hardcoded.c

// hardcoded.c
#include <stdio.h>
#include <stdlib.h>

#define sqr(x) ((x) * (x))

typedef struct { double r; double i; } complex;

void interpret(complex *registers, char const *code) {
  complex *a = &registers[0];
  complex *b = &registers[1];
  double r, i;
  r = b->r * b->r - b->i * b->i;
  i = b->r * b->i + b->i * b->r;
  b->r = r;
  b->i = i;
  b->r += a->r;
  b->i += a->i;
}

int main(int argc, char **argv) {
  complex registers[4];
  int i, x, y;
  char line[1600];
  printf("P5\n%d %d\n%d\n", 1600, 900, 255);
  for (y = 0; y < 900; ++y) {
    for (x = 0; x < 1600; ++x) {
      registers[0].r = 2 * 1.6 * (x / 1600.0 - 0.5);
      registers[0].i = 2 * 0.9 * (y /  900.0 - 0.5);
      for (i = 1; i < 4; ++i) registers[i].r = registers[i].i = 0;
      for (i = 0; i < 256 && sqr(registers[1].r) + sqr(registers[1].i) < 4; ++i)
        interpret(registers, argv[1]);
      line[x] = i;
    }
    fwrite(line, 1, sizeof(line), stdout);
  }
  return 0;
}