@woosh/meep-engine/src/core/math/frexp.js

Pure JavaScript game engine. Fully featured and production ready.

/**
 * Decomposes a floating-point number into a normalized fraction and an integer power of 2.
 * Similar to C's frexp function.
 * @param {number} value - The floating-point number to decompose
 * @param {number[]} exponent where to write exponent to
 * @param {number} [offset_into_exponent]
 * @returns {number} mantissa
 */
export function frexp(
    value,
    exponent,
    offset_into_exponent = 0
) {

    if (value === 0 || Number.isFinite(value) === false) {
        exponent[offset_into_exponent] = 0;
        return value;
    }

    let abs_value = Math.abs(value);

    // Get the exponent using Math.log2 and floor it
    let exp = Math.max(
        -1023,
        Math.floor(Math.log2(abs_value)) + 1
    );

    // Calculate the mantissa by dividing by 2^exp
    let mantissa = abs_value * Math.pow(2, -exp);

    // These while loops compensate for rounding errors that sometimes occur because of ECMAScript's Math.log2's undefined precision
    // and also works around the issue of Math.pow(2, -exp) === Infinity when exp <= -1024
    while (mantissa < 0.5) {
        mantissa *= 2;
        exp--;
    }
    while (mantissa >= 1) {
        mantissa *= 0.5;
        exp++;
    }

    if (value < 0) {
        mantissa = -mantissa;
    }

    exponent[offset_into_exponent] = exp;

    return mantissa;
}