/*
 * Copyright (c) 1989, 1993
 *      The Regents of the University of California.  All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * Rick Adams.
 *
 * Authors:
 *      Stan King, John Eldridge, based on algorithm suggested by
 *      Bob Morris
 * 29-Sep-82
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#include <errno.h>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <unistd.h>

#define LINELENGTH      2048
#define ROTATE(ch, perm) \
     isascii(ch) ? ( \
        isupper(ch) ? ('A' + (ch - 'A' + perm) % 26) : \
            islower(ch) ? ('a' + (ch - 'a' + perm) % 26) : ch) : ch

/*
 * 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,
};

static void printit(char *);

int
main(int argc, char **argv)
{
        int ch, dot, i, nread, winnerdot = 0;
        char *inbuf;
        int obs[26], try, winner;

        if (argc > 1)
                printit(argv[1]);

        if (!(inbuf = malloc((size_t)LINELENGTH))) {
                (void)fprintf(stderr, "caesar: out of memory.\n");
                exit(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);

        /* zero out observation table */
        bzero(obs, 26 * sizeof(int));

        if ((nread = read(STDIN_FILENO, inbuf, (size_t)LINELENGTH)) < 0) {
                (void)fprintf(stderr, "caesar: %s\n", strerror(errno));
                exit(1);
        }
        for (i = nread; i--;) {
                ch = (unsigned char) inbuf[i];
                if (isascii(ch)) {
                        if (islower(ch))
                                ++obs[ch - 'a'];
                        else if (isupper(ch))
                                ++obs[ch - 'A'];
                }
        }

        /*
         * now "dot" the freqs with the observed letter freqs
         * and keep track of best fit
         */
        for (try = winner = 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;
                }
        }

        for (;;) {
                for (i = 0; i < nread; ++i) {
                        ch = (unsigned char) inbuf[i];
                        putchar(ROTATE(ch, winner));
                }
                if (nread < LINELENGTH)
                        break;
                if ((nread = read(STDIN_FILENO, inbuf, (size_t)LINELENGTH)) < 0) {
                        (void)fprintf(stderr, "caesar: %s\n", strerror(errno));
                        exit(1);
                }
        }
        exit(0);
}

static void
printit(char *arg)
{
        int ch, rot;

        if ((rot = atoi(arg)) < 0) {
                (void)fprintf(stderr, "caesar: bad rotation value.\n");
                exit(1);
        }
        while ((ch = getchar()) != EOF)
                putchar(ROTATE(ch, rot));
        exit(0);
}