c++ - Fast Arc Cos algorithm?

Question

I have my own, very fast cos function:

float sine(float x)
{
    const float B = 4/pi;
    const float C = -4/(pi*pi);

    float y = B * x + C * x * abs(x);

    //  const float Q = 0.775;
    const float P = 0.225;

    y = P * (y * abs(y) - y) + y;   // Q * y + P * y * abs(y)


    return y;
}

float cosine(float x)
{
    return sine(x + (pi / 2));
}

But now when I profile, I see that acos() is killing the processor. I don't need intense precision. What is a fast way to calculate acos(x) Thanks.

Answer 1

A simple cubic approximation, the Lagrange polynomial for x ∈ {-1, -?, 0, ?, 1}, is:

double acos(x) {
   return (-0.69813170079773212 * x * x - 0.87266462599716477) * x + 1.5707963267948966;
}

It has a maximum error of about 0.18 rad.