usr/src/games/caesar/caesar.c

/*
 * program to to decrypt caesar(tm) cypher
 * (caesar is a trademark of the roman empire)
 *
 * to compile:
 *
 *      cc decrypt.c -lm -o decrypt.c
 *
 * usage:
 *
 *      decrypt [n] < file
 *
 * where n is an optional forced rotation.
 *
 * authors: Stan King, John Eldridge, based on algorithm suggested by
 *              Bob Morris
 * 29-Sep-82
 *
 */

#ifdef SCCSID
static char     *SccsId = "@(#)caesar.c 1.7     4/16/85";
#endif /* SCCSID */

#include <stdio.h>
#include <ctype.h>
#include <math.h>
extern char *calloc();

main(argc, argv)
int argc;
char *argv[];
{
        /* letter frequencies (taken from some unix(tm) documentation) */
        /* (unix is a trademark of Bell Laboratories) */
        static double stdf[ 26 ] =
        {
                7.97, 1.35, 3.61, 4.78, 12.37, 2.01, 1.46, 4.49,
                6.39, 0.04, 0.42, 3.81, 2.69, 5.92, 6.96, 2.91,
                0.08, 6.63, 8.77, 9.68, 2.62, 0.81, 1.88, 0.23,
                2.07, 0.06,
        };
        int obs[26];
        int bufsize;
        int c, i, try;
        double dot, winnerdot;  /* .. */
        int winner, forced = 0;
        char *inbuf;

        bufsize = 0;
        if( argc > 1 )
                sscanf( argv[1], "%d", &forced );
        if( forced == 0 )
                forced = -1000;

        inbuf = calloc( BUFSIZ, 1 );

        /* adjust frequency table to weight low probs REAL low */
        for (i=0; i<26; i++)    {
                stdf[i] = log(stdf[i]) + log(26.0/100.0);
        }

        /* Decode each line separately */
        for (;;) {
                for (i=0; i<=25; obs[i++]=0)
                        ;

                /* get a sample of the text */
                for( i = 0; i < BUFSIZ; i++ ) {
                        if( (c = getchar()) == EOF ) {
                                exit(0);
                        }
                        inbuf[i] = c;
                        if (c == '\n') {
                                bufsize = i+1;
                                break;
                        }
                        if (islower(c))
                                obs[c-'a'] += 1;
                        else if (isupper(c))
                                obs[c-'A'] += 1;
                }

                /* now "dot" the freqs with the observed letter freqs */
                /*      and keep track of best fit */
                winner = 0;
                for (try = 0; try<26; try+=13) {
                        dot = 0;
                        for ( i=0; i<26; i++ ) {
                                dot += obs[i] * stdf[ (i+try) % 26 ];
                                }
                        /* initialize winning score */
                        if( try == 0 )
                                winnerdot = dot;
                        if( dot > winnerdot ) {
                                /* got a new winner! */
                                winner = try;
                                winnerdot = dot;
                        }
                }

                if (forced != -1000)
                        winner = forced;

                /* print out sample buffer */
                for( i = 0; i < bufsize; i++ )
                        putchar( rotate( inbuf[i], winner ) );
        }
 }


static int
rotate( c, perm )
char c;
int perm;
{
        if (isupper(c)) {
                return 'A' + (c - 'A' + perm) % 26 ;
        }
        else if (islower(c)) {
                return 'a' + (c-'a'+perm) % 26 ;
        }
        else return c;
}