zig/std/math/log2.zig

const math = @import("index.zig");
const assert = @import("../debug.zig").assert;

// TODO issue #393
pub const log2 = log2_workaround;

pub fn log2_workaround(x: var) -> @typeOf(x) {
    const T = @typeOf(x);
    switch (T) {
        f32 => @inlineCall(log2_32, x),
        f64 => @inlineCall(log2_64, x),
        else => @compileError("log2 not implemented for " ++ @typeName(T)),
    }
}

fn log2_32(x_: f32) -> f32 {
    const ivln2hi: f32 =  1.4428710938e+00;
    const ivln2lo: f32 = -1.7605285393e-04;
    const Lg1: f32 = 0xaaaaaa.0p-24;
    const Lg2: f32 = 0xccce13.0p-25;
    const Lg3: f32 = 0x91e9ee.0p-25;
    const Lg4: f32 = 0xf89e26.0p-26;

    var x = x_;
    var u = @bitCast(u32, x);
    var ix = u;
    var k: i32 = 0;

    // x < 2^(-126)
    if (ix < 0x00800000 or ix >> 31 != 0) {
        // log(+-0) = -inf
        if (ix << 1 == 0) {
            return -1 / (x * x);
        }
        // log(-#) = nan
        if (ix >> 31 != 0) {
            return (x - x) / 0.0
        }

        k -= 25;
        x *= 0x1.0p25;
        ix = @bitCast(u32, x);
    } else if (ix >= 0x7F800000) {
        return x;
    } else if (ix == 0x3F800000) {
        return 0;
    }

    // x into [sqrt(2) / 2, sqrt(2)]
    ix += 0x3F800000 - 0x3F3504F3;
    k += i32(ix >> 23) - 0x7F;
    ix = (ix & 0x007FFFFF) + 0x3F3504F3;
    x = @bitCast(f32, ix);

    const f = x - 1.0;
    const s = f / (2.0 + f);
    const z = s * s;
    const w = z * z;
    const t1 = w * (Lg2 + w * Lg4);
    const t2 = z * (Lg1 + w * Lg3);
    const R = t2 + t1;
    const hfsq = 0.5 * f * f;

    var hi = f - hfsq;
    u = @bitCast(u32, hi);
    u &= 0xFFFFF000;
    hi = @bitCast(f32, u);
    const lo = f - hi - hfsq + s * (hfsq + R);
    (lo + hi) * ivln2lo + lo * ivln2hi + hi * ivln2hi + f32(k)
}

fn log2_64(x_: f64) -> f64 {
    const ivln2hi: f64 = 1.44269504072144627571e+00;
    const ivln2lo: f64 = 1.67517131648865118353e-10;
    const Lg1: f64 = 6.666666666666735130e-01;
    const Lg2: f64 = 3.999999999940941908e-01;
    const Lg3: f64 = 2.857142874366239149e-01;
    const Lg4: f64 = 2.222219843214978396e-01;
    const Lg5: f64 = 1.818357216161805012e-01;
    const Lg6: f64 = 1.531383769920937332e-01;
    const Lg7: f64 = 1.479819860511658591e-01;

    var x = x_;
    var ix = @bitCast(u64, x);
    var hx = u32(ix >> 32);
    var k: i32 = 0;

    if (hx < 0x00100000 or hx >> 31 != 0) {
        // log(+-0) = -inf
        if (ix << 1 == 0) {
            return -1 / (x * x);
        }
        // log(-#) = nan
        if (hx >> 31 != 0) {
            return (x - x) / 0.0;
        }

        // subnormal, scale x
        k -= 54;
        x *= 0x1.0p54;
        hx = u32(@bitCast(u64, x) >> 32);
    }
    else if (hx >= 0x7FF00000) {
        return x;
    }
    else if (hx == 0x3FF00000 and ix << 32 == 0) {
        return 0;
    }

    // x into [sqrt(2) / 2, sqrt(2)]
    hx += 0x3FF00000 - 0x3FE6A09E;
    k += i32(hx >> 20) - 0x3FF;
    hx = (hx & 0x000FFFFF) + 0x3FE6A09E;
    ix = (u64(hx) << 32) | (ix & 0xFFFFFFFF);
    x = @bitCast(f64, ix);

    const f = x - 1.0;
    const hfsq = 0.5 * f * f;
    const s = f / (2.0 + f);
    const z = s * s;
    const w = z * z;
    const t1 = w * (Lg2 + w * (Lg4 + w * Lg6));
    const t2 = z * (Lg1 + w * (Lg3 + w * (Lg5 + w * Lg7)));
    const R = t2 + t1;

    // hi + lo = f - hfsq + s * (hfsq + R) ~ log(1 + f)
    var hi = f - hfsq;
    var hii = @bitCast(u64, hi);
    hii &= @maxValue(u64) << 32;
    hi = @bitCast(f64, hii);
    const lo = f - hi - hfsq + s * (hfsq + R);

    var val_hi = hi * ivln2hi;
    var val_lo = (lo + hi) * ivln2lo + lo * ivln2hi;

    // spadd(val_hi, val_lo, y)
    const y = f64(k);
    const ww = y + val_hi;
    val_lo += (y - ww) + val_hi;
    val_hi = ww;

    val_lo + val_hi
}

test "math.log2" {
    assert(log2(f32(0.2)) == log2_32(0.2));
    assert(log2(f64(0.2)) == log2_64(0.2));
}

test "math.log2_32" {
    const epsilon = 0.000001;

    assert(math.approxEq(f32, log2_32(0.2), -2.321928, epsilon));
    assert(math.approxEq(f32, log2_32(0.8923), -0.164399, epsilon));
    assert(math.approxEq(f32, log2_32(1.5), 0.584962, epsilon));
    assert(math.approxEq(f32, log2_32(37.45), 5.226894, epsilon));
    assert(math.approxEq(f32, log2_32(123123.234375), 16.909744, epsilon));
}

test "math.log2_64" {
    const epsilon = 0.000001;

    assert(math.approxEq(f64, log2_64(0.2), -2.321928, epsilon));
    assert(math.approxEq(f64, log2_64(0.8923), -0.164399, epsilon));
    assert(math.approxEq(f64, log2_64(1.5), 0.584962, epsilon));
    assert(math.approxEq(f64, log2_64(37.45), 5.226894, epsilon));
    assert(math.approxEq(f64, log2_64(123123.234375), 16.909744, epsilon));
}