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+// Copyright (c) 2006-2008 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.
+//
+// A JSON parser.  Converts strings of JSON into a Value object (see
+// base/values.h).
+// http://www.ietf.org/rfc/rfc4627.txt?number=4627
+//
+// Known limitations/deviations from the RFC:
+// - Only knows how to parse ints within the range of a signed 32 bit int and
+//   decimal numbers within a double.
+// - Assumes input is encoded as UTF8.  The spec says we should allow UTF-16
+//   (BE or LE) and UTF-32 (BE or LE) as well.
+// - We limit nesting to 100 levels to prevent stack overflow (this is allowed
+//   by the RFC).
+// - A Unicode FAQ ("http://unicode.org/faq/utf_bom.html") writes a data
+//   stream may start with a Unicode Byte-Order-Mark (U+FEFF), i.e. the input
+//   UTF-8 string for the JSONReader::JsonToValue() function may start with a
+//   UTF-8 BOM (0xEF, 0xBB, 0xBF).
+//   To avoid the function from mis-treating a UTF-8 BOM as an invalid
+//   character, the function skips a Unicode BOM at the beginning of the
+//   Unicode string (converted from the input UTF-8 string) before parsing it.
+//
+// TODO(tc): Add a parsing option to to relax object keys being wrapped in
+//   double quotes
+// TODO(tc): Add an option to disable comment stripping
+// TODO(aa): Consider making the constructor public and the static Read() method
+// only a convenience for the common uses with more complex configuration going
+// on the instance.
+
+#ifndef BASE_JSON_JSON_READER_H_
+#define BASE_JSON_JSON_READER_H_
+
+#include <string>
+
+#include "base/basictypes.h"
+#include "testing/gtest/include/gtest/gtest_prod.h"
+
+class Value;
+
+namespace base {
+
+class JSONReader {
+ public:
+  // A struct to hold a JS token.
+  class Token {
+   public:
+    enum Type {
+     OBJECT_BEGIN,           // {
+     OBJECT_END,             // }
+     ARRAY_BEGIN,            // [
+     ARRAY_END,              // ]
+     STRING,
+     NUMBER,
+     BOOL_TRUE,              // true
+     BOOL_FALSE,             // false
+     NULL_TOKEN,             // null
+     LIST_SEPARATOR,         // ,
+     OBJECT_PAIR_SEPARATOR,  // :
+     END_OF_INPUT,
+     INVALID_TOKEN,
+    };
+    Token(Type t, const wchar_t* b, int len)
+      : type(t), begin(b), length(len) {}
+
+    Type type;
+
+    // A pointer into JSONReader::json_pos_ that's the beginning of this token.
+    const wchar_t* begin;
+
+    // End should be one char past the end of the token.
+    int length;
+
+    // Get the character that's one past the end of this token.
+    wchar_t NextChar() {
+      return *(begin + length);
+    }
+  };
+
+  // Error messages that can be returned.
+  static const char* kBadRootElementType;
+  static const char* kInvalidEscape;
+  static const char* kSyntaxError;
+  static const char* kTrailingComma;
+  static const char* kTooMuchNesting;
+  static const char* kUnexpectedDataAfterRoot;
+  static const char* kUnsupportedEncoding;
+  static const char* kUnquotedDictionaryKey;
+
+  JSONReader();
+
+  // Reads and parses |json|, returning a Value. The caller owns the returned
+  // instance. If |json| is not a properly formed JSON string, returns NULL.
+  // If |allow_trailing_comma| is true, we will ignore trailing commas in
+  // objects and arrays even though this goes against the RFC.
+  static Value* Read(const std::string& json, bool allow_trailing_comma);
+
+  // Reads and parses |json| like Read(). |error_message_out| is optional. If
+  // specified and NULL is returned, |error_message_out| will be populated with
+  // a string describing the error. Otherwise, |error_message_out| is
+  // unmodified.
+  static Value* ReadAndReturnError(const std::string& json,
+                                   bool allow_trailing_comma,
+                                   std::string* error_message_out);
+
+  // Returns the error message if the last call to JsonToValue() failed. If the
+  // last call did not fail, returns a valid empty string.
+  std::string error_message() { return error_message_; }
+
+  // Reads and parses |json|, returning a Value. The caller owns the returned
+  // instance. If |json| is not a properly formed JSON string, returns NULL and
+  // a detailed error can be retrieved from |error_message()|.
+  // If |check_root| is true, we require that the root object be an object or
+  // array. Otherwise, it can be any valid JSON type.
+  // If |allow_trailing_comma| is true, we will ignore trailing commas in
+  // objects and arrays even though this goes against the RFC.
+  Value* JsonToValue(const std::string& json, bool check_root,
+                     bool allow_trailing_comma);
+
+ private:
+  static std::string FormatErrorMessage(int line, int column,
+                                        const char* description);
+
+  DISALLOW_COPY_AND_ASSIGN(JSONReader);
+
+  FRIEND_TEST(JSONReaderTest, Reading);
+  FRIEND_TEST(JSONReaderTest, ErrorMessages);
+
+  // Recursively build Value.  Returns NULL if we don't have a valid JSON
+  // string.  If |is_root| is true, we verify that the root element is either
+  // an object or an array.
+  Value* BuildValue(bool is_root);
+
+  // Parses a sequence of characters into a Token::NUMBER. If the sequence of
+  // characters is not a valid number, returns a Token::INVALID_TOKEN. Note
+  // that DecodeNumber is used to actually convert from a string to an
+  // int/double.
+  Token ParseNumberToken();
+
+  // Try and convert the substring that token holds into an int or a double. If
+  // we can (ie., no overflow), return the value, else return NULL.
+  Value* DecodeNumber(const Token& token);
+
+  // Parses a sequence of characters into a Token::STRING. If the sequence of
+  // characters is not a valid string, returns a Token::INVALID_TOKEN. Note
+  // that DecodeString is used to actually decode the escaped string into an
+  // actual wstring.
+  Token ParseStringToken();
+
+  // Convert the substring into a value string.  This should always succeed
+  // (otherwise ParseStringToken would have failed).
+  Value* DecodeString(const Token& token);
+
+  // Grabs the next token in the JSON stream.  This does not increment the
+  // stream so it can be used to look ahead at the next token.
+  Token ParseToken();
+
+  // Increments |json_pos_| past leading whitespace and comments.
+  void EatWhitespaceAndComments();
+
+  // If |json_pos_| is at the start of a comment, eat it, otherwise, returns
+  // false.
+  bool EatComment();
+
+  // Checks if |json_pos_| matches str.
+  bool NextStringMatch(const std::wstring& str);
+
+  // Creates the error message that will be returned to the caller. The current
+  // line and column are determined and added into the final message.
+  void SetErrorMessage(const char* description, const wchar_t* error_pos);
+
+  // Pointer to the starting position in the input string.
+  const wchar_t* start_pos_;
+
+  // Pointer to the current position in the input string.
+  const wchar_t* json_pos_;
+
+  // Used to keep track of how many nested lists/dicts there are.
+  int stack_depth_;
+
+  // A parser flag that allows trailing commas in objects and arrays.
+  bool allow_trailing_comma_;
+
+  // Contains the error message for the last call to JsonToValue(), if any.
+  std::string error_message_;
+};
+
+}  // namespace base
+
+#endif  // BASE_JSON_JSON_READER_H_