Generate reindex tables.

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

/* Lazy impl of the extended Euclidean algorithm. */
int invmod(int a, int n)
{
    int t = 0, r = n, newt = 1, newr = a;
    while (newr != 0) {
        int tmp, q = (r / newr);
        tmp = newt; newt = t - q * newt; t = tmp;
        tmp = newr; newr = r - q * newr; r = tmp;
    }
    if (r > 1) t = 0; //printf("a is not invertible!\n");
    if (t < 0) t += n;
    return t;
}

/*
 * Can adapt this to m x n PFA splitting by replacing 15 with a strtoul on a second argument.
 * Note that m and n have to be coprime for this to work.
 */
int main(int argc, char **argv)
{
    int i, j, k = 0, Ntot, n = strtoul(argv[1], NULL, 10);
    int inv_1, inv_2;
    n = (1U << n);
    inv_1 = invmod(n, 15);
    inv_2 = invmod(15, n);
    Ntot = 15*n;
    printf("inv_1: %u\n", inv_2);
    printf("const uint16_t pfa15_fwd_reindex[%u] = {\n  ", Ntot);
    for (i = 0; i < n; i++) {
        for (j = 0; j < 15; j++) {
            printf("%2u, ", (15*i + n*j)%Ntot);
            k++;
        }
        if (i+1 != n)
            printf("\n  ");
    }
    printf("\n};\n\nconst uint16_t pfa15_inv_reindex[%u] = {\n  ", Ntot);
    k = 0;
    for (j = 0; j < 15; j += 1) {
        for (i = 0; i < n; i++) {
            printf("%2u, ", (i*inv_2*15 + j*inv_1*n)%Ntot);
            k++;
        }
        if (j+1 != 15)
            printf("\n  ");
    }
    printf("\n};\n\n");
    return 0;
}