module math

const (
    tanh_p = [
        -9.64399179425052238628e-1,
        -9.92877231001918586564e+1,
        -1.61468768441708447952e+3,
    ]
    tanh_q = [
        1.12811678491632931402e+2,
        2.23548839060100448583e+3,
        4.84406305325125486048e+3,
    ]
)

// tanh returns the hyperbolic tangent of x.
//
// special cases are:
// tanh(±0) = ±0
// tanh(±inf) = ±1
// tanh(nan) = nan
pub fn tanh(x f64) f64 {
    maxlog := 8.8029691931113054295988e+01 // log(2**127)
    mut z := abs(x)
    if z > 0.5 * maxlog {
        if x < 0 {
            return f64(-1)
        }
        return 1.0
    } else if z >= 0.625 {
        s := exp(2.0 * z)
        z = 1.0 - 2.0 / (s + 1.0)
        if x < 0 {
            z = -z
        }
    } else {
        if x == 0 {
            return x
        }
        s := x * x
        z = x + x * s * ((math.tanh_p[0] * s + math.tanh_p[1]) * s + math.tanh_p[2]) / (((s +
            math.tanh_q[0]) * s + math.tanh_q[1]) * s + math.tanh_q[2])
    }
    return z
}