Valid search backwards - find EOI quicker if its there.

BUG=210
TEST=out\release\libyuv_unittest --gtest_filter=*Jpeg*
Review URL: https://webrtc-codereview.appspot.com/1271004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@627 16f28f9a-4ce2-e073-06de-1de4eb20be90

README.chromium

 Name: libyuv
 URL: http://code.google.com/p/libyuv/
-Version: 626
+Version: 627
 License: BSD
 License File: LICENSE
 

include/libyuv/version.h

@@ -11,6 +11,6 @@
 #ifndef INCLUDE_LIBYUV_VERSION_H_  // NOLINT
 #define INCLUDE_LIBYUV_VERSION_H_
 
-#define LIBYUV_VERSION 626
+#define LIBYUV_VERSION 627
 
 #endif  // INCLUDE_LIBYUV_VERSION_H_  NOLINT

source/mjpeg_decoder.cc

@@ -76,35 +76,23 @@ MJpegDecoder::~MJpegDecoder() {
 }
 
 // Helper function to validate the jpeg looks ok.
-// TODO(fbarchard): Improve performance. Scan backward for EOI?
+// TODO(fbarchard): Optimize case where SOI is found but EOI is not.
 bool ValidateJpeg(const uint8* sample, size_t sample_size) {
   if (sample_size < 64) {
     // ERROR: Invalid jpeg size: sample_size
     return false;
   }
-  if (sample[0] != 0xff || sample[1] != 0xd8) {
+  if (sample[0] != 0xff || sample[1] != 0xd8) {  // Start Of Image
     // ERROR: Invalid jpeg initial start code
     return false;
   }
-  bool soi = true;
-  int total_eoi = 0;
-  for (int i = 2; i < static_cast<int>(sample_size) - 1; ++i) {
-    if (sample[i] == 0xff) {
-      if (sample[i + 1] == 0xd8) {  // Start Of Image
-        soi = true;
-      } else if (sample[i + 1] == 0xd9) {  // End Of Image
-        if (soi) {
-          ++total_eoi;
-        }
-        soi = false;
-      }
+  for (int i = static_cast<int>(sample_size) - 1; i > 2; --i) {
+    if (sample[i - 1] == 0xff && sample[i] == 0xd9) {  // End Of Image
+      return true;
     }
   }
-  if (!total_eoi) {
-    // ERROR: Invalid jpeg end code not found. Size sample_size
-    return false;
-  }
-  return true;
+  // ERROR: Invalid jpeg end code not found. Size sample_size
+  return false;
 }
 
 bool MJpegDecoder::LoadFrame(const uint8* src, size_t src_len) {