Implement fast path in UTF8ToUTF16 for pure ASCII strings

base/strings/utf_string_conversions.cc

 // Copyright (c) 2010 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/strings/utf_string_conversions.h"
 
 #include "base/strings/string_piece.h"
 #include "base/strings/string_util.h"
 #include "base/strings/utf_string_conversion_utils.h"
 
@@ skipping 38 matching lines @@
 
 std::string WideToUTF8(const std::wstring& wide) {
   std::string ret;
   // Ignore the success flag of this call, it will do the best it can for
   // invalid input, which is what we want here.
   WideToUTF8(wide.data(), wide.length(), &ret);
   return ret;
 }
 
 bool UTF8ToWide(const char* src, size_t src_len, std::wstring* output) {
-  PrepareForUTF16Or32Output(src, src_len, output);
-  return ConvertUnicode(src, src_len, output);
+  if (IsStringASCII(StringPiece(src, src_len))) {
+    output->assign(src, src + src_len);
+    return true;
+  } else {
+    PrepareForUTF16Or32Output(src, src_len, output);
+    return ConvertUnicode(src, src_len, output);
+  }
 }
 
 std::wstring UTF8ToWide(const StringPiece& utf8) {
+  if (IsStringASCII(utf8)) {
+    return std::wstring(utf8.begin(), utf8.end());
+  }
+
   std::wstring ret;
-  UTF8ToWide(utf8.data(), utf8.length(), &ret);
+  PrepareForUTF16Or32Output(utf8.data(), utf8.length(), &ret);
+  ConvertUnicode(utf8.data(), utf8.length(), &ret);
   return ret;
 }
 
 // UTF-16 <-> Wide -------------------------------------------------------------
 
 #if defined(WCHAR_T_IS_UTF16)
 
 // When wide == UTF-16, then conversions are a NOP.
 bool WideToUTF16(const wchar_t* src, size_t src_len, string16* output) {
   output->assign(src, src_len);
@@ skipping 43 matching lines @@
   return ret;
 }
 
 #endif  // defined(WCHAR_T_IS_UTF32)
 
 // UTF16 <-> UTF8 --------------------------------------------------------------
 
 #if defined(WCHAR_T_IS_UTF32)
 
 bool UTF8ToUTF16(const char* src, size_t src_len, string16* output) {
-  PrepareForUTF16Or32Output(src, src_len, output);
-  return ConvertUnicode(src, src_len, output);
+  if (IsStringASCII(StringPiece(src, src_len))) {
+    output->assign(src, src + src_len);
+    return true;
+  } else {
+    PrepareForUTF16Or32Output(src, src_len, output);
+    return ConvertUnicode(src, src_len, output);
+  }
 }
 
 string16 UTF8ToUTF16(const StringPiece& utf8) {
+  if (IsStringASCII(utf8)) {
+    return string16(utf8.begin(), utf8.end());
+  }
+
   string16 ret;
+  PrepareForUTF16Or32Output(utf8.data(), utf8.length(), &ret);
   // Ignore the success flag of this call, it will do the best it can for
   // invalid input, which is what we want here.
-  UTF8ToUTF16(utf8.data(), utf8.length(), &ret);
+  ConvertUnicode(utf8.data(), utf8.length(), &ret);
   return ret;
 }
 
 bool UTF16ToUTF8(const char16* src, size_t src_len, std::string* output) {
   PrepareForUTF8Output(src, src_len, output);
   return ConvertUnicode(src, src_len, output);
 }
 
 std::string UTF16ToUTF8(const string16& utf16) {
   std::string ret;
@@ skipping 33 matching lines @@
   DCHECK(IsStringASCII(ascii)) << ascii;
   return string16(ascii.begin(), ascii.end());
 }
 
 std::string UTF16ToASCII(const string16& utf16) {
   DCHECK(IsStringASCII(utf16)) << UTF16ToUTF8(utf16);
   return std::string(utf16.begin(), utf16.end());
 }
 
 }  // namespace base