Rewrite base::JSONReader to be 35-40% faster, depending on the input string.

base/json/json_parser.cc

Index: base/json/json_parser.cc
diff --git a/base/json/json_parser.cc b/base/json/json_parser.cc
new file mode 100644
index 0000000000000000000000000000000000000000..de2f55fabfe4fa0fd413f8eb343c5b8b559929b9
--- /dev/null
+++ b/base/json/json_parser.cc
@@ -0,0 +1,972 @@
+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "base/json/json_parser.h"
+
+#include "base/float_util.h"
+#include "base/logging.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/string_number_conversions.h"
+#include "base/string_util.h"
+#include "base/stringprintf.h"
+#include "base/third_party/icu/icu_utf.h"
+#include "base/utf_string_conversion_utils.h"
+#include "base/utf_string_conversions.h"
+#include "base/values.h"
+
+namespace base {
+namespace internal {
+
+namespace {
+
+const int kStackMaxDepth = 100;
+
+const int32 kExtendedASCIIStart = 0x80;
+
+// This and the class below are used to own the JSON input string for when
+// string tokens are stored as StringPiece instead of std::string. This
+// optimization avoids about 2/3rds of string memory copies. The constructor
+// takes the input string and swaps its data into the new instance. The real
+// root value is also Swap()ed into the new instance.
+class DictionaryHiddenRootValue : public base::DictionaryValue {
+ public:
+  DictionaryHiddenRootValue(std::string* json, Value* root) {
+    DCHECK(root->IsType(Value::TYPE_DICTIONARY));
+    DictionaryValue::Swap(static_cast<DictionaryValue*>(root));
+    json->swap(json_);
+  }
+
+  virtual void Swap(DictionaryValue* other) OVERRIDE {
+    DVLOG(1) << "Swap()ing a DictionaryValue inefficiently.";
+
+    // First deep copy to convert JSONStringValue to std::string and swap that
+    // copy with |other|, which contains the new contents of |this|.
+    scoped_ptr<base::DictionaryValue> copy(DeepCopy());
+    copy->Swap(other);
+
+    // Then erase the contents of the current dictionary and swap in the
+    // new contents, originally from |other|.
+    Clear();
+    json_.clear();
+    DictionaryValue::Swap(copy.get());
+  }
+
+  // Not overriding DictionaryValue::Remove because it just calls through to
+  // the method below.
+
+  virtual bool RemoveWithoutPathExpansion(const std::string& key,
+                                          Value** out) OVERRIDE {
+    // If the caller won't take ownership of the removed value, just call up.
+    if (!out)
+      return DictionaryValue::RemoveWithoutPathExpansion(key, out);
+
+    DVLOG(1) << "Remove()ing from a DictionaryValue inefficiently.";
+
+    // Otherwise, remove the value while its still "owned" by this and copy it
+    // to convert any JSONStringValues to std::string.
+    Value* out_owned = NULL;
+    if (!DictionaryValue::RemoveWithoutPathExpansion(key, &out_owned))
+      return false;
+
+    *out = out_owned->DeepCopy();
+    delete out_owned;
+
+    return true;
+  }
+
+ private:
+  std::string json_;
+
+  DISALLOW_COPY_AND_ASSIGN(DictionaryHiddenRootValue);
+};
+
+class ListHiddenRootValue : public base::ListValue {
+ public:
+  ListHiddenRootValue(std::string* json, Value* root) {
+    DCHECK(root->IsType(Value::TYPE_LIST));
+    ListValue::Swap(static_cast<ListValue*>(root));
+    json->swap(json_);
+  }
+
+  virtual void Swap(ListValue* other) OVERRIDE {
+    DVLOG(1) << "Swap()ing a ListValue inefficiently.";
+
+    // First deep copy to convert JSONStringValue to std::string and swap that
+    // copy with |other|, which contains the new contents of |this|.
+    scoped_ptr<base::ListValue> copy(DeepCopy());
+    copy->Swap(other);
+
+    // Then erase the contents of the current list and swap in the new contents,
+    // originally from |other|.
+    Clear();
+    json_.clear();
+    ListValue::Swap(copy.get());
+  }
+
+  virtual bool Remove(size_t index, Value** out) OVERRIDE {
+    // If the caller won't take ownership of the removed value, just call up.
+    if (!out)
+      return ListValue::Remove(index, out);
+
+    DVLOG(1) << "Remove()ing from a ListValue inefficiently.";
+
+    // Otherwise, remove the value while its still "owned" by this and copy it
+    // to convert any JSONStringValues to std::string.
+    Value* out_owned = NULL;
+    if (!ListValue::Remove(index, &out_owned))
+      return false;
+
+    *out = out_owned->DeepCopy();
+    delete out_owned;
+
+    return true;
+  }
+
+ private:
+  std::string json_;
+
+  DISALLOW_COPY_AND_ASSIGN(ListHiddenRootValue);
+};
+
+// A variant on StringValue that uses StringPiece instead of copying the string
+// into the Value. This can only be stored in a child of hidden root (above),
+// otherwise the referenced string will not be guaranteed to outlive it.
+class JSONStringValue : public base::Value {
+ public:
+  explicit JSONStringValue(const base::StringPiece& piece)
+      : Value(TYPE_STRING),
+        string_piece_(piece) {
+  }
+
+  // Value:
+  bool GetAsString(std::string* out_value) const OVERRIDE {
+    string_piece_.CopyToString(out_value);
+    return true;
+  }
+  bool GetAsString(string16* out_value) const OVERRIDE {
+    *out_value = UTF8ToUTF16(string_piece_);
+    return true;
+  }
+  virtual Value* DeepCopy() const OVERRIDE {
+    return Value::CreateStringValue(string_piece_.as_string());
+  }
+  virtual bool Equals(const Value* other) const OVERRIDE {
+    std::string other_string;
+    return other->IsType(TYPE_STRING) && other->GetAsString(&other_string) &&
+        StringPiece(other_string) == string_piece_;
+  }
+
+ private:
+  // The location in the original input stream.
+  base::StringPiece string_piece_;
+
+  DISALLOW_COPY_AND_ASSIGN(JSONStringValue);
+};
+
+// Simple class that checks for maximum recursion/"stack overflow."
+class StackMarker {
+ public:
+  explicit StackMarker(int* depth) : depth_(depth) {
+    ++(*depth_);
+    DCHECK_LE(*depth_, kStackMaxDepth);
+  }
+  ~StackMarker() {
+    --(*depth_);
+  }
+
+  bool IsTooDeep() const {
+    return *depth_ >= kStackMaxDepth;
+  }
+
+ private:
+  int* const depth_;
+
+  DISALLOW_COPY_AND_ASSIGN(StackMarker);
+};
+
+}  // namespace
+
+JSONParser::JSONParser(int options)
+    : options_(options),
+      start_pos_(NULL),
+      pos_(NULL),
+      end_pos_(NULL),
+      index_(0),
+      stack_depth_(0),
+      line_number_(0),
+      index_last_line_(0),
+      error_code_(JSONReader::JSON_NO_ERROR),
+      error_line_(0),
+      error_column_(0) {
+}
+
+JSONParser::~JSONParser() {
+}
+
+Value* JSONParser::Parse(const std::string& input) {
+  // TODO(rsesek): Windows has problems with StringPiece/hidden roots. Fix
+  // <http://crbug.com/126107> when my Windows box arrives.
+#if defined(OS_WIN)
+  options_ |= JSON_DETACHABLE_CHILDREN;
+#endif
+
+  std::string input_copy;
+  // If the children of a JSON root can be detached, then hidden roots cannot
+  // be used, so do not bother copying the input because StringPiece will not
+  // be used anywhere.
+  if (!(options_ & JSON_DETACHABLE_CHILDREN)) {
+    input_copy = input;
+    start_pos_ = input_copy.data();
+  } else {
+    start_pos_ = input.data();
+  }
+  pos_ = start_pos_;
+  end_pos_ = start_pos_ + input.length();
+  index_ = 0;
+  line_number_ = 1;
+  index_last_line_ = 0;
+
+  error_code_ = JSONReader::JSON_NO_ERROR;
+  error_line_ = 0;
+  error_column_ = 0;
+
+  // When the input JSON string starts with a UTF-8 Byte-Order-Mark
+  // <0xEF 0xBB 0xBF>, advance the start position to avoid the
+  // ParseNextToken function mis-treating a Unicode BOM as an invalid
+  // character and returning NULL.
+  if (CanConsume(3) && static_cast<uint8>(*pos_) == 0xEF &&
+      static_cast<uint8>(*(pos_ + 1)) == 0xBB &&
+      static_cast<uint8>(*(pos_ + 2)) == 0xBF) {
+    NextNChars(3);
+  }
+
+  // Parse the first and all subsequent tokens.
+  scoped_ptr<Value> root(ParseNextToken());
+  if (!root.get())
+    return NULL;
+
+  // Make sure the input stream is at an end.
+  if (GetNextToken() != T_END_OF_INPUT) {
+    if (!CanConsume(1) || (NextChar() && GetNextToken() != T_END_OF_INPUT)) {
+      ReportError(JSONReader::JSON_UNEXPECTED_DATA_AFTER_ROOT, 1);
+      return NULL;
+    }
+  }
+
+  // Dictionaries and lists can contain JSONStringValues, so wrap them in a
+  // hidden root.
+  if (!(options_ & JSON_DETACHABLE_CHILDREN)) {
+    if (root->IsType(Value::TYPE_DICTIONARY)) {
+      return new DictionaryHiddenRootValue(&input_copy, root.release());
+    } else if (root->IsType(Value::TYPE_LIST)) {
+      return new ListHiddenRootValue(&input_copy, root.release());
+    } else if (root->IsType(Value::TYPE_STRING)) {
+      // A string type could be a JSONStringValue, but because there's no
+      // corresponding HiddenRootValue, the memory will be lost. Deep copy to
+      // preserve it.
+      return root->DeepCopy();
+    }
+  }
+
+  // All other values can be returned directly.
+  return root.release();
+}
+
+JSONReader::JsonParseError JSONParser::error_code() const {
+  return error_code_;
+}
+
+std::string JSONParser::GetErrorMessage() const {
+  return FormatErrorMessage(error_line_, error_column_,
+      JSONReader::ErrorCodeToString(error_code_));
+}
+
+// StringBuilder ///////////////////////////////////////////////////////////////
+
+JSONParser::StringBuilder::StringBuilder()
+    : pos_(NULL),
+      length_(0),
+      string_(NULL) {
+}
+
+JSONParser::StringBuilder::StringBuilder(const char* pos)
+    : pos_(pos),
+      length_(0),
+      string_(NULL) {
+}
+
+void JSONParser::StringBuilder::Swap(StringBuilder* other) {
+  std::swap(other->string_, string_);
+  std::swap(other->pos_, pos_);
+  std::swap(other->length_, length_);
+}
+
+JSONParser::StringBuilder::~StringBuilder() {
+  delete string_;
+}
+
+void JSONParser::StringBuilder::Append(const char& c) {
+  DCHECK_GE(c, 0);
+  DCHECK_LT(c, 128);
+
+  if (string_)
+    string_->push_back(c);
+  else
+    ++length_;
+}
+
+void JSONParser::StringBuilder::AppendString(const std::string& str) {
+  DCHECK(string_);
+  string_->append(str);
+}
+
+void JSONParser::StringBuilder::Convert() {
+  if (string_)
+    return;
+  string_  = new std::string(pos_, length_);
+}
+
+bool JSONParser::StringBuilder::CanBeStringPiece() const {
+  return !string_;
+}
+
+StringPiece JSONParser::StringBuilder::AsStringPiece() {
+  if (string_)
+    return StringPiece();
+  return StringPiece(pos_, length_);
+}
+
+const std::string& JSONParser::StringBuilder::AsString() {
+  if (!string_)
+    Convert();
+  return *string_;
+}
+
+// JSONParser private //////////////////////////////////////////////////////////
+
+inline bool JSONParser::CanConsume(int length) {
+  return pos_ + length <= end_pos_;
+}
+
+const char* JSONParser::NextChar() {
+  DCHECK(CanConsume(1));
+  ++index_;
+  ++pos_;
+  return pos_;
+}
+
+void JSONParser::NextNChars(int n) {
+  DCHECK(CanConsume(n));
+  index_ += n;
+  pos_ += n;
+}
+
+JSONParser::Token JSONParser::GetNextToken() {
+  EatWhitespaceAndComments();
+  if (!CanConsume(1))
+    return T_END_OF_INPUT;
+
+  switch (*pos_) {
+    case '{':
+      return T_OBJECT_BEGIN;
+    case '}':
+      return T_OBJECT_END;
+    case '[':
+      return T_ARRAY_BEGIN;
+    case ']':
+      return T_ARRAY_END;
+    case '"':
+      return T_STRING;
+    case '0':
+    case '1':
+    case '2':
+    case '3':
+    case '4':
+    case '5':
+    case '6':
+    case '7':
+    case '8':
+    case '9':
+    case '-':
+      return T_NUMBER;
+    case 't':
+      return T_BOOL_TRUE;
+    case 'f':
+      return T_BOOL_FALSE;
+    case 'n':
+      return T_NULL;
+    case ',':
+      return T_LIST_SEPARATOR;
+    case ':':
+      return T_OBJECT_PAIR_SEPARATOR;
+    default:
+      return T_INVALID_TOKEN;
+  }
+}
+
+void JSONParser::EatWhitespaceAndComments() {
+  while (pos_ < end_pos_) {
+    switch (*pos_) {
+      case '\r':
+      case '\n':
+        index_last_line_ = index_;
+        ++line_number_;
+        // Fall through.
+      case ' ':
+      case '\t':
+        NextChar();
+        break;
+      case '/':
+        if (!EatComment())
+          return;
+        break;
+      default:
+        return;
+    }
+  }
+}
+
+bool JSONParser::EatComment() {
+  if (*pos_ != '/' || !CanConsume(1))
+    return false;
+
+  char next_char = *NextChar();
+  if (next_char == '/') {
+    // Single line comment, read to newline.
+    while (CanConsume(1)) {
+      char next_char = *NextChar();
+      if (next_char == '\n' || next_char == '\r')
+        return true;
+    }
+  } else if (next_char == '*') {
+    // Block comment, read until end marker.
+    while (CanConsume(2)) {
+      if (*NextChar() == '*' && *NextChar() == '/') {
+        // EatWhitespaceAndComments will inspect pos_, which will still be on
+        // the last / of the comment, so advance once more (which may also be
+        // end of input).
+        NextChar();
+        return true;
+      }
+    }
+
+    // If the comment is unterminated, GetNextToken will report T_END_OF_INPUT.
+  }
+
+  return false;
+}
+
+Value* JSONParser::ParseNextToken() {
+  return ParseToken(GetNextToken());
+}
+
+Value* JSONParser::ParseToken(Token token) {
+  switch (token) {
+    case T_OBJECT_BEGIN:
+      return ConsumeDictionary();
+    case T_ARRAY_BEGIN:
+      return ConsumeList();
+    case T_STRING:
+      return ConsumeString();
+    case T_NUMBER:
+      return ConsumeNumber();
+    case T_BOOL_TRUE:
+    case T_BOOL_FALSE:
+    case T_NULL:
+      return ConsumeLiteral();
+    default:
+      ReportError(JSONReader::JSON_UNEXPECTED_TOKEN, 1);
+      return NULL;
+  }
+}
+
+Value* JSONParser::ConsumeDictionary() {
+  if (*pos_ != '{') {
+    ReportError(JSONReader::JSON_UNEXPECTED_TOKEN, 1);
+    return NULL;
+  }
+
+  StackMarker depth_check(&stack_depth_);
+  if (depth_check.IsTooDeep()) {
+    ReportError(JSONReader::JSON_TOO_MUCH_NESTING, 1);
+    return NULL;
+  }
+
+  scoped_ptr<DictionaryValue> dict(new DictionaryValue);
+
+  NextChar();
+  Token token = GetNextToken();
+  while (token != T_OBJECT_END) {
+    if (token != T_STRING) {
+      ReportError(JSONReader::JSON_UNQUOTED_DICTIONARY_KEY, 1);
+      return NULL;
+    }
+
+    // First consume the key.
+    StringBuilder key;
+    if (!ConsumeStringRaw(&key)) {
+      return NULL;
+    }
+
+    // Read the separator.
+    NextChar();
+    token = GetNextToken();
+    if (token != T_OBJECT_PAIR_SEPARATOR) {
+      ReportError(JSONReader::JSON_SYNTAX_ERROR, 1);
+      return NULL;
+    }
+
+    // The token is the value. Ownership transfers to |dict|.
+    NextChar();
+    Value* value = ParseNextToken();
+    if (!value) {
+      return NULL;
+    }
+
+    dict->SetWithoutPathExpansion(key.AsString(), value);
+
+    NextChar();
+    token = GetNextToken();
+    if (token == T_LIST_SEPARATOR) {
+      NextChar();
+      token = GetNextToken();
+      if (token == T_OBJECT_END && !(options_ & JSON_ALLOW_TRAILING_COMMAS)) {
+        ReportError(JSONReader::JSON_TRAILING_COMMA, 1);
+        return NULL;
+      }
+    } else if (token != T_OBJECT_END) {
+      ReportError(JSONReader::JSON_SYNTAX_ERROR, 0);
+      return NULL;
+    }
+  }
+
+  if (token != T_OBJECT_END)
+    return NULL;
+
+  return dict.release();
+}
+
+Value* JSONParser::ConsumeList() {
+  if (*pos_ != '[') {
+    ReportError(JSONReader::JSON_UNEXPECTED_TOKEN, 1);
+    return NULL;
+  }
+
+  StackMarker depth_check(&stack_depth_);
+  if (depth_check.IsTooDeep()) {
+    ReportError(JSONReader::JSON_TOO_MUCH_NESTING, 1);
+    return NULL;
+  }
+
+  scoped_ptr<ListValue> list(new ListValue);
+
+  NextChar();
+  Token token = GetNextToken();
+  while (token != T_ARRAY_END) {
+    Value* item = ParseToken(token);
+    if (!item) {
+      // ReportError from deeper level.
+      return NULL;
+    }
+
+    list->Append(item);
+
+    NextChar();
+    token = GetNextToken();
+    if (token == T_LIST_SEPARATOR) {
+      NextChar();
+      token = GetNextToken();
+      if (token == T_ARRAY_END && !(options_ & JSON_ALLOW_TRAILING_COMMAS)) {
+        ReportError(JSONReader::JSON_TRAILING_COMMA, 1);
+        return NULL;
+      }
+    } else if (token != T_ARRAY_END) {
+      ReportError(JSONReader::JSON_SYNTAX_ERROR, 1);
+      return NULL;
+    }
+  }
+
+  if (token != T_ARRAY_END)
+    return NULL;
+
+  return list.release();
+}
+
+Value* JSONParser::ConsumeString() {
+  StringBuilder string;
+  if (!ConsumeStringRaw(&string))
+    return NULL;
+
+  // Create the Value representation, either using a hidden root, if configured
+  // to do so, and the string can be represented by StringPiece.
+  if (string.CanBeStringPiece() && !(options_ & JSON_DETACHABLE_CHILDREN)) {
+    return new JSONStringValue(string.AsStringPiece());
+  } else {
+    if (string.CanBeStringPiece())
+      string.Convert();
+    return new StringValue(string.AsString());
+  }
+}
+
+bool JSONParser::ConsumeStringRaw(StringBuilder* out) {
+  if (*pos_ != '"') {
+    ReportError(JSONReader::JSON_UNEXPECTED_TOKEN, 1);
+    return false;
+  }
+
+  // StringBuilder will internally build a StringPiece unless a UTF-16
+  // conversion occurs, at which point it will perform a copy into a
+  // std::string.
+  StringBuilder string(NextChar());
+
+  int length = end_pos_ - start_pos_;
+  int32 next_char = 0;
+
+  while (CanConsume(1)) {
+    pos_ = start_pos_ + index_;  // CBU8_NEXT is postcrement.
+    CBU8_NEXT(start_pos_, index_, length, next_char);
+    if (next_char < 0 || !IsValidCharacter(next_char)) {
+      ReportError(JSONReader::JSON_UNSUPPORTED_ENCODING, 1);
+      return false;
+    }
+
+    // If this character is an escape sequence...
+    if (next_char == '\\') {
+      // The input string will be adjusted (either by combining the two
+      // characters of an encoded escape sequence, or with a UTF conversion),
+      // so using StringPiece isn't possible -- force a conversion.
+      string.Convert();
+
+      if (!CanConsume(1)) {
+        ReportError(JSONReader::JSON_INVALID_ESCAPE, 0);
+        return false;
+      }
+
+      switch (*NextChar()) {
+        // Allowed esape sequences:
+        case 'x': {  // UTF-8 sequence.
+          // UTF-8 \x escape sequences are not allowed in the spec, but they
+          // are supported here for backwards-compatiblity with the old parser.
+          if (!CanConsume(2)) {
+            ReportError(JSONReader::JSON_INVALID_ESCAPE, 1);
+            return false;
+          }
+
+          int hex_digit = 0;
+          if (!HexStringToInt(StringPiece(NextChar(), 2), &hex_digit)) {
+            ReportError(JSONReader::JSON_INVALID_ESCAPE, -1);
+            return false;
+          }
+          NextChar();
+
+          if (hex_digit < kExtendedASCIIStart)
+            string.Append(hex_digit);
+          else
+            DecodeUTF8(hex_digit, &string);
+          break;
+        }
+        case 'u': {  // UTF-16 sequence.
+          // UTF units are of the form \uXXXX.
+          if (!CanConsume(5)) {  // 5 being 'u' and four HEX digits.
+            ReportError(JSONReader::JSON_INVALID_ESCAPE, 0);
+            return false;
+          }
+
+          // Skip the 'u'.
+          NextChar();
+
+          std::string utf8_units;
+          if (!DecodeUTF16(&utf8_units)) {
+            ReportError(JSONReader::JSON_INVALID_ESCAPE, -1);
+            return false;
+          }
+
+          string.AppendString(utf8_units);
+          break;
+        }
+        case '"':
+          string.Append('"');
+          break;
+        case '\\':
+          string.Append('\\');
+          break;
+        case '/':
+          string.Append('/');
+          break;
+        case 'b':
+          string.Append('\b');
+          break;
+        case 'f':
+          string.Append('\f');
+          break;
+        case 'n':
+          string.Append('\n');
+          break;
+        case 'r':
+          string.Append('\r');
+          break;
+        case 't':
+          string.Append('\t');
+          break;
+        case 'v':  // Not listed as valid escape sequence in the RFC.
+          string.Append('\v');
+          break;
+        // All other escape squences are illegal.
+        default:
+          ReportError(JSONReader::JSON_INVALID_ESCAPE, 0);
+          return false;
+      }
+    } else if (next_char == '"') {
+      --index_;  // Rewind by one because of CBU8_NEXT.
+      out->Swap(&string);
+      return true;
+    } else {
+      if (next_char < kExtendedASCIIStart)
+        string.Append(next_char);
+      else
+        DecodeUTF8(next_char, &string);
+    }
+  }
+
+  ReportError(JSONReader::JSON_SYNTAX_ERROR, 0);
+  return false;
+}
+
+// Entry is at the first X in \uXXXX.
+bool JSONParser::DecodeUTF16(std::string* dest_string) {
+  if (!CanConsume(4))
+    return false;
+
+  // This is a 32-bit field because the shift operations in the
+  // conversion process below cause MSVC to error about "data loss."
+  // This only stores UTF-16 code units, though.
+  // Consume the UTF-16 code unit, which may be a high surrogate.
+  int code_unit16_high = 0;
+  if (!HexStringToInt(StringPiece(pos_, 4), &code_unit16_high))
+    return false;
+
+  // Only add 3, not 4, because at the end of this iteration, the parser has
+  // finished working with the last digit of the UTF sequence, meaning that
+  // the next iteration will advance to the next byte.
+  NextNChars(3);
+
+  // Used to convert the UTF-16 code units to a code point and then to a UTF-8
+  // code unit sequence.
+  char code_point[8] = { 0 };
+  size_t offset = 0;
+
+  // If this is a high surrogate, consume the next code unit to get the
+  // low surrogate.
+  if (CBU16_IS_SURROGATE(code_unit16_high)) {
+    // Make sure this is the high surrogate. If not, it's an encoding
+    // error.
+    if (!CBU16_IS_SURROGATE_LEAD(code_unit16_high))
+      return false;
+
+    // Make sure that the token has more characters to consume the
+    // lower surrogate.
+    if (!CanConsume(6))  // 6 being '\' 'u' and four HEX digits.
+      return false;
+    if (*NextChar() != '\\' || *NextChar() != 'u')
+      return false;
+
+    NextChar();  // Read past 'u'.
+    int code_unit16_low = 0;
+    if (!HexStringToInt(StringPiece(pos_, 4), &code_unit16_low))
+      return false;
+
+    NextNChars(3);
+
+    if (!CBU16_IS_TRAIL(code_unit16_low)) {
+      return false;
+    }
+
+    uint32 code_unit32 = CBU16_GET_SUPPLEMENTARY(code_unit16_high,
+                                                 code_unit16_low);
+    offset = 0;
+    CBU8_APPEND_UNSAFE(code_point, offset, code_unit32);
+  } else {
+    // Not a surrogate.
+    DCHECK(CBU16_IS_SINGLE(code_unit16_high));
+    CBU8_APPEND_UNSAFE(code_point, offset, code_unit16_high);
+  }
+
+  dest_string->append(code_point);
+  return true;
+}
+
+void JSONParser::DecodeUTF8(const int32& point, StringBuilder* dest) {
+  // Anything outside of the basic ASCII plane will need to be decomposed from
+  // int32 to a multi-byte sequence.
+  if (point < kExtendedASCIIStart) {
+    dest->Append(point);
+  } else {
+    char utf8_units[4] = { 0 };
+    int offset = 0;
+    CBU8_APPEND_UNSAFE(utf8_units, offset, point);
+    dest->Convert();
+    dest->AppendString(utf8_units);
+  }
+}
+
+Value* JSONParser::ConsumeNumber() {
+  const char* num_start = pos_;
+  const int start_index = index_;
+  int end_index = start_index;
+
+  if (*pos_ == '-')
+    NextChar();
+
+  if (!ReadInt(false)) {
+    ReportError(JSONReader::JSON_SYNTAX_ERROR, 1);
+    return NULL;
+  }
+  end_index = index_;
+
+  // The optional fraction part.
+  if (*pos_ == '.') {
+    if (!CanConsume(1)) {
+      ReportError(JSONReader::JSON_SYNTAX_ERROR, 1);
+      return NULL;
+    }
+    NextChar();
+    if (!ReadInt(true)) {
+      ReportError(JSONReader::JSON_SYNTAX_ERROR, 1);
+      return NULL;
+    }
+    end_index = index_;
+  }
+
+  // Optional exponent part.
+  if (*pos_ == 'e' || *pos_ == 'E') {
+    NextChar();
+    if (*pos_ == '-' || *pos_ == '+')
+      NextChar();
+    if (!ReadInt(true)) {
+      ReportError(JSONReader::JSON_SYNTAX_ERROR, 1);
+      return NULL;
+    }
+    end_index = index_;
+  }
+
+  // ReadInt is greedy because numbers have no easily detectable sentinel,
+  // so save off where the parser should be on exit (see Consume invariant at
+  // the top of the header), then make sure the next token is one which is
+  // valid.
+  const char* exit_pos = pos_ - 1;
+  int exit_index = index_ - 1;
+
+  switch (GetNextToken()) {
+    case T_OBJECT_END:
+    case T_ARRAY_END:
+    case T_LIST_SEPARATOR:
+    case T_END_OF_INPUT:
+      break;
+    default:
+      ReportError(JSONReader::JSON_SYNTAX_ERROR, 1);
+      return NULL;
+  }
+
+  pos_ = exit_pos;
+  index_ = exit_index;
+
+  StringPiece num_string(num_start, end_index - start_index);
+
+  int num_int;
+  if (StringToInt(num_string, &num_int))
+    return Value::CreateIntegerValue(num_int);
+
+  double num_double;
+  if (base::StringToDouble(num_string.as_string(), &num_double) &&
+      IsFinite(num_double)) {
+    return Value::CreateDoubleValue(num_double);
+  }
+
+  return NULL;
+}
+
+bool JSONParser::ReadInt(bool allow_leading_zeros) {
+  char first = *pos_;
+  int len = 0;
+
+  char c = first;
+  while (CanConsume(1) && IsAsciiDigit(c)) {
+    c = *NextChar();
+    ++len;
+  }
+
+  if (len == 0)
+    return false;
+
+  if (!allow_leading_zeros && len > 1 && first == '0')
+    return false;
+
+  return true;
+}
+
+Value* JSONParser::ConsumeLiteral() {
+  switch (*pos_) {
+    case 't': {
+      const char* kTrueLiteral = "true";
+      const int kTrueLen = static_cast<int>(strlen(kTrueLiteral));
+      if (!CanConsume(kTrueLen - 1) ||
+          !StringsAreEqual(pos_, kTrueLiteral, kTrueLen)) {
+        ReportError(JSONReader::JSON_SYNTAX_ERROR, 1);
+        return NULL;
+      }
+      NextNChars(kTrueLen - 1);
+      return Value::CreateBooleanValue(true);
+    }
+    case 'f': {
+      const char* kFalseLiteral = "false";
+      const int kFalseLen = static_cast<int>(strlen(kFalseLiteral));
+      if (!CanConsume(kFalseLen - 1) ||
+          !StringsAreEqual(pos_, kFalseLiteral, kFalseLen)) {
+        ReportError(JSONReader::JSON_SYNTAX_ERROR, 1);
+        return NULL;
+      }
+      NextNChars(kFalseLen - 1);
+      return Value::CreateBooleanValue(false);
+    }
+    case 'n': {
+      const char* kNullLiteral = "null";
+      const int kNullLen = static_cast<int>(strlen(kNullLiteral));
+      if (!CanConsume(kNullLen - 1) ||
+          !StringsAreEqual(pos_, kNullLiteral, kNullLen)) {
+        ReportError(JSONReader::JSON_SYNTAX_ERROR, 1);
+        return NULL;
+      }
+      NextNChars(kNullLen - 1);
+      return Value::CreateNullValue();
+    }
+    default:
+      ReportError(JSONReader::JSON_UNEXPECTED_TOKEN, 1);
+      return NULL;
+  }
+}
+
+// static
+bool JSONParser::StringsAreEqual(const char* one, const char* two, size_t len) {
+  return strncmp(one, two, len) == 0;
+}
+
+void JSONParser::ReportError(JSONReader::JsonParseError code,
+                             int column_adjust) {
+  error_code_ = code;
+  error_line_ = line_number_;
+  error_column_ = index_ - index_last_line_ + column_adjust;
+}
+
+// static
+std::string JSONParser::FormatErrorMessage(int line, int column,
+                                           const std::string& description) {
+  if (line || column) {
+    return StringPrintf("Line: %i, column: %i, %s",
+        line, column, description.c_str());
+  }
+  return description;
+}
+
+}  // namespace internal
+}  // namespace base