aureooms/js-lempel-ziv

import {push, iter} from '@functional-data-structure/persistent-stack';
import {Trie, ObjectNode} from '@trie-data-structure/uncompressed-trie';

/**
 * Simple Lempel-Ziv lossless data compression algorithm implementation.
 */

// TODO read input as async tape
// TODO await trie operations
// TODO use persistent trie

export function* encode({trie, table}, symbols) {
    let chunkNumber = table.length - 1;
    const itSymbols = symbols[Symbol.iterator]();
    while (true) {
        const [part, node] = trie.getClosestAncestor(itSymbols);
        if (part === undefined) {
            yield [node.value(), undefined];
            break;
        }

        node.set(part, ++chunkNumber);

        yield [node.value(), part];
    }
}

const reify = (chunk) => [...iter(chunk)].reverse().join('');

// TODO use persistent table

export function* decode({table}, tokens) {
    for (const [parent, child] of tokens) {
        const previous = table[parent];

        if (child === undefined) {
            yield reify(previous);
        } else {
            const newChunk = push(previous, child);
            yield reify(newChunk);
            table.push(newChunk);
        }
    }
}

// TODO use persistent dict (hence a persistent trie and table)
export const dict = (symbols = ['']) => {
    const trie = makeTrie(symbols);
    const table = makeTable(trie);
    return {trie, table};
};

const makeTable = (trie) => {
    const table = [];
    for (const [key, value] of trie) table[value] = key;
    return table;
};

const emptyTrie = () => {
    const root = new ObjectNode();
    const trie = new Trie(root);
    return trie;
};

const makeTrie = (symbols) => {
    const trie = emptyTrie();
    let i = 0;
    for (const x of symbols) trie.set(x, i++);
    return trie;
};