/*
  Author:  Pate Williams (c) 1997

  Exercise 1.1
  "Below are given four examples of cipehertext,
  one obtained from a Substitution Cipher, one
  from a Vigenere Cipher, one from an Affine
  Cipher, and one unspecified. In each case, the
  task is to determine the plaintext."
  -Douglas R. Stinson-
  See "Cryptography: Theory and Practice" by
  Douglas R. Stinson page 40.
*/

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

struct code {double rfreq; long alpha, count;};
struct digram {double rfreq; long alpha1, alpha2, count;};

int main(void)
{
  char cipher[11][24] = {"EMGLOSUDCGDNCUSWYSFHNSF",
                         "CYKDPUMLWGYICOXYSIPJCK",
                         "QPKUGKMGOLICGINCGACKSNI",
                         "SACYKZSCKXECJCKSHYSXCG",
                         "OIDPKZCNKSHICGIWYGKKGKG",
                         "OLDSILKGOIUSIGLEDSPWZU",
                         "GFZCCNDGYYSFUSZCNXEOJNC",
                         "GYEOWEUPXEZGACGNFGLKNS",
                         "ACIGOIYCKXCJUCIUZCFZCCN",
                         "DGYYSFEUEKUZCSOCFZCCNC",
                         "IACZEJNCSHFZEJZEGMXCYHC"};
  char ch, ciphertext[500], lt_ch, rt_ch;
  char c_a, c_e, c_o, c_r, c_s, c_w;
  char answer[256], plaintext[500];
  long frequency[26] = {0}, count = 0, i, j, k = 0, l;
  long di_freq[26][26] = {{0}}, d_count = 0, line;
  struct code t, c[26];
  struct digram *d, dt;

  for (i = 0; i < 11; i++) {
    for (j = 0; j < strlen(cipher[i]); j++) {
      ch = ciphertext[count++] = cipher[i][j];
      frequency[ch - 'A']++;
    }
  }
  for (i = 0; i < 26; i++) {
    c[i].alpha = i + 'A';
    c[i].count = frequency[i];
  }
  for (i = 0; i < 25; i++)
    for (j = i + 1; j < 26; j++)
      if (c[i].count < c[j].count)
        t = c[i], c[i] = c[j], c[j] = t;
  for (i = 0; i < 26; i++) {
    c[i].rfreq = c[i].count / (double) count;
    if (c[i].rfreq != 0.0)
      printf("%c %lf\n", c[i].alpha, c[i].rfreq);
  }
  lt_ch = ciphertext[0];
  for (i = 1; i < count - 1; i++) {
    rt_ch = ciphertext[i];
    di_freq[lt_ch - 'A'][rt_ch - 'A']++;
    lt_ch = rt_ch;
  }
  for (i = 0; i < 26; i++)
    for (j = 0; j < 26; j++)
      if (di_freq[i][j] != 0) d_count++;
  d = calloc(d_count, sizeof(struct digram));
  for (i = 0; i < 26; i++) {
    for (j = 0; j < 26; j++) {
      if (di_freq[i][j] != 0) {
        d[k].alpha1 = i + 'A';
        d[k].alpha2 = j + 'A';
        d[k++].count = di_freq[i][j];
      }
    }
  }
  for (i = 0; i < d_count - 1; i++)
    for (j = i + 1; j < d_count; j++)
      if (d[i].count < d[j].count)
        dt = d[i], d[i] = d[j], d[j] = dt;
  for (i = 0; i < d_count; i++)
    if (d[i].count != 0)
      printf("%3ld %c %c %ld\n", i, d[i].alpha1,
             d[i].alpha2, d[i].count);
  c_e = (char) c[0].alpha;
  c_a = (char) c[1].alpha;
  c_o = (char) c[2].alpha;
  c_r = (char) c[5].alpha;
  c_s = (char) c[3].alpha;
  c_w = 'F';
  for (i = 0; i < count; i++) {
    ch = ciphertext[i];
    if (ch == c_a) plaintext[i] = 'A';
    else if (ch == c_e) plaintext[i] = 'E';
    else if (ch == c_o) plaintext[i] = 'O';
    else if (ch == c_r) plaintext[i] = 'R';
    else if (ch == c_s) plaintext[i] = 'S';
    else if (ch == c_w) plaintext[i] = 'W';
    else plaintext[i] = '-';
  }
  line = count / 25;
  if (line % 25 != 0) line++;
  do {
    i = j = 0;
    for (k = 0; k < line; k++) {
      printf("P: ");
      for (l = 0; l < 25; l++)
        if (i < count)
          printf("%c", plaintext[i++]);
      printf("\nC: ");
      for (l = 0; l < 25; l++)
        if (j < count)
          printf("%c", ciphertext[j++]);
      printf("\n");
    }
    do {
      printf("Command: M(ap) Q(uit) U(nmap)? ");
      scanf("%s", answer);
      ch = (char) tolower(answer[0]);
    } while (ch != 'm' && ch != 'q' && ch != 'u');
    if (ch != 'q') {
      if (ch == 'm') {
        printf("Ciphertext character: ");
        scanf("%s", answer);
        c_a = (char) toupper(answer[0]);
        printf("Plaintext  character: ");
        scanf("%s", answer);
        c_e = (char) toupper(answer[0]);
        for (i = 0; i < count; i++)
          if (c_a == ciphertext[i])
            plaintext[i] = c_e;
      }
      if (ch == 'u') {
        printf("Plaintext character: ");
        scanf("%s", answer);
        c_a = (char) toupper(answer[0]);
        for (i = 0; i < count; i++)
          if (c_a == plaintext[i])
            plaintext[i] = '-';
      }
    }
  } while (ch != 'q');
  return 0;
}