    // 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/coshf.c

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

    const std = @import("../std.zig");
    const math = std.math;
    const expo2 = @import("expo2.zig").expo2;
    const expect = std.testing.expect;
    const maxInt = std.math.maxInt;

    /// Returns the hyperbolic cosine of x.
    ///
    /// Special Cases:
    ///  - cosh(+-0)   = 1
    ///  - cosh(+-inf) = +inf
    ///  - cosh(nan)   = nan
    pub fn cosh(x: anytype) @TypeOf(x) {
        const T = @TypeOf(x);
        return switch (T) {
            f32 => cosh32(x),
            f64 => cosh64(x),
            else => @compileError("cosh not implemented for " ++ @typeName(T)),
        };
    }

    // cosh(x) = (exp(x) + 1 / exp(x)) / 2

    //         = 1 + 0.5 * (exp(x) - 1) * (exp(x) - 1) / exp(x)

    //         = 1 + (x * x) / 2 + o(x^4)

    fn cosh32(x: f32) f32 {
        const u = @bitCast(u32, x);
        const ux = u & 0x7FFFFFFF;
        const ax = @bitCast(f32, ux);

        // |x| < log(2)

        if (ux < 0x3F317217) {
            if (ux < 0x3F800000 - (12 << 23)) {
                math.raiseOverflow();
                return 1.0;
            }
            const t = math.expm1(ax);
            return 1 + t * t / (2 * (1 + t));
        }

        // |x| < log(FLT_MAX)

        if (ux < 0x42B17217) {
            const t = @exp(ax);
            return 0.5 * (t + 1 / t);
        }

        // |x| > log(FLT_MAX) or nan

        return expo2(ax);
    }

    fn cosh64(x: f64) f64 {
        const u = @bitCast(u64, x);
        const w = @intCast(u32, u >> 32) & (maxInt(u32) >> 1);
        const ax = @bitCast(f64, u & (maxInt(u64) >> 1));

        // TODO: Shouldn't need this explicit check.

        if (x == 0.0) {
            return 1.0;
        }

        // |x| < log(2)

        if (w < 0x3FE62E42) {
            if (w < 0x3FF00000 - (26 << 20)) {
                if (x != 0) {
                    math.raiseInexact();
                }
                return 1.0;
            }
            const t = math.expm1(ax);
            return 1 + t * t / (2 * (1 + t));
        }

        // |x| < log(DBL_MAX)

        if (w < 0x40862E42) {
            const t = @exp(ax);
            // NOTE: If x > log(0x1p26) then 1/t is not required.

            return 0.5 * (t + 1 / t);
        }

        // |x| > log(CBL_MAX) or nan

        return expo2(ax);
    }

    test "math.cosh" {
        try expect(cosh(@as(f32, 1.5)) == cosh32(1.5));
        try expect(cosh(@as(f64, 1.5)) == cosh64(1.5));
    }

    test "math.cosh32" {
        const epsilon = 0.000001;

        try expect(math.approxEqAbs(f32, cosh32(0.0), 1.0, epsilon));
        try expect(math.approxEqAbs(f32, cosh32(0.2), 1.020067, epsilon));
        try expect(math.approxEqAbs(f32, cosh32(0.8923), 1.425225, epsilon));
        try expect(math.approxEqAbs(f32, cosh32(1.5), 2.352410, epsilon));
        try expect(math.approxEqAbs(f32, cosh32(-0.0), 1.0, epsilon));
        try expect(math.approxEqAbs(f32, cosh32(-0.2), 1.020067, epsilon));
        try expect(math.approxEqAbs(f32, cosh32(-0.8923), 1.425225, epsilon));
        try expect(math.approxEqAbs(f32, cosh32(-1.5), 2.352410, epsilon));
    }

    test "math.cosh64" {
        const epsilon = 0.000001;

        try expect(math.approxEqAbs(f64, cosh64(0.0), 1.0, epsilon));
        try expect(math.approxEqAbs(f64, cosh64(0.2), 1.020067, epsilon));
        try expect(math.approxEqAbs(f64, cosh64(0.8923), 1.425225, epsilon));
        try expect(math.approxEqAbs(f64, cosh64(1.5), 2.352410, epsilon));
        try expect(math.approxEqAbs(f64, cosh64(-0.0), 1.0, epsilon));
        try expect(math.approxEqAbs(f64, cosh64(-0.2), 1.020067, epsilon));
        try expect(math.approxEqAbs(f64, cosh64(-0.8923), 1.425225, epsilon));
        try expect(math.approxEqAbs(f64, cosh64(-1.5), 2.352410, epsilon));
    }

    test "math.cosh32.special" {
        try expect(cosh32(0.0) == 1.0);
        try expect(cosh32(-0.0) == 1.0);
        try expect(math.isPositiveInf(cosh32(math.inf(f32))));
        try expect(math.isPositiveInf(cosh32(-math.inf(f32))));
        try expect(math.isNan(cosh32(math.nan(f32))));
    }

    test "math.cosh64.special" {
        try expect(cosh64(0.0) == 1.0);
        try expect(cosh64(-0.0) == 1.0);
        try expect(math.isPositiveInf(cosh64(math.inf(f64))));
        try expect(math.isPositiveInf(cosh64(-math.inf(f64))));
        try expect(math.isNan(cosh64(math.nan(f64))));
    }