Gets the JPEG size from the array of data passed to the function

jpegDimensions.go

func jpegDimensions(data []byte) (int32, int32) {
  var width, height, i int

  data_size := cap(data)

  if data[i] == 0xFF && data[i+1] == 0xD8 && data[i+2] == 0xFF && data[i+3] == 0xE0 {
    i += 4
    if data[i+2] == 'J' && data[i+3] == 'F' && data[i+4] == 'I' && data[i+5] == 'F' && data[i+6] == 0x00 {
      block_length := int(data[i])*256 + int(data[i+1])
      for i < data_size {
        i += block_length //Increase the file index to get to the next block

        if i >= data_size {
          return -1, -1 //Check to protect against segmentation faults
        }

        if data[i] != 0xFF {
          return -1, -1 //Check that we are truly at the start of another block
        }

        if data[i+1] == 0xC0 {
          //0xFFC0 is the "Start of frame" marker which contains the file size
          //The structure of the 0xFFC0 block is quite simple [0xFFC0][ushort length][uchar precision][ushort x][ushort y]
          height = int(data[i+5])*256 + int(data[i+6])
          width = int(data[i+7])*256 + int(data[i+8])
          return int32(width), int32(height)
        } else {
          i += 2                                           //Skip the block marker
          block_length = int(data[i])*256 + int(data[i+1]) //Go to the next block
        }
      }
    }
  }

  return 0, 0
}