Tortoise tutorial

/* Simple backend for a Logo like tortoise drawer.  */
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <math.h>
#include <libguile.h>

static const int WIDTH = 10;
static const int HEIGHT = 10;
static FILE* global_output;
static double x = 0.0;
static double y = 0.0;
static double direction = 0.0;
static int pendown = 1;

static FILE* start_gnuplot ();
static void draw_line (FILE* output, double x1, double y1, double x2, double y2);
static SCM tortoise_reset ();
static SCM tortoise_pendown ();
static SCM tortoise_penup ();
static SCM tortoise_turn (SCM degrees);
static SCM tortoise_move (SCM length);
static void* register_functions (void* data);


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; /* 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;
}



int main (int argc, char* argv[])
{
  global_output = start_gnuplot ();

  scm_with_guile (&register_functions, NULL);
  scm_shell (argc, argv);

  return EXIT_SUCCESS;
}




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

  fprintf (global_output, "clear\n");
  fflush (global_output);

  return SCM_UNSPECIFIED;
}

static SCM tortoise_pendown ()
{
  SCM result = scm_from_bool (pendown);
  pendown = 1;
  return result;
}

static SCM tortoise_penup ()
{
  SCM result = scm_from_bool (pendown);
  pendown = 0;
  return result;
}

static SCM tortoise_turn (SCM degrees)
{
  const double value = scm_to_double (degrees);
  direction += M_PI / 180.0 * value;
  return scm_from_double (direction * 180.0 / M_PI);
}

static SCM tortoise_move (SCM length)
{
  const double value = scm_to_double (length);
  double newX, newY;

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

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

  x = newX;
  y = newY;

  return scm_list_2 (scm_from_double (x), scm_from_double (y));
}


static void* register_functions (void* data)
{
  scm_c_define_gsubr ("tortoise-reset", 0, 0, 0, &tortoise_reset);
  scm_c_define_gsubr ("tortoise-penup", 0, 0, 0, &tortoise_penup);
  scm_c_define_gsubr ("tortoise-pendown", 0, 0, 0, &tortoise_pendown);
  scm_c_define_gsubr ("tortoise-turn", 1, 0, 0, &tortoise_turn);
  scm_c_define_gsubr ("tortoise-move", 1, 0, 0, &tortoise_move);
  return NULL;
}