    // 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/expmf.c

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

    // TODO: Updated recently.

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

    /// Returns e raised to the power of x, minus 1 (e^x - 1). This is more accurate than exp(e, x) - 1
    /// when x is near 0.
    ///
    /// Special Cases:
    ///  - expm1(+inf) = +inf
    ///  - expm1(-inf) = -1
    ///  - expm1(nan)  = nan
    pub fn expm1(x: anytype) @TypeOf(x) {
        const T = @TypeOf(x);
        return switch (T) {
            f32 => expm1_32(x),
            f64 => expm1_64(x),
            else => @compileError("exp1m not implemented for " ++ @typeName(T)),
        };
    }

    fn expm1_32(x_: f32) f32 {
        if (math.isNan(x_))
            return math.nan(f32);

        const o_threshold: f32 = 8.8721679688e+01;
        const ln2_hi: f32 = 6.9313812256e-01;
        const ln2_lo: f32 = 9.0580006145e-06;
        const invln2: f32 = 1.4426950216e+00;
        const Q1: f32 = -3.3333212137e-2;
        const Q2: f32 = 1.5807170421e-3;

        var x = x_;
        const ux = @as(u32, @bitCast(x));
        const hx = ux & 0x7FFFFFFF;
        const sign = hx >> 31;

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

        if (math.isNegativeInf(x)) {
            return -1.0;
        }

        // |x| >= 27 * ln2

        if (hx >= 0x4195B844) {
            // nan

            if (hx > 0x7F800000) {
                return x;
            }
            if (sign != 0) {
                return -1;
            }
            if (x > o_threshold) {
                x *= 0x1.0p127;
                return x;
            }
        }

        var hi: f32 = undefined;
        var lo: f32 = undefined;
        var c: f32 = undefined;
        var k: i32 = undefined;

        // |x| > 0.5 * ln2

        if (hx > 0x3EB17218) {
            // |x| < 1.5 * ln2

            if (hx < 0x3F851592) {
                if (sign == 0) {
                    hi = x - ln2_hi;
                    lo = ln2_lo;
                    k = 1;
                } else {
                    hi = x + ln2_hi;
                    lo = -ln2_lo;
                    k = -1;
                }
            } else {
                var kf = invln2 * x;
                if (sign != 0) {
                    kf -= 0.5;
                } else {
                    kf += 0.5;
                }

                k = @as(i32, @intFromFloat(kf));
                const t = @as(f32, @floatFromInt(k));
                hi = x - t * ln2_hi;
                lo = t * ln2_lo;
            }

            x = hi - lo;
            c = (hi - x) - lo;
        }
        // |x| < 2^(-25)

        else if (hx < 0x33000000) {
            if (hx < 0x00800000) {
                math.doNotOptimizeAway(x * x);
            }
            return x;
        } else {
            k = 0;
        }

        const hfx = 0.5 * x;
        const hxs = x * hfx;
        const r1 = 1.0 + hxs * (Q1 + hxs * Q2);
        const t = 3.0 - r1 * hfx;
        var e = hxs * ((r1 - t) / (6.0 - x * t));

        // c is 0

        if (k == 0) {
            return x - (x * e - hxs);
        }

        e = x * (e - c) - c;
        e -= hxs;

        // exp(x) ~ 2^k (x_reduced - e + 1)

        if (k == -1) {
            return 0.5 * (x - e) - 0.5;
        }
        if (k == 1) {
            if (x < -0.25) {
                return -2.0 * (e - (x + 0.5));
            } else {
                return 1.0 + 2.0 * (x - e);
            }
        }

        const twopk = @as(f32, @bitCast(@as(u32, @intCast((0x7F +% k) << 23))));

        if (k < 0 or k > 56) {
            var y = x - e + 1.0;
            if (k == 128) {
                y = y * 2.0 * 0x1.0p127;
            } else {
                y = y * twopk;
            }

            return y - 1.0;
        }

        const uf = @as(f32, @bitCast(@as(u32, @intCast(0x7F -% k)) << 23));
        if (k < 23) {
            return (x - e + (1 - uf)) * twopk;
        } else {
            return (x - (e + uf) + 1) * twopk;
        }
    }

    fn expm1_64(x_: f64) f64 {
        if (math.isNan(x_))
            return math.nan(f64);

        const o_threshold: f64 = 7.09782712893383973096e+02;
        const ln2_hi: f64 = 6.93147180369123816490e-01;
        const ln2_lo: f64 = 1.90821492927058770002e-10;
        const invln2: f64 = 1.44269504088896338700e+00;
        const Q1: f64 = -3.33333333333331316428e-02;
        const Q2: f64 = 1.58730158725481460165e-03;
        const Q3: f64 = -7.93650757867487942473e-05;
        const Q4: f64 = 4.00821782732936239552e-06;
        const Q5: f64 = -2.01099218183624371326e-07;

        var x = x_;
        const ux = @as(u64, @bitCast(x));
        const hx = @as(u32, @intCast(ux >> 32)) & 0x7FFFFFFF;
        const sign = ux >> 63;

        if (math.isNegativeInf(x)) {
            return -1.0;
        }

        // |x| >= 56 * ln2

        if (hx >= 0x4043687A) {
            // exp1md(nan) = nan

            if (hx > 0x7FF00000) {
                return x;
            }
            // exp1md(-ve) = -1

            if (sign != 0) {
                return -1;
            }
            if (x > o_threshold) {
                math.raiseOverflow();
                return math.inf(f64);
            }
        }

        var hi: f64 = undefined;
        var lo: f64 = undefined;
        var c: f64 = undefined;
        var k: i32 = undefined;

        // |x| > 0.5 * ln2

        if (hx > 0x3FD62E42) {
            // |x| < 1.5 * ln2

            if (hx < 0x3FF0A2B2) {
                if (sign == 0) {
                    hi = x - ln2_hi;
                    lo = ln2_lo;
                    k = 1;
                } else {
                    hi = x + ln2_hi;
                    lo = -ln2_lo;
                    k = -1;
                }
            } else {
                var kf = invln2 * x;
                if (sign != 0) {
                    kf -= 0.5;
                } else {
                    kf += 0.5;
                }

                k = @as(i32, @intFromFloat(kf));
                const t = @as(f64, @floatFromInt(k));
                hi = x - t * ln2_hi;
                lo = t * ln2_lo;
            }

            x = hi - lo;
            c = (hi - x) - lo;
        }
        // |x| < 2^(-54)

        else if (hx < 0x3C900000) {
            if (hx < 0x00100000) {
                math.doNotOptimizeAway(@as(f32, @floatCast(x)));
            }
            return x;
        } else {
            k = 0;
        }

        const hfx = 0.5 * x;
        const hxs = x * hfx;
        const r1 = 1.0 + hxs * (Q1 + hxs * (Q2 + hxs * (Q3 + hxs * (Q4 + hxs * Q5))));
        const t = 3.0 - r1 * hfx;
        var e = hxs * ((r1 - t) / (6.0 - x * t));

        // c is 0

        if (k == 0) {
            return x - (x * e - hxs);
        }

        e = x * (e - c) - c;
        e -= hxs;

        // exp(x) ~ 2^k (x_reduced - e + 1)

        if (k == -1) {
            return 0.5 * (x - e) - 0.5;
        }
        if (k == 1) {
            if (x < -0.25) {
                return -2.0 * (e - (x + 0.5));
            } else {
                return 1.0 + 2.0 * (x - e);
            }
        }

        const twopk = @as(f64, @bitCast(@as(u64, @intCast(0x3FF +% k)) << 52));

        if (k < 0 or k > 56) {
            var y = x - e + 1.0;
            if (k == 1024) {
                y = y * 2.0 * 0x1.0p1023;
            } else {
                y = y * twopk;
            }

            return y - 1.0;
        }

        const uf = @as(f64, @bitCast(@as(u64, @intCast(0x3FF -% k)) << 52));
        if (k < 20) {
            return (x - e + (1 - uf)) * twopk;
        } else {
            return (x - (e + uf) + 1) * twopk;
        }
    }

    test "math.exp1m" {
        try expect(expm1(@as(f32, 0.0)) == expm1_32(0.0));
        try expect(expm1(@as(f64, 0.0)) == expm1_64(0.0));
    }

    test "math.expm1_32" {
        const epsilon = 0.000001;

        try expect(expm1_32(0.0) == 0.0);
        try expect(math.approxEqAbs(f32, expm1_32(0.0), 0.0, epsilon));
        try expect(math.approxEqAbs(f32, expm1_32(0.2), 0.221403, epsilon));
        try expect(math.approxEqAbs(f32, expm1_32(0.8923), 1.440737, epsilon));
        try expect(math.approxEqAbs(f32, expm1_32(1.5), 3.481689, epsilon));
    }

    test "math.expm1_64" {
        const epsilon = 0.000001;

        try expect(expm1_64(0.0) == 0.0);
        try expect(math.approxEqAbs(f64, expm1_64(0.0), 0.0, epsilon));
        try expect(math.approxEqAbs(f64, expm1_64(0.2), 0.221403, epsilon));
        try expect(math.approxEqAbs(f64, expm1_64(0.8923), 1.440737, epsilon));
        try expect(math.approxEqAbs(f64, expm1_64(1.5), 3.481689, epsilon));
    }

    test "math.expm1_32.special" {
        try expect(math.isPositiveInf(expm1_32(math.inf(f32))));
        try expect(expm1_32(-math.inf(f32)) == -1.0);
        try expect(math.isNan(expm1_32(math.nan(f32))));
    }

    test "math.expm1_64.special" {
        try expect(math.isPositiveInf(expm1_64(math.inf(f64))));
        try expect(expm1_64(-math.inf(f64)) == -1.0);
        try expect(math.isNan(expm1_64(math.nan(f64))));
    }