CodeCup 2025 Box tile generator

generator.c

#include <assert.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

static uint8_t random_data[240];
static size_t random_size = 0;

uint8_t GetRandomByte() {
  if (random_size == 0) {
    fprintf(stderr, "Out of entropy!\n");
    exit(1);
  }
  return random_data[--random_size];
}

/* Returns a random integer between 0 and n (exclusive). */
int RandInt(int n) {
  assert(n > 0 && n <= 256);
  int lim = 256 / n * n;
  uint8_t byte;
  do byte = GetRandomByte(); while (byte >= lim);
  return byte % n;
}

void ShuffleTile(char tile[6]) {
  for (size_t i = 5; i > 0; --i) {
    int j = RandInt(i + 1);
    char tmp = tile[i];
    tile[i] = tile[j];
    tile[j] = tmp;
  }
}

int main() {
  random_size = sizeof(random_data);
  if (getentropy(random_data, random_size) != 0) {
    perror("getentropy");
    exit(1);
  }

  int color1 = RandInt(6) + 1;
  int color2 = RandInt(6) + 1;
  printf("%d %d\n", color1, color2);
  char tile[7] = "123456";
  for (int n = 0; n < 40; ++n) {
    ShuffleTile(tile);
    printf("%s\n", tile);
  }
}