/* Generate the Fibonacci word as a texture.
 *
 * The Fibonacci word is the morphic word generated by the limit of
 * the L-system
 *     b where b → a and a → ab
 * but here we are going to use a finite recursion limit.
 *
 * The first few iterations of this L-system play out as follows:
 *
 *    b
 *    a
 *    ab
 *    aba
 *    abaab
 *    abaababa
 *    abaababaabaab
 *    abaababaabaababaababa
 *    abaababaabaababaababaabaababaabaab
 *    abaababaabaababaababaabaababaabaababaababaabaababaababa
 *
 * With recursion limit N, the L-system generates fib(N) letters.  In
 * this case a couple million is more than enough.  fib(34) = 5702887
 * (by some definition) but just to be safe we use 100.
 *
 * Our generate_a and generate_b functions return 0 on failure, as
 * does our append function.  This permits the 100 levels of stack to
 * unwind in linear time rather than exponential time.
 *
 * The resulting texture may be technically nonrepeating but it sure
 * looks repetitive.
 */

#include "ppmp6.h"

enum { ww = 512, hh = 512 };
ppm_p6_color image[hh][ww];

typedef struct {
  ppm_p6_color *outp;
  int remaining_pixels;
  ppm_p6_color palette[256];
} output;

static inline int
append(output *out, char color)
{
  if (!out->remaining_pixels) return 0;
  *out->outp++ = out->palette[(int)(unsigned char)color];
  out->remaining_pixels--;
  return 1;
}

static inline int
generate_a(output *out, int maxdepth);

static inline int
generate_b(output *out, int maxdepth)
{
  if (maxdepth) {
    return generate_a(out, maxdepth-1);
  }

  return append(out, 'b');
}

static inline int
generate_a(output *out, int maxdepth)
{
  if (maxdepth) {
    return generate_a(out, maxdepth-1)
        && generate_b(out, maxdepth-1);
  }

  return append(out, 'a');
}

int main(int argc, char **argv)
{
  output out = {image[0], ww*hh};
  ppm_p6_color white = { r: 1, g: 1, b: 1 };
  out.palette[(int)'a'] = white;
  generate_b(&out, 100);

  ppm_p6_output_image(image[0], ww, hh);
  return 0;
}