/* Cafe wall illusion generator.

   ./cafe_wall <width> <height> > output.pgm

   https://en.wikipedia.org/wiki/Caf%C3%A9_wall_illusion
                                                         */
#include<math.h>
#include<stdio.h>
#include<stdlib.h>
#include<time.h>
#ifdef _WIN32
   #include<fcntl.h>
   #include<io.h>
#endif

#define PI               3.14159265358979323846264338327950288419716939937510
#define MIN_BRICK_COUNT  10.0
#define MAX_BRICK_COUNT  30.0

typedef struct { double x, y; } XY;
typedef struct { double a, b, c, d, e, f; } Transform;

/* Apply transformations to point. */
static double ApplyTransformX(XY *input, Transform *t)
{
   return input->x * t->a + input->y * t->b + t->c;
}
static double ApplyTransformY(XY *input, Transform *t)
{
   return input->x * t->d + input->y * t->e + t->f;
}
static XY ApplyTransform(XY *input, Transform *t)
{
   XY output;
   output.x = ApplyTransformX(input, t);
   output.y = ApplyTransformY(input, t);
   return output;
}


int main(int argc, char **argv)
{
   int width, height, x, y, b;
   double angle, scale;
   Transform wall;
   XY p1, p2;

   if( argc != 3 ||
       (width = atoi(argv[1])) <= 0 ||
       (height = atoi(argv[2])) <= 0 )
   {
      return printf("%s <width> <height>\n", *argv);
   }
   if( width >= 0x8000 || height >= 0x8000 )
      return !puts("Output size too large.");

   #ifdef _WIN32
      setmode(STDOUT_FILENO, O_BINARY);
   #endif

   srand(time(NULL));

   /* Generate transformation from screen coordinates to brick coordinates. */
   scale = (double)rand() / RAND_MAX * (MAX_BRICK_COUNT - MIN_BRICK_COUNT)
         + MIN_BRICK_COUNT;
   scale /= width > height ? width : height;
   angle = ((double)rand() / RAND_MAX) * 0.5 * PI;
   wall.a = cos(angle) * scale;
   wall.d = sin(angle) * scale;
   wall.c = (double)rand() / RAND_MAX * 4.0;
   wall.b = -wall.d;
   wall.e = wall.a;
   wall.f = (double)rand() / RAND_MAX * 2.0;

   /* Render pixels. */
   printf("P5\n%d %d\n255\n", width, height);
   for(y = 0; y < height; y++)
   {
      p1.y = y;
      for(x = 0; x < width; x++)
      {
         p1.x = x;
         p2 = ApplyTransform(&p1, &wall);

         /* Draw mortar lines for Y values near integer boundaries. */
         if( p2.y - floor(p2.y) < 0.1 )
         {
            fputc(128, stdout);
            continue;
         }

         /* Draw black or white bricks based on X value, applying
            a shift based on Y value.                             */
         b = (int)floor(p2.x + sin(floor(p2.y) * PI / 4));
         fputc(b % 2 == 0 ? 255 : 0, stdout);
      }
   }
   return 0;
}