/* 2022-11-06
 *
 * a variation on http://sedcore.eu.org/small/koch.c
 * check it for basic instructions
 */

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

typedef struct {
  unsigned char r, g, b;
} color_t;

unsigned char screen[160*90*4];

void point(int x, int y, color_t color)
{
  y = 89 - y;
  screen[(y * 160 + x) * 4] = color.b;
  screen[(y * 160 + x) * 4 + 1] = color.g;
  screen[(y * 160 + x) * 4 + 2] = color.r;
}

int is_right(int x, int y, float x1, float y1, float x2, float y2)
{
  float v1x = x2 - x1;
  float v1y = y2 - y1;
  float v2x = x - x1;
  float v2y = y - y1;
  float det = v1x * v2y - v2x * v1y;
  if (det <= 0) return 1;
  return 0;
}

void triangle(float x3, float y3, float x4, float y4, float x5, float y5)
{
  int x, y;
  float xmin = x3-1;
  float ymin = y3-1;
  float xmax = x3+1;
  float ymax = y3+1;
  if (x4-1 < xmin) xmin = x4-1;
  if (x5-1 < xmin) xmin = x5-1;
  if (y4-1 < ymin) ymin = y4-1;
  if (y5-1 < ymin) ymin = y5-1;
  if (x4+1 > xmax) xmax = x4+1;
  if (x5+1 > xmax) xmax = x5+1;
  if (y4+1 > ymax) ymax = y4+1;
  if (y5+1 > ymax) ymax = y5+1;
  for (x = xmin; x <= xmax; x++) for (y = ymin; y <= ymax; y++)
    if (is_right(x, y, x3, y3, x4, y4) &&
        is_right(x, y, x4, y4, x5, y5) &&
        is_right(x, y, x5, y5, x3, y3))
      point(x, y, (color_t){192, 0, 0});
}

void koch(float x1, float y1, float x2, float y2, int frame)
{
  float x3, y3;
  float x4, y4;
  float x5, y5;

  if (fabs(x1-x2) < .5 && fabs(y1-y2) < .5)
    return;

  x3 = x1 + (x2 - x1) / 3;
  y3 = y1 + (y2 - y1) / 3;

  x5 = x2 - (x2 - x1) / 3;
  y5 = y2 - (y2 - y1) / 3;

  x4 = x5 - x3;
  y4 = y5 - y3;
  /* cos + isin *  x + iy =   cos x - sin y + i sin x + cos y */
  float c = 1. / 2;
  float s = sqrt(3) / 2;
  float x4n = c * x4 - s * y4;
  float y4n = s * x4 + c * y4;
  x4 = x4n + x3;
  y4 = y4n + y3;

  double angle = atan2(y2-y1, x2-x1);
  double length = sqrt((y2-y1)*(y2-y1) + (x2-x1)*(x2-x1)) * 2./3;

  float f = cos(2 * M_PI * frame / 100);
  float g = sin(2 * M_PI * frame*2 / 100);
  float x = f * length / 2;
  float y = g * length / 2. / 4.;
  c = cos(angle);
  s = sin(angle);
  float xn = c * x - s * y;
  float yn = s * x + c * y;
  x4 += xn;
  y4 += yn;

  triangle(x3, y3, x4, y4, x5, y5);
  koch(x1, y1, x3, y3, frame);
  koch(x3, y3, x4, y4, frame);
  koch(x4, y4, x5, y5, frame);
  koch(x5, y5, x2, y2, frame);
}

void clear(void)
{
  unsigned char *s = screen;
  int i;
  for (i = 0; i < 160*80; i++, s+=4) { s[0] = 203; s[1] = 192; s[2] = 255; s[3] = 255; }
  for (i = 160*80; i < 160*90; i++, s+=4) { s[0] = 0; s[1] = 0; s[2] = 192; s[3] = 255; }
}

void dump(void)
{
  fwrite(screen, 160*90*4, 1, stdout);
}

int main(void)
{
  int frame = 0;
  while (frame < 100) {
    clear();
    koch(0, 10, 160, 10, frame);
    dump();
    frame++;
  }
  fflush(stdout);
  return 0;
}