turtle.cpp

/**
 * This program reads an L-system from standard input and outputs a PGM image.
 * The following characters are possible:
 *   uppercase: Move forward with pen down
 *   lowercase: Move forward with pen up
 *   +: turn left
 *   -: turn right
 *   [: push current turtle state
 *   ]: pop turtle state
 * usage: turtle ANGLE STEP_SIZE > output.pgm
 */

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <stack>

using namespace std;

typedef unsigned char byte;

#define SIZE 800

byte world[SIZE][SIZE];

struct _turtle{
  double x, y;
  double vx, vy;
  int pen_down;
}turtle;

void put_pixel(int x, int y, int which){
  if (x >= 0 && x < SIZE && y >= 0 && y < SIZE){
    world[y][x] = which;
  }
}

void forward(int n){
  if (turtle.pen_down){
    int i;
    double x = turtle.x, y = turtle.y;

    for (i = 0; i < n; i++){
      put_pixel((int)x, (int)y, 255);
      x += turtle.vx;
      y += turtle.vy;
    }
  }

  turtle.x += turtle.vx * n;
  turtle.y += turtle.vy * n;
}

void normalize(){
  double len = sqrt(turtle.vx * turtle.vx + turtle.vy * turtle.vy);

  turtle.vx /= len;
  turtle.vy /= len;
}

void rotate(double n){
  double rad = n / 180.0 * M_PI;

  double vx = turtle.vx;
  turtle.vx = turtle.vx * cos(rad) - turtle.vy * sin(rad);
  turtle.vy = vx * sin(rad) + turtle.vy * cos(rad);
  normalize();
}

void home(){
  turtle.x = turtle.y = SIZE / 2.0;
  turtle.vx = 0.0;
  turtle.vy = -1.0;
  turtle.pen_down = 0;
}

void pen_down(){
  turtle.pen_down = 1;
}

void pen_up(){
  turtle.pen_down = 0;
}

int main(int argc, char * argv[]){
  char c;
  double angle;
  int step_size;
  
  angle = atof(argv[1]);
  step_size = atoi(argv[2]);

  memset(world, 0, sizeof(byte) * SIZE * SIZE);
  home();

  stack<_turtle> states;

  while ((c = getchar())){
    if (c == '+'){
      rotate(-angle);
    }else if (c == '-'){
      rotate(angle);
    }else if (c == '['){
      states.push(turtle);
    }else if (c == ']'){
      turtle = states.top();
      states.pop();
    }else if (c >= 'a' && c <= 'z'){
      pen_up();
      forward(step_size);
    }else if (c >= 'A' && c <= 'Z'){
      pen_down();
      forward(step_size);
      pen_up();
    }
  }

  printf("P2\n%d %d\n255\n", SIZE, SIZE);

  int i, j;
  for (i = 0; i < SIZE; i++){
    for (j = 0; j < SIZE; j++){
      printf("%d ", 255 - world[i][j]);
    }
    printf("\n");
  }
}