Counting DNA codons. Mostly an exploration of multidimensional array pointers. (No input checking.)

codons.c

#include <stdio.h>

#define CHAR_TO_BASE(C) (((C) >> 1) & 3)

enum base {
    A = CHAR_TO_BASE('A'),
    C = CHAR_TO_BASE('C'),
    G = CHAR_TO_BASE('G'),
    T = CHAR_TO_BASE('T'),
};

#define BASE_TO_CHAR(X) ((X) == T ? 'T' : 'A' | ((X) << 1))

_Static_assert(BASE_TO_CHAR(A) == 'A', "A must be correctly represented");
_Static_assert(BASE_TO_CHAR(C) == 'C', "C must be correctly represented");
_Static_assert(BASE_TO_CHAR(G) == 'G', "G must be correctly represented");
_Static_assert(BASE_TO_CHAR(T) == 'T', "T must be correctly represented");

static void count_codons(int (*)[4][4], const char *);
static void print_codons(const int (*)[4][4]);

int
main(int argc, char **argv)
{
    int counts[4][4][4] = {0};

    for (int i = 1; i < argc; i++)
        count_codons(counts, argv[i]);

    print_codons((const int (*)[4][4])counts);

    return 0;
}

void
count_codons(int (*counts)[4][4], const char *s)
{
    if (!s[0] || !s[1])
        return;

    enum base x = CHAR_TO_BASE(s[0]);
    enum base y = CHAR_TO_BASE(s[1]);
    enum base z;

    for (const char *p = &s[2]; *p; x = y, y = z, p++) {
        z = CHAR_TO_BASE(*p);
        counts[x][y][z]++;
    }
}

void
print_codons(const int (*counts)[4][4])
{
    for (int x = 0; x < 4; x++) {
        const char xc = BASE_TO_CHAR(x);
        const int (*xcounts)[4] = counts[x];

        for (int y = 0; y < 4; y++) {
            const char yc = BASE_TO_CHAR(y);
            const int *xycounts = xcounts[y];

            for (int z = 0; z < 4; z++) {
                const char zc = BASE_TO_CHAR(z);
                const int xyzcount = xycounts[z];

                if (xyzcount > 0) {
                    printf("%c%c%c: %d\n", xc, yc, zc, xyzcount);
                }
            }
        }
    }
}