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Apache Lucene (module: suggest)
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/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.lucene.search.suggest.tst;

import java.util.*;

/**
 * Ternary Search Trie implementation.
 * 
 * @see TernaryTreeNode
 */
public class TSTAutocomplete {
  
  TSTAutocomplete() {}

  /**
   * Inserting keys in TST in the order middle,small,big (lexicographic measure)
   * recursively creates a balanced tree which reduces insertion and search
   * times significantly.
   * 
   * @param tokens
   *          Sorted list of keys to be inserted in TST.
   * @param lo
   *          stores the lower index of current list.
   * @param hi
   *          stores the higher index of current list.
   * @param root
   *          a reference object to root of TST.
   */
  public void balancedTree(Object[] tokens, Object[] vals, int lo, int hi,
          TernaryTreeNode root) {
    if (lo > hi) return;
    int mid = (lo + hi) / 2;
    root = insert(root, (String) tokens[mid], vals[mid], 0);
    balancedTree(tokens, vals, lo, mid - 1, root);
    balancedTree(tokens, vals, mid + 1, hi, root);
  }

  /**
   * Inserts a key in TST creating a series of Binary Search Trees at each node.
   * The key is actually stored across the eqKid of each node in a successive
   * manner.
   * 
   * @param currentNode
   *          a reference node where the insertion will take currently.
   * @param s
   *          key to be inserted in TST.
   * @param x
   *          index of character in key to be inserted currently.
   * @return currentNode The new reference to root node of TST
   */
  public TernaryTreeNode insert(TernaryTreeNode currentNode, CharSequence s,
          Object val, int x) {
    if (s == null || s.length() <= x) {
      return currentNode;
    }
    if (currentNode == null) {
      TernaryTreeNode newNode = new TernaryTreeNode();
      newNode.splitchar = s.charAt(x);
      currentNode = newNode;
      if (x < s.length() - 1) {
        currentNode.eqKid = insert(currentNode.eqKid, s, val, x + 1);
      } else {
        currentNode.token = s.toString();
        currentNode.val = val;
        return currentNode;
      }
    } else if (currentNode.splitchar > s.charAt(x)) {
      currentNode.loKid = insert(currentNode.loKid, s, val, x);
    } else if (currentNode.splitchar == s.charAt(x)) {
      if (x < s.length() - 1) {
        currentNode.eqKid = insert(currentNode.eqKid, s, val, x + 1);
      } else {
        currentNode.token = s.toString();
        currentNode.val = val;
        return currentNode;
      }
    } else {
      currentNode.hiKid = insert(currentNode.hiKid, s, val, x);
    }
    return currentNode;
  }

  /**
   * Auto-completes a given prefix query using Depth-First Search with the end
   * of prefix as source node each time finding a new leaf to get a complete key
   * to be added in the suggest list.
   * 
   * @param root
   *          a reference to root node of TST.
   * @param s
   *          prefix query to be auto-completed.
   * @param x
   *          index of current character to be searched while traversing through
   *          the prefix in TST.
   * @return suggest list of auto-completed keys for the given prefix query.
   */
  public ArrayList prefixCompletion(TernaryTreeNode root,
          CharSequence s, int x) {

    TernaryTreeNode p = root;
    ArrayList suggest = new ArrayList<>();

    while (p != null) {
      if (s.charAt(x) < p.splitchar) {
        p = p.loKid;
      } else if (s.charAt(x) == p.splitchar) {
        if (x == s.length() - 1) {
          break;
        } else {
          x++;
        }
        p = p.eqKid;
      } else {
        p = p.hiKid;
      }
    }

    if (p == null) return suggest;
    if (p.eqKid == null && p.token == null) return suggest;
    if (p.eqKid == null && p.token != null) {
      suggest.add(p);
      return suggest;
    }

    if (p.token != null) {
      suggest.add(p);
    }
    p = p.eqKid;

    Stack st = new Stack<>();
    st.push(p);
    while (!st.empty()) {
      TernaryTreeNode top = st.peek();
      st.pop();
      if (top.token != null) {
        suggest.add(top);
      }
      if (top.eqKid != null) {
        st.push(top.eqKid);
      }
      if (top.loKid != null) {
        st.push(top.loKid);
      }
      if (top.hiKid != null) {
        st.push(top.hiKid);
      }
    }
    return suggest;
  }
}