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caesar.c revision 1.2 
      1 /*
      2  * Copyright (c) 1989 The Regents of the University of California.
      3  * All rights reserved.
      4  *
      5  * This code is derived from software contributed to Berkeley by
      6  * Rick Adams.
      7  *
      8  * Authors:
      9  *	Stan King, John Eldridge, based on algorithm suggested by
     10  *	Bob Morris
     11  * 29-Sep-82
     12  *
     13  * Redistribution and use in source and binary forms, with or without
     14  * modification, are permitted provided that the following conditions
     15  * are met:
     16  * 1. Redistributions of source code must retain the above copyright
     17  *    notice, this list of conditions and the following disclaimer.
     18  * 2. Redistributions in binary form must reproduce the above copyright
     19  *    notice, this list of conditions and the following disclaimer in the
     20  *    documentation and/or other materials provided with the distribution.
     21  * 3. All advertising materials mentioning features or use of this software
     22  *    must display the following acknowledgement:
     23  *	This product includes software developed by the University of
     24  *	California, Berkeley and its contributors.
     25  * 4. Neither the name of the University nor the names of its contributors
     26  *    may be used to endorse or promote products derived from this software
     27  *    without specific prior written permission.
     28  *
     29  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
     30  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     31  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     32  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
     33  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
     34  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
     35  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
     36  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
     37  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
     38  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     39  * SUCH DAMAGE.
     40  */
     41 
     42 #ifndef lint
     43 char copyright[] =
     44 "@(#) Copyright (c) 1989 The Regents of the University of California.\n\
     45  All rights reserved.\n";
     46 #endif /* not lint */
     47 
     48 #ifndef lint
     49 /*static char sccsid[] = "from: @(#)caesar.c	5.4 (Berkeley) 6/1/90";*/
     50 static char rcsid[] = "$Id: caesar.c,v 1.2 1993/08/01 18:55:37 mycroft Exp $";
     51 #endif /* not lint */
     52 
     53 #include <math.h>
     54 #include <stdio.h>
     55 #include <ctype.h>
     56 #include <unistd.h>
     57 
     58 #define	LINELENGTH	2048
     59 #define	ROTATE(ch, perm) \
     60 	isupper(ch) ? ('A' + (ch - 'A' + perm) % 26) : \
     61 	    islower(ch) ? ('a' + (ch - 'a' + perm) % 26) : ch
     62 
     63 /*
     64  * letter frequencies (taken from some unix(tm) documentation)
     65  * (unix is a trademark of Bell Laboratories)
     66  */
     67 double stdf[26] = {
     68 	7.97, 1.35, 3.61, 4.78, 12.37, 2.01, 1.46, 4.49, 6.39, 0.04,
     69 	0.42, 3.81, 2.69, 5.92,  6.96, 2.91, 0.08, 6.63, 8.77, 9.68,
     70 	2.62, 0.81, 1.88, 0.23,  2.07, 0.06,
     71 };
     72 
     73 main(argc, argv)
     74 	int argc;
     75 	char **argv;
     76 {
     77 	extern int errno;
     78 	register int ch, dot, i, nread, winnerdot;
     79 	register char *inbuf;
     80 	int obs[26], try, winner;
     81 	char *malloc(), *strerror();
     82 
     83 	if (argc > 1)
     84 		printit(argv[1]);
     85 
     86 	if (!(inbuf = malloc(LINELENGTH))) {
     87 		(void)fprintf(stderr, "caesar: out of memory.\n");
     88 		exit(1);
     89 	}
     90 
     91 	/* adjust frequency table to weight low probs REAL low */
     92 	for (i = 0; i < 26; ++i)
     93 		stdf[i] = log(stdf[i]) + log(26.0 / 100.0);
     94 
     95 	/* zero out observation table */
     96 	bzero(obs, 26 * sizeof(int));
     97 
     98 	if ((nread = read(STDIN_FILENO, inbuf, LINELENGTH)) < 0) {
     99 		(void)fprintf(stderr, "caesar: %s\n", strerror(errno));
    100 		exit(1);
    101 	}
    102 	for (i = nread; i--;) {
    103 		ch = inbuf[i];
    104 		if (islower(ch))
    105 			++obs[ch - 'a'];
    106 		else if (isupper(ch))
    107 			++obs[ch - 'A'];
    108 	}
    109 
    110 	/*
    111 	 * now "dot" the freqs with the observed letter freqs
    112 	 * and keep track of best fit
    113 	 */
    114 	for (try = winner = 0; try < 26; ++try) { /* += 13) { */
    115 		dot = 0;
    116 		for (i = 0; i < 26; i++)
    117 			dot += obs[i] * stdf[(i + try) % 26];
    118 		/* initialize winning score */
    119 		if (try == 0)
    120 			winnerdot = dot;
    121 		if (dot > winnerdot) {
    122 			/* got a new winner! */
    123 			winner = try;
    124 			winnerdot = dot;
    125 		}
    126 	}
    127 
    128 	for (;;) {
    129 		for (i = 0; i < nread; ++i) {
    130 			ch = inbuf[i];
    131 			putchar(ROTATE(ch, winner));
    132 		}
    133 		if (nread < LINELENGTH)
    134 			break;
    135 		if ((nread = read(STDIN_FILENO, inbuf, LINELENGTH)) < 0) {
    136 			(void)fprintf(stderr, "caesar: %s\n", strerror(errno));
    137 			exit(1);
    138 		}
    139 	}
    140 	exit(0);
    141 }
    142 
    143 printit(arg)
    144 	char *arg;
    145 {
    146 	register int ch, rot;
    147 
    148 	if ((rot = atoi(arg)) < 0) {
    149 		(void)fprintf(stderr, "caesar: bad rotation value.\n");
    150 		exit(1);
    151 	}
    152 	while ((ch = getchar()) != EOF)
    153 		putchar(ROTATE(ch, rot));
    154 	exit(0);
    155 }
    156