2x32BitsXBase64.js

Base64 = Base64 || {
    // @param n     Number, 6 bits, e.g. 0b111111
    // @return      String, a char, e.g. "/"
    encode6Bits: function(n) {
        n = Math.min(Math.max(0, n), 63);
        if      (n >=  0 &&  n <= 25)   return String.fromCharCode("A".charCodeAt(0) + n);      // ["A", "Z"]
        else if (n >= 26 &&  n <= 51)   return String.fromCharCode("a".charCodeAt(0) + n - 26); // ["a", "z"]
        else if (n >= 52 &&  n <= 61)   return String.fromCharCode("0".charCodeAt(0) + n - 52); // ["0", "9"]
        else if (n == 62)               return "+";
        else /* if (n == 63) */         return "/";
    },
    // @param c     Number, char code, e.g. 47
    // @return      Number, 6 bits, e.g. 0b111111
    decode6Bits: function(c) { // c: charCode
        if      (c >= "A".charCodeAt(0) && c <= "Z".charCodeAt(0))  return c - "A".charCodeAt(0);      // [ 0, 25]
        else if (c >= "a".charCodeAt(0) && c <= "z".charCodeAt(0))  return c - "a".charCodeAt(0) + 26; // [26, 51]
        else if (c >= "0".charCodeAt(0) && c <= "9".charCodeAt(0))  return c - "0".charCodeAt(0) + 52; // [52, 61]
        else if (c == "+".charCodeAt(0))                            return 62;
        else /* if (c == "/".charCodeAt(0)) */                      return 63;
    },
    // @param bits2x32  Bits2x32, e.g. { high32: 0xFFFFFFFF, low32: 0xFFFFFFFF }
    // @return          String, 1 ~ 11 chars, e.g. "//////////P"
    encode2x32Bits: function(bits2x32) {
        let high32 = bits2x32.high32, low32 = bits2x32.low32;
        let base64 = "";
        while (high32 > 0 || low32 > 0) {
            base64 += this.encode6Bits(low32 & 0b111111); // 0b111111: 6 bits
            low32 = (high32 << (32 - 6) >>> 0) | (low32 >>> 6);
            high32 >>>= 6;
        }
        return base64 || "A";
    },
    // @param base64    String, 1 ~ 11 chars, e.g. "//////////P"
    // @return          Bits2x32, e.g. { high32: 0xFFFFFFFF, low32: 0xFFFFFFFF }
    decode2x32Bits: function(base64) {
        let length = Math.min(base64.length, 11); // 11: 64 bits ~= 11 chars * 6 bits
        let high32 = 0, low32 = 0;
        for (let i = 0; i < length; ++i) {
            let bits06 = this.decode6Bits(base64.charCodeAt(i));
            if (i < 5) {
                low32 |= bits06 << (6 * i) >>> 0;
            }
            else if (i == 5) {
                low32 |= (bits06 & 0b11) << (6 * i) >>> 0;
                high32 |= bits06 >>> 2;
            }
            else {
                high32 |= bits06 << (6 * i % 32) >>> 0;
            }
            low32 >>>= 0;
            high32 >>>= 0;
        }
        return new Bits2x32(high32, low32);
    }
};

function Bits2x32(high32, low32) {
    this.low32 = low32;
    this.high32 = high32;
}

Bits2x32.prototype = {
    getBit: function(index) {
        let key = index < 32 ? "low32" : "high32";
        return (this[key] >>> (index % 32)) & 1;
    },
    setBit: function(index, bool) {
        let key = index < 32 ? "low32" : "high32";
        let mask = 1 << (index % 32); // >>> 0
        this[key] = bool ? ((this[key] | mask) >>> 0) : (this[key] & (0xFFFFFFFF & ~mask));
    },
    encode64Bits: function() {
        return Base64.encode64Bits(this);
    }
};

let bits2x32 = new Bits2x32(0b11111111111111111111111111111111, 0b11111111111111111111111111111111);
let base64 = bits2x32.encode2x32Bits();
console.log(bits2x32);
console.log(base64);
console.log(Base64.decode2x32Bits(base64));

bits2x32 = new Bits2x32(0b11111111111111111111111111111111, 0b11111111111111111111111111111111);
console.log(bits2x32);
bits2x32.setBit(31, 0);
console.log(bits2x32);
bits2x32.setBit(63, 0);
console.log(bits2x32);
bits2x32.setBit(31, 1);
console.log(bits2x32);
bits2x32.setBit(63, 1);
console.log(bits2x32);