'use strict';

// NOTE: we only support baseline and progressive JPGs here // due to the structure of the loader class, we only get a buffer // with a maximum size of 4096 bytes. so if the SOF marker is outside // if this range we can't detect the file size correctly.

function isJPG (buffer) { //, filepath

var SOIMarker = buffer.toString('hex', 0, 2);

return ('ffd8' === SOIMarker);

}

function extractSize (buffer, i) {

return {
  'height' : buffer.readUInt16BE(i),
  'width' : buffer.readUInt16BE(i + 2)
};

}

function validateBuffer (buffer, i) {

// index should be within buffer limits
if (i > buffer.length) {
  throw new TypeError('Corrupt JPG, exceeded buffer limits');
}
// Every JPEG block must begin with a 0xFF
if (buffer[i] !== 0xFF) {
  throw new TypeError('Invalid JPG, marker table corrupted');
}

}

function calculate (buffer) {

// Skip 5 chars, they are for signature
buffer = buffer.slice(4);

var i, next;
while (buffer.length) {
  // read length of the next block
  i = buffer.readUInt16BE(0);

  // ensure correct format
  validateBuffer(buffer, i);

  // 0xFFC0 is baseline(SOF)
  // 0xFFC2 is progressive(SOF2)
  next = buffer[i + 1];
  if (next === 0xC0 || next === 0xC2) {
    return extractSize(buffer, i + 5);
  }

  // move to the next block
  buffer = buffer.slice(i + 2);
}

throw new TypeError('Invalid JPG, no size found');

}

module.exports = {

'detect': isJPG,
'calculate': calculate

};