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  1. #include <stdio.h>
  2.  
  3. unsigned char f(unsigned char x) {
  4.   return ((x << 1) ^ (x>127) ^ x ^ (x >> 1) ^ ((x & 1) << 7)) + x;
  5. }
  6.  
  7. void iterate(unsigned char s[5]) {
  8.   unsigned char new_s[5];
  9.   int i;
  10.   new_s[0] = f(s[3]) + s[4] + s[1] + f(s[2]);
  11.   new_s[1] = f(s[4]) + s[0] + s[2] + f(s[3]);
  12.   new_s[2] = f(s[0]) + s[1] + s[3] + f(s[4]);
  13.   new_s[3] = f(s[1]) + s[2] + s[4] + f(s[0]);
  14.   new_s[4] = f(s[2]) + s[3] + s[0] + f(s[1]);
  15.   for (i=0; i<5; i++) {
  16.     s[i] = new_s[i];
  17.   }
  18. }
  19.  
  20. int main() {
  21.   int i, j;
  22.   unsigned char s[5] = { 0, 1, 2, 3, 4 };
  23.   unsigned char t[5];
  24.  
  25.   /* Start by iterating a million times */
  26.   for (i=0; i<1000000; i++) iterate(s);
  27.  
  28.   /* Store the updated seed values */
  29.   for (i=0; i<5; i++) t[i]=s[i];
  30.  
  31.   /* Now see if they come up again */
  32.   for (j=1; j<1000000000; j++) {
  33.     iterate(s);
  34.     if (s[0]==t[0] && s[1]==t[1] && s[2]==t[2] && s[3]==t[3] && s[4]==t[4]) {
  35.       printf("Stuck in a loop of length %d\n",j);
  36.       for (i=0; i<5; i++) printf("%hhu ",s[i]);
  37.       putchar('\n');
  38.       return 0;
  39.     }
  40.   }
  41.  
  42.   /* We probably won't end up here */
  43.   puts("No repetitions");
  44.  
  45.   return 0;
  46. }
  47.  
Success #stdin #stdout 0.02s 2008KB
comments (?)
stdin 
Standard input is empty
stdout 
Stuck in a loop of length 560
134 132 134 60 60
https://ideone.com/3e4nPH
language:
C (gcc 8.3)
created:
10 years ago
visibility:
secret

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