LZW Implementation

LZW.js

/**
    Namespace for LZW compression and decompression.
    Methods:
        LZW.compress(uncompressed)
        LZW.decompress(compressed)
*/


function createDictionary({ size = 256, inverse = false } = {}) {
    const dictionary = new Map();

    for (let i = 0; i < size; i++) {
        if (inverse) dictionary.set(i, String.fromCharCode(i));
        else dictionary.set(String.fromCharCode(i), i);
    }

    return dictionary;
}

/**
    Perform the LZW compression
    uncompressed - String. The string on which to perform the compression.
*/
function compress(uncompressed) {
    // Initialize dictionary
    const dictionary = createDictionary();

    let word = '';
    const result = [];
    let dictSize = dictionary.size;

    for (let i = 0, len = uncompressed.length; i < len; i++) {
        const curChar = uncompressed[i];
        const joinedWord = word + curChar;

        if (dictionary.has(joinedWord)) {
            word = joinedWord;
        } else {
            result.push(dictionary.get(word));
            // Add wc to the dictionary.
            dictionary.set(joinedWord, dictSize++);
            word = curChar;
        }
    }

    if (word !== '') {
        result.push(dictionary.get(word));
    }

    return String.fromCharCode(...result);
}

/**
    Decompress LZW array generated by LZW.compress()
    compressed - Array. The array that holds LZW compressed data.
*/
function decompress(compressed) {
    // Initialize Dictionary (inverse of compress)
    const dictionary = createDictionary({ inverse: true });

    let word = compressed[0];
    let result = word;
    let entry = '';
    let dictSize = dictionary.size;

    for (let i = 1, len = compressed.length; i < len; i++) {
        const curNumber = compressed.charCodeAt(i);
        if (dictionary.has(curNumber)) {
            entry = dictionary.get(curNumber);
        } else {
            if (curNumber === dictSize) {
                entry = word + word[0];
            } else {
                throw new Error(`Character not in dictionary: ${curNumber}`);
            }
        }

        result += entry;

        // Add word + entry[0] to dictionary
        dictionary.set(dictSize++, word + entry[0]);

        word = entry;
    }

    return result;
}