astrid

astrid is a CLI for playing with L-systems.

Rewriting Strings

L-systems are string-rewriting systems that take a string and replace every character according to some rule. astrid takes a string and some rewrite rules as CLI arguments and will rewrite a string -n times.

$ astrid a a=ab b=a -n 7
     rules: a -> ab
            b -> a
iterations: 7
     input: a
    output: ab
            aba
            abaab
            abaababa
            abaababaabaab
            abaababaabaababaababa
            abaababaabaababaababaabaababaabaab

NOTE: It's a good idea to quote patterns and your rewrite rules so they don't get interpreted as shell special charachters.

Turtle Graphics

astrid can interpret L-systems as Turtle graphics programs and draw an SVG. Any L-system that contains turtle control characters can be drawn

Running astrid x 'x=F[+x]F[-x]+x' 'F=FF' -n 7 --svg -d 20 produces this drawing.

The turtle knows how to interpret the following commands:

Command	Description
F	Move forward, drawing a line.
f	Move forward, without drawing a line.
+	Turn left (counter-clockwise) by a fixed amount
-	Turn right (clockwise) by a fixed amount
[	Save the current turtle's position and direction on the stack. Doesn't move the turtle.
]	Return the turtle to the position and direction it had at the matching [. Doesn't draw a line.