Amelia Surprise Code!

#include <stdio.h>
#include <string.h>
#include <ctype.h>

char decrypt(int in, int key) {
    in = tolower(in) - 'a'; // convert the letter to a number 0-25

    in = (in - key); // subtract the key
    while(in < 0) in += 26; // Add 26 until we are above 0
    // This is because mod doesn't work properly for negatives

    //in = (in + key) % 26; // Add the key and mod 26 to make sure it's a letter

    return (char)in + 'a'; // Convert the 0-25 number back into a letter and return
}

int main(int argc, char *argv[]) {
    if(argc != 2) return 1; // get file in command line

    int a1[] = {1, 3, 1, 3, -1};                 // time
    int a2[] = {2, 1, 0, 2, 2, 0, 1, 8, -1};     // date
    int a3[] = {5, 4, 1, 17, 20, 0, 17, 24, -1}; // month

    char c[2048] = {0}; // ciphertext
    char p[2048] = {0}; // plaintext
    int count = 0, read, total = 0;
    int i,j,k; // array counters
    i=j=k= 0;  // Set all counters to 0

    FILE *file = fopen(argv[1], "r"); // open the file
    if(file == NULL) return 1; // Make sure file opened
    while( (read = fread(c + total, 1, 2048-total, file)) > 0) // read the file into buffer
        total += read; // add number of bytes read to the total
    fclose(file); // close the file

    while(count < total) { // loop until end of ciphertext
        if(!isalpha(c[count])) { // If the character is non-alphabetic
            p[count] = c[count]; // Set the plaintext
            count ++; // Move to next character
            continue; // keep looping
        }


        i++;
        if(a1[i] == -1) { // check if a1 is at end
            j++;
            if(a2[j] == -1) { // check if a2 is at end
                k++;
                if(a3[k] == -1) { // check if a3 is at end
                    k = 0; // reset a3
                }
                j = 0; // reset a2
            }
            i = 0; // reset a1
        }

        // decrypt the ciphertext by multiplying all keys together
        p[count] = decrypt(c[count], a1[i] * a2[j] * a3[k]);
        count++; // Move to next character
    }

    // Print the result
    printf("%s\n", p);

    return 0;
}