Tips for manipulating bits 52-32 within Javascript code, without utilizing string methods

Here is my functional prototype code that is currently operational:

function int2str(int, bits) {
    const str = int.toString(2);
    var ret = '';
    for (var i = 0; i < (bits - str.length); ++i) {
        ret += '0';
    }
    ret += str;
    return ret;
}

function high32(u64) {
    return parseInt(int2str(u64, 64).substr(0, 32), 2);
}

function low32(u64) {
    return parseInt(int2str(u64, 64).substr(32, 32), 2);
}

function combine(low, high) {
    return parseInt(int2str(high, 32) + int2str(low, 32), 2);
}

Is there a more efficient way to achieve this without the use of strings?

(In Javascript, bitwise operators cast to a 32-bit integer, rendering them ineffective.)

Answer №1

Simply disregards the sign and allows it to pass through. Utilizes division and conversion to a 32-bit integer to obtain the high bits, and modulus for the low bits.

const POW2_32 = (1<<16)*(1<<16)

function high32(u64) {
    return (u64/POW2_32)|0
}

function low32(u64) {
    return u64%POW2_32
}

function combine(low, high) {
    return high * POW2_32 + low
}

If you require acceptance of signed numbers:

function high32(u64) {
    return (u64/((1<<16)*(1<<16)))
}

function low32(u64) {
    return Math.abs(u64%((1<<16)*(1<<16)))
}

function combine(low, high) {    
    return (high<0?-1:1) * // if high32 is neg, whole number is neg
           (Math.abs(high * POW2_32) + Math.abs(low|((low<0)<<15)))
                                       // if low bit is neg, set highest low bit
}

If you need to capture more than 32 bits for the high bits using Math.trunc, and you will need to convert everything to use BigInt (so 1n<<16n instead of 1<<16)

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