Caesar Cipher

#include <stdio.h>
#include <string.h>
#include <stdbool.h>
#include <assert.h>

bool isAlphabetValid(char* alphabet) {
    assert(alphabet);
    if (!*alphabet)
        return false;
    for (; *alphabet; alphabet++)
        if (strchr(alphabet + 1, *alphabet))
            return false;
    return true;
}
bool isStringValid(char* string, char* alphabet, char* exclude) {
    assert(string);
    assert(alphabet);
    if (!*string)
        return false;

    if (exclude)
        for (; *string; string++) {
            if (strchr(exclude, *string))
                continue;
            if (!strchr(alphabet, *string))
                return false;
        }
    else
        for (; *string; string++)
            if (!strchr(alphabet, *string))
                return false;
    return true;
}
inline ptrdiff_t getIndex(char* string, int c) {
    return strchr(string, c) - string;
}

void Caesar_Cipher(char* string, char* alphabet, char* exclude, unsigned shift) {
    if (!isAlphabetValid(alphabet) || !isStringValid(string, alphabet, exclude))
        return;

    size_t alphabet_size = strlen(alphabet);

    if (exclude)
        for (; *string; string++) {
            if (strchr(exclude, *string))
                continue;
            *string = alphabet[(getIndex(alphabet, *string) + shift) % alphabet_size];
        }
    else
        for (; *string; string++)
            *string = alphabet[(getIndex(alphabet, *string) + shift) % alphabet_size];
}

char alphabet[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
char exclude[] = " ,.!?";

char MyString[] = "HELLO, MY NAME IS ALEX! WHATS YOU?";

int main()
{
    printf("\"%s\"\n", MyString);
    Caesar_Cipher(MyString, alphabet, NULL, 3);
    printf("\"%s\"\n", MyString);
    Caesar_Cipher(MyString, alphabet, exclude, 1);
    printf("\"%s\"\n", MyString);
    Caesar_Cipher(MyString, alphabet, exclude, 0);
    printf("\"%s\"\n", MyString);
    Caesar_Cipher(MyString, alphabet, exclude, 25);
    printf("\"%s\"\n", MyString);
}