Ultra fast IEEE754 double-precision mode sin function

gistfile1.c

double cc_sin(double x) {
    const double a =  0.00735246819687011731341356165096815;
    const double b = -0.16528911397014738207016302002888890;
    const double c =  0.99969198629596757779830113868360584;

    double x2;

    union {
        double d;
        long i;
    } dtoi_hack;

    /* find offset of x from the range -pi to pi */
    dtoi_hack.d = CC_1_PI * x + 103079215104.5;
    dtoi_hack.i >>= 16;

    x -= dtoi_hack.i * CC_PI;
    x2 = x*x;

    x *= (c + x2*(b + a*x2));

    /* if x is in an odd pi count we must flip */
    x -= (2 * (dtoi_hack.i & 1)) * x; /* trick for r = (k % 2) == 0 ? r : -r; */

    return x;
}