#lang scheme/base

(provide (all-defined-out))

; try all the rules on this character - 
; returns #f if none are matched
(define (run-rules char rules)
    (foldl
        (lambda (rule str)
            (if str ; if str is not #f
                str ; then we have already found a rule, return it
                (if (char=? char (string-ref (car rule) 0)) ; check this rule
                    (cadr rule) ; return the string
                    #f))) ; no match
        #f
        rules))

; runs the lsystem rules on every character in a string, 
; returns the new string
(define (search-replace str rules pos result)
    (cond 
        ((>= pos (string-length str)) result)
        (else
            (let ((ret (run-rules (string-ref str pos) rules)))
                (if (string? ret)
                    (search-replace str rules (+ pos 1) 
                        (string-append result ret))
                    (search-replace str rules (+ pos 1) 
                        (string-append result (string (string-ref str pos)))))))))

; runs the search-replace multiple (n) times on a string
(define (lsystem-generate n str rules)
    (cond 
        ((zero? n) str)
        (else
            (lsystem-generate (- n 1) 
                (search-replace str rules 0 "") rules))))