//  caesar.c -- example for CS 2505 notes
//  Usage:  caesar <shift distance> <file name>
//
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <ctype.h>

int checkParams(int nparams, char** params);
int checkShiftAmt(char* src);
int setShiftAmt(char* src);
int processFile(int shiftAmt, char* fileName);
char applyShift(char Original, int shiftAmt);

#define WRONG_NUMBER_OF_PARAMS    1
#define INVALID_SHIFT_SPECIFIED   2
#define FILE_NOT_FOUND            3

int main(int argc, char** argv) {

   int ckStatus;
   if ( ( ckStatus = checkParams(argc, argv) ) != 0 ) {
      return ckStatus;
   }

   int shiftAmt = setShiftAmt(argv[1]);
   printf("Shifting alphabetic input text by %d positions.\n", shiftAmt);

   int charsShifted = processFile(shiftAmt, argv[2]);
   printf("Shifted %d alphabetic characters.\n", charsShifted);
   return 0;
}

int processFile(int shiftAmt, char* fileName) {

   shiftAmt = ((shiftAmt % 26) + 26) % 26;

   int nChars = 0;
   FILE *In = fopen(fileName, "r");
   char nextIn, nextOut;
   while ( fscanf(In, "%c", &nextIn) == 1 ) {

      if ( isalpha(nextIn) ) {
         ++nChars;
         nextOut = applyShift(nextIn, shiftAmt);
      }
      else
         nextOut = nextIn;
      
      printf("%c", nextOut);
   }

   fclose(In);
   return nChars;
}

char applyShift(char Original, int shiftAmt) {

   char Modified = Original;
   //. . .
   return Modified;
}

int checkParams(int nparams, char** params) {

   if ( nparams != 3 ) {
      printf("Invoke as: caesar <shift distance> <file name>\n");
      return WRONG_NUMBER_OF_PARAMS;
   }
   if ( !checkShiftAmt(params[1]) ) {
      return INVALID_SHIFT_SPECIFIED;
   }
   FILE* fp;
   if ( (fp = fopen(params[2], "r") ) == 0 ) {
      printf("The file %s could not be found.\n", params[2]);
      return FILE_NOT_FOUND;
   }
   fclose(fp);
   return 0;
}

int checkShiftAmt(char* src) {

   char *p;
   errno = 0;
   int result = (int) strtol(src, &p, 10);
   return (errno == 0 && *p == 0 && p != src);
}

int setShiftAmt(char* src) {

   char *p;
   int shiftAmt = strtol(src, &p, 10);
   return shiftAmt;
}