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smile-nlp
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/*
 * Copyright (c) 2010-2021 Haifeng Li. All rights reserved.
 *
 * Smile is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * Smile is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with Smile.  If not, see .
 */

package smile.nlp;

import java.util.HashMap;
import java.util.LinkedList;

/**
 * A trie, also called digital tree or prefix tree, is an ordered tree data
 * structure that is used to store a dynamic set or associative array where
 * the keys are usually strings. Unlike a binary search tree, no node in the
 * tree stores the key associated with that node; instead, its position in
 * the tree defines the key with which it is associated. All the descendants
 * of a node have a common prefix of the string associated with that node,
 * and the root is associated with the empty string. Values are normally
 * not associated with every node, only with leaves and some inner nodes
 * that correspond to keys of interest.
 *
 * @param  the data type of key.
 * @param  the data type of value.
 *
 * @author Haifeng Li
 */
public class Trie {

    /**
     * The root node is specialized as a hash map for 
     * quick search. In case of Named Entities, the root
     * node will contains a lot of children. A plain list
     * will be slow for search.
     */
    private final HashMap root;
    /**
     * The number of entries.
     */
    private int size;

    /** The nodes in the trie. */
    public class Node {

        private final K key;
        private V value;
        private final LinkedList children;

        /**
         * Constructor.
         * @param key the key.
         */
        public Node(K key) {
            this.key = key;
            this.value = null;
            this.children = new LinkedList<>();
        }

        /**
         * Returns the key.
         * @return the key.
         */
        public K getKey() {
            return key;
        }

        /**
         * Returns the value.
         * @return the value.
         */
        public V getValue() {
            return value;
        }

        /**
         * Returns the value matching the key sequence.
         * @param key the key sequence.
         * @param index the index of current element in the key sequence.
         * @return the value.
         */
        public V getChild(K[] key, int index) {
            if (index >= key.length) {
                return value;
            }

            K k = key[index];
            for (Node child : children) {
                if (child.key.equals(k)) {
                    return child.getChild(key, index + 1);
                }
            }

            return null;
        }

        /**
         * Returns the child with the key.
         * @param key the key.
         * @return the child.
         */
        public Node getChild(K key) {
            for (Node child : children) {
                if (child.key.equals(key)) {
                    return child;
                }
            }

            return null;
        }

        /**
         * Adds a child.
         * @param key the key sequence.
         * @param value the value.
         * @param index the index of current element in the key sequence.
         */
        public void addChild(K[] key, V value, int index) {
            if (index >= key.length) {
                if (this.value == null) {
                    size++;
                }
                this.value = value;
                return;
            }

            for (Node child : children) {
                if (child.key.equals(key[index])) {
                    child.addChild(key, value, index + 1);
                    return;
                }
            }

            Node child = new Node(key[index]);
            children.addFirst(child);
            child.addChild(key, value, index + 1);
        }

    }

    /**
     * Constructor.
     */
    public Trie() {
        root = new HashMap<>();
    }

    /**
     * Constructor.
     * @param initialCapacity the initial capacity of root node.
     */
    public Trie(int initialCapacity) {
        root = new HashMap<>(initialCapacity);
    }

    /**
     * Add a key with associated value to the trie.
     * @param key the key.
     * @param value the value.
     */
    public void put(K[] key, V value) {
        Node child = root.get(key[0]);
        if (child == null) {
            child = new Node(key[0]);
            root.put(key[0], child);
        }
        child.addChild(key, value, 1);
    }

    /**
     * Returns the associated value of a given key.
     * Returns null if the key doesn't exist in the trie.
     * @param key the key.
     * @return the associated value or null.
     */
    public V get(K[] key) {
        Node child = root.get(key[0]);
        if (child != null) {
            return child.getChild(key, 1);
        }
        return null;
    }
    
    /**
     * Returns the node of a given key.
     * Returns null if the key doesn't exist in the trie.
     * @param key the key.
     * @return the node of the given key or null.
     */
    public Node get(K key) {
        return root.get(key);
    }
    
    /**
     * Returns the number of entries.
     * @return the number of entries.
     */
    public int size() {
        return size;
    }
}