Finite State Machine Implementation in Clojure

A finite state machine is set of states, one being the start state, each state has a list of transitions, transitions in turn has conditions and actions whenever a condition for a transition is met FSM performs the action and enters the new state. FSM are widely used for game and robotic AI. Most games are just a bunch of FSMs running little chunks of code reacting to state changes. An NPC for example when instantiated can be in patrol state and as soon as the player approaches, it might cause it to transition into attack state which might cause it to run towards you.

(ns state-machine.core
  (:use [clojure.contrib.seq-utils :only [find-first flatten]]))

(defn state-machine [transition-table initial-state]
  (ref initial-state :meta transition-table))

(defn- switch-state? [conds]
  (if (empty? conds)
    (not (some false? (reduce #(conj %1 (if (fn? %2) (%2) %2)) [] conds)))))

(defn- first-valid-transition [ts]
  (find-first #(= (second %) true)
              (map #(let [{conds :conditions 
                           transition :transition
                           on-success :on-success} %]
                      [transition (switch-state? conds) on-success]) ts)))

(defn update-state [state]
  (let [transition-list ((meta state) @state)
        [transition _ on-success] (first-valid-transition transition-list)]
    (if-not (nil? transition)
        (if-not (nil? on-success)
        (dosync (ref-set state transition))))))

(defmacro until-state [s c & body] 
  `(while (not= (deref ~s) ~c) 
     (update-state ~s)))

A state machine is a ref holding the current state, transition table containing the list of states and transition rules are attached as meta data. Transition table is represented as a map containing states as keys and vector of maps containing condition, action and transition information. Every time we try to update state machines state, first we get the list of transition rules for the current state, then we start checking conditions for transition in the order they appear in the vector first transition that returns true for all its conditions is picked, if it has a on-success function it will be executed and reference will be set to the new state.

(def traffic-light
  {:green [{:conditions [] :transition :yellow}]
   :yellow  [{:conditions [] :transition :red}]
   :red [{:conditions [] :transition :green}]})

(let [sm (state-machine traffic-light :green)] 
  (dotimes [_ 4]
    (println @sm)
    (update-state sm)))
state-machine.core=> :green

Above example shows how traffic light state machine iterates through its states. A more complicated and famous example is a find-lisp state machine that would search for the word lisp in a character sequence,

(defn pop-char [char-seq]
  (dosync (ref-set char-seq (rest @char-seq))))

(defn find-lisp [char-seq]
  (let [start-trans {:conditions []
                     :on-success #(pop-char char-seq)
                     :transition :start}
        found-l-trans {:conditions [#(= (first @char-seq) \l)] 
                       :on-success #(pop-char char-seq)
                       :transition :found-l}]

    {:start [found-l-trans

     :found-l [found-l-trans
               {:conditions [#(= (first @char-seq) \i)] 
                :on-success #(pop-char char-seq)
                :transition :found-i}

     :found-i [found-l-trans
               {:conditions [#(= (first @char-seq) \s)] 
                :on-success #(pop-char char-seq)
                :transition :found-s}

     :found-s [found-l-trans
               {:conditions [#(= (first @char-seq) \p)] 
                :on-success #(do (println "Found Lisp")
                                 (pop-char char-seq))
                :transition :start}

When we run it, it will print Found Lisp every time we find the sequence of characters l,i,s,p in this particular order,

(let [char-seq (ref "ablislasllllispsslis")
      sm (state-machine (find-lisp char-seq) :start)] 
  (dotimes [_ (count @char-seq)]
    (update-state sm)))
state-machine.core=> Found Lisp

Even though it is not designed for this but Vijual works for quick and dirty visualization of state machines,

(defn prepare-nodes [state]
  (let [table (meta state)]
     2 (flatten 
        (map (fn [s]
               (let [[name transitions] s
                     transitions (flatten (map :transition transitions))]
                 (map #(vector name %) transitions))) table)))))
(use 'vijual)
(do (println )
    (draw-graph (prepare-nodes (state-machine traffic-light :start))))
+--------+   +-------+
|        |   |       |
| yellow |---|       |
|        |   | green |
+--------+   |       |
  |          |       |
  |          +-------+
  |            |
  |   +--------+
  |   |
| red |