Modifying Behaviors Using Decorators

My previous post on behavior trees made use of composite and action types, this posts serves as an example on how to improve behaviors using decorators. They take their name from the software design pattern. In the context of behavior trees a decorator node is a node with a single child, it modifies the behavior of the branch in some way, i.e. you don't want your NPC to keep kicking the door forever when it is blocked or replay an animation without completing the cycle.

The problem with my previous Robocode tree was that every time we execute it, it searched through the whole tree and as a consequence it kept switching targets, using decorators we can create loops in the tree that way we don't switch targets unless the robot we are going after dies.

behavior tree decorator

Kill All sequence demonstrates two decorators used for looping, since at the beginning of the match we don't have any enemies to fight we scan around until we find a robot to kill, until-success decorator will keep executing its child until it returns success meaning we found at least one robot to fight. Next in the sequence is Attack branch, using until-fail decorator it will pick a target to attack, keep attacking until the enemy is dead then move on to a new target until select-target fails which means all the robots in the arena are dead, having loops in the tree allows us to execute the tree once unlike the previous example which kept executing the tree forever in a while loop.

In order to combat robots that track our movement, two non deterministic composite nodes exists which shuffle their children prior to execution such as the one used in the Strafe branch which gives us some degree of non determinism.

(ns gez
  (:refer-clojure :exclude [sequence])
  (:use [alter-ego.core])
  (:import (java.awt Color)
           (robocode.util Utils))
  (:gen-class :extends robocode.AdvancedRobot :init init :state board))

(def safety 200)

(defn execute [robot]
  (.execute robot) true)

(defn back [robot]
  (action "Back"
          (.setBack robot 100)
          (execute robot)))

(defn forward [robot]
  (action "Forward"
          (.setAhead robot 100)
          (execute robot)))

(defn lock-gun [robot bb]
  (action (let [target ((:enemies @bb) (:target @bb))
                self-bearing (.getHeadingRadians robot)
                target-bearing (Math/toRadians (:bearing target))
                gun-heading (.getGunHeadingRadians robot)
                abs-bearing (+ self-bearing target-bearing)
                sin (Math/sin (- target-bearing abs-bearing))
                vel (/ (* (:velocity target) sin) 13)
                angle (Utils/normalRelativeAngle 
                       (+ (- abs-bearing gun-heading) vel))]
            (.setTurnGunRight robot (Math/toDegrees angle))
            (execute robot))))

(defn too-close-to-wall? [robot]
  (action "Too Close to Wall?"
          (let [margin 50 x (.getX robot) y (.getY robot)
                w (.getBattleFieldWidth robot) 
                h (.getBattleFieldHeight robot)]
            (or (<= x margin) (>= x (- w margin))
                (<= y margin) (>= y (- h margin))))))

(defn fire [robot]
  (action "Fire" (.fire robot 3) true))

(defn move [robot board]
  (selector "Move"
            (sequence "Approach Target"
                       (action "In Range?"
                               (let [distance (:distance ((:enemies @board)
                                                          (:target @board)))]
                                 (< distance safety))))
                      (action "Face Target"
                              (.setTurnRight robot (:bearing ((:enemies @board)
                                                              (:target @board))))
                              (execute robot))
                      (forward robot))

            (sequence "Attack Target"
                      (action "Face Sideways"
                              (let [bearing (:bearing ((:enemies @board)
                                                       (:target @board)))] 
                                (.setTurnRight robot (+ 90 bearing))
                                (execute robot)))

                      (selector "Strafe/Fire"
                                (sequence "Strafe"
                                           (too-close-to-wall? robot))
                                           (forward robot)
                                           (back robot))
                                          (fire robot))
                                (fire robot)))))

(defn scan [robot]
  (action "Scan"
          (.setTurnRadarLeft robot 360)
          (execute robot)))

(defn look-for-enemy? [robot board]
   (sequence "Look for Enemy?"
             (scan robot)
             (action "Enemy Found?"
                     (not (empty? (:enemies @board)))))))

(defn attack [robot board]
   (sequence "Attack"
             (scan robot)
             (action "Target Not Dead?"
                     (if (nil? (:target @board))
                       false (not (contains? (:dead @board)
                                             (:target @board)))))
             (lock-gun robot board)
             (move robot board))))

(defn next-target [board]
  (let [{:keys [enemies dead]} @board
        alive (filter #(not (contains? dead (key %))) enemies)] 
    (first (first (sort-by #(:distance (val %)) alive)))))

(defn tree [robot board]
  (sequence "Kill All"
            (look-for-enemy? robot board)
             (sequence "Until No Enemy Left"
                       (action "Select Target"
                               (dosync (alter board assoc 
                                              :target (next-target board))))
                       (attack robot board)))))

(defn -init []
  [[] (ref {:enemies {} :dead #{} :scanned [0 0]})])

(defn setup [robot]
  (doto robot
    (.setColors Color/RED Color/WHITE Color/RED)
    (.setAdjustGunForRobotTurn true)
    (.setAdjustRadarForGunTurn true)
    (.setAdjustRadarForRobotTurn true))
  (dosync (alter (.board robot) assoc :robot robot)))

(defn -run [robot]
  (setup robot)
  (exec (tree robot (.board robot))))

(defn -onPaint [robot g]
  (let [[x y] (:scanned @(.board robot))] 
    (.setColor g (java.awt.Color. 0xff 0x00 0x00 0x80))
    (.fillRect g (- x 20) (- y 20) 40 40)))

(defn -onRobotDeath [robot event]
  (dosync (alter (.board robot) assoc 
                 :dead (conj (:dead @(.board robot)) (.getName event)))))

(defn -onScannedRobot [robot event]
  (let [distance (.getDistance event)
        name (.getName event)
        bearing (.getBearing event)
        velocity (.getVelocity event)
        distance (.getDistance event)
        target {:distance distance :bearing bearing :velocity velocity}
        heading (.getHeading robot)
        angle (Math/toRadians (mod (+ heading bearing) 360))
        x (+ (.getX robot) (* (Math/sin angle) distance))
        y (+ (.getY robot) (* (Math/cos angle) distance))]
    (dosync (alter (.board robot) assoc-in [:enemies name] target)
            (alter (.board robot) assoc :scanned [x y]))))