encodeVarDeltaRunAsWords function

dynamic encodeVarDeltaRunAsWords(dynamic deltas, dynamic offset, dynamic result)

Implementation

encodeVarDeltaRunAsWords(deltas, offset, result) {
    var runLength = 0;
    var numDeltas = deltas.length;
    var pos = offset;
    while (pos < numDeltas && runLength < 64) {
        var value = deltas[pos];

        // Within a word-encoded run of deltas, it is easiest to start
        // a new run (with a different encoding) whenever we encounter
        // a zero value. For example, the sequence [0x6666, 0, 0x7777]
        // needs 7 bytes when storing the zero inside the current run
        // (42 66 66 00 00 77 77), and equally 7 bytes when starting a
        // new run (40 66 66 80 40 77 77).
        if (value == 0) {
            break;
        }

        // Within a word-encoded run of deltas, a single value in the
        // range (-128..127) should be encoded within the current run
        // because it is more compact. For example, the sequence
        // [0x6666, 2, 0x7777] becomes 7 bytes when storing the value
        // literally (42 66 66 00 02 77 77), but 8 bytes when starting
        // a new run (40 66 66 00 02 40 77 77).
        if (isByteEncodable(value) && pos + 1 < numDeltas && isByteEncodable(deltas[pos + 1])) {
            break;
        }

        ++pos;
        ++runLength;
    }
    result.push(0x40 | (runLength - 1));
    for (var i = offset; i < pos; ++i) {
        var val = deltas[i];
        result.push(((val + 0x10000) >> 8) & 0xff, (val + 0x100) & 0xff);
    }
    return pos;
}