// Simple turtle based graphics. The turtle starts out at 128, 128, looking North, pen down.

// The input string is read left to right (but see 'r' below). Commands are a single character,

// sometimes followed by additional arguments:

//

// u : Raises the pen

// d : Lowers the pen

// + : Reads integer N, rotates turtle N degrees clockwise

// - : Reads integer N, rotates turtle N degrees counterclockwise

// f : Reads integer N, moves turtle forwards N units

// r : Reads integer N, then separator character C. C should be some non-digit, non-command

//     character. Repeats all commands after C until the next instance of C, N times. Can be

//     nested.

//const char* input = "r2[r3(f50+90f50+90f50+90f50(+45uf50d[";

//const char* input = "r360|f1+1|";

const char* input = "uf100+91dr180|f3+2|+89uf60+90r2$f20-90dr60|f1+6|u-90f20$-90f50-90f30d+180f60uf1000";

​

struct Turtle { float x; float y; float h; bool p; } t;

​

const SkPaint& p() {

  static SkPaint paint;

  paint.setColor(SK_ColorBLACK);

  paint.setAntiAlias(true);

  paint.setStyle(SkPaint::kStroke_Style);

  paint.setStrokeWidth(0);

  return paint;

}

​

const char* eval(SkCanvas* canvas, const char* s, char e, float& dist, float& l, bool pt) {

  while (*s != e) {

    switch(*s++) {

      case 'u': t.p = false; break;

      case 'd': t.p = true; break;

      case '+': t.h += atoi(s); break;

      case '-': t.h -= atoi(s); break;

      case 'f': {

        float d = atoi(s);

        d = std::min(d, l);

        dist += d; l -= d;

        float r = t.h * 0.01745329f;

        Turtle nt = { t.x + sinf(r) * d, t.y - cosf(r) * d, t.h, t.p };

        if (pt && t.p) canvas->drawLine(t.x, t.y, nt.x, nt.y, p());

        t = nt;

        break;

      }

      case 'r': {

        int c = atoi(s);

        while (*s >= '0' && *s <= '9') { ++s; }

        auto n = s+1;

        for (int i = 0; i < c; ++i) { n = eval(canvas, s+1, *s, dist, l, pt); }

        s = n;

      }

    }

  }

  return s+1;

}

​

void draw(SkCanvas* canvas) {

  canvas->clear(SK_ColorWHITE);

​

  t = { 128, 128, 0, true };

  float totalDist = 0;

  float l = 1E9f;

  eval(canvas, input, 0, totalDist, l, false);

​

  l = frame * totalDist;

  t = { 128, 128, 0, true };

  eval(canvas, input, 0, totalDist, l, true);

}