zig/std/math/modf.zig

// Special Cases:
//
// - modf(+-inf) = +-inf, nan
// - modf(nan)   = nan, nan

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

fn modf_result(comptime T: type) -> type {
    struct {
        fpart: T,
        ipart: T,
    }
}
pub const modf32_result = modf_result(f32);
pub const modf64_result = modf_result(f64);

pub fn modf(x: var) -> modf_result(@typeOf(x)) {
    const T = @typeOf(x);
    switch (T) {
        f32 => @inlineCall(modf32, x),
        f64 => @inlineCall(modf64, x),
        else => @compileError("modf not implemented for " ++ @typeName(T)),
    }
}

fn modf32(x: f32) -> modf32_result {
    var result: modf32_result = undefined;

    const u = @bitCast(u32, x);
    const e = i32((u >> 23) & 0xFF) - 0x7F;
    const us = u & 0x80000000;

    // TODO: Shouldn't need this.
    if (math.isInf(x)) {
        result.ipart = x;
        result.fpart = math.nan(f32);
        return result;
    }

    // no fractional part
    if (e >= 23) {
        result.ipart = x;
        if (e == 0x80 and u << 9 != 0) { // nan
            result.fpart = x;
        } else {
            result.fpart = @bitCast(f32, us);
        }
        return result;
    }

    // no integral part
    if (e < 0) {
        result.ipart = @bitCast(f32, us);
        result.fpart = x;
        return result;
    }

    const mask = u32(0x007FFFFF) >> u5(e);
    if (u & mask == 0) {
        result.ipart = x;
        result.fpart = @bitCast(f32, us);
        return result;
    }

    const uf = @bitCast(f32, u & ~mask);
    result.ipart = uf;
    result.fpart = x - uf;
    result
}

fn modf64(x: f64) -> modf64_result {
    var result: modf64_result = undefined;

    const u = @bitCast(u64, x);
    const e = i32((u >> 52) & 0x7FF) - 0x3FF;
    const us = u & (1 << 63);

    if (math.isInf(x)) {
        result.ipart = x;
        result.fpart = math.nan(f64);
        return result;
    }

    // no fractional part
    if (e >= 52) {
        result.ipart = x;
        if (e == 0x400 and u << 12 != 0) { // nan
            result.fpart = x;
        } else {
            result.fpart = @bitCast(f64, us);
        }
        return result;
    }

    // no integral part
    if (e < 0) {
        result.ipart = @bitCast(f64, us);
        result.fpart = x;
        return result;
    }

    const mask = u64(@maxValue(u64) >> 12) >> u6(e);
    if (u & mask == 0) {
        result.ipart = x;
        result.fpart = @bitCast(f64, us);
        return result;
    }

    const uf = @bitCast(f64, u & ~mask);
    result.ipart = uf;
    result.fpart = x - uf;
    result
}

test "math.modf" {
    const a = modf(f32(1.0));
    const b = modf32(1.0);
    // NOTE: No struct comparison on generic return type function? non-named, makes sense, but still.
    assert(a.ipart == b.ipart and a.fpart == b.fpart);

    const c = modf(f64(1.0));
    const d = modf64(1.0);
    assert(a.ipart == b.ipart and a.fpart == b.fpart);
}

test "math.modf32" {
    const epsilon = 0.000001;
    var r: modf32_result = undefined;

    r = modf32(1.0);
    assert(math.approxEq(f32, r.ipart, 1.0, epsilon));
    assert(math.approxEq(f32, r.fpart, 0.0, epsilon));

    r = modf32(2.545);
    assert(math.approxEq(f32, r.ipart, 2.0, epsilon));
    assert(math.approxEq(f32, r.fpart, 0.545, epsilon));

    r = modf32(3.978123);
    assert(math.approxEq(f32, r.ipart, 3.0, epsilon));
    assert(math.approxEq(f32, r.fpart, 0.978123, epsilon));

    r = modf32(43874.3);
    assert(math.approxEq(f32, r.ipart, 43874, epsilon));
    assert(math.approxEq(f32, r.fpart, 0.300781, epsilon));

    r = modf32(1234.340780);
    assert(math.approxEq(f32, r.ipart, 1234, epsilon));
    assert(math.approxEq(f32, r.fpart, 0.340820, epsilon));
}

test "math.modf64" {
    const epsilon = 0.000001;
    var r: modf64_result = undefined;

    r = modf64(1.0);
    assert(math.approxEq(f64, r.ipart, 1.0, epsilon));
    assert(math.approxEq(f64, r.fpart, 0.0, epsilon));

    r = modf64(2.545);
    assert(math.approxEq(f64, r.ipart, 2.0, epsilon));
    assert(math.approxEq(f64, r.fpart, 0.545, epsilon));

    r = modf64(3.978123);
    assert(math.approxEq(f64, r.ipart, 3.0, epsilon));
    assert(math.approxEq(f64, r.fpart, 0.978123, epsilon));

    r = modf64(43874.3);
    assert(math.approxEq(f64, r.ipart, 43874, epsilon));
    assert(math.approxEq(f64, r.fpart, 0.3, epsilon));

    r = modf64(1234.340780);
    assert(math.approxEq(f64, r.ipart, 1234, epsilon));
    assert(math.approxEq(f64, r.fpart, 0.340780, epsilon));
}

test "math.modf32.special" {
    var r: modf32_result = undefined;

    r = modf32(math.inf(f32));
    assert(math.isPositiveInf(r.ipart) and math.isNan(r.fpart));

    r = modf32(-math.inf(f32));
    assert(math.isNegativeInf(r.ipart) and math.isNan(r.fpart));

    r = modf32(math.nan(f32));
    assert(math.isNan(r.ipart) and math.isNan(r.fpart));
}

test "math.modf64.special" {
    var r: modf64_result = undefined;

    r = modf64(math.inf(f64));
    assert(math.isPositiveInf(r.ipart) and math.isNan(r.fpart));

    r = modf64(-math.inf(f64));
    assert(math.isNegativeInf(r.ipart) and math.isNan(r.fpart));

    r = modf64(math.nan(f64));
    assert(math.isNan(r.ipart) and math.isNan(r.fpart));
}