Improve JSONReader performance by up to 55% by using std::string instead of wstring.

base/json/json_reader.cc

 // Copyright (c) 2011 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_reader.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"
+#include "base/string_piece.h"
 #include "base/string_util.h"
+#include "base/third_party/icu/icu_utf.h"
 #include "base/utf_string_conversions.h"
 #include "base/values.h"
 
 namespace {
 
-const wchar_t kNullString[] = L"null";
-const wchar_t kTrueString[] = L"true";
-const wchar_t kFalseString[] = L"false";
+const char kNullString[] = "null";
+const char kTrueString[] = "true";
+const char kFalseString[] = "false";
 
 const int kStackLimit = 100;
 
 // A helper method for ParseNumberToken.  It reads an int from the end of
 // token.  The method returns false if there is no valid integer at the end of
 // the token.
 bool ReadInt(base::JSONReader::Token& token, bool can_have_leading_zeros) {
-  wchar_t first = token.NextChar();
+  char first = token.NextChar();
   int len = 0;
 
   // Read in more digits.
-  wchar_t c = first;
+  char c = first;
   while ('\0' != c && IsAsciiDigit(c)) {
     ++token.length;
     ++len;
     c = token.NextChar();
   }
   // We need at least 1 digit.
   if (len == 0)
     return false;
 
   if (!can_have_leading_zeros && len > 1 && '0' == first)
     return false;
 
   return true;
 }
 
 // A helper method for ParseStringToken.  It reads |digits| hex digits from the
 // token. If the sequence if digits is not valid (contains other characters),
 // the method returns false.
 bool ReadHexDigits(base::JSONReader::Token& token, int digits) {
   for (int i = 1; i <= digits; ++i) {
-    wchar_t c = *(token.begin + token.length + i);
+    char c = *(token.begin + token.length + i);
     if (c == '\0' || !IsHexDigit(c))
       return false;
   }
 
   token.length += digits;
   return true;
 }
 
 }  // namespace
 
@@ skipping 12 matching lines @@
 const char* JSONReader::kUnexpectedDataAfterRoot =
     "Unexpected data after root element.";
 const char* JSONReader::kUnsupportedEncoding =
     "Unsupported encoding. JSON must be UTF-8.";
 const char* JSONReader::kUnquotedDictionaryKey =
     "Dictionary keys must be quoted.";
 
 JSONReader::JSONReader()
     : start_pos_(NULL),
       json_pos_(NULL),
+      end_pos_(NULL),
       stack_depth_(0),
       allow_trailing_comma_(false),
       error_code_(JSON_NO_ERROR),
       error_line_(0),
       error_col_(0) {}
 
 // static
 Value* JSONReader::Read(const std::string& json,
                         bool allow_trailing_comma) {
   return ReadAndReturnError(json, allow_trailing_comma, NULL, NULL);
@@ skipping 45 matching lines @@
 }
 
 std::string JSONReader::GetErrorMessage() const {
   return FormatErrorMessage(error_line_, error_col_,
                             ErrorCodeToString(error_code_));
 }
 
 Value* JSONReader::JsonToValue(const std::string& json, bool check_root,
                                bool allow_trailing_comma) {
   // The input must be in UTF-8.
-  if (!IsStringUTF8(json.c_str())) {
+  if (!IsStringUTF8(json.data())) {
     error_code_ = JSON_UNSUPPORTED_ENCODING;
     return NULL;
   }
 
-  // The conversion from UTF8 to wstring removes null bytes for us
-  // (a good thing).
-  std::wstring json_wide(UTF8ToWide(json));
-  start_pos_ = json_wide.c_str();
+  start_pos_ = json.data();
+  end_pos_ = start_pos_ + json.size();
 
-  // When the input JSON string starts with a UTF-8 Byte-Order-Mark
-  // (0xEF, 0xBB, 0xBF), the UTF8ToWide() function converts it to a Unicode
-  // BOM (U+FEFF). To avoid the JSONReader::BuildValue() function from
-  // mis-treating a Unicode BOM as an invalid character and returning NULL,
-  // skip a converted Unicode BOM if it exists.
-  if (!json_wide.empty() && start_pos_[0] == 0xFEFF) {
-    ++start_pos_;
+  // When the input JSON string starts with a UTF-8 Byte-Order-Mark (U+FEFF)
+  // or <0xEF 0xBB 0xBF>, advance the start position to avoid the
+  // JSONReader::BuildValue() function from mis-treating a Unicode BOM as an
+  // invalid character and returning NULL.
+  if (json.size() >= 3 && start_pos_[0] == 0xEF &&
+      start_pos_[1] == 0xBB && start_pos_[2] == 0xBF) {
+    start_pos_ += 3;
   }
 
   json_pos_ = start_pos_;
   allow_trailing_comma_ = allow_trailing_comma;
   stack_depth_ = 0;
   error_code_ = JSON_NO_ERROR;
 
   scoped_ptr<Value> root(BuildValue(check_root));
   if (root.get()) {
     if (ParseToken().type == Token::END_OF_INPUT) {
@@ skipping 171 matching lines @@
   json_pos_ += token.length;
 
   --stack_depth_;
   return node.release();
 }
 
 JSONReader::Token JSONReader::ParseNumberToken() {
   // We just grab the number here.  We validate the size in DecodeNumber.
   // According   to RFC4627, a valid number is: [minus] int [frac] [exp]
   Token token(Token::NUMBER, json_pos_, 0);
-  wchar_t c = *json_pos_;
+  char c = *json_pos_;
   if ('-' == c) {
     ++token.length;
     c = token.NextChar();
   }
 
   if (!ReadInt(token, false))
     return Token::CreateInvalidToken();
 
   // Optional fraction part
   c = token.NextChar();
@@ skipping 13 matching lines @@
       c = token.NextChar();
     }
     if (!ReadInt(token, true))
       return Token::CreateInvalidToken();
   }
 
   return token;
 }
 
 Value* JSONReader::DecodeNumber(const Token& token) {
-  const std::wstring num_string(token.begin, token.length);
+  const std::string num_string(token.begin, token.length);
 
   int num_int;
-  if (StringToInt(WideToUTF8(num_string), &num_int))
+  if (StringToInt(num_string, &num_int))
     return Value::CreateIntegerValue(num_int);
 
   double num_double;
-  if (StringToDouble(WideToUTF8(num_string), &num_double) &&
-      base::IsFinite(num_double))
+  if (StringToDouble(num_string, &num_double) && base::IsFinite(num_double))
     return Value::CreateDoubleValue(num_double);
 
   return NULL;
 }
 
 JSONReader::Token JSONReader::ParseStringToken() {
   Token token(Token::STRING, json_pos_, 1);
-  wchar_t c = token.NextChar();
-  while ('\0' != c) {
+  char c = token.NextChar();
+  while (json_pos_ + token.length < end_pos_) {
     if ('\\' == c) {
       ++token.length;
       c = token.NextChar();
       // Make sure the escaped char is valid.
       switch (c) {
         case 'x':
           if (!ReadHexDigits(token, 2)) {
             SetErrorCode(JSON_INVALID_ESCAPE, json_pos_ + token.length);
             return Token::CreateInvalidToken();
           }
@@ skipping 22 matching lines @@
       ++token.length;
       return token;
     }
     ++token.length;
     c = token.NextChar();
   }
   return Token::CreateInvalidToken();
 }
 
 Value* JSONReader::DecodeString(const Token& token) {
-  std::wstring decoded_str;
+  std::string decoded_str;
   decoded_str.reserve(token.length - 2);
 
   for (int i = 1; i < token.length - 1; ++i) {
-    wchar_t c = *(token.begin + i);
+    char c = *(token.begin + i);
     if ('\\' == c) {
       ++i;
       c = *(token.begin + i);
       switch (c) {
         case '"':
         case '/':
         case '\\':
           decoded_str.push_back(c);
           break;
         case 'b':
           decoded_str.push_back('\b');
           break;
         case 'f':
           decoded_str.push_back('\f');
           break;
         case 'n':
           decoded_str.push_back('\n');
           break;
         case 'r':
           decoded_str.push_back('\r');
           break;
         case 't':
           decoded_str.push_back('\t');
           break;
         case 'v':
           decoded_str.push_back('\v');
           break;
 
-        case 'x':
-          decoded_str.push_back((HexDigitToInt(*(token.begin + i + 1)) << 4) +
-                                HexDigitToInt(*(token.begin + i + 2)));
+        case 'x': {
+          if (i + 2 >= token.length)
+            return NULL;
+          int hex_digit = 0;
+          if (!HexStringToInt(StringPiece(token.begin + i + 1, 2), &hex_digit))
+            return NULL;
+          decoded_str.push_back(hex_digit);
           i += 2;
           break;
+        }
         case 'u':
-          decoded_str.push_back((HexDigitToInt(*(token.begin + i + 1)) << 12 ) +
-                                (HexDigitToInt(*(token.begin + i + 2)) << 8) +
-                                (HexDigitToInt(*(token.begin + i + 3)) << 4) +
-                                HexDigitToInt(*(token.begin + i + 4)));
-          i += 4;
+          if (!ConvertUTF16Units(token, &i, &decoded_str))
+            return NULL;
           break;
 
         default:
           // We should only have valid strings at this point.  If not,
           // ParseStringToken didn't do it's job.
           NOTREACHED();
           return NULL;
       }
     } else {
       // Not escaped
       decoded_str.push_back(c);
     }
   }
-  return Value::CreateStringValue(WideToUTF16Hack(decoded_str));
+  return Value::CreateStringValue(decoded_str);
+}
+
+bool JSONReader::ConvertUTF16Units(const Token& token,
+                                   int* i,
+                                   std::string* dest_string) {
+  if (*i + 4 >= token.length)
+    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(token.begin + *i + 1, 4), &code_unit16_high))
+    return false;
+  *i += 4;
+
+  // 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 (*i + 6 >= token.length)
+      return false;
+    if (*(++(*i) + token.begin) != '\\' || *(++(*i) + token.begin) != 'u')
+      return false;
+
+    if (!HexStringToInt(StringPiece(token.begin + *i + 1, 4), &code_unit16_low))
+      return false;
+    *i += 4;
+    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;
 }
 
 JSONReader::Token JSONReader::ParseToken() {
   EatWhitespaceAndComments();
 
   Token token(Token::INVALID_TOKEN, 0, 0);
   switch (*json_pos_) {
     case '\0':
       token.type = Token::END_OF_INPUT;
       break;
@@ skipping 52 matching lines @@
       break;
 
     case '"':
       token = ParseStringToken();
       break;
   }
   return token;
 }
 
 void JSONReader::EatWhitespaceAndComments() {
-  while ('\0' != *json_pos_) {
+  while (json_pos_ != end_pos_) {
     switch (*json_pos_) {
       case ' ':
       case '\n':
       case '\r':
       case '\t':
         ++json_pos_;
         break;
       case '/':
         // TODO(tc): This isn't in the RFC so it should be a parser flag.
         if (!EatComment())
           return;
         break;
       default:
         // Not a whitespace char, just exit.
         return;
     }
   }
 }
 
 bool JSONReader::EatComment() {
   if ('/' != *json_pos_)
     return false;
 
-  wchar_t next_char = *(json_pos_ + 1);
+  char next_char = *(json_pos_ + 1);
   if ('/' == next_char) {
     // Line comment, read until \n or \r
     json_pos_ += 2;
-    while ('\0' != *json_pos_) {
+    while (json_pos_ != end_pos_) {
       switch (*json_pos_) {
         case '\n':
         case '\r':
           ++json_pos_;
           return true;
         default:
           ++json_pos_;
       }
     }
   } else if ('*' == next_char) {
     // Block comment, read until */
     json_pos_ += 2;
-    while ('\0' != *json_pos_) {
+    while (json_pos_ != end_pos_) {
       if ('*' == *json_pos_ && '/' == *(json_pos_ + 1)) {
         json_pos_ += 2;
         return true;
       }
       ++json_pos_;
     }
   } else {
     return false;
   }
   return true;
 }
 
-bool JSONReader::NextStringMatch(const wchar_t* str, size_t length) {
-  return wcsncmp(json_pos_, str, length) == 0;
+bool JSONReader::NextStringMatch(const char* str, size_t length) {
+  return strncmp(json_pos_, str, length) == 0;
 }
 
 void JSONReader::SetErrorCode(JsonParseError error,
-                              const wchar_t* error_pos) {
+                              const char* error_pos) {
   int line_number = 1;
   int column_number = 1;
 
   // Figure out the line and column the error occured at.
-  for (const wchar_t* pos = start_pos_; pos != error_pos; ++pos) {
-    if (*pos == '\0') {
+  for (const char* pos = start_pos_; pos != error_pos; ++pos) {
+    if (pos > end_pos_) {
       NOTREACHED();
       return;
     }
 
     if (*pos == '\n') {
       ++line_number;
       column_number = 1;
     } else {
       ++column_number;
     }
   }
 
   error_line_ = line_number;
   error_col_ = column_number;
   error_code_ = error;
 }
 
 }  // namespace base