/****************************************************************************/
/**                                                                        **/
/**  Vig.c --- performs vignere encryption on standard input.              **/
/**                                                                        **/
/**  To compile:  gcc -ansi -o vignere vig.c                               **/
/**  To run:      vig keyword < input.txt                                  **/
/**                                                                        **/
/**  "vig -d keyword" performs decryption.                                 **/
/**  "vig -t keyword" strips away all non-letters.                         **/
/**                                                                        **/
/****************************************************************************/

#include <stdio.h>
#include <ctype.h>
#define  INDEX(c) ( ((c)|' ')-'a' )
#define  BASE(c)  ( ((c)&' ')?'a':'A' )

char *password;
int  keysize;
int  onlytext=0, decrypt=0;
int  everything_is_cool( int argc, char ** argv );


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

   int count=0;
   unsigned char ch, key;

   if( everything_is_cool( argc, argv ) ) {

      ch =  getchar();  /* get first character */

      while( !feof(stdin) ) {
          if( isalpha(ch) ) {
               key = INDEX( password[count] )+1;
               if( decrypt ) key = 26-key;
			
               ch = BASE(ch) + (INDEX(ch)+key)%26;
			
               putchar(ch);
               count++; if(count==keysize) count=0;
          }
          else if( !onlytext ) putchar(ch);

          ch =  getchar();  /* get next character */
      }
   }
   return 0;
}

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

	int i, j, coolness=0;

	for( i=1, password=NULL; i<argc; i++ ) 
        {
		if( argv[i][0]=='-' )  /* if argument is a flag */
		  for( j=1; j<strlen(argv[i]); j++ )
                  switch( argv[i][j] ) 
                  {
		    case 't' : onlytext = 1; break;
		    case 'd' : decrypt  = 1; break;
		    default  : break;
		  }
		else if(password==NULL) { 
			password = argv[i];
			keysize  = strlen(password);
			coolness = 1;
			for( j=0; j<keysize; j++ )
				if( !isalpha(password[i]) )
					coolness = 0; 
		}
		else    coolness = 0; 
	}
	if( coolness ) {
		fprintf(stderr,"\nYou chose the password %s\n", password );
		if( decrypt )
		  fprintf(stderr,"Decrypting instead of encrypting (-d)\n" );
		if( onlytext )
		  fprintf(stderr,"Only text will be output (-t)\n" );
	} else
        { 
	      printf( "\nUsage:  %s [flags] password\n", argv[0] );
	      printf( "\nFlags:  -d:  decrypt instead of encrypt\n");
	      printf( "        -t:  only output letters.\n");
	      printf( "\nThe password can only be letters.\n");

	} 
	return coolness;
}