(define-module (hello-lor)
  #:use-module (ice-9 regex)
  #:use-module (srfi srfi-1))

(define data
  "a → b, c, d, z
b → d, e, f, z
c → d, e, f, z
d → e, f, z
e → f, z
f → g, h, j, z
g → h, i, j, z
h → i, j, k, z
i → j, k, l, z
j → k, m, z
k → l, n, o, z
l → m, z
m → n, o, z
n → o, z
o → p, z
p → q, z
q → r, z
r → s, z
s → t, u, z
t → u, v, z
u → v, w, z
v → w, z
w → x, y, z
x → y, z
y → z")

;; graph[to] <- [from1, from2,fromN]
(define graph (make-hash-table 26))
;; counter[to] <- num of paths from a to to
(define counter (make-hash-table 26))
(hash-set! counter "a" 1)

(define (add-from-path from to)
  (let ((froms (hash-ref graph to #nil)))
    (hash-set! graph to (cons from froms))))

(define (register-path path)
  (let ((from (car path))
    (tos (cdr path)))
    (map
     (λ (to)
       (add-from-path from to))
     tos)))

(define (build-graph data)
  (let ((from->to-list (map
             (λ (from->tos)
               (map match:substring (list-matches "(\\w)" from->tos)))
             (string-split data #\Newline))))
    (map register-path from->to-list)))

(define (count-paths to)
  (or
   (hash-ref counter to)
   (hash-set! counter to
          (fold
           (λ (to acc)
         (+ acc (count-paths to)))
           0
           (hash-ref graph to)))))