substitution cipher

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <limits.h>

#define ALPHABET 26

char encode (int, const char *);
void test_encode (void);

int main (int argc, char *argv[]) {
    test_encode();

    printf ("Test passed\n");

    char permutation[ALPHABET];

    int plain_char = getchar();
    while (plain_char != EOF) {
        int encoded_char = encode (plain_char, permutation);
        putchar (encoded_char);
        plain_char = getchar();
    }

    return EXIT_SUCCESS;
}

void test_encode (void) {
    int i;
    for (i = 0; i < CHAR_MAX; i++) {
        assert (encode ((char)i,"abcdefghijklmnopqrstuvwxyz") == (char)i);
    }

    assert (encode ('a',"bcdefghijklmnopqrstuvwxyza") == 'b');
    assert (encode ('m',"bcdefghijklmnopqrstuvwxyza") == 'n');
    assert (encode ('z',"bcdefghijklmnopqrstuvwxyza") == 'a');

    assert (encode ('a',"qwertyuiopasdfghjklzxcvbnm") == 'q');
    assert (encode ('b',"qwertyuiopasdfghjklzxcvbnm") == 'w');
    assert (encode ('z',"qwertyuiopasdfghjklzxcvbnm") == 'm');
}

char encode (int plain_char, const char *permutation) {
    //todo
    // only need lowercase

    return plain_char;
}