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

base/json/json_parser.cc

+// 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/stringprintf.h"
+#include "base/string_number_conversions.h"

[Mark Mentovai, 2012/04/19 16:40:11]

'_' < 'p'

I would have stayed quiet about it, but on lines 14 and 15, you sorted '_' <
's', so this is inconsistent.

[Robert Sesek, 2012/05/03 15:34:52]

Done.

+#include "base/string_util.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 {
+
+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) {
+    CHECK(root->IsType(Value::TYPE_DICTIONARY));

[Mark Mentovai, 2012/04/19 16:40:11]

Can this be a DCHECK?

Same on line 50.

[Robert Sesek, 2012/05/03 15:34:52]

Done.

+    Swap(static_cast<DictionaryValue*>(root));
+    json->swap(json_);
+  }
+
+  virtual base::DictionaryValue* DeepCopy() const OVERRIDE {
+    scoped_ptr<base::Value> data(base::DictionaryValue::DeepCopy());
+    std::string json(json_);
+    return new DictionaryHiddenRootValue(&json, data.get());
+  }
+
+ private:
+  std::string json_;

[Mark Mentovai, 2012/04/19 16:40:11]

DISALLOW_COPY_AND_ASSIGN? Maybe not. Don’t know how you’re using it yet.

Same on line 62.

[Robert Sesek, 2012/05/03 15:34:52]

Done.

+};
+
+class ListHiddenRootValue : public base::ListValue {
+ public:
+  ListHiddenRootValue(std::string* json, Value* root) {
+    CHECK(root->IsType(Value::TYPE_LIST));
+    Swap(static_cast<ListValue*>(root));
+    json->swap(json_);
+  }
+
+  virtual base::ListValue* DeepCopy() const OVERRIDE {
+    scoped_ptr<base::Value> data(base::ListValue::DeepCopy());
+    std::string json(json_);
+    return new ListHiddenRootValue(&json, data.get());
+  }
+
+ private:
+  std::string json_;
+};
+
+// 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) &&

[Mark Mentovai, 2012/04/19 16:40:11]

Isn’t it cheaper to do the comparison of other and string_piece_ as StringPieces
instead of forcing everyone to be a std::string?

[Robert Sesek, 2012/05/03 15:34:52]

Yes it is. Good idea.

+        other_string == string_piece_.as_string();
+  }
+
+ private:
+  // The location in the original input stream.
+  base::StringPiece string_piece_;

[Mark Mentovai, 2012/04/19 16:40:11]

DISALLOW_COPY_AND_ASSIGN? Maybe not. Don’t know how you’re using it yet.

[Robert Sesek, 2012/05/03 15:34:52]

Done.

+};
+
+// Simple class that checks for maximum recursion/"stack overflow."
+class StackMarker {
+ public:
+  StackMarker(int* depth) : depth_(depth) {

[Mark Mentovai, 2012/04/19 16:40:11]

explicit

[Robert Sesek, 2012/05/03 15:34:52]

Done.

+    ++(*depth_);
+  }
+  ~StackMarker() {
+    --(*depth_);
+  }
+
+  bool IsTooDeep() {

[Mark Mentovai, 2012/04/19 16:40:11]

Can be const.

[Robert Sesek, 2012/05/03 15:34:52]

Done.

+    return *depth_ >= kStackMaxDepth;
+  }
+
+ private:
+  int* depth_;

[Mark Mentovai, 2012/04/19 16:40:11]

DISALLOW_COPY_AND_ASSIGN

[Mark Mentovai, 2012/04/19 16:40:11]

The pointer (not value) can be const, which is nice in cases like this because
it keeps you from accidentally incrementing or decrementing the pointer itself.

[Robert Sesek, 2012/05/03 15:34:52]

Done.

[Robert Sesek, 2012/05/03 15:34:52]

Done.

+};
+
+}  // namespace
+
+namespace base {
+namespace internal {
+
+JSONParser::JSONParser(int options)
+    : options_(options),
+      start_pos_(NULL),
+      pos_(0),
+      index_(0),

[Mark Mentovai, 2012/04/19 16:40:11]

Is end_pos_ missing intentionally?

[Robert Sesek, 2012/05/03 15:34:52]

Nope.

+      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) {

[Mark Mentovai, 2012/04/19 16:40:11]

Perhaps this can even accept StringPiece input, potentially saving another copy
(depending on the callers).

+  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 (U+FEFF)

[Mark Mentovai, 2012/04/19 16:40:11]

That’s a UTF-16 BOM.

Your comment makes it sound like this will accept a UTF-16 BOM at the beginning,
but that’s not true. (And if it were true, and it accepted a UTF-16 BOM and then
continued to parse UTF-8, it’d be pretty weird.)

[Robert Sesek, 2012/05/03 15:34:52]

Isn't U+FEFF the BOM code point, which in UTF-16 is a single unit that's the
same as the point, but in UTF-8 is those three? That's what I was trying to say,
but I can see why that'd be confusing, so fixed.

+  // or <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);

[Mark Mentovai, 2012/04/19 16:40:11]

What’s the “, 1” doing here? The unexpected data might not be one column after
what ends the expected root.

+      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) {
+  if (other->string_) {

[Mark Mentovai, 2012/04/19 16:40:12]

This implementation seems wrong. What if other->string_ is set, string_ is not,
and pos_ and length_ are set?

Shouldn’t you just be swapping all three fields without regard to whether
other->string_ is set?

[Robert Sesek, 2012/05/03 15:34:52]

Done.

+    std::swap(other->string_, string_);
+  } else {
+    std::swap(other->pos_, pos_);
+    std::swap(other->length_, length_);
+  }
+}
+
+JSONParser::StringBuilder::~StringBuilder() {
+  delete string_;
+}
+
+void JSONParser::StringBuilder::Append(const int32& c) {
+  if (string_) {
+    string_->push_back(c);

[Mark Mentovai, 2012/04/19 16:40:12]

The header never said what restrictions were placed on the value of “c”, but
using an int32 and not having any checks around the value, not to mention the
“else” branch of this conditional, seems to imply that you want to accept
arbitrary Unicode code points.

You can’t push a code point (int32) onto a std::string (effectively char*) like
this. Shouldn’t you be pushing one to four characters onto the string depending
on the value of c, like you do below? Shouldn’t you be UTF-8-encoding c?

+  } else {
+    // A code point is 32 bits/4 bytes, which needs to be mapped to 1-byte
+    // |char|s.
+    if (c < kExtendedASCIIStart) {
+      ++length_;
+    } else if (c < 0x0800) {
+      length_ += 2;
+    } else if (c < 0x1000) {
+      length_ += 3;
+    } else {
+      length_ += 4;
+    }
+  }
+}
+
+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() {
+  return !string_;

[Mark Mentovai, 2012/04/19 16:40:12]

Oh, so once something is converted to a string, it can never be a StringPiece
again? That wasn’t clear to me based on the header.

[Robert Sesek, 2012/05/03 15:34:52]

Clarified.

+}
+
+StringPiece JSONParser::StringBuilder::AsStringPiece() {
+  if (string_)
+    return StringPiece();
+  return StringPiece(pos_, length_);
+}
+
+std::string JSONParser::StringBuilder::AsString() {
+  if (!string_)
+    Convert();
+  return *string_;

[Mark Mentovai, 2012/04/19 16:40:12]

I may have said this in the other file, but if this can return a const&, that’d
be better.

[Robert Sesek, 2012/05/03 15:34:52]

Done.

+}
+
+// JSONParser private //////////////////////////////////////////////////////////

[Mark Mentovai, 2012/04/19 16:40:12]

Stopping here for lunch.

[Robert Sesek, 2012/05/03 15:34:52]

The flatbreads were good today, yes?

+
+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;
+      }
+    }
+  }
+
+  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;
+
+  DCHECK_EQ(*pos_, *(start_pos_ + index_));
+
+  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.
+          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();
+
+          string.Append(hex_digit);
+          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 {
+      // Anything outside of the basic ASCII plane will need to be
+      // decomposed from int32 to a multi-byte sequence.
+      char utf8_units[4] = { 0 };
+      int offset = 0;
+      string.Convert();
+      CBU8_APPEND_UNSAFE(utf8_units, offset, next_char);
+      string.AppendString(utf8_units);
+    }
+  }
+
+  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 spin of the loop will advance to the next byte.
+  NextNChars(3);
+
+  // If this is a high surrogate, consume the next code unit to get the
+  // low surrogate.
+  int code_unit16_low = 0;
+  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'.
+    if (!HexStringToInt(StringPiece(pos_, 4), &code_unit16_low))
+      return false;
+
+    NextNChars(3);
+
+    if (!CBU16_IS_SURROGATE(code_unit16_low) ||
+        !CBU16_IS_TRAIL(code_unit16_low)) {
+      return false;
+    }
+  } else if (!CBU16_IS_SINGLE(code_unit16_high)) {
+    // If this is not a code point, it's an encoding error.
+    return false;
+  }
+
+  // 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 (!code_unit16_low) {
+    CBU8_APPEND_UNSAFE(code_point, offset, code_unit16_high);
+  } else {
+    uint32 code_unit32 = CBU16_GET_SUPPLEMENTARY(code_unit16_high,
+                                                 code_unit16_low);
+    offset = 0;
+    CBU8_APPEND_UNSAFE(code_point, offset, code_unit32);
+  }
+  dest_string->append(code_point);
+  return true;
+}
+
+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 faction 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':
+      if (!CanConsume(3) || !StringsAreEqual(pos_, "true", 4)) {
+        ReportError(JSONReader::JSON_SYNTAX_ERROR, 1);
+        return NULL;
+      }
+      NextNChars(3);
+      return Value::CreateBooleanValue(true);
+    case 'f':
+      if (!CanConsume(4) || !StringsAreEqual(pos_, "false", 5)) {
+        ReportError(JSONReader::JSON_SYNTAX_ERROR, 1);
+        return NULL;
+      }
+      NextNChars(4);
+      return Value::CreateBooleanValue(false);
+    case 'n':
+      if (!CanConsume(3) || !StringsAreEqual(pos_, "null", 4)) {
+        ReportError(JSONReader::JSON_SYNTAX_ERROR, 1);
+        return NULL;
+      }
+      NextNChars(3);
+      return Value::CreateNullValue();
+    default:
+      ReportError(JSONReader::JSON_UNEXPECTED_TOKEN, 1);
+      return NULL;
+  }
+}
+
+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 base::StringPrintf(

[tfarina, 2012/04/19 22:54:47]

nit: base:: here is not necessary as we are in base namespace.

[Robert Sesek, 2012/05/03 15:34:52]

Done.

+        "Line: %i, column: %i, %s", line, column, description.c_str());
+  }
+  return description;
+}
+
+}  // namespace internal
+}  // namespace base