    const std = @import("../../std.zig");
    const testing = std.testing;
    const math = std.math;
    const cmath = math.complex;
    const Complex = cmath.Complex;

    /// Returns the arc-cosine of z.
    pub fn acos(z: anytype) Complex(@TypeOf(z.re)) {
        const T = @TypeOf(z.re);
        const q = cmath.asin(z);
        return Complex(T).init(@as(T, math.pi) / 2 - q.re, -q.im);
    }

    const epsilon = 0.0001;

    test "complex.cacos" {
        const a = Complex(f32).init(5, 3);
        const c = acos(a);

        try testing.expect(math.approxEqAbs(f32, c.re, 0.546975, epsilon));
        try testing.expect(math.approxEqAbs(f32, c.im, -2.452914, epsilon));
    }