    // Ported from go, which is licensed under a BSD-3 license.

    // https://golang.org/LICENSE

    //

    // https://golang.org/src/math/pow.go

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

    /// Returns x raised to the power of y (x^y).
    ///
    /// Special Cases:
    ///  - pow(x, +-0)    = 1 for any x
    ///  - pow(1, y)      = 1 for any y
    ///  - pow(x, 1)      = x for any x
    ///  - pow(nan, y)    = nan
    ///  - pow(x, nan)    = nan
    ///  - pow(+-0, y)    = +-inf for y an odd integer < 0
    ///  - pow(+-0, -inf) = +inf
    ///  - pow(+-0, +inf) = +0
    ///  - pow(+-0, y)    = +inf for finite y < 0 and not an odd integer
    ///  - pow(+-0, y)    = +-0 for y an odd integer > 0
    ///  - pow(+-0, y)    = +0 for finite y > 0 and not an odd integer
    ///  - pow(-1, +-inf) = 1
    ///  - pow(x, +inf)   = +inf for |x| > 1
    ///  - pow(x, -inf)   = +0 for |x| > 1
    ///  - pow(x, +inf)   = +0 for |x| < 1
    ///  - pow(x, -inf)   = +inf for |x| < 1
    ///  - pow(+inf, y)   = +inf for y > 0
    ///  - pow(+inf, y)   = +0 for y < 0
    ///  - pow(-inf, y)   = pow(-0, -y)
    ///  - pow(x, y)      = nan for finite x < 0 and finite non-integer y
    pub fn pow(comptime T: type, x: T, y: T) T {
        if (@typeInfo(T) == .Int) {
            return math.powi(T, x, y) catch unreachable;
        }

        if (T != f32 and T != f64) {
            @compileError("pow not implemented for " ++ @typeName(T));
        }

        // pow(x, +-0) = 1      for all x

        // pow(1, y) = 1        for all y

        if (y == 0 or x == 1) {
            return 1;
        }

        // pow(nan, y) = nan    for all y

        // pow(x, nan) = nan    for all x

        if (math.isNan(x) or math.isNan(y)) {
            return math.nan(T);
        }

        // pow(x, 1) = x        for all x

        if (y == 1) {
            return x;
        }

        if (x == 0) {
            if (y < 0) {
                // pow(+-0, y) = +- 0   for y an odd integer

                if (isOddInteger(y)) {
                    return math.copysign(math.inf(T), x);
                }
                // pow(+-0, y) = +inf   for y an even integer

                else {
                    return math.inf(T);
                }
            } else {
                if (isOddInteger(y)) {
                    return x;
                } else {
                    return 0;
                }
            }
        }

        if (math.isInf(y)) {
            // pow(-1, inf) = 1     for all x

            if (x == -1) {
                return 1.0;
            }
            // pow(x, +inf) = +0    for |x| < 1

            // pow(x, -inf) = +0    for |x| > 1

            else if ((@fabs(x) < 1) == math.isPositiveInf(y)) {
                return 0;
            }
            // pow(x, -inf) = +inf  for |x| < 1

            // pow(x, +inf) = +inf  for |x| > 1

            else {
                return math.inf(T);
            }
        }

        if (math.isInf(x)) {
            if (math.isNegativeInf(x)) {
                return pow(T, 1 / x, -y);
            }
            // pow(+inf, y) = +0    for y < 0

            else if (y < 0) {
                return 0;
            }
            // pow(+inf, y) = +0    for y > 0

            else if (y > 0) {
                return math.inf(T);
            }
        }

        // special case sqrt

        if (y == 0.5) {
            return @sqrt(x);
        }

        if (y == -0.5) {
            return 1 / @sqrt(x);
        }

        const r1 = math.modf(@fabs(y));
        var yi = r1.ipart;
        var yf = r1.fpart;

        if (yf != 0 and x < 0) {
            return math.nan(T);
        }
        if (yi >= 1 << (@typeInfo(T).Float.bits - 1)) {
            return @exp(y * @log(x));
        }

        // a = a1 * 2^ae

        var a1: T = 1.0;
        var ae: i32 = 0;

        // a *= x^yf

        if (yf != 0) {
            if (yf > 0.5) {
                yf -= 1;
                yi += 1;
            }
            a1 = @exp(yf * @log(x));
        }

        // a *= x^yi

        const r2 = math.frexp(x);
        var xe = r2.exponent;
        var x1 = r2.significand;

        var i = @as(std.meta.Int(.signed, @typeInfo(T).Float.bits), @intFromFloat(yi));
        while (i != 0) : (i >>= 1) {
            const overflow_shift = math.floatExponentBits(T) + 1;
            if (xe < -(1 << overflow_shift) or (1 << overflow_shift) < xe) {
                // catch xe before it overflows the left shift below

                // Since i != 0 it has at least one bit still set, so ae will accumulate xe

                // on at least one more iteration, ae += xe is a lower bound on ae

                // the lower bound on ae exceeds the size of a float exp

                // so the final call to Ldexp will produce under/overflow (0/Inf)

                ae += xe;
                break;
            }
            if (i & 1 == 1) {
                a1 *= x1;
                ae += xe;
            }
            x1 *= x1;
            xe <<= 1;
            if (x1 < 0.5) {
                x1 += x1;
                xe -= 1;
            }
        }

        // a *= a1 * 2^ae

        if (y < 0) {
            a1 = 1 / a1;
            ae = -ae;
        }

        return math.scalbn(a1, ae);
    }

    fn isOddInteger(x: f64) bool {
        const r = math.modf(x);
        return r.fpart == 0.0 and @as(i64, @intFromFloat(r.ipart)) & 1 == 1;
    }

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

        try expect(math.approxEqAbs(f32, pow(f32, 0.0, 3.3), 0.0, epsilon));
        try expect(math.approxEqAbs(f32, pow(f32, 0.8923, 3.3), 0.686572, epsilon));
        try expect(math.approxEqAbs(f32, pow(f32, 0.2, 3.3), 0.004936, epsilon));
        try expect(math.approxEqAbs(f32, pow(f32, 1.5, 3.3), 3.811546, epsilon));
        try expect(math.approxEqAbs(f32, pow(f32, 37.45, 3.3), 155736.703125, epsilon));
        try expect(math.approxEqAbs(f32, pow(f32, 89.123, 3.3), 2722489.5, epsilon));

        try expect(math.approxEqAbs(f64, pow(f64, 0.0, 3.3), 0.0, epsilon));
        try expect(math.approxEqAbs(f64, pow(f64, 0.8923, 3.3), 0.686572, epsilon));
        try expect(math.approxEqAbs(f64, pow(f64, 0.2, 3.3), 0.004936, epsilon));
        try expect(math.approxEqAbs(f64, pow(f64, 1.5, 3.3), 3.811546, epsilon));
        try expect(math.approxEqAbs(f64, pow(f64, 37.45, 3.3), 155736.7160616, epsilon));
        try expect(math.approxEqAbs(f64, pow(f64, 89.123, 3.3), 2722490.231436, epsilon));
    }

    test "math.pow.special" {
        const epsilon = 0.000001;

        try expect(pow(f32, 4, 0.0) == 1.0);
        try expect(pow(f32, 7, -0.0) == 1.0);
        try expect(pow(f32, 45, 1.0) == 45);
        try expect(pow(f32, -45, 1.0) == -45);
        try expect(math.isNan(pow(f32, math.nan(f32), 5.0)));
        try expect(math.isPositiveInf(pow(f32, -math.inf(f32), 0.5)));
        try expect(math.isPositiveInf(pow(f32, -0, -0.5)));
        try expect(pow(f32, -0, 0.5) == 0);
        try expect(math.isNan(pow(f32, 5.0, math.nan(f32))));
        try expect(math.isPositiveInf(pow(f32, 0.0, -1.0)));
        //expect(math.isNegativeInf(pow(f32, -0.0, -3.0))); TODO is this required?

        try expect(math.isPositiveInf(pow(f32, 0.0, -math.inf(f32))));
        try expect(math.isPositiveInf(pow(f32, -0.0, -math.inf(f32))));
        try expect(pow(f32, 0.0, math.inf(f32)) == 0.0);
        try expect(pow(f32, -0.0, math.inf(f32)) == 0.0);
        try expect(math.isPositiveInf(pow(f32, 0.0, -2.0)));
        try expect(math.isPositiveInf(pow(f32, -0.0, -2.0)));
        try expect(pow(f32, 0.0, 1.0) == 0.0);
        try expect(pow(f32, -0.0, 1.0) == -0.0);
        try expect(pow(f32, 0.0, 2.0) == 0.0);
        try expect(pow(f32, -0.0, 2.0) == 0.0);
        try expect(math.approxEqAbs(f32, pow(f32, -1.0, math.inf(f32)), 1.0, epsilon));
        try expect(math.approxEqAbs(f32, pow(f32, -1.0, -math.inf(f32)), 1.0, epsilon));
        try expect(math.isPositiveInf(pow(f32, 1.2, math.inf(f32))));
        try expect(math.isPositiveInf(pow(f32, -1.2, math.inf(f32))));
        try expect(pow(f32, 1.2, -math.inf(f32)) == 0.0);
        try expect(pow(f32, -1.2, -math.inf(f32)) == 0.0);
        try expect(pow(f32, 0.2, math.inf(f32)) == 0.0);
        try expect(pow(f32, -0.2, math.inf(f32)) == 0.0);
        try expect(math.isPositiveInf(pow(f32, 0.2, -math.inf(f32))));
        try expect(math.isPositiveInf(pow(f32, -0.2, -math.inf(f32))));
        try expect(math.isPositiveInf(pow(f32, math.inf(f32), 1.0)));
        try expect(pow(f32, math.inf(f32), -1.0) == 0.0);
        //expect(pow(f32, -math.inf(f32), 5.0) == pow(f32, -0.0, -5.0)); TODO support negative 0?

        try expect(pow(f32, -math.inf(f32), -5.2) == pow(f32, -0.0, 5.2));
        try expect(math.isNan(pow(f32, -1.0, 1.2)));
        try expect(math.isNan(pow(f32, -12.4, 78.5)));
    }

    test "math.pow.overflow" {
        try expect(math.isPositiveInf(pow(f64, 2, 1 << 32)));
        try expect(pow(f64, 2, -(1 << 32)) == 0);
        try expect(math.isNegativeInf(pow(f64, -2, (1 << 32) + 1)));
        try expect(pow(f64, 0.5, 1 << 45) == 0);
        try expect(math.isPositiveInf(pow(f64, 0.5, -(1 << 45))));
    }