Base85 编码和解码

base85.js

const charset = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz!#$%&()*+-;<=>?@^_`{|}~';
const charsetReverse = charset
    .split('')
    .reduce((acc, cur, idx) => {
        acc[cur] = idx;
        return acc;
    }, {});

/**
 * @param {Uint8Array} data
 * @returns {String}
 */
const base85Encode = data => {
    const paddingLength = 4 - (data.length % 4 || 4);
    const paddedData = new Uint8Array(data.length + paddingLength);
    paddedData.set(data);
    let result = '';
    for (let i = 0; i < paddedData.length; i += 4) {
        let group4 = 0;
        for (let j = 0; j < 4; j++) {
            group4 |= paddedData[i + j] << ((3 - j) << 3);
            group4 >>>= 0;
        }
        let part85 = '';
        for (let j = 0; j < 5; j++) {
            let k = group4 % 85;
            group4 = (group4 - k) / 85;
            part85 = charset[k] + part85;
        }
        result += part85;
    }
    return result.substr(0, result.length - paddingLength);
};

/**
 * @param {String} data
 * @returns {Uint8Array}
 */
const base85Decode = data => {
    const paddingLength = 5 - (data.length % 5 || 5);
    data += '~'.repeat(paddingLength);
    const result = new Uint8Array(data.length / 5 * 4 - paddingLength);
    for (let i = 0, j = 0; i < data.length; i += 5, j += 4) {
        let group5 = 0;
        let base = 52200625; // 85 ** 4
        for (let k = 0; k < 5; k++) {
            group5 += base * charsetReverse[data[i + k]];
            base /= 85;
        }
        for (let k = 0; k < 4; k++) {
            result[j + k] = group5 >> ((3 - k) << 3);
        }
    }
    return result;
};

for (let i = 0; i < 256; i++) {
    const a = new Uint8Array(Math.random() * 262144);
    for (let j = 0; j < a.length; j++) {
        a[j] = Math.random() * 256;
    }
    const b = base85Decode(base85Encode(a));
    if (a.length !== b.length) throw new Error('test failed');
    for (let j = 0; j < a.length; j++) {
        if (a[j] !== b[j]) throw new Error('test failed');
    }
}