fast_sinus.c

#include <math.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdint.h>

#ifndef M_PI
    #define M_PI 3.14159265358979323846
#endif

double fast_sin(double x)
{
    const double x_pi = x / M_PI;
    const double floor_x_pi = floor(x_pi);
    const double sign4 = (long long)floor_x_pi % 2 == 0 ? -4.0 : 4.0;
    return sign4 * (x_pi - ceil(x_pi)) * (x_pi - floor_x_pi);
}

int main(int argc, char* argv[])
{
    if (argc < 3)
    {
	    fprintf(stderr, "No argument specified\n");
	    return 1;
    }

    const int n = atoi(argv[2]);

    if (!n)
    {
        fprintf(stderr, "Invalid size '%s'!\n", argv[2]);
        return 1;
    }

    volatile double x = -(n/3);

    if (!strcmp(argv[1], "error"))
    {
        double abs_error = 0.0;

        for (int i = 0; i < n; i++)
        {
            abs_error += fabs(sin(x) - fast_sin(x));
            x += 0.3;
        }

        abs_error /= (double)n;

        printf("Average absolute error: %f\n", abs_error);
        return 0;
    }

    volatile double* sins = malloc(sizeof(double) * n);

    if (!sins)
    {
        fprintf(stderr, "Failed to allocate the array!\n");
        return 1;
    }

    double (*f)(double);
    
    if (!strcmp(argv[1], "sin"))
	    f = sin;
    else if (!strcmp(argv[1], "fast"))
        f = fast_sin;
    else
    {
	    fprintf(
            stderr, 
            "Invalid argument. Options are 'sin' | 'fast' | 'error' \n"
        );
	    return 1;
    }

    for (int i = 0; i < n; i++)
    {
        sins[i] = f(x);
        x += 0.3;
    }

    free((double*)sins);
}