具体情况

在OS X下，计算得的正确md5值是：63e3fbe6 a0438f26 e49e5dab 1c4af0d3。

NSData *temp = [[NSData alloc] initWithContentsOfFile:@"/Users/apple/Pictures/temp/tu7.png"];
NSLog(@"md5: %@", [temp md5]);

在iOS中，UIImagePickerControllerDelegate协议，imagePickerController: didFinishPickingMediaWithInfo:方法，计算的的md5值是：55ea2f62 5df31067 683aa9a8 3a06d420。

UIImage *image = info[UIImagePickerControllerOriginalImage];
NSData *temp = UIImagePNGRepresentation(image);
NSLog(@"md5: %@", [temp md5]);

两个md5值结果不一样，我希望计算图片整个文件的md5，因为要拿这个md5值与服务器上贮存的图片进行比对，服务器上图片文件计算得md5与通过绝对路径计算得md5相同，所以我尝试获取被选中图片的路径，像/Users/apple/Pictures/temp/tu7.png这种形式的，我知道通过info可以获取asset library url。

NSURL *imgurl = [info objectForKey:UIImagePickerControllerReferenceURL];

获得的结果是像assets-library://asset/asset.JPG?id=00000000-0000-0000-0000-00000000007E&ext=JPG这种形式的。

问

1. 有什么方法可以把asset library url变成绝对路径，也就是 形式assets-library://asset/asset.JPG?id=00000000-0000-0000-0000-00000000007E&ext=JPG -> 形式/Users/apple/Pictures/temp/tu7.png 的转变？
2. 如果没有，有什么方法可以决解这个问题？能否提供提供决解思路？

• (NSString *)fileMD5WithAsset:(ALAsset *)asset
  {
  if (!asset) {
  return nil;
  }

  ALAssetRepresentation *rep = [asset defaultRepresentation];
  unsigned long readStep = 256;
  uint8_t *buffer = calloc(readStep, sizeof(*buffer));
  unsigned long long offset = 0;
  unsigned long long bytesRead = 0;
  NSError *error = nil;
  unsigned long long fileSize = [rep size];
  int chunks = (int)((fileSize + readStep - 1)/readStep);
  unsigned long long lastChunkSize = fileSize%readStep;

  CC_MD5_CTX md5;
  CC_MD5_Init(&md5);
  BOOL isExp = NO;
  int currentChunk = 0;
  while(!isExp && currentChunk < chunks){
  @try {
  if(currentChunk < chunks - 1){
  bytesRead = [rep getBytes:buffer fromOffset:offset length:(unsigned long)readStep error:&error];
  }else{
  bytesRead = [rep getBytes:buffer fromOffset:offset length:(unsigned long)lastChunkSize error:&error];
  }
  NSData * fileData = [NSData dataWithBytesNoCopy:buffer length:(unsigned long)bytesRead freeWhenDone:NO];
  CC_MD5_Update(&md5, [fileData bytes], (CC_LONG)[fileData length]);
  offset += readStep;
  } @catch(NSException exception) {
  isExp = YES;
  free(buffer);
  }
  currentChunk += 1;
  }
  unsigned char digest[CC_MD5_DIGEST_LENGTH];
  CC_MD5_Final(digest, &md5);
  NSString s = [NSString stringWithFormat: @"%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x",
  digest[0], digest[1],
  digest[2], digest[3],
  digest[4], digest[5],
  digest[6], digest[7],
  digest[8], digest[9],
  digest[10], digest[11],
  digest[12], digest[13],
  digest[14], digest[15]];
  return s;
  }