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diff --git a/Firestore/core/src/firebase/firestore/nanopb/writer.h b/Firestore/core/src/firebase/firestore/nanopb/writer.h
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+/*
+ * Copyright 2018 Google
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_NANOPB_WRITER_H_
+#define FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_NANOPB_WRITER_H_
+
+#include <pb.h>
+#include <pb_encode.h>
+
+#include <cstdint>
+#include <functional>
+#include <string>
+#include <vector>
+
+#include "Firestore/core/src/firebase/firestore/nanopb/tag.h"
+#include "Firestore/core/src/firebase/firestore/util/status.h"
+
+namespace firebase {
+namespace firestore {
+namespace nanopb {
+
+/**
+ * Docs TODO(rsgowman). But currently, this just wraps the underlying nanopb
+ * pb_ostream_t. Also doc how to check status.
+ */
+class Writer {
+ public:
+  /**
+   * Creates an output stream that writes to the specified vector. Note that
+   * this vector pointer must remain valid for the lifetime of this Writer.
+   *
+   * (This is roughly equivalent to the nanopb function
+   * pb_ostream_from_buffer())
+   *
+   * @param out_bytes where the output should be serialized to.
+   */
+  static Writer Wrap(std::vector<std::uint8_t>* out_bytes);
+
+  /**
+   * Creates a non-writing output stream used to calculate the size of
+   * the serialized output.
+   */
+  static Writer Sizing() {
+    return Writer(PB_OSTREAM_SIZING);
+  }
+
+  /**
+   * Writes a message type to the output stream.
+   *
+   * This essentially wraps calls to nanopb's pb_encode_tag() method.
+   */
+  void WriteTag(Tag tag);
+
+  /**
+   * Writes a nanopb message to the output stream.
+   *
+   * This essentially wraps calls to nanopb's `pb_encode()` method. If we didn't
+   * use `oneof`s in our protos, this would be the primary way of encoding
+   * messages.
+   */
+  void WriteNanopbMessage(const pb_field_t fields[], const void* src_struct);
+
+  void WriteSize(size_t size);
+  void WriteNull();
+  void WriteBool(bool bool_value);
+  void WriteInteger(std::int64_t integer_value);
+
+  void WriteString(const std::string& string_value);
+
+  /**
+   * Writes a message and its length.
+   *
+   * When writing a top level message, protobuf doesn't include the length
+   * (since you can get that already from the length of the binary output.) But
+   * when writing a sub/nested message, you must include the length in the
+   * serialization.
+   *
+   * Call this method when writing a nested message. Provide a function to
+   * write the message itself. This method will calculate the size of the
+   * written message (using the provided function with a non-writing sizing
+   * stream), write out the size (and perform sanity checks), and then serialize
+   * the message by calling the provided function a second time.
+   */
+  void WriteNestedMessage(const std::function<void(Writer*)>& write_message_fn);
+
+  size_t bytes_written() const {
+    return stream_.bytes_written;
+  }
+
+  util::Status status() const {
+    return status_;
+  }
+
+ private:
+  util::Status status_ = util::Status::OK();
+
+  /**
+   * Creates a new Writer, based on the given nanopb pb_ostream_t. Note that
+   * a shallow copy will be taken. (Non-null pointers within this struct must
+   * remain valid for the lifetime of this Writer.)
+   */
+  explicit Writer(const pb_ostream_t& stream) : stream_(stream) {
+  }
+
+  /**
+   * Writes a "varint" to the output stream.
+   *
+   * This essentially wraps calls to nanopb's pb_encode_varint() method.
+   *
+   * Note that (despite the value parameter type) this works for bool, enum,
+   * int32, int64, uint32 and uint64 proto field types.
+   *
+   * Note: This is not expected to be called directly, but rather only
+   * via the other Write* methods (i.e. WriteBool, WriteLong, etc)
+   *
+   * @param value The value to write, represented as a uint64_t.
+   */
+  void WriteVarint(std::uint64_t value);
+
+  pb_ostream_t stream_;
+};
+
+}  // namespace nanopb
+}  // namespace firestore
+}  // namespace firebase
+
+#endif  // FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_NANOPB_WRITER_H_