added turtle2d

docs/lib-turtle2d.md

@@ -0,0 +1,86 @@
+# turtle2d
+
+An OpenSCAD implementation of Turtle Graphics. It moves on the xy plane. You can get the cooridinate `[x, y]` or `angle` of its current position.
+
+## Parameters
+
+- `cmd` : A string command. Different commands use different numbers of arguments. 
+    - `"create"` : Creates a turtle data. The command needs three arguments `x`, `y` and `angle`. For example, use `turtle2d("create", 5, 10, 30)` to create a turtle located at `[0, 0]` with an angle `30` degrees.
+    - `"set_x"` : Sets the `x` coordinate of a turtle. The command needs two arguments. The first one is a turtle data, and the second one is the `x` coordinate. For example, `turtle2d("set_x", turtle, 20)`.
+    - `"set_y"` : Sets the `y` coordinate of a turtle. The command needs two arguments. The first one is a turtle data, and the second one is the `y` coordinate. For example, `turtle2d("set_y", turtle, 20)`.
+    - `"set_a"` : Sets the angle of a turtle. The command needs two arguments. The first one is a turtle data, and the second one is the angle. For example, `turtle2d("set_a", turtle, 45)`.
+    - `"set_pt"` : Sets `[x, y]` of a turtle. The command needs two arguments. The first one is a turtle data, and the second one is `[x, y]`. For example, `turtle2d("set_pt", turtle, [x, y])`.
+    - `"forward"` : Forwards a turtle. The command needs two arguments. The first one is a turtle data, and the second one is the length. For example, `turtle2d("forward", turtle, 100)`.
+    - `"forward"` : Turns a turtle. The command needs two arguments. The first one is a turtle data, and the second one is the angle. For example, `turtle2d("turn", turtle, 180)`.
+    - `"get_x"`, `"get_y"`, `"get_a"`, `"get_pt"` : All these commands needs only one argument, the turtle data.  
+
+## Examples
+    
+	include <line2d.scad>;
+    include <turtle2d.scad>;
+	
+	module turtle_spiral(t_before, times, side_leng, angle, width) {
+        $fn = 24;
+	    if(times != 0) {
+	        t_after_tr = turtle2d("turn", t_before, angle);
+	        t_after_fd = turtle2d("forward", t_after_tr, side_leng);
+	      
+	        line2d(
+	            turtle2d("get_pt", t_before),
+	            turtle2d("get_pt", t_after_fd),
+	            width,
+	            p1Style = "CAP_ROUND", 
+	            p2Style =  "CAP_ROUND"
+	        );
+	        
+	        turtle_spiral(t_after_fd, times - 1, side_leng, angle, width);
+	    }
+	
+	}
+	
+	side_leng = 10;
+	angle = 144;
+	width = 1;
+	times = 5;
+	
+	turtle_spiral(turtle2d("create", 0, 0, 0), times, side_leng, angle, width);
+
+![turtle2d](images/lib-turtle2d-1.JPG)
+	
+	include <line2d.scad>;
+	
+	module turtle_spiral(t_before, side_leng, d_step, min_leng, angle, width) {
+	    $fn = 24;
+	    if(side_leng > min_leng) {
+	        t_after = turtle2d("forward", turtle2d("turn", t_before, angle), side_leng);
+	
+	        line2d(
+	            turtle2d("get_pt", t_before),
+	            turtle2d("get_pt", t_after),
+	            width,
+	            p1Style = "CAP_ROUND", 
+	            p2Style =  "CAP_ROUND"
+	        );
+	        
+	        
+	        turtle_spiral(t_after, side_leng - d_step, d_step, min_leng, angle, width);
+	    }
+	
+	}
+	
+	side_leng = 50;
+	d_step = 1;
+	min_leng = 1;
+	angle = 90;
+	width = 1;
+	
+	turtle_spiral(
+	    turtle2d("create", 0, 0, 0), 
+	    side_leng, 
+	    d_step, 
+	    min_leng, 
+	    angle, 
+	    width
+	);
+
+![turtle2d](images/lib-turtle2d-2.JPG)