    // 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/asinf.c

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

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

    /// Returns the arc-sin of x.
    ///
    /// Special Cases:
    ///  - asin(+-0) = +-0
    ///  - asin(x)   = nan if x < -1 or x > 1
    pub fn asin(x: anytype) @TypeOf(x) {
        const T = @TypeOf(x);
        return switch (T) {
            f32 => asin32(x),
            f64 => asin64(x),
            else => @compileError("asin 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 asin32(x: f32) f32 {
        const pio2 = 1.570796326794896558e+00;

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

        // |x| >= 1

        if (ix >= 0x3F800000) {
            // |x| >= 1

            if (ix == 0x3F800000) {
                return x * pio2 + 0x1.0p-120; // asin(+-1) = +-pi/2 with inexact

            } else {
                return math.nan(f32); // asin(|x| > 1) is nan

            }
        }

        // |x| < 0.5

        if (ix < 0x3F000000) {
            // 0x1p-126 <= |x| < 0x1p-12

            if (ix < 0x39800000 and ix >= 0x00800000) {
                return x;
            } else {
                return x + x * r32(x * x);
            }
        }

        // 1 > |x| >= 0.5

        const z = (1 - @fabs(x)) * 0.5;
        const s = @sqrt(z);
        const fx = pio2 - 2 * (s + s * r32(z));

        if (hx >> 31 != 0) {
            return -fx;
        } else {
            return fx;
        }
    }

    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 asin64(x: f64) f64 {
        const pio2_hi: f64 = 1.57079632679489655800e+00;
        const pio2_lo: f64 = 6.12323399573676603587e-17;

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

        // |x| >= 1 or nan

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

            // asin(1) = +-pi/2 with inexact

            if ((ix - 0x3FF00000) | lx == 0) {
                return x * pio2_hi + 0x1.0p-120;
            } else {
                return math.nan(f64);
            }
        }

        // |x| < 0.5

        if (ix < 0x3FE00000) {
            // if 0x1p-1022 <= |x| < 0x1p-26 avoid raising overflow

            if (ix < 0x3E500000 and ix >= 0x00100000) {
                return x;
            } else {
                return x + x * r64(x * x);
            }
        }

        // 1 > |x| >= 0.5

        const z = (1 - @fabs(x)) * 0.5;
        const s = @sqrt(z);
        const r = r64(z);
        var fx: f64 = undefined;

        // |x| > 0.975

        if (ix >= 0x3FEF3333) {
            fx = pio2_hi - 2 * (s + s * r);
        } else {
            const jx = @as(u64, @bitCast(s));
            const df = @as(f64, @bitCast(jx & 0xFFFFFFFF00000000));
            const c = (z - df * df) / (s + df);
            fx = 0.5 * pio2_hi - (2 * s * r - (pio2_lo - 2 * c) - (0.5 * pio2_hi - 2 * df));
        }

        if (hx >> 31 != 0) {
            return -fx;
        } else {
            return fx;
        }
    }

    test "math.asin" {
        try expect(asin(@as(f32, 0.0)) == asin32(0.0));
        try expect(asin(@as(f64, 0.0)) == asin64(0.0));
    }

    test "math.asin32" {
        const epsilon = 0.000001;

        try expect(math.approxEqAbs(f32, asin32(0.0), 0.0, epsilon));
        try expect(math.approxEqAbs(f32, asin32(0.2), 0.201358, epsilon));
        try expect(math.approxEqAbs(f32, asin32(-0.2), -0.201358, epsilon));
        try expect(math.approxEqAbs(f32, asin32(0.3434), 0.350535, epsilon));
        try expect(math.approxEqAbs(f32, asin32(0.5), 0.523599, epsilon));
        try expect(math.approxEqAbs(f32, asin32(0.8923), 1.102415, epsilon));
    }

    test "math.asin64" {
        const epsilon = 0.000001;

        try expect(math.approxEqAbs(f64, asin64(0.0), 0.0, epsilon));
        try expect(math.approxEqAbs(f64, asin64(0.2), 0.201358, epsilon));
        try expect(math.approxEqAbs(f64, asin64(-0.2), -0.201358, epsilon));
        try expect(math.approxEqAbs(f64, asin64(0.3434), 0.350535, epsilon));
        try expect(math.approxEqAbs(f64, asin64(0.5), 0.523599, epsilon));
        try expect(math.approxEqAbs(f64, asin64(0.8923), 1.102415, epsilon));
    }

    test "math.asin32.special" {
        try expect(asin32(0.0) == 0.0);
        try expect(asin32(-0.0) == -0.0);
        try expect(math.isNan(asin32(-2)));
        try expect(math.isNan(asin32(1.5)));
    }

    test "math.asin64.special" {
        try expect(asin64(0.0) == 0.0);
        try expect(asin64(-0.0) == -0.0);
        try expect(math.isNan(asin64(-2)));
        try expect(math.isNan(asin64(1.5)));
    }