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,但是我的实现(使用gensym来模拟指针,基本上)让我的嘴里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目前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)))))
地图和集合是一种极好的数据结构,因为它们在开发过程中是可见的,并且可以使用
zipper library轻松操作,您可能会发现这种类型的问题很有用.
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