adding HillCliming

Author: BusyByte

core/src/main/scala/aima/core/search/local/HillClimbing.scala

@@ -0,0 +1,54 @@
+package aima.core.search.local
+
+import aima.core.agent.Action
+import aima.core.search.{Problem, ProblemSearch, State, StateNode}
+
+import scala.annotation.tailrec
+
+/**
+  *
+  * <pre>
+  * function HILL-CLIMBING(problem) returns a state that is a local maximum
+  *
+  *   current &larr; MAKE-NODE(problem.INITIAL-STATE)
+  *   loop do
+  *     neighbor &larr; a highest-valued successor of current
+  *     if neighbor.VALUE &le; current.VALUE then return current.STATE
+  *     current &larr; neighbor
+  * </pre>
+  * <p>
+  *
+  * @author Shawn Garner
+  */
+object HillClimbing {
+
+  final case class StateValueNode(state: State, value: Double) extends State
+
+  def apply(stateToValue: State => Double)(problem: Problem): State = {
+
+    def makeNode(state: State) = StateValueNode(state, stateToValue(state))
+
+    def highestValuedSuccessor(current: StateValueNode): StateValueNode = {
+      val successors = problem.actions(current).map(a => problem.result(current, a)).map(makeNode)
+
+      if (successors.isEmpty) {
+        current
+      } else {
+        successors.maxBy(_.value)
+      }
+
+    }
+
+    @tailrec def recurse(current: StateValueNode): State = {
+      val neighbor = highestValuedSuccessor(current)
+      if (neighbor.value <= current.value) {
+        current.state
+      } else {
+        recurse(neighbor)
+      }
+    }
+
+    recurse(makeNode(problem.initialState))
+  }
+
+}