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QuakeIIIArena bit-hack code written in featurescript

joshtargo
joshtargo Member Posts: 640 EDU
in FeatureScript

https://cad.onshape.com/documents/29e6ce5316fbadb2fa98702b/w/61d5a536b165313813af25ac/e/32631c72aee224385208e1ac

image.png 
FeatureScript 3029;
import(path : "onshape/std/common.fs", version : "3029.0");

const TWO_23 = 8388608;      // 2^23
const TWO_31 = 2147483648;   // 2^31
const BIAS = 127;
const MAGIC = 1597463007;    // 0x5f3759df

annotation { "Feature Type Name" : "Fast Inverse Sqrt", "Feature Type Description" : "" }
export const testFastInverseSqrt = defineFeature(function(context is Context, id is Id, definition is map)
precondition
{
    annotation { "Name" : "Number" }
    isReal(definition.inputValue, POSITIVE_REAL_BOUNDS);
}
{
    var result = fastInverseSqrt(definition.inputValue);
    var actual = 1 / sqrt(definition.inputValue);

    println("input = " ~ definition.inputValue);
    println("fastInverseSqrt = " ~ roundToPrecision(result, 6));
    println("actual 1/sqrt   = " ~ roundToPrecision(actual, 6));
    println("error           = " ~ roundToPrecision(abs(result - actual) / actual,6));
});
    
// Line-for-line port of Q_rsqrt
function fastInverseSqrt(inputValue is number) returns number
{
    var threeHalfs = 1.5;
    var x2 = inputValue * 0.5;
    var y = inputValue;

    var i = floatToBits(y);                // i  = *(long*)&y

    i = MAGIC - floor(i / 2);
    y = bitsToFloat(i);                     // y  = *(float*)&i

    y = y * (threeHalfs - (x2 * y * y));    // 1st Newton iteration

    return y;
}

// Simulates: i = *(long*)&y
function floatToBits(x is number) returns number
{
    if (x == 0)
    {
        return 0;
    }

    var sign = (x < 0) ? 1 : 0;
    var absX = abs(x);

    var exponent = floor(log(absX) / log(2));
    var mantissaFrac = absX / (2^exponent) - 1;
    var mantissaBits = round(mantissaFrac * TWO_23);

    if (mantissaBits >= TWO_23)
    {
        mantissaBits = 0;
        exponent = exponent + 1;
    }

    var biasedExponent = exponent + BIAS;
    return sign * TWO_31 + biasedExponent * TWO_23 + mantissaBits;
}

// Simulates: y = *(float*)&i
function bitsToFloat(bits is number) returns number
{
    var sign = floor(bits / TWO_31);
    var remaining = bits - sign * TWO_31;

    var biasedExponent = floor(remaining / TWO_23);
    var mantissaBits = remaining - biasedExponent * TWO_23;

    var exponent = biasedExponent - BIAS;
    var mantissaFrac = mantissaBits / TWO_23;

    var value = (1 + mantissaFrac) * (2^exponent);
    return (sign == 1) ? -value : value;
}

/*
function bitsToBinaryString(bits is number) returns string
{
    var remaining = bits;

    // simulate unsigned 32-bit wraparound: if negative, add 2^32
    if (remaining < 0)
    {
        remaining = remaining + (2^32);
    }

    var result = "";
    for (var bitIndex = 31; bitIndex >= 0; bitIndex -= 1)
    {
        var placeValue = (2^bitIndex);
        if (remaining >= placeValue)
        {
            result = result ~ "1";
            remaining = remaining - placeValue;
        }
        else
        {
            result = result ~ "0";
        }

        if (bitIndex % 8 == 0 && bitIndex != 0)
        {
            result = result ~ " ";
        }
    }

    return result;
}

*/

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