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

FILE *openFile(char filename[]);
FILE *createFile(char filename[]);
void printUsage();

int main(int argc, char *argv[])
{
    FILE *source; /* source file */
    FILE *dist; /* destination file */
    char arr[500]; /* a place to store lines from files */
    int i=0;
    
    /* if wrong number of arguments passed */
    if (argc != 3)
        printUsage(argv[0]);
    
    else {
        source = openFile(argv[1]);
        dist = createFile(argv[2]);
        
        /* scan each line and apply the cipher */
        while ((fgets(arr, 500, source)) != NULL) {
            for (i = 0; i < strlen(arr); i++)
                if (isalpha(arr[i]) && islower(arr[i]))
                    arr[i] = (arr[i] + 13) > 'z' ? arr[i] - 13 : arr[i] + 13;
                else if (isalpha(arr[i]) && isupper(arr[i]))
                    arr[i] = (arr[i] + 13) > 'Z' ? arr[i] - 13 : arr[i] + 13;

            fputs(arr, dist);
        }
    }
    fclose(source);
    fclose(dist);
    
    return EXIT_SUCCESS;
}

void printUsage(char programName[])
{
    printf("USAGE: %s <filename1.txt> <filename2.txt>\n\n", programName);
    exit(1);
}

FILE *openFile(char filename[]){
    FILE *fp;
    fp = fopen(filename, "r");
    if (fp == NULL) {
        printf("\nERROR: file \"%s\" does not exist.\n", filename);
        exit(1);
    }
    else
        return fp;
}

FILE *createFile(char filename[]){
    FILE *fp;
    char response;
    
    /* check to see if distination file already exists */
    fp = fopen(filename, "r");
    
    if (fp != NULL) {
        while(1) {
            printf("Destination file already exits;");
            printf(" overwrite? [y/n] ");
            scanf("%c", &response);
            
            if ((tolower(response)) == 'y'){
                fclose(fp);
                fp = fopen(filename, "w");
                return fp;
            }
            
            else if ((tolower(response)) == 'n')
                exit(1);

            else {
                printf("invalid response.\n");
                scanf("%c", &response);
                /* this line picks up the '\n' so it doesn't
                get re-scanned in the next loop */
            }
        }
    }
    
    /* if file doesn't exist, create it */
    else {
        fp = fopen(filename, "w");
        if (fp == NULL) {
            printf("Unable to create destination file. make");
            printf(" sure you have proper permissions.\n");
            exit(1);
        }
        else
            return fp;
    }
}
// 'a' + (c-'a')%('z'-'a'+1)