/* exported getImageSize */
/* global BlobView */
/* global parseJPEGMetadata */

/*
 * Determine the pixel dimensions of an image without actually
 * decoding the image. Passes an object of metadata to the callback
 * function on success or an error message to the error function on
 * failure. The metadata object will include type, width and height
 * properties. Supported image types are GIF, PNG and JPEG. JPEG
 * metadata may also include information about an EXIF preview image.
 *
 * Because of shortcomings in the way Gecko handles images, the
 * Gallery app will crash with an OOM error if it attempts to decode
 * and display an image that is too big. Images require 4 bytes per
 * pixel, so a 10 megapixel photograph requires 40 megabytes of image
 * memory. This function gives the gallery app a way to reject images
 * that are too large.
 *
 * Requires the BlobView class from shared/js/blobview.js and the
 * parseJPEGMetadata() function from shared/js/media/jpeg_metadata_parser.js
 */
function getImageSize(blob, callback, error) {
  'use strict';

  BlobView.get(blob, 0, Math.min(1024, blob.size), function(data) {
    // Make sure we are at least 8 bytes long before reading the first 8 bytes
    if (data.byteLength <= 8) {
      error('corrupt image file');
      return;
    }
    var magic = data.getBinaryText(0, 8);
    if (magic.substring(0, 4) === 'GIF8') {
      try {
        callback({
          type: 'gif',
          width: data.getUint16(6, true),
          height: data.getUint16(8, true)
        });
      }
      catch (e) {
        error(e.toString());
      }
    }
    else if (magic.substring(0, 8) === '\x89PNG\r\n\x1A\n') {
      try {
        callback({
          type: 'png',
          width: data.getUint32(16, false),
          height: data.getUint32(20, false)
        });
      }
      catch (e) {
        error(e.toString());
      }
    }
    else if (magic.substring(0, 2) === 'BM' &&
             data.getUint32(2, true) === blob.size) {
      // This is a BMP file
      try {
        var width, height;

        if (data.getUint16(14, true) === 12) { // check format version
          width = data.getUint16(18, true);  // 16-bit little endian width
          height = data.getUint16(20, true); // 16-bit little endian height
        }
        else { // newer versions of the format use 32-bit ints
          width = data.getUint32(18, true);  // 32-bit little endian width
          height = data.getUint32(22, true); // 32-bit little endian height
        }

        callback({
          type: 'bmp',
          width: width,
          height: height
        });
      }
      catch (e) {
        error(e.toString());
      }
    }
    else if (magic.substring(0, 2) === '\xFF\xD8') {
      parseJPEGMetadata(blob,
                        function(metadata) {
                          if (metadata.progressive) {
                            // If the jpeg parser tells us that this is
                            // a progressive jpeg, then treat that as a
                            // distinct image type because pjpegs have
                            // such different memory requirements than
                            // regular jpegs.
                            delete metadata.progressive;
                            metadata.type = 'pjpeg';
                          }
                          else {
                            metadata.type = 'jpeg';
                          }
                          callback(metadata);
                        },
                        error);
    }
    else {
      error('unknown image type');
    }
  });
}