Question

我试图通过移植玩具NFA regexp matcher来学习一些clojure。显然，我的主要问题是表示和操纵图形。我找到了一个有效的解决方案，但我的实现（使用gensym来模拟指针，基本上）让我觉得口中有一种令人讨厌的味道。

关注表示图表以及一般可读性和习语的改进？（最初的命令式解决方案比我现在的解决方案更具可读性。）

干杯！

; This is a port of Matt Might's toy regexp library described in
; http://matt.might.net/articles/implementation-of-nfas-and-regular-expressions-in-java/

; char_transitions is a map that associates a character with a set 
; of target state names
; empty_transitions is a set of target state names
(defrecord NfaState [final char_transitions empty_transitions])

; 'entry' and 'exit' are state names
; 'states' is a map that associates state names with states 
(defrecord Nfa [entry exit states])

(defn- add-empty-edge [nfa curr_state_name next_state_name]
  (let [curr_state (curr_state_name (:states nfa))]
    (Nfa. (:entry nfa) (:exit nfa) (assoc (:states nfa) curr_state_name (NfaState. (:final curr_state) (:char_transitions curr_state) (conj (:empty_transitions curr_state) next_state_name))))))

(defn- nfa-matches? 
  ([nfa nfa_state_name string] (nfa-matches? nfa nfa_state_name string #{}))
  ([nfa nfa_state_name string visited]
   (let [nfa_state (nfa_state_name (:states nfa))
         new_visited (conj visited nfa_state_name)]
     (cond
       (contains? visited nfa_state_name) false
       (empty? string) (or (:final nfa_state)
                           (some #(nfa-matches? nfa % string new_visited) 
                                 (:empty_transitions nfa_state)))
       (not (empty? string)) (or 
                               (some #(nfa-matches? nfa % (.substring string 1)) (get (:char_transitions nfa_state) (.charAt string 0) #{}))
                               (some #(nfa-matches? nfa % string new_visited) (:empty_transitions nfa_state)))
       :else false))))

(defn matches? [nfa string]
  "Matches a string against an NFA"
  (nfa-matches? nfa (:entry nfa) string))

(defn c [ch]
  "Creates an NFA which matches the character 'c'"
  (let [in (gensym)
        out (gensym)]
    (Nfa. in out {in (NfaState. false {ch #{out}} #{}) out (NfaState. true {} #{})})))

(defn e []
  "Creates an NFA which matches the empty string"
  (let [in (gensym)
        out (gensym)]
    (Nfa. in out {in (NfaState. false {} #{out}) out (NfaState. true {} #{})})))

(defn rep [nfa]
  "Creates an NFA which matches zero or more repetitions of the given NFA"
  (add-empty-edge (add-empty-edge nfa (:entry nfa) (:exit nfa)) (:exit nfa) (:entry nfa)))

(defn- update-final [nfastate new_final]
  (NfaState. new_final (:char_transitions nfastate) (:empty_transitions nfastate)))

(defn s [first second]
  "Creates an NFA which matches a sequence of two provided NFAs"
  (add-empty-edge (Nfa. (:entry first) (:exit second) (merge (update-in (:states first) [(:exit first)] update-final false) (update-in (:states second) [(:exit second)] update-final true))) (:exit first) (:entry second)))

(defn ou [first second]
  "Creates an NFA which matches either provided NFA"
  (let [in (gensym)
        out (gensym)]
    (add-empty-edge (add-empty-edge (Nfa. in out (assoc (merge (update-in (:states first) [(:exit first)] update-final false) (update-in (:states second) [(:exit second)] update-final false)) in (NfaState. false {} #{(:entry first) (:entry second)}) out (NfaState. true {} #{}))) (:exit first) out) (:exit second) out)))

; sugar
(defn st [string]
  "Creates an NFA which matches the provided string"
  (if (empty? string)
    (e)
    (s (c (.charAt string 0)) (st (.substring string 1)))))

Answer 1

地图和集合是一种优秀的数据结构，因为它们在开发过程中是可见的，并且可以使用您可能会发现对此类问题有用的zipper library轻松操作。

代表在clojure的图表

1 个答案: