/* Simple backend for a Logo like tortoise drawer. */

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

static const int WIDTH = 10;
static const int HEIGHT = 10;
static double x, y;
static double direction;
static int pendown;


static FILE*
start_gnuplot() {
  FILE* output;
  int pipes[2];
  pid_t pid;

  pipe(pipes);
  pid = fork();

  if (!pid) {
    dup2 (pipes[0], STDIN_FILENO);
    execlp ("gnuplot", NULL);
    return 0; /* Not reached. */
  }

  output = fdopen(pipes[1], "w");

  fprintf (output, "set multiplot\n");
  fprintf (output, "set parametric\n");
  fprintf (output, "set xrange [-%d:%d]\n", WIDTH, WIDTH);
  fprintf (output, "set yrange [-%d:%d]\n", HEIGHT, HEIGHT);
  fprintf (output, "set size ratio -1\n");
  fprintf (output, "unset xtics\n");
  fprintf (output, "unset ytics\n");
  fflush (output);

  return output;
 }

static FILE* global_output;

static void tortoise_reset () {
  x = y = 0.0;
  direction = 0.0;
  pendown = 1;

  fprintf(global_output, "clear\n");
  fflush(global_output);
}
static void draw_line(FILE *output, double x1, double y1, double x2,
                      double y2) {
  fprintf(output, "plot [0:1] %f + %f * t, %f + %f * t notitle\n",
	  x1, x2 -x1, y1, y2 -y1);
  fflush(output);
}

static void tortoise_pendown() {
  pendown = 1;
}

static void tortoise_penup() {
  pendown = 0;
}

static void tortoise_turn (double degrees)
{
  direction += M_PI / 180.0 * degrees;
}

static void tortoise_move(double length) {
  double newX, newY;

  newX = x + length * cos (direction);
  newY = y + length * sin (direction);

  if (pendown)
    draw_line(global_output, x, y, newX, newY);

  x = newX;
  y = newY;
}


int main(int argc, char *argv[]) {
  global_output = start_gnuplot();
  tortoise_reset ();
  int i;
  tortoise_pendown();
  for (i = 1; i <= 4; ++i) {
    tortoise_move(3.0);
    tortoise_turn(90.0); 
  };
  return EXIT_SUCCESS;
}