using a stack :)

Author: sebastianhaeni

gradle/wrapper/gradle-wrapper.jar

@@ -1,6 +1,6 @@
-#Mon Nov 14 19:33:06 CET 2016
+#Wed Nov 30 22:32:28 CET 2016
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-3.1-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-3.1-bin.zip

gradle/wrapper/gradle-wrapper.properties

@@ -161,4 +161,9 @@ function splitJvmOpts() {
 eval splitJvmOpts $DEFAULT_JVM_OPTS $JAVA_OPTS $GRADLE_OPTS
 JVM_OPTS[${#JVM_OPTS[*]}]="-Dorg.gradle.appname=$APP_BASE_NAME"
 
+# by default we should be in the correct project dir, but when run from Finder on Mac, the cwd is wrong
+if [[ "$(uname)" == "Darwin" ]] && [[ "$HOME" == "$PWD" ]]; then
+ cd "$(dirname "$0")"
+fi
+
 exec "$JAVACMD" "${JVM_OPTS[@]}" -classpath "$CLASSPATH" org.gradle.wrapper.GradleWrapperMain "$@"

gradlew

@@ -49,7 +49,6 @@ goto fail
 @rem Get command-line arguments, handling Windows variants
 
 if not "%OS%" == "Windows_NT" goto win9xME_args
-if "%@eval[2+2]" == "4" goto 4NT_args
 
 :win9xME_args
 @rem Slurp the command line arguments.
@@ -60,11 +59,6 @@ set _SKIP=2
 if "x%~1" == "x" goto execute
 
 set CMD_LINE_ARGS=%*
-goto execute
-
-:4NT_args
-@rem Get arguments from the 4NT Shell from JP Software
-set CMD_LINE_ARGS=%$
 
 :execute
 @rem Setup the command line

gradlew.bat

@@ -1,103 +1,79 @@
 package ch.sebastianhaeni.pancake;
 
-import java.util.Arrays;
-import java.util.List;
-
 import ch.sebastianhaeni.pancake.dto.Node;
-import ch.sebastianhaeni.pancake.dto.SearchResult;
+
+import java.util.Arrays;
+import java.util.Stack;
 
 public class RecursiveSolver {
 
+ private static Stack<Node> stack = new Stack<>();
+ private static int candidateBound = Integer.MAX_VALUE;
+
  private RecursiveSolver() {
  }
 
  public static void main(String[] args) {
 
- // Node root = new Node(new int[]{46, 53, 18, 4, 56, 11, 57, 59, 55, 25, 6, 34, 41, 13, 39, 52, 7, 33, 43, 21, 32, 51, 5, 1, 58, 49, 54, 9, 24, 17, 19, 27, 50, 42, 22, 30, 20, 3, 14, 28, 40, 10, 12, 29, 37, 8, 36, 2, 26, 38, 16, 60, 15, 48, 35, 45, 31, 44, 23, 47});
+ Node root = new Node(new int[]{46, 53, 18, 4, 56, 11, 57, 59, 55, 25, 6, 34, 41, 13, 39, 52, 7, 33, 43, 21, 32, 51, 5, 1, 58, 49, 54, 9, 24, 17, 19, 27, 50, 42, 22, 30, 20, 3, 14, 28, 40, 10, 12, 29, 37, 8, 36, 2, 26, 38, 16, 60, 15, 48, 35, 45, 31, 44, 23, 47});
  // Node root = new Node(new int[]{2, 6, 10, 5, 1, 3, 8, 4, 7, 9});
  // Node root = new Node(new int[]{3, 1, 2});
  // Node root = new Node(new int[]{11, 7, 15, 6, 17, 3, 19, 16, 4, 10, 8, 12, 20, 14, 2, 5, 1, 9, 13, 18});
  // Node root = new Node(new int[]{5, 2, 7, 10, 13, 16, 14, 6, 8, 18, 15, 11, 1, 12, 3, 4, 9, 17});
- Node root = new Node(new int[] { 4, 25, 15, 7, 28, 17, 6, 5, 23, 29, 19, 27, 13, 21, 9, 1, 26, 22, 11, 18, 16, 12, 10, 30, 2, 3, 8, 14, 24, 20 });
+// Node root = new Node(new int[] { 4, 25, 15, 7, 28, 17, 6, 5, 23, 29, 19, 27, 13, 21, 9, 1, 26, 22, 11, 18, 16, 12, 10, 30, 2, 3, 8, 14, 24, 20 });
+// Node root = new Node(new int[]{2, 1, 4, 3, 6, 5, 8, 7, 10, 9, 12, 11});
 
  System.out.format("Solving a pancake pile of height %d.\n", root.getState().length);
 
  long start = System.currentTimeMillis();
- Node solution = solve(root);
+ solve(root);
  long end = System.currentTimeMillis();
 
- System.out.format("%dms passed\n", (end - start));
+ showSolution(start, end);
+ }
 
- if (solution == null) {
- System.out.println("No solution found");
- return;
- }
- System.out.format("Found solution after %d flips.\n", solution.getDepth());
+ private static void showSolution(long start, long end) {
+ System.out.format("%dms passed\n", (end - start));
+ System.out.format("Found solution after %d flips.\n", stack.size());
 
  StringBuilder sb = new StringBuilder();
- Node current = solution;
- while (current.getParent() != null) {
- sb.insert(0, Arrays.toString(current.getParent().getState()) + '\n');
- current = current.getParent();
+
+ while (!stack.isEmpty()) {
+ sb.insert(0, Arrays.toString(stack.pop().getState()) + '\n');
  }
 
  System.out.println(sb.toString());
  }
 
- private static Node solve(Node root) {
- if (root.isSolution()) {
- return root;
- }
-
- Node solutionNode = null;
- int bound = root.getOptimisticDistanceToSolution();
- int maxBound = bound * 10;
-
- while (solutionNode == null) {
- System.out.println("Searching with bound " + bound);
- SearchResult result = search(root, bound);
-
- if (result.getSolutionNode() != null) {
- solutionNode = result.getSolutionNode();
- }
-
- if (result.getBound() >= maxBound) {
- return null;
+ private static void solve(Node root) {
+ root = root.augment();
+ stack.push(root);
+ stack.peek().calcDistance();
+ stack.peek().nextNodes();
+ int bound = root.getDistance();
+
+ int stateBound;
+
+ while (stack.peek().getDistance() != 0) {
+ if (stack.peek().getDistance() + stack.peek().getDepth() > bound) {
+ stateBound = stack.peek().getDepth() + stack.peek().getDistance();
+ candidateBound = stateBound < candidateBound ? stateBound : candidateBound;
+ stack.pop();
+ } else if (stack.peek().getChildren().empty()) {
+ if (stack.peek().getDepth() == 0) {
+ bound = candidateBound;
+ System.out.println("Searching with bound " + candidateBound);
+ candidateBound = Integer.MAX_VALUE;
+ stack.peek().nextNodes();
+ } else {
+ stack.pop();
+ }
+ } else {
+ stack.push(stack.peek().getChildren().pop());
+ stack.peek().nextNodes();
  }
-
- bound = result.getBound();
  }
-
- return solutionNode;
  }
 
- private static SearchResult search(Node node, int bound) {
- int newBound = node.getDepth() + node.getOptimisticDistanceToSolution();
-
- if (newBound > bound) {
- return new SearchResult(newBound);
- }
-
- if (node.isSolution()) {
- return new SearchResult(node);
- }
-
- int min = Integer.MAX_VALUE;
- List<Node> successors = node.nextNodes();
-
- for (Node successor : successors) {
- SearchResult result = search(successor, bound);
-
- if (result.getSolutionNode() != null) {
- return result;
- }
-
- if (result.getBound() < min) {
- min = result.getBound();
- }
- }
-
- return new SearchResult(min);
- }
 
 }

src/main/java/ch/sebastianhaeni/pancake/RecursiveSolver.java

@@ -1,9 +1,7 @@
 package ch.sebastianhaeni.pancake.dto;
 
 import java.io.Serializable;
-import java.util.AbstractList;
-import java.util.ArrayList;
-import java.util.List;
+import java.util.Stack;
 
 /**
  * A node in a search tree.
@@ -25,80 +23,90 @@ public class Node implements Serializable {
  * The parent node. This is only used to figure out the solution path after the solution has been found.
  */
  private final Node parent;
+
+ /**
+ * Position this node's parent was flipped at.
+ */
  private final int flipPosition;
 
+ private Stack<Node> children = new Stack<>();
+
+ /**
+ * Pancake pile size.
+ */
+ private final int size;
+ private int distance;
+
  public Node(int[] state) {
- this(state, 0, null, -1);
+ this(state, 0, null, -1, 0);
  }
 
- private Node(int[] state, int depth, Node parent, int flipPosition) {
+ private Node(int[] state, int depth, Node parent, int flipPosition,int distance) {
  this.state = state;
  this.depth = depth;
  this.parent = parent;
  this.flipPosition = flipPosition;
+ this.size = state.length;
+ this.distance=distance;
  }
 
- public int getOptimisticDistanceToSolution() {
- int distance = 0;
- int current = 1;
+ public void calcDistance() {
+ distance = 0;
 
- for (int i = 1; i < state.length; i++) {
- int pancake = state[i];
- if (Math.abs(pancake - current) != 1) {
+ for (int i = 0; i < size - 1; i++) {
+ if (Math.abs(state[i] - state[i + 1]) > 1) {
  distance++;
  }
- current = pancake;
  }
-
- // 1.07 from https://en.wikipedia.org/wiki/Pancake_sorting
- return (int) (distance * 1.07);
  }
 
  public boolean isSolution() {
- int current = 1;
-
- for (int i = 1; i < state.length; i++) {
- int pancake = state[i];
- if (pancake - current != 1) {
+ for (int i = 0; i < size - 1; i++) {
+ if (state[i] != state[i + 1] - 1) {
  return false;
  }
- current = pancake;
  }
 
  return true;
  }
 
- public List<Node> nextNodes() {
- AbstractList<Node> list = new ArrayList<>();
-
- for (int i = 1; i < state.length; i++) {
- int flipPosition = i + 1;
- if (flipPosition != this.flipPosition) {
- Node flip = flip(flipPosition);
- list.add(flip);
+ public void nextNodes() {
+ for (int i = 2; i < size; i++) {
+ if (Math.abs(state[i - 1] - state[i]) > 1) {
+  if (Math.abs(state[i] - state[0]) > 1) {
+  children.push(flip(i, distance));
+  } else {
+  children.push(flip(i, distance - 1));
+ }
  }
  }
-
- return list;
  }
 
  /**
  * Flips the prefix at the defined flip position.
  *
  * @param flipPosition the position where the state shall be reversed
+ * @param distance predetermined optimistic distance
  * @return prefix reversed state
  */
- Node flip(int flipPosition) {
+ Node flip(int flipPosition, int distance) {
 
- int[] flipped = new int[state.length];
+ int[] flipped = new int[size];
 
  for (int i = 0; i < flipPosition; i++) {
  flipped[i] = state[flipPosition - i - 1];
  }
 
- System.arraycopy(state, flipPosition, flipped, flipPosition, state.length - flipPosition);
+ System.arraycopy(state, flipPosition, flipped, flipPosition, size - flipPosition);
+
+ return new Node(flipped, getDepth() + 1, this, flipPosition, distance);
+ }
 
- return new Node(flipped, getDepth() + 1, this, flipPosition);
+ public Node augment() {
+ int[] augmentedState = new int[size + 1];
+ System.arraycopy(state, 0, augmentedState, 0, size);
+ augmentedState[size] = size + 1;
+ return new Node(augmentedState);
  }
 
  public int getDepth() {
@@ -116,4 +124,12 @@ public Node getParent() {
  public int getFlipPosition() {
  return flipPosition;
  }
+
+ public int getDistance() {
+ return distance;
+ }
+
+ public Stack<Node> getChildren() {
+ return children;
+ }
 }

src/main/java/ch/sebastianhaeni/pancake/dto/Node.java

@@ -138,7 +138,7 @@ private Node solve(Node root) throws InterruptedException {
  }
 
  Node solutionNode = null;
- int bound = root.getOptimisticDistanceToSolution();
+ int bound = root.getDistance();
  int maxBound = bound * 2;
 
  while (solutionNode == null) {
@@ -165,7 +165,7 @@ private Node solve(Node root) throws InterruptedException {
  }
 
  private SearchResult search(Node node, int bound) throws InterruptedException {
- int newBound = node.getDepth() + node.getOptimisticDistanceToSolution();
+ int newBound = node.getDepth() + node.getDistance();
 
  if (newBound > bound) {
  return new SearchResult(newBound);
@@ -175,11 +175,11 @@ private SearchResult search(Node node, int bound) throws InterruptedException {
  return new SearchResult(node);
  }
 
- List<Node> successors = node.nextNodes();
-
- outstandingWork.addAll(successors.stream()
- .map(successor -> new WorkPacket(successor, bound))
- .collect(Collectors.toList()));
+// List<Node> successors = node.nextNodes();
+//
+// outstandingWork.addAll(successors.stream()
+// .map(successor -> new WorkPacket(successor, bound))
+// .collect(Collectors.toList()));
 
  if (!distributor.isStarted()) {
  distributor.start();

src/main/java/ch/sebastianhaeni/pancake/processor/Controller.java

@@ -1,6 +1,7 @@
 package ch.sebastianhaeni.pancake.processor;
 
 import java.text.DecimalFormat;
+import java.util.ArrayList;
 import java.util.List;
 
 import org.apache.logging.log4j.LogManager;
@@ -91,7 +92,7 @@ private void listenToSplitCommand() {
 
  private SearchResult compute(Node node, int bound) {
  computeCount++;
- int newBound = node.getDepth() + node.getOptimisticDistanceToSolution();
+ int newBound = node.getDepth() + node.getDistance();
 
  if (newBound > bound) {
  return new SearchResult(newBound);
@@ -101,7 +102,8 @@ private SearchResult compute(Node node, int bound) {
  return new SearchResult(node);
  }
 
- List<Node> successors = node.nextNodes();
+// List<Node> successors = node.nextNodes();
+ List<Node> successors = new ArrayList<>();
 
  if (splitCommand != null && splitCommand.Test() != null) {
  listenToSplitCommand();