worker.cjs

let lz4, BLOCK_SIZE, ChunkFlags;
let importPromises = [
    import('./lz4/lz4.js').then(x => lz4 = x),
    import('./LZ4Helper.js').then(x => { BLOCK_SIZE = x.BLOCK_SIZE; ChunkFlags = x.ChunkFlags; }),
];
let writePosition = 0;
let currentOutput = new Array(0);

addEventListener('message', e => {
    Promise.all(importPromises).then(() => {
        writePosition = 0;

        let { index, bytes } = e.data;
        currentOutput = new Array(bytes.byteLength);

        let compressed = new Array(BLOCK_SIZE);
        let compressedLength = lz4.compressBlock(bytes, compressed, 0, bytes.byteLength, new Uint32Array(1 << 14));
        compressed = compressed.slice(0, compressedLength);

        if (compressedLength <= 0 || compressedLength >= BLOCK_SIZE) {
            // incompressible block
            compressed = bytes;
            compressedLength = bytes.byteLength;
        }

        let isCompressed = compressedLength < BLOCK_SIZE;
        let flags = ChunkFlags.None;

        if (isCompressed) flags |= ChunkFlags.Compressed;
        WriteVarInt(flags);
        WriteVarInt(bytes.byteLength);
        if (isCompressed) WriteVarInt(compressedLength);

        let finalOutput = new Uint8Array(writePosition + compressedLength);
        finalOutput.set(new Uint8Array(currentOutput.slice(0, writePosition)), 0);
        finalOutput.set(new Uint8Array(compressed), writePosition);

        postMessage({ index, bytes: finalOutput });
    });
});

/**
 * @param {number} value 
 */
function WriteVarInt(value) {
    let bufferVal;

    while (true) {
        let b = (value & 0x7F);
        value >>= 7;
        bufferVal = (b | (value == 0 ? 0 : 0x80));
        currentOutput[writePosition] = bufferVal;
        writePosition++;
        if (value == 0) break;
    }
}