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

base/json/json_parser.h

Index: base/json/json_parser.h
diff --git a/base/json/json_parser.h b/base/json/json_parser.h
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+// 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.
+
+#ifndef BASE_JSON_JSON_PARSER_H_
+#define BASE_JSON_JSON_PARSER_H_
+#pragma once
+
+#include <string>
+
+#include "base/base_export.h"
+#include "base/basictypes.h"
+#include "base/compiler_specific.h"
+#include "base/json/json_reader.h"
+#include "base/string_piece.h"
+
+#if !defined(OS_CHROMEOS)
+#include "base/gtest_prod_util.h"
+#endif
+
+namespace base {
+class Value;
+}
+
+#if defined(OS_CHROMEOS)
+// Chromium and Chromium OS check out gtest to different places, so this is
+// unable to compile on both if gtest_prod.h is included here. Instead, include
+// its only contents -- this will need to be updated if the macro ever changes.
+#define FRIEND_TEST(test_case_name, test_name)\
+friend class test_case_name##_##test_name##_Test
+
+#define FRIEND_TEST_ALL_PREFIXES(test_case_name, test_name) \
+  FRIEND_TEST(test_case_name, test_name); \
+  FRIEND_TEST(test_case_name, DISABLED_##test_name); \
+  FRIEND_TEST(test_case_name, FLAKY_##test_name); \
+  FRIEND_TEST(test_case_name, FAILS_##test_name)
+#endif  // OS_CHROMEOS
+
+namespace base {
+namespace internal {
+
+class JSONParserTest;
+
+// The implementation behind the JSONReader interface. This class is not meant
+// to be used directly; it encapsulates logic that need not be exposed publicly.
+//
+// This parser guarantees O(n) time through the input string. It also optimizes
+// base::StringValue by using StringPiece where possible when returning Value
+// objects by using "hidden roots," discussed in the implementation.
+//
+// Iteration happens on the byte level, with the functions CanConsume and
+// NextChar. The conversion from byte to JSON token happens without advancing
+// the parser in GetNextToken/ParseToken, that is tokenization operates on
+// the current parser position without advancing.
+//
+// Built on top of these are a family of Consume functions that iterate
+// internally. Invariant: on entry of a Consume function, the parser is wound
+// to the first byte of a valid JSON token. On exit, it is on the last byte
+// of a token, such that the next iteration of the parser will be at the byte
+// immediately following the token, which would likely be the first byte of the
+// next token.
+class BASE_EXPORT_PRIVATE JSONParser {
+ public:
+  explicit JSONParser(int options);
+  ~JSONParser();
+
+  // Parses the input string according to the set options and returns the
+  // result as a Value owned by the caller.
+  Value* Parse(const std::string& input);
+
+  // Returns the error code.
+  JSONReader::JsonParseError error_code() const;
+
+  // Returns the human-friendly error message.
+  std::string GetErrorMessage() const;
+
+ private:
+  enum Token {
+    T_OBJECT_BEGIN,           // {
+    T_OBJECT_END,             // }
+    T_ARRAY_BEGIN,            // [
+    T_ARRAY_END,              // ]
+    T_STRING,
+    T_NUMBER,
+    T_BOOL_TRUE,              // true
+    T_BOOL_FALSE,             // false
+    T_NULL,                   // null
+    T_LIST_SEPARATOR,         // ,
+    T_OBJECT_PAIR_SEPARATOR,  // :
+    T_END_OF_INPUT,
+    T_INVALID_TOKEN,
+  };
+
+  // A helper class used for parsing strings. One optimization performed is to
+  // create base::Value with a StringPiece to avoid unnecessary std::string
+  // copies. This is not possible if the input string needs to be decoded from
+  // UTF-16 to UTF-8, or if an escape sequence causes characters to be skipped.
+  // This class centralizes that logic.
+  class StringBuilder {
+   public:
+    // Empty constructor. Used for creating a builder with which to Swap().
+    StringBuilder();
+
+    // |pos| is the beginning of an input string, excluding the |"|.
+    explicit StringBuilder(const char* pos);
+
+    ~StringBuilder();
+
+    // Swaps the contents of |other| with this.
+    void Swap(StringBuilder* other);
+
+    // Either increases the |length_| of the string or copies the character if
+    // the StringBuilder has been converted. |c| must be in the basic ASCII
+    // plane; all other characters need to be in UTF-8 units, appended with
+    // AppendString below.
+    void Append(const char& c);
+
+    // Appends a string to the std::string. Must be Convert()ed to use.
+    void AppendString(const std::string& str);
+
+    // Converts the builder from its default StringPiece to a full std::string,
+    // performing a copy. Once a builder is converted, it cannot be made a
+    // StringPiece again.
+    void Convert();
+
+    // Returns whether the builder can be converted to a StringPiece.
+    bool CanBeStringPiece() const;
+
+    // Returns the StringPiece representation. Returns an empty piece if it
+    // cannot be converted.
+    StringPiece AsStringPiece();
+
+    // Returns the builder as a std::string.
+    const std::string& AsString();
+
+   private:
+    // The beginning of the input string.
+    const char* pos_;
+
+    // Number of bytes in |pos_| that make up the string being built.
+    size_t length_;
+
+    // The copied string representation. NULL until Convert() is called.
+    // Strong. scoped_ptr<T> has too much of an overhead here.
+    std::string* string_;
+  };
+
+  // Quick check that the stream has capacity to consume |length| more bytes.
+  bool CanConsume(int length);
+
+  // The basic way to consume a single character in the stream. Consumes one
+  // byte of the input stream and returns a pointer to the rest of it.
+  const char* NextChar();
+
+  // Performs the equivalent of NextChar N times.
+  void NextNChars(int n);
+
+  // Skips over whitespace and comments to find the next token in the stream.
+  // This does not advance the parser for non-whitespace or comment chars.
+  Token GetNextToken();
+
+  // Consumes whitespace characters and comments until the next non-that is
+  // encountered.
+  void EatWhitespaceAndComments();
+  // Helper function that consumes a comment, assuming that the parser is
+  // currently wound to a '/'.
+  bool EatComment();
+
+  // Calls GetNextToken() and then ParseToken(). Caller owns the result.
+  Value* ParseNextToken();
+
+  // Takes a token that represents the start of a Value ("a structural token"
+  // in RFC terms) and consumes it, returning the result as an object the
+  // caller owns.
+  Value* ParseToken(Token token);
+
+  // Assuming that the parser is currently wound to '{', this parses a JSON
+  // object into a DictionaryValue.
+  Value* ConsumeDictionary();
+
+  // Assuming that the parser is wound to '[', this parses a JSON list into a
+  // ListValue.
+  Value* ConsumeList();
+
+  // Calls through ConsumeStringRaw and wraps it in a value.
+  Value* ConsumeString();
+
+  // Assuming that the parser is wound to a double quote, this parses a string,
+  // decoding any escape sequences and converts UTF-16 to UTF-8. Returns true on
+  // success and Swap()s the result into |out|. Returns false on failure with
+  // error information set.
+  bool ConsumeStringRaw(StringBuilder* out);
+  // Helper function for ConsumeStringRaw() that consumes the next four or 10
+  // bytes (parser is wound to the first character of a HEX sequence, with the
+  // potential for consuming another \uXXXX for a surrogate). Returns true on
+  // success and places the UTF8 code units in |dest_string|, and false on
+  // failure.
+  bool DecodeUTF16(std::string* dest_string);
+  // Helper function for ConsumeStringRaw() that takes a single code point,
+  // decodes it into UTF-8 units, and appends it to the given builder. The
+  // point must be valid.
+  void DecodeUTF8(const int32& point, StringBuilder* dest);
+
+  // Assuming that the parser is wound to the start of a valid JSON number,
+  // this parses and converts it to either an int or double value.
+  Value* ConsumeNumber();
+  // Helper that reads characters that are ints. Returns true if a number was
+  // read and false on error.
+  bool ReadInt(bool allow_leading_zeros);
+
+  // Consumes the literal values of |true|, |false|, and |null|, assuming the
+  // parser is wound to the first character of any of those.
+  Value* ConsumeLiteral();
+
+  // Compares two string buffers of a given length.
+  static bool StringsAreEqual(const char* left, const char* right, size_t len);
+
+  // Sets the error information to |code| at the current column, based on
+  // |index_| and |index_last_line_|, with an optional positive/negative
+  // adjustment by |column_adjust|.
+  void ReportError(JSONReader::JsonParseError code, int column_adjust);
+
+  // Given the line and column number of an error, formats one of the error
+  // message contants from json_reader.h for human display.
+  static std::string FormatErrorMessage(int line, int column,
+                                        const std::string& description);
+
+  // base::JSONParserOptions that control parsing.
+  int options_;
+
+  // Pointer to the start of the input data.
+  const char* start_pos_;
+
+  // Pointer to the current position in the input data. Equivalent to
+  // |start_pos_ + index_|.
+  const char* pos_;
+
+  // Pointer to the last character of the input data.
+  const char* end_pos_;
+
+  // The index in the input stream to which the parser is wound.
+  int index_;
+
+  // The number of times the parser has recursed (current stack depth).
+  int stack_depth_;
+
+  // The line number that the parser is at currently.
+  int line_number_;
+
+  // The last value of |index_| on the previous line.
+  int index_last_line_;
+
+  // Error information.
+  JSONReader::JsonParseError error_code_;
+  int error_line_;
+  int error_column_;
+
+  friend class JSONParserTest;
+  FRIEND_TEST_ALL_PREFIXES(JSONParserTest, NextChar);
+  FRIEND_TEST_ALL_PREFIXES(JSONParserTest, ConsumeDictionary);
+  FRIEND_TEST_ALL_PREFIXES(JSONParserTest, ConsumeList);
+  FRIEND_TEST_ALL_PREFIXES(JSONParserTest, ConsumeString);
+  FRIEND_TEST_ALL_PREFIXES(JSONParserTest, ConsumeLiterals);
+  FRIEND_TEST_ALL_PREFIXES(JSONParserTest, ConsumeNumbers);
+  FRIEND_TEST_ALL_PREFIXES(JSONParserTest, ErrorMessages);
+
+  DISALLOW_COPY_AND_ASSIGN(JSONParser);
+};
+
+}  // namespace internal
+}  // namespace base
+
+#endif  // BASE_JSON_JSON_PARSER_H_