synchsafe conversion functions

synchsafe.js

/* 
    Synchsafe integers are used in ID3 tags
    http://id3.org/id3v2.4.0-structure (6.2)
    https://en.wikipedia.org/wiki/Synchsafe

    The why of synchsafe:
    http://stackoverflow.com/a/5652842

    Example:
    255 (%11111111) encoded as a 16 bit synchsafe integer is 383 (%00000001 01111111).
*/

function synchsafe(input) {
	var out, mask = 0x7F;
	while (mask ^ 0x7FFFFFFF) {
		out = input & ~mask;
		out = out << 1;
		out = out | (input & mask);
		mask = ((mask + 1) << 8) - 1;
		input = out;
	}
	return out;
}

function unsynchsafe(input) {
  var out = 0, mask = 0x7F000000;
  while (mask) {
    out = out >> 1;
    out = out | (input & mask)
    mask = mask >> 8;
  }
  return out;
}