    // Ported from musl, which is licensed under the MIT license:

    // https://git.musl-libc.org/cgit/musl/tree/COPYRIGHT

    //

    // https://git.musl-libc.org/cgit/musl/tree/src/math/acosf.c

    // https://git.musl-libc.org/cgit/musl/tree/src/math/acos.c

    const std = @import("../std.zig");
    const math = std.math;
    const expect = std.testing.expect;

    /// Returns the arc-cosine of x.
    ///
    /// Special cases:
    ///  - acos(x)   = nan if x < -1 or x > 1
    pub fn acos(x: anytype) @TypeOf(x) {
        const T = @TypeOf(x);
        return switch (T) {
            f32 => acos32(x),
            f64 => acos64(x),
            else => @compileError("acos not implemented for " ++ @typeName(T)),
        };
    }

    fn r32(z: f32) f32 {
        const pS0 = 1.6666586697e-01;
        const pS1 = -4.2743422091e-02;
        const pS2 = -8.6563630030e-03;
        const qS1 = -7.0662963390e-01;

        const p = z * (pS0 + z * (pS1 + z * pS2));
        const q = 1.0 + z * qS1;
        return p / q;
    }

    fn acos32(x: f32) f32 {
        const pio2_hi = 1.5707962513e+00;
        const pio2_lo = 7.5497894159e-08;

        const hx: u32 = @bitCast(u32, x);
        const ix: u32 = hx & 0x7FFFFFFF;

        // |x| >= 1 or nan

        if (ix >= 0x3F800000) {
            if (ix == 0x3F800000) {
                if (hx >> 31 != 0) {
                    return 2.0 * pio2_hi + 0x1.0p-120;
                } else {
                    return 0.0;
                }
            } else {
                return math.nan(f32);
            }
        }

        // |x| < 0.5

        if (ix < 0x3F000000) {
            if (ix <= 0x32800000) { // |x| < 2^(-26)

                return pio2_hi + 0x1.0p-120;
            } else {
                return pio2_hi - (x - (pio2_lo - x * r32(x * x)));
            }
        }

        // x < -0.5

        if (hx >> 31 != 0) {
            const z = (1 + x) * 0.5;
            const s = @sqrt(z);
            const w = r32(z) * s - pio2_lo;
            return 2 * (pio2_hi - (s + w));
        }

        // x > 0.5

        const z = (1.0 - x) * 0.5;
        const s = @sqrt(z);
        const jx = @bitCast(u32, s);
        const df = @bitCast(f32, jx & 0xFFFFF000);
        const c = (z - df * df) / (s + df);
        const w = r32(z) * s + c;
        return 2 * (df + w);
    }

    fn r64(z: f64) f64 {
        const pS0: f64 = 1.66666666666666657415e-01;
        const pS1: f64 = -3.25565818622400915405e-01;
        const pS2: f64 = 2.01212532134862925881e-01;
        const pS3: f64 = -4.00555345006794114027e-02;
        const pS4: f64 = 7.91534994289814532176e-04;
        const pS5: f64 = 3.47933107596021167570e-05;
        const qS1: f64 = -2.40339491173441421878e+00;
        const qS2: f64 = 2.02094576023350569471e+00;
        const qS3: f64 = -6.88283971605453293030e-01;
        const qS4: f64 = 7.70381505559019352791e-02;

        const p = z * (pS0 + z * (pS1 + z * (pS2 + z * (pS3 + z * (pS4 + z * pS5)))));
        const q = 1.0 + z * (qS1 + z * (qS2 + z * (qS3 + z * qS4)));
        return p / q;
    }

    fn acos64(x: f64) f64 {
        const pio2_hi: f64 = 1.57079632679489655800e+00;
        const pio2_lo: f64 = 6.12323399573676603587e-17;

        const ux = @bitCast(u64, x);
        const hx = @intCast(u32, ux >> 32);
        const ix = hx & 0x7FFFFFFF;

        // |x| >= 1 or nan

        if (ix >= 0x3FF00000) {
            const lx = @intCast(u32, ux & 0xFFFFFFFF);

            // acos(1) = 0, acos(-1) = pi

            if ((ix - 0x3FF00000) | lx == 0) {
                if (hx >> 31 != 0) {
                    return 2 * pio2_hi + 0x1.0p-120;
                } else {
                    return 0;
                }
            }

            return math.nan(f32);
        }

        // |x| < 0.5

        if (ix < 0x3FE00000) {
            // |x| < 2^(-57)

            if (ix <= 0x3C600000) {
                return pio2_hi + 0x1.0p-120;
            } else {
                return pio2_hi - (x - (pio2_lo - x * r64(x * x)));
            }
        }

        // x < -0.5

        if (hx >> 31 != 0) {
            const z = (1.0 + x) * 0.5;
            const s = @sqrt(z);
            const w = r64(z) * s - pio2_lo;
            return 2 * (pio2_hi - (s + w));
        }

        // x > 0.5

        const z = (1.0 - x) * 0.5;
        const s = @sqrt(z);
        const jx = @bitCast(u64, s);
        const df = @bitCast(f64, jx & 0xFFFFFFFF00000000);
        const c = (z - df * df) / (s + df);
        const w = r64(z) * s + c;
        return 2 * (df + w);
    }

    test "math.acos" {
        try expect(acos(@as(f32, 0.0)) == acos32(0.0));
        try expect(acos(@as(f64, 0.0)) == acos64(0.0));
    }

    test "math.acos32" {
        const epsilon = 0.000001;

        try expect(math.approxEqAbs(f32, acos32(0.0), 1.570796, epsilon));
        try expect(math.approxEqAbs(f32, acos32(0.2), 1.369438, epsilon));
        try expect(math.approxEqAbs(f32, acos32(0.3434), 1.220262, epsilon));
        try expect(math.approxEqAbs(f32, acos32(0.5), 1.047198, epsilon));
        try expect(math.approxEqAbs(f32, acos32(0.8923), 0.468382, epsilon));
        try expect(math.approxEqAbs(f32, acos32(-0.2), 1.772154, epsilon));
    }

    test "math.acos64" {
        const epsilon = 0.000001;

        try expect(math.approxEqAbs(f64, acos64(0.0), 1.570796, epsilon));
        try expect(math.approxEqAbs(f64, acos64(0.2), 1.369438, epsilon));
        try expect(math.approxEqAbs(f64, acos64(0.3434), 1.220262, epsilon));
        try expect(math.approxEqAbs(f64, acos64(0.5), 1.047198, epsilon));
        try expect(math.approxEqAbs(f64, acos64(0.8923), 0.468382, epsilon));
        try expect(math.approxEqAbs(f64, acos64(-0.2), 1.772154, epsilon));
    }

    test "math.acos32.special" {
        try expect(math.isNan(acos32(-2)));
        try expect(math.isNan(acos32(1.5)));
    }

    test "math.acos64.special" {
        try expect(math.isNan(acos64(-2)));
        try expect(math.isNan(acos64(1.5)));
    }