+  const upb_fielddef* value_field = map_field_value(field);
+
+  if (!RB_TYPE_P(val, T_DATA) || !RTYPEDDATA_P(val) ||
+      RTYPEDDATA_TYPE(val) != &Map_type) {
+    rb_raise(rb_eTypeError, "Expected Map instance");
+  }
+
+  Map* self = ruby_to_Map(val);
+  if (self->key_type != upb_fielddef_type(key_field)) {
+    rb_raise(rb_eTypeError, "Map key type does not match field's key type");
+  }
+  if (self->value_type != upb_fielddef_type(value_field)) {
+    rb_raise(rb_eTypeError, "Map value type does not match field's value type");
+  }
+  if (upb_fielddef_type(value_field) == UPB_TYPE_MESSAGE ||
+      upb_fielddef_type(value_field) == UPB_TYPE_ENUM) {
+    if (self->value_type_class !=
+        get_def_obj(upb_fielddef_subdef(value_field))) {
+      rb_raise(rb_eTypeError,
+               "Map value type has wrong message/enum class");
+    }
+  }
+}
+
+
 void layout_set(MessageLayout* layout,
                 void* storage,
                 const upb_fielddef* field,
                 VALUE val) {
   void* memory = ((uint8_t *)storage) +
       layout->offsets[upb_fielddef_index(field)];
-  if (upb_fielddef_label(field) == UPB_LABEL_REPEATED) {
+  if (is_map_field(field)) {
+    check_map_field_type(val, field);
+    DEREF(memory, VALUE) = val;
+  } else if (upb_fielddef_label(field) == UPB_LABEL_REPEATED) {
     check_repeated_field_type(val, field);
-    *((VALUE *)memory) = val;
+    DEREF(memory, VALUE) = val;
   } else {
-    native_slot_set(upb_fielddef_type(field), get_type_class(field),
+    native_slot_set(upb_fielddef_type(field), field_type_class(field),
                     memory, val);
   }
 }
@@ -434,9 +498,34 @@ void layout_init(MessageLayout* layout,
     void* memory = ((uint8_t *)storage) +
         layout->offsets[upb_fielddef_index(field)];
 
-    if (upb_fielddef_label(field) == UPB_LABEL_REPEATED) {
+    if (is_map_field(field)) {
+      VALUE map = Qnil;
+
+      const upb_fielddef* key_field = map_field_key(field);
+      const upb_fielddef* value_field = map_field_value(field);
+      VALUE type_class = field_type_class(value_field);
+
+      if (type_class != Qnil) {
+        VALUE args[3] = {
+          fieldtype_to_ruby(upb_fielddef_type(key_field)),
+          fieldtype_to_ruby(upb_fielddef_type(value_field)),
+          type_class,
+        };
+        map = rb_class_new_instance(3, args, cMap);
+      } else {
+        VALUE args[2] = {
+          fieldtype_to_ruby(upb_fielddef_type(key_field)),
+          fieldtype_to_ruby(upb_fielddef_type(value_field)),
+        };
+        map = rb_class_new_instance(2, args, cMap);
+      }
+
+      DEREF(memory, VALUE) = map;
+    } else if (upb_fielddef_label(field) == UPB_LABEL_REPEATED) {
       VALUE ary = Qnil;
-      VALUE type_class = get_type_class(field);
+
+      VALUE type_class = field_type_class(field);
+
       if (type_class != Qnil) {
         VALUE args[2] = {
           fieldtype_to_ruby(upb_fielddef_type(field)),
@@ -447,7 +536,8 @@ void layout_init(MessageLayout* layout,
         VALUE args[1] = { fieldtype_to_ruby(upb_fielddef_type(field)) };
         ary = rb_class_new_instance(1, args, cRepeatedField);
       }
-      *((VALUE *)memory) = ary;
+
+      DEREF(memory, VALUE) = ary;
     } else {
       native_slot_init(upb_fielddef_type(field), memory);
     }
@@ -464,7 +554,7 @@ void layout_mark(MessageLayout* layout, void* storage) {
         layout->offsets[upb_fielddef_index(field)];
 
     if (upb_fielddef_label(field) == UPB_LABEL_REPEATED) {
-      rb_gc_mark(*((VALUE *)memory));
+      rb_gc_mark(DEREF(memory, VALUE));
     } else {
       native_slot_mark(upb_fielddef_type(field), memory);
     }
@@ -482,8 +572,10 @@ void layout_dup(MessageLayout* layout, void* to, void* from) {
     void* from_memory = ((uint8_t *)from) +
         layout->offsets[upb_fielddef_index(field)];
 
-    if (upb_fielddef_label(field) == UPB_LABEL_REPEATED) {
-      *((VALUE *)to_memory) = RepeatedField_dup(*((VALUE *)from_memory));
+    if (is_map_field(field)) {
+      DEREF(to_memory, VALUE) = Map_dup(DEREF(from_memory, VALUE));
+    } else if (upb_fielddef_label(field) == UPB_LABEL_REPEATED) {
+      DEREF(to_memory, VALUE) = RepeatedField_dup(DEREF(from_memory, VALUE));
     } else {
       native_slot_dup(upb_fielddef_type(field), to_memory, from_memory);
     }
@@ -501,8 +593,12 @@ void layout_deep_copy(MessageLayout* layout, void* to, void* from) {
     void* from_memory = ((uint8_t *)from) +
         layout->offsets[upb_fielddef_index(field)];
 
-    if (upb_fielddef_label(field) == UPB_LABEL_REPEATED) {
-      *((VALUE *)to_memory) = RepeatedField_deep_copy(*((VALUE *)from_memory));
+    if (is_map_field(field)) {
+      DEREF(to_memory, VALUE) =
+          Map_deep_copy(DEREF(from_memory, VALUE));
+    } else if (upb_fielddef_label(field) == UPB_LABEL_REPEATED) {
+      DEREF(to_memory, VALUE) =
+          RepeatedField_deep_copy(DEREF(from_memory, VALUE));
     } else {
       native_slot_deep_copy(upb_fielddef_type(field), to_memory, from_memory);
     }
@@ -520,11 +616,12 @@ VALUE layout_eq(MessageLayout* layout, void* msg1, void* msg2) {
     void* msg2_memory = ((uint8_t *)msg2) +
         layout->offsets[upb_fielddef_index(field)];
 
-    if (upb_fielddef_label(field) == UPB_LABEL_REPEATED) {
-      if (RepeatedField_eq(*((VALUE *)msg1_memory),
-                           *((VALUE *)msg2_memory)) == Qfalse) {
-        return Qfalse;
-      }
+    if (is_map_field(field)) {
+      return Map_eq(DEREF(msg1_memory, VALUE),
+                    DEREF(msg2_memory, VALUE));
+    } else if (upb_fielddef_label(field) == UPB_LABEL_REPEATED) {
+      return RepeatedField_eq(DEREF(msg1_memory, VALUE),
+                              DEREF(msg2_memory, VALUE));
     } else {
       if (!native_slot_eq(upb_fielddef_type(field),
                           msg1_memory, msg2_memory)) {
diff --git a/ruby/ext/google/protobuf_c/upb.c b/ruby/ext/google/protobuf_c/upb.c
index c9f47195..0015aad1 100644
--- a/ruby/ext/google/protobuf_c/upb.c
+++ b/ruby/ext/google/protobuf_c/upb.c
@@ -1269,6 +1269,7 @@ upb_msgdef *upb_msgdef_new(const void *owner) {
   if (!upb_def_init(UPB_UPCAST(m), UPB_DEF_MSG, &vtbl, owner)) goto err2;
   if (!upb_inttable_init(&m->itof, UPB_CTYPE_PTR)) goto err2;
   if (!upb_strtable_init(&m->ntof, UPB_CTYPE_PTR)) goto err1;
+  m->map_entry = false;
   return m;
 
 err1:
@@ -1283,6 +1284,7 @@ upb_msgdef *upb_msgdef_dup(const upb_msgdef *m, const void *owner) {
   if (!newm) return NULL;
   bool ok = upb_def_setfullname(UPB_UPCAST(newm),
                                 upb_def_fullname(UPB_UPCAST(m)), NULL);
+  newm->map_entry = m->map_entry;
   UPB_ASSERT_VAR(ok, ok);
   upb_msg_iter i;
   for(upb_msg_begin(&i, m); !upb_msg_done(&i); upb_msg_next(&i)) {
@@ -1386,6 +1388,15 @@ int upb_msgdef_numfields(const upb_msgdef *m) {
   return upb_strtable_count(&m->ntof);
 }
 
+void upb_msgdef_setmapentry(upb_msgdef *m, bool map_entry) {
+  assert(!upb_msgdef_isfrozen(m));
+  m->map_entry = map_entry;
+}
+
+bool upb_msgdef_mapentry(const upb_msgdef *m) {
+  return m->map_entry;
+}
+
 void upb_msg_begin(upb_msg_iter *iter, const upb_msgdef *m) {
   upb_inttable_begin(iter, &m->itof);
 }
@@ -3401,31 +3412,28 @@ int log2ceil(uint64_t v) {
 }
 
 char *upb_strdup(const char *s) {
-  size_t n = strlen(s) + 1;
+  return upb_strdup2(s, strlen(s));
+}
+
+char *upb_strdup2(const char *s, size_t len) {
+  // Always null-terminate, even if binary data; but don't rely on the input to
+  // have a null-terminating byte since it may be a raw binary buffer.
+  size_t n = len + 1;
   char *p = malloc(n);
-  if (p) memcpy(p, s, n);
+  if (p) memcpy(p, s, len);
+  p[len] = 0;
   return p;
 }
 
 // A type to represent the lookup key of either a strtable or an inttable.
-// This is like upb_tabkey, but can carry a size also to allow lookups of
-// non-NULL-terminated strings (we don't store string lengths in the table).
 typedef struct {
   upb_tabkey key;
-  uint32_t len;  // For string keys only.
 } lookupkey_t;
 
-static lookupkey_t strkey(const char *str) {
-  lookupkey_t k;
-  k.key.str = (char*)str;
-  k.len = strlen(str);
-  return k;
-}
-
 static lookupkey_t strkey2(const char *str, size_t len) {
   lookupkey_t k;
-  k.key.str = (char*)str;
-  k.len = len;
+  k.key.s.str = (char*)str;
+  k.key.s.length = len;
   return k;
 }
 
@@ -3607,11 +3615,12 @@ static size_t begin(const upb_table *t) {
 // A simple "subclass" of upb_table that only adds a hash function for strings.
 
 static uint32_t strhash(upb_tabkey key) {
-  return MurmurHash2(key.str, strlen(key.str), 0);
+  return MurmurHash2(key.s.str, key.s.length, 0);
 }
 
 static bool streql(upb_tabkey k1, lookupkey_t k2) {
-  return strncmp(k1.str, k2.key.str, k2.len) == 0 && k1.str[k2.len] == '\0';
+  return k1.s.length == k2.key.s.length &&
+         memcmp(k1.s.str, k2.key.s.str, k1.s.length) == 0;
 }
 
 bool upb_strtable_init(upb_strtable *t, upb_ctype_t ctype) {
@@ -3620,7 +3629,7 @@ bool upb_strtable_init(upb_strtable *t, upb_ctype_t ctype) {
 
 void upb_strtable_uninit(upb_strtable *t) {
   for (size_t i = 0; i < upb_table_size(&t->t); i++)
-    free((void*)t->t.entries[i].key.str);
+    free((void*)t->t.entries[i].key.s.str);
   uninit(&t->t);
 }
 
@@ -3631,26 +3640,30 @@ bool upb_strtable_resize(upb_strtable *t, size_t size_lg2) {
   upb_strtable_iter i;
   upb_strtable_begin(&i, t);
   for ( ; !upb_strtable_done(&i); upb_strtable_next(&i)) {
-    upb_strtable_insert(
-        &new_table, upb_strtable_iter_key(&i), upb_strtable_iter_value(&i));
+    upb_strtable_insert2(
+        &new_table,
+        upb_strtable_iter_key(&i),
+        upb_strtable_iter_keylength(&i),
+        upb_strtable_iter_value(&i));
   }
   upb_strtable_uninit(t);
   *t = new_table;
   return true;
 }
 
-bool upb_strtable_insert(upb_strtable *t, const char *k, upb_value v) {
+bool upb_strtable_insert2(upb_strtable *t, const char *k, size_t len,
+                          upb_value v) {
   if (isfull(&t->t)) {
     // Need to resize.  New table of double the size, add old elements to it.
     if (!upb_strtable_resize(t, t->t.size_lg2 + 1)) {
       return false;
     }
   }
-  if ((k = upb_strdup(k)) == NULL) return false;
+  if ((k = upb_strdup2(k, len)) == NULL) return false;
 
-  lookupkey_t key = strkey(k);
-  uint32_t hash = MurmurHash2(key.key.str, key.len, 0);
-  insert(&t->t, strkey(k), v, hash, &strhash, &streql);
+  lookupkey_t key = strkey2(k, len);
+  uint32_t hash = MurmurHash2(key.key.s.str, key.key.s.length, 0);
+  insert(&t->t, key, v, hash, &strhash, &streql);
   return true;
 }
 
@@ -3660,11 +3673,12 @@ bool upb_strtable_lookup2(const upb_strtable *t, const char *key, size_t len,
   return lookup(&t->t, strkey2(key, len), v, hash, &streql);
 }
 
-bool upb_strtable_remove(upb_strtable *t, const char *key, upb_value *val) {
+bool upb_strtable_remove2(upb_strtable *t, const char *key, size_t len,
+                         upb_value *val) {
   uint32_t hash = MurmurHash2(key, strlen(key), 0);
   upb_tabkey tabkey;
-  if (rm(&t->t, strkey(key), val, &tabkey, hash, &streql)) {
-    free((void*)tabkey.str);
+  if (rm(&t->t, strkey2(key, len), val, &tabkey, hash, &streql)) {
+    free((void*)tabkey.s.str);
     return true;
   } else {
     return false;
@@ -3693,7 +3707,12 @@ bool upb_strtable_done(const upb_strtable_iter *i) {
 
 const char *upb_strtable_iter_key(upb_strtable_iter *i) {
   assert(!upb_strtable_done(i));
-  return str_tabent(i)->key.str;
+  return str_tabent(i)->key.s.str;
+}
+
+size_t upb_strtable_iter_keylength(upb_strtable_iter *i) {
+  assert(!upb_strtable_done(i));
+  return str_tabent(i)->key.s.length;
 }
 
 upb_value upb_strtable_iter_value(const upb_strtable_iter *i) {
diff --git a/ruby/ext/google/protobuf_c/upb.h b/ruby/ext/google/protobuf_c/upb.h
index 150aef10..0e98bbab 100644
--- a/ruby/ext/google/protobuf_c/upb.h
+++ b/ruby/ext/google/protobuf_c/upb.h
@@ -600,6 +600,9 @@ typedef struct {
 
 // Like strdup(), which isn't always available since it's not ANSI C.
 char *upb_strdup(const char *s);
+// Variant that works with a length-delimited rather than NULL-delimited string,
+// as supported by strtable.
+char *upb_strdup2(const char *s, size_t len);
 
 UPB_INLINE void _upb_value_setval(upb_value *v, _upb_value val,
                                   upb_ctype_t ctype) {
@@ -654,12 +657,24 @@ FUNCS(fptr,     fptr,         upb_func*,    UPB_CTYPE_FPTR);
 
 typedef union {
   uintptr_t num;
-  const char *str;  // We own, nullz.
+  struct {
+    // We own this. NULL-terminated but may also contain binary data; see
+    // explicit length below.
+    // TODO: move the length to the start of the string in order to reduce
+    // tabkey's size (to one machine word) in a way that supports static
+    // initialization.
+    const char *str;
+    size_t length;
+  } s;
 } upb_tabkey;
 
 #define UPB_TABKEY_NUM(n) {n}
 #ifdef UPB_C99
-#define UPB_TABKEY_STR(s) {.str = s}
+// Given that |s| is a string literal, sizeof(s) gives us a
+// compile-time-constant strlen(). We must ensure that this works for static
+// data initializers.
+#define UPB_TABKEY_STR(strval) { .s = { .str = strval,                    \
+                                        .length = sizeof(strval) - 1 } }
 #endif
 // TODO(haberman): C++
 #define UPB_TABKEY_NONE {0}
@@ -765,7 +780,14 @@ UPB_INLINE size_t upb_strtable_count(const upb_strtable *t) {
 // If a table resize was required but memory allocation failed, false is
 // returned and the table is unchanged.
 bool upb_inttable_insert(upb_inttable *t, uintptr_t key, upb_value val);
-bool upb_strtable_insert(upb_strtable *t, const char *key, upb_value val);
+bool upb_strtable_insert2(upb_strtable *t, const char *key, size_t len,
+                          upb_value val);
+
+// For NULL-terminated strings.
+UPB_INLINE bool upb_strtable_insert(upb_strtable *t, const char *key,
+                                    upb_value val) {
+  return upb_strtable_insert2(t, key, strlen(key), val);
+}
 
 // Looks up key in this table, returning "true" if the key was found.
 // If v is non-NULL, copies the value for this key into *v.
@@ -782,7 +804,14 @@ UPB_INLINE bool upb_strtable_lookup(const upb_strtable *t, const char *key,
 // Removes an item from the table.  Returns true if the remove was successful,
 // and stores the removed item in *val if non-NULL.
 bool upb_inttable_remove(upb_inttable *t, uintptr_t key, upb_value *val);
-bool upb_strtable_remove(upb_strtable *t, const char *key, upb_value *val);
+bool upb_strtable_remove2(upb_strtable *t, const char *key, size_t len,
+                          upb_value *val);
+
+// For NULL-terminated strings.
+UPB_INLINE bool upb_strtable_remove(upb_strtable *t, const char *key,
+                                    upb_value *v) {
+  return upb_strtable_remove2(t, key, strlen(key), v);
+}
 
 // Updates an existing entry in an inttable.  If the entry does not exist,
 // returns false and does nothing.  Unlike insert/remove, this does not
@@ -876,6 +905,7 @@ void upb_strtable_begin(upb_strtable_iter *i, const upb_strtable *t);
 void upb_strtable_next(upb_strtable_iter *i);
 bool upb_strtable_done(const upb_strtable_iter *i);
 const char *upb_strtable_iter_key(upb_strtable_iter *i);
+size_t upb_strtable_iter_keylength(upb_strtable_iter *i);
 upb_value upb_strtable_iter_value(const upb_strtable_iter *i);
 void upb_strtable_iter_setdone(upb_strtable_iter *i);
 bool upb_strtable_iter_isequal(const upb_strtable_iter *i1,
@@ -1777,6 +1807,10 @@ UPB_DEFINE_DEF(upb::MessageDef, msgdef, MSG, UPB_QUOTE(
   // just be moved into symtab.c?
   MessageDef* Dup(const void* owner) const;
 
+  // Is this message a map entry?
+  void setmapentry(bool map_entry);
+  bool mapentry() const;
+
   // Iteration over fields.  The order is undefined.
   class iterator : public std::iterator<std::forward_iterator_tag, FieldDef*> {
    public:
@@ -1823,6 +1857,11 @@ UPB_DEFINE_STRUCT(upb_msgdef, upb_def,
   upb_inttable itof;  // int to field
   upb_strtable ntof;  // name to field
 
+  // Is this a map-entry message?
+  // TODO: set this flag properly for static descriptors; regenerate
+  // descriptor.upb.c.
+  bool map_entry;
+
   // TODO(haberman): proper extension ranges (there can be multiple).
 ));
 
@@ -1830,7 +1869,7 @@ UPB_DEFINE_STRUCT(upb_msgdef, upb_def,
                         refs, ref2s)                                          \
   {                                                                           \
     UPB_DEF_INIT(name, UPB_DEF_MSG, refs, ref2s), selector_count,             \
-        submsg_field_count, itof, ntof                                        \
+        submsg_field_count, itof, ntof, false                                 \
   }
 
 UPB_BEGIN_EXTERN_C  // {
@@ -1878,6 +1917,9 @@ UPB_INLINE upb_fielddef *upb_msgdef_ntof_mutable(upb_msgdef *m,
   return (upb_fielddef *)upb_msgdef_ntof(m, name, len);
 }
 
+void upb_msgdef_setmapentry(upb_msgdef *m, bool map_entry);
+bool upb_msgdef_mapentry(const upb_msgdef *m);
+
 // upb_msg_iter i;
 // for(upb_msg_begin(&i, m); !upb_msg_done(&i); upb_msg_next(&i)) {
 //   upb_fielddef *f = upb_msg_iter_field(&i);
@@ -2331,6 +2373,12 @@ inline const FieldDef *MessageDef::FindFieldByName(const char *name,
 inline MessageDef* MessageDef::Dup(const void *owner) const {
   return upb_msgdef_dup(this, owner);
 }
+inline void MessageDef::setmapentry(bool map_entry) {
+  upb_msgdef_setmapentry(this, map_entry);
+}
+inline bool MessageDef::mapentry() const {
+  return upb_msgdef_mapentry(this);
+}
 inline MessageDef::iterator MessageDef::begin() { return iterator(this); }
 inline MessageDef::iterator MessageDef::end() { return iterator::end(this); }
 inline MessageDef::const_iterator MessageDef::begin() const {
diff --git a/ruby/tests/basic.rb b/ruby/tests/basic.rb
index 05b3a0fa..2620fa9b 100644
--- a/ruby/tests/basic.rb
+++ b/ruby/tests/basic.rb
@@ -36,23 +36,43 @@ module BasicTest
     add_message "TestMessage2" do
       optional :foo, :int32, 1
     end
+
     add_message "Recursive1" do
       optional :foo, :message, 1, "Recursive2"
     end
     add_message "Recursive2" do
       optional :foo, :message, 1, "Recursive1"
     end
+
     add_enum "TestEnum" do
       value :Default, 0
       value :A, 1
       value :B, 2
       value :C, 3
     end
+
     add_message "BadFieldNames" do
       optional :dup, :int32, 1
       optional :class, :int32, 2
       optional :"a.b", :int32, 3
     end
+
+    add_message "MapMessage" do
+      map :map_string_int32, :string, :int32, 1
+      map :map_string_msg, :string, :message, 2, "TestMessage2"
+    end
+    add_message "MapMessageWireEquiv" do
+      repeated :map_string_int32, :message, 1, "MapMessageWireEquiv_entry1"
+      repeated :map_string_msg, :message, 2, "MapMessageWireEquiv_entry2"
+    end
+    add_message "MapMessageWireEquiv_entry1" do
+      optional :key, :string, 1
+      optional :value, :int32, 2
+    end
+    add_message "MapMessageWireEquiv_entry2" do
+      optional :key, :string, 1
+      optional :value, :message, 2, "TestMessage2"
+    end
   end
 
   TestMessage = pool.lookup("TestMessage").msgclass
@@ -61,6 +81,12 @@ module BasicTest
   Recursive2 = pool.lookup("Recursive2").msgclass
   TestEnum = pool.lookup("TestEnum").enummodule
   BadFieldNames = pool.lookup("BadFieldNames").msgclass
+  MapMessage = pool.lookup("MapMessage").msgclass
+  MapMessageWireEquiv = pool.lookup("MapMessageWireEquiv").msgclass
+  MapMessageWireEquiv_entry1 =
+    pool.lookup("MapMessageWireEquiv_entry1").msgclass
+  MapMessageWireEquiv_entry2 =
+    pool.lookup("MapMessageWireEquiv_entry2").msgclass
 
 # ------------ test cases ---------------
 
@@ -300,7 +326,7 @@ module BasicTest
       l.push :B
       l.push :C
       assert l.count == 3
-      assert_raise NameError do
+      assert_raise RangeError do
         l.push :D
       end
       assert l[0] == :A
@@ -324,12 +350,240 @@ module BasicTest
       end
     end
 
+    def test_map_basic
+      # allowed key types:
+      # :int32, :int64, :uint32, :uint64, :bool, :string, :bytes.
+
+      m = Google::Protobuf::Map.new(:string, :int32)
+      m["asdf"] = 1
+      assert m["asdf"] == 1
+      m["jkl;"] = 42
+      assert m == { "jkl;" => 42, "asdf" => 1 }
+      assert m.has_key?("asdf")
+      assert !m.has_key?("qwerty")
+      assert m.length == 2
+
+      m2 = m.dup
+      assert m == m2
+      assert m.hash != 0
+      assert m.hash == m2.hash
+
+      collected = {}
+      m.each { |k,v| collected[v] = k }
+      assert collected == { 42 => "jkl;", 1 => "asdf" }
+
+      assert m.delete("asdf") == 1
+      assert !m.has_key?("asdf")
+      assert m["asdf"] == nil
+      assert !m.has_key?("asdf")
+
+      # We only assert on inspect value when there is one map entry because the
+      # order in which elements appear is unspecified (depends on the internal
+      # hash function). We don't want a brittle test.
+      assert m.inspect == "{\"jkl;\" => 42}"
+
+      assert m.keys == ["jkl;"]
+      assert m.values == [42]
+
+      m.clear
+      assert m.length == 0
+      assert m == {}
+
+      assert_raise TypeError do
+        m[1] = 1
+      end
+      assert_raise RangeError do
+        m["asdf"] = 0x1_0000_0000
+      end
+    end
+
+    def test_map_ctor
+      m = Google::Protobuf::Map.new(:string, :int32,
+                                    {"a" => 1, "b" => 2, "c" => 3})
+      assert m == {"a" => 1, "c" => 3, "b" => 2}
+    end
+
+    def test_map_keytypes
+      m = Google::Protobuf::Map.new(:int32, :int32)
+      m[1] = 42
+      m[-1] = 42
+      assert_raise RangeError do
+        m[0x8000_0000] = 1
+      end
+      assert_raise TypeError do
+        m["asdf"] = 1
+      end
+
+      m = Google::Protobuf::Map.new(:int64, :int32)
+      m[0x1000_0000_0000_0000] = 1
+      assert_raise RangeError do
+        m[0x1_0000_0000_0000_0000] = 1
+      end
+      assert_raise TypeError do
+        m["asdf"] = 1
+      end
+
+      m = Google::Protobuf::Map.new(:uint32, :int32)
+      m[0x8000_0000] = 1
+      assert_raise RangeError do
+        m[0x1_0000_0000] = 1
+      end
+      assert_raise RangeError do
+        m[-1] = 1
+      end
+
+      m = Google::Protobuf::Map.new(:uint64, :int32)
+      m[0x8000_0000_0000_0000] = 1
+      assert_raise RangeError do
+        m[0x1_0000_0000_0000_0000] = 1
+      end
+      assert_raise RangeError do
+        m[-1] = 1
+      end
+
+      m = Google::Protobuf::Map.new(:bool, :int32)
+      m[true] = 1
+      m[false] = 2
+      assert_raise TypeError do
+        m[1] = 1
+      end
+      assert_raise TypeError do
+        m["asdf"] = 1
+      end
+
+      m = Google::Protobuf::Map.new(:string, :int32)
+      m["asdf"] = 1
+      assert_raise TypeError do
+        m[1] = 1
+      end
+      assert_raise TypeError do
+        bytestring = ["FFFF"].pack("H*")
+        m[bytestring] = 1
+      end
+
+      m = Google::Protobuf::Map.new(:bytes, :int32)
+      bytestring = ["FFFF"].pack("H*")
+      m[bytestring] = 1
+      assert_raise TypeError do
+        m["asdf"] = 1
+      end
+      assert_raise TypeError do
+        m[1] = 1
+      end
+    end
+
+    def test_map_msg_enum_valuetypes
+      m = Google::Protobuf::Map.new(:string, :message, TestMessage)
+      m["asdf"] = TestMessage.new
+      assert_raise TypeError do
+        m["jkl;"] = TestMessage2.new
+      end
+
+      m = Google::Protobuf::Map.new(:string, :message, TestMessage,
+                                    { "a" => TestMessage.new(:optional_int32 => 42),
+                                      "b" => TestMessage.new(:optional_int32 => 84) })
+      assert m.length == 2
+      assert m.values.map{|msg| msg.optional_int32}.sort == [42, 84]
+
+      m = Google::Protobuf::Map.new(:string, :enum, TestEnum,
+                                    { "x" => :A, "y" => :B, "z" => :C })
+      assert m.length == 3
+      assert m["z"] == :C
+      m["z"] = 2
+      assert m["z"] == :B
+      m["z"] = 4
+      assert m["z"] == 4
+      assert_raise RangeError do
+        m["z"] = :Z
+      end
+      assert_raise TypeError do
+        m["z"] = "z"
+      end
+    end
+
+    def test_map_dup_deep_copy
+      m = Google::Protobuf::Map.new(:string, :message, TestMessage,
+                                    { "a" => TestMessage.new(:optional_int32 => 42),
+                                      "b" => TestMessage.new(:optional_int32 => 84) })
+
+      m2 = m.dup
+      assert m == m2
+      assert m.object_id != m2.object_id
+      assert m["a"].object_id == m2["a"].object_id
+      assert m["b"].object_id == m2["b"].object_id
+
+      m2 = Google::Protobuf.deep_copy(m)
+      assert m == m2
+      assert m.object_id != m2.object_id
+      assert m["a"].object_id != m2["a"].object_id
+      assert m["b"].object_id != m2["b"].object_id
+    end
+
+    def test_map_field
+      m = MapMessage.new
+      assert m.map_string_int32 == {}
+      assert m.map_string_msg == {}
+
+      m = MapMessage.new(:map_string_int32 => {"a" => 1, "b" => 2},
+                         :map_string_msg => {"a" => TestMessage2.new(:foo => 1),
+                                             "b" => TestMessage2.new(:foo => 2)})
+      assert m.map_string_int32.keys.sort == ["a", "b"]
+      assert m.map_string_int32["a"] == 1
+      assert m.map_string_msg["b"].foo == 2
+
+      m.map_string_int32["c"] = 3
+      assert m.map_string_int32["c"] == 3
+      m.map_string_msg["c"] = TestMessage2.new(:foo => 3)
+      assert m.map_string_msg["c"] == TestMessage2.new(:foo => 3)
+      m.map_string_msg.delete("b")
+      m.map_string_msg.delete("c")
+      assert m.map_string_msg == { "a" => TestMessage2.new(:foo => 1) }
+
+      assert_raise TypeError do
+        m.map_string_msg["e"] = TestMessage.new # wrong value type
+      end
+      # ensure nothing was added by the above
+      assert m.map_string_msg == { "a" => TestMessage2.new(:foo => 1) }
+
+      m.map_string_int32 = Google::Protobuf::Map.new(:string, :int32)
+      assert_raise TypeError do
+        m.map_string_int32 = Google::Protobuf::Map.new(:string, :int64)
+      end
+      assert_raise TypeError do
+        m.map_string_int32 = {}
+      end
+
+      assert_raise TypeError do
+        m = MapMessage.new(:map_string_int32 => { 1 => "I am not a number" })
+      end
+    end
+
+    def test_map_encode_decode
+      m = MapMessage.new(:map_string_int32 => {"a" => 1, "b" => 2},
+                         :map_string_msg => {"a" => TestMessage2.new(:foo => 1),
+                                             "b" => TestMessage2.new(:foo => 2)})
+      m2 = MapMessage.decode(MapMessage.encode(m))
+      assert m == m2
+
+      m3 = MapMessageWireEquiv.decode(MapMessage.encode(m))
+      assert m3.map_string_int32.length == 2
+
+      kv = {}
+      m3.map_string_int32.map { |msg| kv[msg.key] = msg.value }
+      assert kv == {"a" => 1, "b" => 2}
+
+      kv = {}
+      m3.map_string_msg.map { |msg| kv[msg.key] = msg.value }
+      assert kv == {"a" => TestMessage2.new(:foo => 1),
+                    "b" => TestMessage2.new(:foo => 2)}
+    end
+
     def test_enum_field
       m = TestMessage.new
       assert m.optional_enum == :Default
       m.optional_enum = :A
       assert m.optional_enum == :A
-      assert_raise NameError do
+      assert_raise RangeError do
         m.optional_enum = :ASDF
       end
       m.optional_enum = 1