/*
 * Program to do *very* simple encryption (rotate alphabet N positions).
 *
 * N, and input and output filenames, are given by command-line arguments.
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char* lc_alphabet = "abcdefghijklmnopqrstuvwxyz";
char* uc_alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

int rotate(int inchar, int positions_to_rotate);

/* main program */
int main(int argc, char* argv[]) {

    if (argc < 4) {
        fprintf(stderr, "usage %s pos_to_rotate infile outfile\n", argv[0]);
        return EXIT_FAILURE;
    }

    char* endptr;
    int n = strtol(argv[1], &endptr, 10);
    if ((*endptr != '\0') || n <= 0) {
        fprintf(stderr, "%s not a positive integer\n", argv[1]);
        return EXIT_FAILURE;
    }

    FILE* infile = fopen(argv[2], "r");
    if (infile == NULL) {
        fprintf(stderr, "cannot open input file %s\n", argv[2]);
        return EXIT_FAILURE;
    }

    FILE* outfile = fopen(argv[3], "w");
    if (outfile == NULL) {
        fprintf(stderr, "cannot open output file %s\n", argv[3]);
        return EXIT_FAILURE;
    }

    int inchar;
    while ((inchar = fgetc(infile)) != EOF) {
        fputc(rotate(inchar, n), outfile);
    }

    return EXIT_SUCCESS;
}

/* encode single character */
int rotate(int inchar, int positions_to_rotate) {
    char* in_lc_alphabet = strchr(lc_alphabet, inchar);
    char* in_uc_alphabet = strchr(uc_alphabet, inchar);
    if (in_lc_alphabet != NULL) {
        int pos = in_lc_alphabet - lc_alphabet;
        return lc_alphabet[(pos + positions_to_rotate) % strlen(lc_alphabet)];
    }
    else if (in_uc_alphabet != NULL) {
        int pos = in_uc_alphabet - uc_alphabet;
        return uc_alphabet[(pos + positions_to_rotate) % strlen(uc_alphabet)];
    }
    else {
        return inchar;
    }
}