Merge pull request #635 from firebase/mikelehen/merge-master-to-firestore-api-changes

Merge master to firestore api changes

11 files changed, 737 insertions, 2 deletions

diff --git a/Firestore/core/src/firebase/firestore/base/port.h b/Firestore/core/src/firebase/firestore/base/port.h
index 37d1041..5e3959d 100644
--- a/Firestore/core/src/firebase/firestore/base/port.h
+++ b/Firestore/core/src/firebase/firestore/base/port.h
@@ -18,7 +18,7 @@
 #define FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_BASE_PORT_H_
 
 #if defined(__APPLE__)
-// On Apple platforms we support building via Cocoapods without CMake. When
+// On Apple platforms we support building via CocoaPods without CMake. When
 // building this way we can't test the presence of features so predefine all
 // the platform-support feature macros to their expected values.
 
diff --git a/Firestore/core/src/firebase/firestore/util/CMakeLists.txt b/Firestore/core/src/firebase/firestore/util/CMakeLists.txt
index ce81363..3028a95 100644
--- a/Firestore/core/src/firebase/firestore/util/CMakeLists.txt
+++ b/Firestore/core/src/firebase/firestore/util/CMakeLists.txt
@@ -12,8 +12,73 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+# firebase_firestore_util is the interface of this module. The rest of the
+# libraries in here are an implementation detail of making this a
+# mutli-platform build.
+
+add_library(
+  firebase_firestore_util_base
+  secure_random_arc4random.cc
+  string_printf.cc
+)
+target_link_libraries(
+  firebase_firestore_util_base
+  PUBLIC
+  absl_base
+)
+
+# stdio-dependent bits can be built and tested everywhere
+add_library(
+  firebase_firestore_util_stdio
+  assert_stdio.cc
+  log_stdio.cc
+)
+target_link_libraries(
+  firebase_firestore_util_stdio
+  PUBLIC
+  firebase_firestore_util_base
+)
+
+# apple-dependent bits can only built and tested on apple plaforms
+if(APPLE)
+  add_library(
+    firebase_firestore_util_apple
+    assert_apple.mm
+    log_apple.mm
+  )
+  target_compile_options(
+    firebase_firestore_util_apple
+    PRIVATE
+    ${OBJC_FLAGS}
+  )
+  target_link_libraries(
+    firebase_firestore_util_apple
+    PUBLIC
+    FirebaseCore
+  )
+endif(APPLE)
+
 add_library(
   firebase_firestore_util
   autoid.cc
-  secure_random_arc4random.cc
 )
+
+# Export a dependency on the correct logging library for this platform. All
+# buildable libraries are built and tested but only the best fit is exported.
+if(APPLE)
+  target_link_libraries(
+    firebase_firestore_util
+    PUBLIC
+    firebase_firestore_util_apple
+    firebase_firestore_util_base
+  )
+
+else(NOT APPLE)
+  target_link_libraries(
+    firebase_firestore_util
+    PUBLIC
+    firebase_firestore_util_stdio
+    firebase_firestore_util_base
+  )
+
+endif(APPLE)
diff --git a/Firestore/core/src/firebase/firestore/util/assert_apple.mm b/Firestore/core/src/firebase/firestore/util/assert_apple.mm
new file mode 100644
index 0000000..0447d6c
--- /dev/null
+++ b/Firestore/core/src/firebase/firestore/util/assert_apple.mm
@@ -0,0 +1,44 @@
+/*
+ * 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.
+ */
+
+#include "Firestore/core/src/firebase/firestore/util/firebase_assert.h"
+
+#import <Foundation/Foundation.h>
+
+#include <string.h>
+
+#include "Firestore/core/src/firebase/firestore/util/string_apple.h"
+
+namespace firebase {
+namespace firestore {
+namespace util {
+
+void FailAssert(const char* file, const char* func, const int line, const char* format, ...) {
+  va_list args;
+  va_start(args, format);
+  NSString *description = [[NSString alloc] initWithFormat:WrapNSStringNoCopy(format) arguments:args];
+  va_end(args);
+  [[NSAssertionHandler currentHandler]
+      handleFailureInFunction:WrapNSStringNoCopy(func)
+                         file:WrapNSStringNoCopy(file)
+                   lineNumber:line
+     description:@"FIRESTORE INTERNAL ASSERTION FAILED: %@", description];
+  abort();
+}
+
+}  // namespace util
+}  // namespace firestore
+}  // namespace firebase
diff --git a/Firestore/core/src/firebase/firestore/util/assert_stdio.cc b/Firestore/core/src/firebase/firestore/util/assert_stdio.cc
new file mode 100644
index 0000000..b5d0b7c
--- /dev/null
+++ b/Firestore/core/src/firebase/firestore/util/assert_stdio.cc
@@ -0,0 +1,53 @@
+/*
+ * 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.
+ */
+
+#include "Firestore/core/src/firebase/firestore/util/firebase_assert.h"
+
+#include <stdarg.h>
+
+#include <exception>
+#include <string>
+
+#include <absl/base/config.h>
+
+#include "Firestore/core/src/firebase/firestore/util/string_printf.h"
+
+namespace firebase {
+namespace firestore {
+namespace util {
+
+void FailAssert(const char* file, const char* func, const int line,
+                const char* format, ...) {
+  std::string message;
+  StringAppendF(&message, "ASSERT: %s(%d) %s: ", file, line, func);
+
+  va_list args;
+  va_start(args, format);
+  StringAppendV(&message, format, args);
+  va_end(args);
+
+#if ABSL_HAVE_EXCEPTIONS
+  throw std::logic_error(message);
+
+#else
+  fprintf(stderr, "%s\n", message.c_str());
+  std::terminate();
+#endif
+}
+
+}  // namespace util
+}  // namespace firestore
+}  // namespace firebase
diff --git a/Firestore/core/src/firebase/firestore/util/firebase_assert.h b/Firestore/core/src/firebase/firestore/util/firebase_assert.h
new file mode 100644
index 0000000..9f1bce8
--- /dev/null
+++ b/Firestore/core/src/firebase/firestore/util/firebase_assert.h
@@ -0,0 +1,102 @@
+/*
+ * 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.
+ */
+
+// To avoid naming-collision, this header is called firebase_assert.h instead
+// of assert.h.
+
+#ifndef FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_FIREBASE_ASSERT_H_
+#define FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_FIREBASE_ASSERT_H_
+
+#include <stdlib.h>
+
+#include "Firestore/core/src/firebase/firestore/util/log.h"
+
+#define FIREBASE_EXPAND_STRINGIFY_(X) #X
+#define FIREBASE_EXPAND_STRINGIFY(X) FIREBASE_EXPAND_STRINGIFY_(X)
+
+// FIREBASE_ASSERT_* macros are not compiled out of release builds. They should
+// be used for assertions that need to be propagated to end-users of SDKs.
+// FIREBASE_DEV_ASSERT_* macros are compiled out of release builds, similar to
+// the C assert() macro. They should be used for internal assertions that are
+// only shown to SDK developers.
+
+// Assert condition is true, if it's false log an assert with the specified
+// expression as a string.
+#define FIREBASE_ASSERT_WITH_EXPRESSION(condition, expression)               \
+  do {                                                                       \
+    if (!(condition)) {                                                      \
+      firebase::firestore::util::FailAssert(                                 \
+          __FILE__, __PRETTY_FUNCTION__, __LINE__,                           \
+          FIREBASE_EXPAND_STRINGIFY(expression));                            \
+    }                                                                        \
+  } while(0)
+
+// Assert condition is true, if it's false log an assert with the specified
+// expression as a string. Compiled out of release builds.
+#if defined(NDEBUG)
+#define FIREBASE_DEV_ASSERT_WITH_EXPRESSION(condition, expression) \
+  { (void)(condition); }
+#else
+#define FIREBASE_DEV_ASSERT_WITH_EXPRESSION(condition, expression) \
+  FIREBASE_ASSERT_WITH_EXPRESSION(condition, expression)
+#endif  // !defined(NDEBUG)
+
+// Custom assert() implementation that is not compiled out in release builds.
+#define FIREBASE_ASSERT(expression) \
+  FIREBASE_ASSERT_WITH_EXPRESSION(expression, expression)
+
+// Custom assert() implementation that is compiled out in release builds.
+// Compiled out of release builds.
+#define FIREBASE_DEV_ASSERT(expression) \
+  FIREBASE_DEV_ASSERT_WITH_EXPRESSION(expression, expression)
+
+// Assert condition is true otherwise display the specified expression,
+// message and abort.
+#define FIREBASE_ASSERT_MESSAGE_WITH_EXPRESSION(condition, expression, ...)  \
+  do {                                                                       \
+    if (!(condition)) {                                                      \
+      firebase::firestore::util::LogError(                                   \
+          FIREBASE_EXPAND_STRINGIFY(expression));                            \
+      firebase::firestore::util::FailAssert(                                 \
+          __FILE__, __PRETTY_FUNCTION__, __LINE__, __VA_ARGS__);             \
+    }                                                                        \
+  } while(0)
+
+// Assert condition is true otherwise display the specified expression,
+// message and abort. Compiled out of release builds.
+#if defined(NDEBUG)
+#define FIREBASE_DEV_ASSERT_MESSAGE_WITH_EXPRESSION(condition, expression, \
+                                                    ...)                   \
+  { (void)(condition); }
+#else
+#define FIREBASE_DEV_ASSERT_MESSAGE_WITH_EXPRESSION(condition, expression, \
+                                                    ...)                   \
+  FIREBASE_ASSERT_MESSAGE_WITH_EXPRESSION(condition, expression, __VA_ARGS__)
+#endif  // !defined(NDEBUG)
+
+namespace firebase {
+namespace firestore {
+namespace util {
+
+// A no-return helper function. To raise an assertion, use Macro instead.
+void FailAssert(const char* file, const char* func, const int line,
+                const char* format, ...);
+
+}  // namespace util
+}  // namespace firestore
+}  // namespace firebase
+
+#endif  // FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_FIREBASE_ASSERT_H_
diff --git a/Firestore/core/src/firebase/firestore/util/log.h b/Firestore/core/src/firebase/firestore/util/log.h
new file mode 100644
index 0000000..d0cff4d
--- /dev/null
+++ b/Firestore/core/src/firebase/firestore/util/log.h
@@ -0,0 +1,63 @@
+/*
+ * Copyright 2017 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_UTIL_LOG_H_
+#define FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_LOG_H_
+
+#include <stdarg.h>
+
+namespace firebase {
+namespace firestore {
+namespace util {
+
+/// @brief Levels used when logging messages.
+enum LogLevel {
+  /// Verbose Log Level
+  kLogLevelVerbose = 0,
+  /// Debug Log Level
+  kLogLevelDebug,
+  /// Info Log Level
+  kLogLevelInfo,
+  /// Warning Log Level
+  kLogLevelWarning,
+  /// Error Log Level
+  kLogLevelError,
+};
+
+// Common log methods.
+
+// All messages at or above the specified log level value are displayed.
+void LogSetLevel(LogLevel level);
+// Get the currently set log level.
+LogLevel LogGetLevel();
+// Log a debug message to the system log.
+void LogDebug(const char* format, ...);
+// Log an info message to the system log.
+void LogInfo(const char* format, ...);
+// Log a warning to the system log.
+void LogWarning(const char* format, ...);
+// Log an error to the system log.
+void LogError(const char* format, ...);
+// Log a firebase message (implemented by the platform specific logger).
+void LogMessageV(LogLevel log_level, const char* format, va_list args);
+// Log a firebase message via LogMessageV().
+void LogMessage(LogLevel log_level, const char* format, ...);
+
+}  // namespace util
+}  // namespace firestore
+}  // namespace firebase
+
+#endif  // FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_LOG_H_
diff --git a/Firestore/core/src/firebase/firestore/util/log_apple.mm b/Firestore/core/src/firebase/firestore/util/log_apple.mm
new file mode 100644
index 0000000..cb2c58e
--- /dev/null
+++ b/Firestore/core/src/firebase/firestore/util/log_apple.mm
@@ -0,0 +1,123 @@
+/*
+ * Copyright 2017 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.
+ */
+
+#include "Firestore/core/src/firebase/firestore/util/log.h"
+
+#import <FirebaseCore/FIRLogger.h>
+#import <Foundation/Foundation.h>
+
+#include <string>
+
+#include "Firestore/core/src/firebase/firestore/util/string_apple.h"
+
+namespace firebase {
+namespace firestore {
+namespace util {
+
+namespace {
+
+// Translates a C++ LogLevel to the equivalent Objective-C FIRLoggerLevel
+FIRLoggerLevel ToFIRLoggerLevel(LogLevel level) {
+  switch (level) {
+    case kLogLevelVerbose:  // fall through
+    case kLogLevelDebug:
+      return FIRLoggerLevelDebug;
+    case kLogLevelInfo:
+      return FIRLoggerLevelInfo;
+    case kLogLevelWarning:
+      return FIRLoggerLevelWarning;
+    case kLogLevelError:
+      return FIRLoggerLevelError;
+    default:
+      // Unsupported log level. FIRSetLoggerLevel will deal with it.
+      return static_cast<FIRLoggerLevel>(-1);
+  }
+}
+
+}  // namespace
+
+void LogSetLevel(LogLevel level) {
+  FIRSetLoggerLevel(ToFIRLoggerLevel(level));
+}
+
+LogLevel LogGetLevel() {
+  // We return the true log level. True log level is what the SDK used to
+  // determine whether to log instead of what parameter is used in the last call
+  // of LogSetLevel().
+  if (FIRIsLoggableLevel(FIRLoggerLevelInfo, NO)) {
+    if (FIRIsLoggableLevel(FIRLoggerLevelDebug, NO)) {
+      // FIRLoggerLevelMax is actually kLogLevelDebug right now. We do not check
+      // further.
+      return kLogLevelDebug;
+    } else {
+      return kLogLevelInfo;
+    }
+  } else {
+    if (FIRIsLoggableLevel(FIRLoggerLevelWarning, NO)) {
+      return kLogLevelWarning;
+    } else {
+      return kLogLevelError;
+    }
+  }
+}
+
+void LogDebug(const char* format, ...) {
+  va_list list;
+  va_start(list, format);
+  FIRLogBasic(FIRLoggerLevelDebug, kFIRLoggerFirestore, @"I-FST000001",
+              WrapNSStringNoCopy(format), list);
+  va_end(list);
+}
+
+void LogInfo(const char* format, ...) {
+  va_list list;
+  va_start(list, format);
+  FIRLogBasic(FIRLoggerLevelInfo, kFIRLoggerFirestore, @"I-FST000001",
+              WrapNSStringNoCopy(format), list);
+  va_end(list);
+}
+
+void LogWarning(const char* format, ...) {
+  va_list list;
+  va_start(list, format);
+  FIRLogBasic(FIRLoggerLevelWarning, kFIRLoggerFirestore, @"I-FST000001",
+              WrapNSStringNoCopy(format), list);
+  va_end(list);
+}
+
+void LogError(const char* format, ...) {
+  va_list list;
+  va_start(list, format);
+  FIRLogBasic(FIRLoggerLevelError, kFIRLoggerFirestore, @"I-FST000001",
+              WrapNSStringNoCopy(format), list);
+  va_end(list);
+}
+
+void LogMessageV(LogLevel log_level, const char* format, va_list args) {
+  FIRLogBasic(ToFIRLoggerLevel(log_level), kFIRLoggerFirestore, @"I-FST000001",
+              WrapNSStringNoCopy(format), args);
+}
+
+void LogMessage(LogLevel log_level, const char* format, ...) {
+  va_list list;
+  va_start(list, format);
+  LogMessageV(log_level, format, list);
+  va_end(list);
+}
+
+}  // namespace util
+}  // namespace firestore
+}  // namespace firebase
diff --git a/Firestore/core/src/firebase/firestore/util/log_stdio.cc b/Firestore/core/src/firebase/firestore/util/log_stdio.cc
new file mode 100644
index 0000000..bca2dc9
--- /dev/null
+++ b/Firestore/core/src/firebase/firestore/util/log_stdio.cc
@@ -0,0 +1,97 @@
+/*
+ * Copyright 2017 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.
+ */
+
+#include "Firestore/core/src/firebase/firestore/util/log.h"
+
+#include <stdio.h>
+#include <string>
+
+namespace firebase {
+namespace firestore {
+namespace util {
+
+LogLevel g_log_level = kLogLevelInfo;
+
+void LogSetLevel(LogLevel level) {
+  g_log_level = level;
+}
+
+LogLevel LogGetLevel() {
+  return g_log_level;
+}
+
+void LogDebug(const char* format, ...) {
+  va_list list;
+  va_start(list, format);
+  LogMessageV(kLogLevelDebug, format, list);
+  va_end(list);
+}
+
+void LogInfo(const char* format, ...) {
+  va_list list;
+  va_start(list, format);
+  LogMessageV(kLogLevelInfo, format, list);
+  va_end(list);
+}
+
+void LogWarning(const char* format, ...) {
+  va_list list;
+  va_start(list, format);
+  LogMessageV(kLogLevelWarning, format, list);
+  va_end(list);
+}
+
+void LogError(const char* format, ...) {
+  va_list list;
+  va_start(list, format);
+  LogMessageV(kLogLevelError, format, list);
+  va_end(list);
+}
+
+void LogMessageV(LogLevel log_level, const char* format, va_list args) {
+  if (log_level < g_log_level) {
+    return;
+  }
+  switch (log_level) {
+    case kLogLevelVerbose:
+      printf("VERBOSE: ");
+      break;
+    case kLogLevelDebug:
+      printf("DEBUG: ");
+      break;
+    case kLogLevelInfo:
+      break;
+    case kLogLevelWarning:
+      printf("WARNING: ");
+      break;
+    case kLogLevelError:
+      printf("ERROR: ");
+      break;
+  }
+  vprintf(format, args);
+  printf("\n");
+}
+
+void LogMessage(LogLevel log_level, const char* format, ...) {
+  va_list list;
+  va_start(list, format);
+  LogMessageV(log_level, format, list);
+  va_end(list);
+}
+
+}  // namespace util
+}  // namespace firestore
+}  // namespace firebase
diff --git a/Firestore/core/src/firebase/firestore/util/string_apple.h b/Firestore/core/src/firebase/firestore/util/string_apple.h
new file mode 100644
index 0000000..42b51dd
--- /dev/null
+++ b/Firestore/core/src/firebase/firestore/util/string_apple.h
@@ -0,0 +1,39 @@
+/*
+ * 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_UTIL_STRING_APPLE_H_
+#define FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_STRING_APPLE_H_
+
+#import <Foundation/Foundation.h>
+
+namespace firebase {
+namespace firestore {
+namespace util {
+
+// Translates a C string to the equivalent NSString without making a copy.
+inline NSString* WrapNSStringNoCopy(const char* c_str) {
+  return [[NSString alloc] initWithBytesNoCopy:const_cast<void*>(static_cast<const void*>(c_str))
+                                        length:strlen(c_str)
+                                      encoding:NSUTF8StringEncoding
+                                  freeWhenDone:NO];
+}
+
+
+}  // namespace util
+}  // namespace firestore
+}  // namespace firebase
+
+#endif  // FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_STRING_APPLE_H_
diff --git a/Firestore/core/src/firebase/firestore/util/string_printf.cc b/Firestore/core/src/firebase/firestore/util/string_printf.cc
new file mode 100644
index 0000000..60cc564
--- /dev/null
+++ b/Firestore/core/src/firebase/firestore/util/string_printf.cc
@@ -0,0 +1,101 @@
+/*
+ * 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.
+ */
+
+#include "Firestore/core/src/firebase/firestore/util/string_printf.h"
+
+#include <stdio.h>
+
+namespace firebase {
+namespace firestore {
+namespace util {
+
+void StringAppendV(std::string* dst, const char* format, va_list ap) {
+  // First try with a small fixed size buffer
+  static const int kSpaceLength = 1024;
+  char space[kSpaceLength];
+
+  // It's possible for methods that use a va_list to invalidate
+  // the data in it upon use.  The fix is to make a copy
+  // of the structure before using it and use that copy instead.
+  va_list backup_ap;
+  va_copy(backup_ap, ap);
+  int result = vsnprintf(space, kSpaceLength, format, backup_ap);
+  va_end(backup_ap);
+
+  if (result < kSpaceLength) {
+    if (result >= 0) {
+      // Normal case -- everything fit.
+      dst->append(space, result);
+      return;
+    }
+
+#ifdef _MSC_VER
+    // Error or MSVC running out of space.  MSVC 8.0 and higher
+    // can be asked about space needed with the special idiom below:
+    va_copy(backup_ap, ap);
+    result = vsnprintf(nullptr, 0, format, backup_ap);
+    va_end(backup_ap);
+#endif
+
+    if (result < 0) {
+      // Just an error.
+      return;
+    }
+  }
+
+  // Increase the buffer size to the size requested by vsnprintf,
+  // plus one for the closing \0.
+  size_t initial_size = dst->size();
+  size_t target_size = initial_size + result;
+
+  dst->resize(target_size + 1);
+  char* buf = &(*dst)[initial_size];
+  int buf_remain = result + 1;
+
+  // Restore the va_list before we use it again
+  va_copy(backup_ap, ap);
+  result = vsnprintf(buf, buf_remain, format, backup_ap);
+  va_end(backup_ap);
+
+  if (result >= 0 && result < buf_remain) {
+    // It fit and vsnprintf copied in directly. Resize down one to
+    // remove the trailing \0.
+    dst->resize(target_size);
+  } else {
+    // Didn't fit. Leave the original string unchanged.
+    dst->resize(initial_size);
+  }
+}
+
+std::string StringPrintf(const char* format, ...) {
+  va_list ap;
+  va_start(ap, format);
+  std::string result;
+  StringAppendV(&result, format, ap);
+  va_end(ap);
+  return result;
+}
+
+void StringAppendF(std::string* dst, const char* format, ...) {
+  va_list ap;
+  va_start(ap, format);
+  StringAppendV(dst, format, ap);
+  va_end(ap);
+}
+
+}  // namespace util
+}  // namespace firestore
+}  // namespace firebase
diff --git a/Firestore/core/src/firebase/firestore/util/string_printf.h b/Firestore/core/src/firebase/firestore/util/string_printf.h
new file mode 100644
index 0000000..9e2b9c0
--- /dev/null
+++ b/Firestore/core/src/firebase/firestore/util/string_printf.h
@@ -0,0 +1,48 @@
+/*
+ * 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_UTIL_STRING_PRINTF_H_
+#define FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_STRING_PRINTF_H_
+
+#include <stdarg.h>
+
+#include <string>
+
+#include <absl/base/attributes.h>
+
+namespace firebase {
+namespace firestore {
+namespace util {
+
+/** Return a C++ string. */
+std::string StringPrintf(const char* format, ...)
+    ABSL_PRINTF_ATTRIBUTE(1, 2);
+
+/** Append result to a supplied string. */
+void StringAppendF(std::string* dst, const char* format, ...)
+    ABSL_PRINTF_ATTRIBUTE(2, 3);
+
+/**
+ * Lower-level routine that takes a va_list and appends to a specified
+ * string.  All other routines are just convenience wrappers around it.
+ */
+void StringAppendV(std::string* dst, const char* format, va_list ap);
+
+}  // namespace util
+}  // namespace firestore
+}  // namespace firebase
+
+#endif  // FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_STRING_FORMAT_H_