sinpi.c

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/*
 * Implement sin(pi * x) and cos(pi * x) for real x. Since the periods
 * of these functions are integral (and thus representable in double
 * precision), it's possible to compute them with greater accuracy
 * than sin(x) and cos(x).
 */
#include "mconf.h"
 
 
/* Compute sin(pi * x). */
double sinpi(double x)
{
    double s = 1.0;
    double r;
 
    if (x < 0.0) {
        x = -x;
        s = -1.0;
    }
 
    r = fmod(x, 2.0);
    if (r < 0.5) {
        return s*sin(M_PI*r);
    }
    else if (r > 1.5) {
        return s*sin(M_PI*(r - 2.0));
    }
    else {
        return -s*sin(M_PI*(r - 1.0));
    }
}
 
 
/* Compute cos(pi * x) */
double cospi(double x)
{
    double r;
 
    if (x < 0.0) {
        x = -x;
    }
 
    r = fmod(x, 2.0);
    if (r == 0.5) {
        // We don't want to return -0.0
        return 0.0;
    }
    if (r < 1.0) {
        return -sin(M_PI*(r - 0.5));
    }
    else {
        return sin(M_PI*(r - 1.5));
    }
}