/* RISC-V LFSR test harness
 *
 * I want to test my LFSR function written in assembly.
 *
 * So this is a test harness that includes a version written in C.
 *
 * It computes the 15-bit LFSR whose characteristic polynomial
 * is x¹⁵ + x¹⁴ + 1, whose bit sequence, as it turns out, can be
 * computed by a 1-dimensional cellular automaton which produces the
 * shape of a Sierpiński triangle.
 */
#include <stdio.h>

int lfsr(unsigned char (*b)[16]);

__attribute__((weak)) int lfsr(unsigned char (*b)[16])
{
  int i = (*b)[0];
  int c = (*b)[i+1] ^= (*b)[(i+1) % 15 + 1];
  i++;
  (*b)[0] = i % 15;
  return c;
}

int main()
{
  unsigned char buf[16] = {0, 255};
  for (size_t i = 0; i < 1024; i++) {
    for (size_t j = 0; j < 16; j++) {
      if (i % 15 == 0) printf("%3d ", buf[j]);
    }

    int c = lfsr(&buf);
    if (i % 15 == 0) printf("(%d)\n", c);
  }

  return 0;
}