// Building a simple rotation cipher.  This is just practice
// since my c/c++ skillz are lacking again.
// author: Nick Soracco <nsoracco!AT!cs.cur.edu>
// Licence: public domain

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

// pull in a single char and a shift value
void rotate( char *, int );

int main( int argc, char **argv ) {
	if( argc != 3 ) {
		printf( "Usage: %s shift message\n", argv[0] );
		return 1;
	}

	int shift = atoi( argv[1] );
	if( shift == 0 || shift > 25 ) {
		printf("Invalid shift.\n");
		return 2;
	}

	printf( "subject: %s Shift: %d\n", argv[2], shift );
	rotate( argv[2], shift );
	printf( "Ciphertext: %s\n", argv[2] );

}

// By reducing the symbol's ASCII value to it's symbolic from 0 representation
// and then adding the shift value and modulo by the number of symbols in the
// alphabet and then adding the base back in to get the ASCII value of the
// shifted value back.  The easier way (and works with all symbols!) is to do
// something like this: shifted = (((symbol + shift) % symbol ) + offset) 
// where:	offset = distance from 0 to the first symbol's ascii value
// 		symbol = ascii value of the symbol in question
//		shift  = how many places up to move.
void rotate( char *cleartext, int shift ) {
	int length = strlen(cleartext);
	char temp;
	int i;	// how do I turn on C99 mode in gcc4? :(
	for( i=0; i < length; ++i ) {
		temp = cleartext[i];
		if( temp >= 65 && temp <= 90 ) {
			temp = (((temp - 65 + shift) % 26) + 65);
		} else if ( temp >= 97 && temp <= 122 ) {
			temp = (((temp - 97 + shift) % 26) + 97);
		}
		cleartext[i] = temp;
	}
}