[ios] Adds support for parsing Text-to-Speech search results.

ios/chrome/browser/voice/text_to_speech_parser.mm

+// Copyright 2015 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.
+
+#import "ios/chrome/browser/voice/text_to_speech_parser.h"
+
+#include "base/logging.h"
+#import "ios/web/public/web_state/js/crw_js_injection_receiver.h"
+#include "ios/web/public/web_state/web_state.h"
+#import "third_party/google_toolbox_for_mac/src/Foundation/GTMStringEncoding.h"
+
+namespace {
+
+// The start and end tags that delimit TTS audio data.
+NSString* const kTTSStartTag = @"function(){var _a_tts='";
+NSString* const kTTSEndTag = @"'";
+
+// When |kTTSAudioDataExtractorScriptFormat| is evaluated on a Google voice
+// search results page, this script will extract the innerHTML from the script
+// element containing TTS data.  The format takes one parameter, which is the
+// start tag from the TTS config singleton.
+NSString* const kTTSAudioDataExtractorScriptFormat =
+    @"(function(){"
+     "  var start_tag = \"%@\";"
+     "  var script_elements = document.getElementsByTagName(\"script\");"
+     "  for (var i = 0; i < script_elements.length; ++i) {"
+     "    var script_html = script_elements[i].innerHTML;"
+     "    if (script_html.indexOf(start_tag) > 0)"
+     "      return script_html;"
+     "  }"
+     "  return \"\";"
+     "})()";
+// Escaped encoding for GWS Voice Search service's trailing '='.
+NSString* const kTrailingEqualEncoding = @"\\x3d";
+// The maximum number of trailing '=' characters in a Voice Search SRP.
+const NSUInteger kMaxTrailingEqualsCount = 2;
+
+}  // namespace
+
+NSData* ExtractVoiceSearchAudioDataFromPageHTML(NSString* page_html) {
+  if (!page_html.length)
+    return nil;
+
+  // The data should be near the end of the page, so search backwards.
+  NSRange data_start_tag_range =
+      [page_html rangeOfString:kTTSStartTag options:NSBackwardsSearch];
+  if (data_start_tag_range.location == NSNotFound) {
+    DLOG(ERROR) << "Did not find base tts tag in search output. "
+                << page_html.length;
+    return nil;
+  }
+
+  // The base64-encoded data will be everything between
+  // |audioDataStartTag| and |audioDataEndTag|.
+  NSUInteger start_position =
+      data_start_tag_range.location + data_start_tag_range.length;
+  NSRange data_range =
+      NSMakeRange(start_position, page_html.length - start_position);
+  NSRange data_end_tag_range =
+      [page_html rangeOfString:kTTSEndTag options:0 range:data_range];
+  if (data_end_tag_range.location == NSNotFound ||
+      data_end_tag_range.location == start_position) {
+    DLOG(ERROR) << "Could not find encoded data before tts closing tag.";
+    return nil;
+  }
+
+  // Extract the data between the tags.
+  NSRange audio_data_range =
+      NSMakeRange(start_position, data_end_tag_range.location - start_position);
+  NSString* raw_base64_encoded_audio_string =
+      [page_html substringWithRange:audio_data_range];
+  if (!raw_base64_encoded_audio_string) {
+    DLOG(ERROR) << "Could not find encoded data between tags.";
+    return nil;
+  }
+
+  // GWS is escaping the trailing '=' characters to \x3d.
+  // Clean these up before passing the string to the base64 decoder.
+  // Note: there are at most 2 encoded trailing '=' characters, so limit the
+  // string replacement to the last characters of the string.
+  NSUInteger search_range_length =
+      std::min(kMaxTrailingEqualsCount * kTrailingEqualEncoding.length,
+               raw_base64_encoded_audio_string.length);
+  NSRange search_range =
+      NSMakeRange(raw_base64_encoded_audio_string.length - search_range_length,
+                  search_range_length);
+  NSString* base64_encoded_audio_string = [raw_base64_encoded_audio_string
+      stringByReplacingOccurrencesOfString:kTrailingEqualEncoding
+                                withString:@"="
+                                   options:0
+                                     range:search_range];
+
+  GTMStringEncoding* base64 = [GTMStringEncoding rfc4648Base64StringEncoding];
+  return [base64 decode:base64_encoded_audio_string];
+}
+
+void ExtractVoiceSearchAudioDataFromWebState(
+    web::WebState* webState,
+    TextToSpeechCompletion completion) {
+  DCHECK(webState);
+  DCHECK(completion);
+  NSString* tts_extraction_script = [NSString
+      stringWithFormat:kTTSAudioDataExtractorScriptFormat, kTTSStartTag];
+  [webState->GetJSInjectionReceiver()
+      executeJavaScript:tts_extraction_script
+      completionHandler:^(id result, NSError* error) {
+        completion(ExtractVoiceSearchAudioDataFromPageHTML(result));
+      }];
+}