zapdb-kv/src/packages/arithmetic-coder-bitmask/index.js

"use strict";

const createArithmeticCoder = require("../arithmetic-coder");

function encodeBooleans(booleans) {
	let n = 1n;
	let bitmask = 0n;

	for (let boolean of booleans) {
		if (boolean === true) {
			bitmask |= n;
		}

		n *= 2n;
	}

	return bitmask;
}

function decodeBooleans(bitmask, count) {
	let n = 1n;
	let booleans = [];

	for (let i = 0; i < count; i++) {
		booleans.push((bitmask & n) !== 0n);
		n *= 2n;
	}

	return booleans;
}

module.exports = function createBitmaskArithmeticCoder(fields) {
	// NOTE: We *always* store the bitmask as the very first field, to ensure that it doesn't interfere with binary sorting order
	let fieldCount = BigInt(fields.length);
	let maximumBitmaskValue = 2n ** fieldCount; // NOTE: Exclusive

	let coder = createArithmeticCoder([
		{ name: "__bitmask", minimum: 0, maximum: maximumBitmaskValue },
		... fields
	]);

	return {
		bits: coder.bits,
		encode: function (data) {
			let fieldPresence = fields.map((field) => data[field.name] != null);
			
			return coder.encode({
				... data,
				__bitmask: encodeBooleans(fieldPresence)
			});
		},
		decode: function (data) {
			let decoded = coder.decode(data);
			let fieldPresence = decodeBooleans(decoded.__bitmask, fields.length);

			fields.forEach((field, i) => {
				if (fieldPresence[i] === false) {
					decoded[field.name] = undefined;
				}
			});

			delete decoded.__bitmask;

			return decoded;
		}
	};
};