lianli-wireless/tuz.js

// Minimal tinyuz encoder (greedy LZ77), stream format of github.com/sisong/tinyuz.
// Output decodes with the reference tuz_decompress_mem.
const kMinDictMatchLen = 2;
const kBigPosForLen = (1 << 11) + (1 << 9) + (1 << 7) - 1;
const kMaxDist = 4096;
const kCtrlStreamEnd = 3;

function tuzCompress(data) {
	const code = [0, 0, 0, 0]; // dict size, patched at the end
	let typesIndex = 0;
	let typeCount = 0;
	let haveDataBack = false;
	let posBack = 1;
	let dictSizeMax = 1;

	const outType = (bit) => {
		if (typeCount === 0) {
			typesIndex = code.length;
			code.push(0);
		}
		code[typesIndex] |= bit << typeCount;
		typeCount = (typeCount + 1) & 7;
	};

	const outLen = (v, packBit) => {
		let count = 1;
		while (v >= 2 ** (count * packBit)) {
			v -= 2 ** (count * packBit);
			count++;
		}
		while (count--) {
			for (let i = 0; i < packBit; i++) {
				outType(Math.floor(v / 2 ** (count * packBit + i)) & 1);
			}
			outType(count > 0 ? 1 : 0);
		}
	};

	const outDictPos = (pos) => {
		const big = pos >= 128 ? 1 : 0;
		if (big) {
			pos -= 128;
		}
		code.push((pos & 0x7f) | (big << 7));
		if (big) {
			outLen(pos >> 7, 2);
		}
	};

	const outLiteral = (b) => {
		outType(1);
		code.push(b);
		haveDataBack = true;
	};

	const outDict = (len, dist) => {
		outType(0);
		const same = haveDataBack && dist === posBack;
		let saved = len - kMinDictMatchLen;
		if (!same && dist > kBigPosForLen) {
			saved--;
		}
		outLen(saved, 1);
		if (haveDataBack) {
			outType(same ? 1 : 0);
		}
		if (!same) {
			outDictPos(dist);
		}
		dictSizeMax = Math.max(dictSizeMax, dist);
		haveDataBack = false;
		posBack = dist;
	};

	let i = 0;
	while (i < data.length) {
		let bestLen = 0;
		let bestDist = 0;
		for (let dist = 1; dist <= Math.min(i, kMaxDist); dist++) {
			let len = 0;
			while (i + len < data.length && data[i + len] === data[i + len - dist]) {
				len++;
			}
			if (len > bestLen) {
				bestLen = len;
				bestDist = dist;
			}
		}
		const minLen = bestDist > kBigPosForLen ? 3 : kMinDictMatchLen;
		if (bestLen >= minLen && (bestLen > 2 || bestDist < 128)) {
			outDict(bestLen, bestDist);
			i += bestLen;
		} else {
			outLiteral(data[i]);
			i++;
		}
	}

	// stream end control code
	outType(0);
	outLen(kCtrlStreamEnd, 1);
	if (haveDataBack) {
		outType(0);
	}
	outDictPos(0);

	for (let k = 0; k < 4; k++) {
		code[k] = (dictSizeMax >> (8 * k)) & 0xff;
	}
	return code;
}

if (typeof module !== "undefined") {
	module.exports = { tuzCompress };
}