PNG decoder

function pad(byte) {
  "use strict";
  return byte.toString().replace(/^(.(..)*)$/, '0$1');
}

function stringy(byte) {
  "use strict";
  return pad(byte.toString(16));
}

function buildString(str, len) {
  "use strict";
  var reg = new RegExp('.{1,' + len + '}', 'g');

  str = str.match(reg);
  str.forEach(function (m, idx) {
    str[idx] += "\n";
  });

  return str.join('');
}

function decompress(code) {
  "use strict";
  var mode = 0, copy, i = 8, lua = '', codelen = (code.charCodeAt(4) << 8) | code.charCodeAt(5);
  var dict = "\n 0123456789abcdefghijklmnopqrstuvwxyz!#%(){}[]<>+=/*:;.,~_".split('');
  var byte, offset, length, buffer;

  while (lua.length < codelen) {
    byte = code.charCodeAt(i);

    if (mode === 1) {
      lua += code.charAt(i);
      mode = 0;
    } else if (mode === 2) {
      offset = lua.length - ((copy - 60) * 16 + (byte & 15));
      length = (byte >> 4) + 2;
      buffer = lua.substring(offset, offset + length);
      lua   += buffer;
      mode   = 0;
    } else if (byte === 0) {
      mode = 1;
    } else if (byte <= 59) {
      lua += dict[byte - 1];
    } else if (byte >= 60) {
      mode = 2;
      copy = byte;
    }

    i++;
  }

  return lua;
}

function getData(bmp) {
  "use strict";
  var gfx = '', map = '', gff = '', music = '', sfx = '', lua  = '', final, imgData = bmp.bitmap, dataLen = imgData.length, i = 0, n = 0, r, g, b, a, byte, compressed, loop, lastbyte, loops = [], mode, speed, start, end, notes = '', tmp_music = [], track, step, note, version;

  if (bmp.width !== 160 || bmp.height !== 205) {
    return alert('Image is the wrong size.');
  }

  while (i < dataLen) {
    // get the last 2 bytes of each value and shift left if necessary
    r = (imgData[i++] & 3) << 4;
    g = (imgData[i++] & 3) << 2;
    b = (imgData[i++] & 3);
    a = (imgData[i++] & 3) << 6;

    // compile new byte, convert to hex and pad left if needed
    byte = (r | g | b | a);

    if (n < 8192) {
      // change endianness and append to output string
      gfx += stringy(byte).split('').reverse().join('');
    } else if (n < 12288) {
      map += stringy(byte);
    } else if (n < 12544) {
      gff += stringy(byte);
    } else if (n < 12800) {
      track = Math.floor((n - 12544) / 4);
      note  = stringy(byte & 127);

      if (n % 4 === 0) {
        tmp_music.push([null, '']);
      }

      tmp_music[track][0] = stringy(((byte & 128) >> 7 - n % 4) | tmp_music[track][0]);
      tmp_music[track][1] += note;
    } else if (n < 17152) {
      step = (n - 12800) % 68;

      if (step < 64 && n % 2 === 1) {
        note = (byte << 8) + lastbyte;
        notes += (stringy(note & 63) + ((note & 448) >> 6).toString(16) + ((note & 3584) >> 9).toString(16) + ((note & 28672) >> 12).toString(16));
      } else if (step === 64) {
        mode = pad(byte);
      } else if (step === 65) {
        speed = byte;
      } else if (step === 66) {
        start = byte;
      } else if (step === 67) {
        end = byte;
        sfx += mode + stringy(speed) + stringy(start) + stringy(end) + notes + "\n";
        notes = '';
      }
    } else if (n < 32768) {
      if (n === 17152) {
        compressed = (byte === 58);
      }

      lua += String.fromCharCode(byte);
    } else if (n === 32768) {
      version = byte;
    }

    lastbyte = byte;
    n++;
  }

  if (compressed) lua = decompress(lua);
  gfx = buildString(gfx, 128);
  gff = buildString(gff, 256);
  map = buildString(map, 256);

  tmp_music.forEach(function (m) {
    music += m[0] + ' ' + m[1] + "\n";
  });

  final  = "pico-8 cartridge // http://www.pico-8.com\n";
  final += 'version ' + version + "\n";
  final += "__lua__\n" + lua + "\n__gfx__\n" + gfx + "__gff__\n" + gff + "__map__\n" + map + "__sfx__\n" + sfx + "__music__\n" + music + "\n\n";

  return final;
}