一个字符串搜索的Aho-Corasick算法

Aho和Corasick对KMP算法(Knuth–Morris–Pratt algorithm)进行了改进，Aho-Corasick算法(Aho-Corasick algorithm)利用构建树，总时间复杂度是O(n)。原理图如下(摘自Aho-Corasick string matching in C#)：

 

Building of the keyword tree (figure 1 - after the first step, figure 2 - tree with the fail function)

C#版本的实现代码可以从Aho-Corasick string matching in C#得到，也可以点击这里获得该算法的PDF文档。

这是一个应用示例：

它能将载入的RTF文档中的搜索关键字高亮，检索速度较快，示例没有实现全字匹配，算法代码简要如下：

/* Aho-Corasick text search algorithm implementation
 *
 * For more information visit
 *      - http://www.cs.uku.fi/~kilpelai/BSA05/lectures/slides04.pdf
 */
using System;
using System.Collections;
namespace EeekSoft.Text
{
    /// <summary>
    /// Interface containing all methods to be implemented
    /// by string search algorithm
    /// </summary>
    public interface IStringSearchAlgorithm
    {
        #region Methods & Properties
        /// <summary>
        /// Ignore case of letters
        /// </summary>
        bool IgnoreCase { get; set; }
        /// <summary>
        /// List of keywords to search for
        /// </summary>
        string[] Keywords { get; set; }

        /// <summary>
        /// Searches passed text and returns all occurrences of any keyword
        /// </summary>
        /// <param name="text">Text to search</param>
        /// <returns>Array of occurrences</returns>
        StringSearchResult[] FindAll(string text);
        /// <summary>
        /// Searches passed text and returns first occurrence of any keyword
        /// </summary>
        /// <param name="text">Text to search</param>
        /// <returns>First occurrence of any keyword (or StringSearchResult.Empty if text doesn't contain any keyword)</returns>
        StringSearchResult FindFirst(string text);
        /// <summary>
        /// Searches passed text and returns true if text contains any keyword
        /// </summary>
        /// <param name="text">Text to search</param>
        /// <returns>True when text contains any keyword</returns>
        bool ContainsAny(string text);
        #endregion
    }
    /// <summary>
    /// Structure containing results of search
    /// (keyword and position in original text)
    /// </summary>
    public struct StringSearchResult
    {
        #region Members
        private int _index;
        private string _keyword;
        /// <summary>
        /// Initialize string search result
        /// </summary>
        /// <param name="index">Index in text</param>
        /// <param name="keyword">Found keyword</param>
        public StringSearchResult(int index, string keyword)
        {
            _index = index; _keyword = keyword;
        }

        /// <summary>
        /// Returns index of found keyword in original text
        /// </summary>
        public int Index
        {
            get { return _index; }
        }

        /// <summary>
        /// Returns keyword found by this result
        /// </summary>
        public string Keyword
        {
            get { return _keyword; }
        }

        /// <summary>
        /// Returns empty search result
        /// </summary>
        public static StringSearchResult Empty
        {
            get { return new StringSearchResult(-1, ""); }
        }
        #endregion
    }

    /// <summary>
    /// Class for searching string for one or multiple
    /// keywords using efficient Aho-Corasick search algorithm
    /// </summary>
    public class StringSearch : IStringSearchAlgorithm
    {
        #region Objects
        /// <summary>
        /// Tree node representing character and its
        /// transition and failure function
        /// </summary>
        class TreeNode
        {
            #region Constructor & Methods
            /// <summary>
            /// Initialize tree node with specified character
            /// </summary>
            /// <param name="parent">Parent node</param>
            /// <param name="c">Character</param>
            public TreeNode(TreeNode parent, char c)
            {
                _char = c; _parent = parent;
                _results = new ArrayList();
                _resultsAr = new string[] { };
                _transitionsAr = new TreeNode[] { };
                _transHash = new Hashtable();
            }

            /// <summary>
            /// Adds pattern ending in this node
            /// </summary>
            /// <param name="result">Pattern</param>
            public void AddResult(string result)
            {
                if (_results.Contains(result)) return;
                _results.Add(result);
                _resultsAr = (string[])_results.ToArray(typeof(string));
            }
            /// <summary>
            /// Adds trabsition node
            /// </summary>
            /// <param name="node">Node</param>
            //public void AddTransition(TreeNode node)
            //{
            //    AddTransition(node, false);
            //}
            /// <summary>
            /// Adds trabsition node
            /// </summary>
            /// <param name="node">Node</param>
            /// <param name="ignoreCase">Ignore case of letters</param>
            public void AddTransition(TreeNode node, bool ignoreCase)
            {
                if (ignoreCase) _transHash.Add(char.ToLower(node.Char), node);
                else _transHash.Add(node.Char, node);
                TreeNode[] ar = new TreeNode[_transHash.Values.Count];
                _transHash.Values.CopyTo(ar, 0);
                _transitionsAr = ar;
            }
            /// <summary>
            /// Returns transition to specified character (if exists)
            /// </summary>
            /// <param name="c">Character</param>
            /// <param name="ignoreCase">Ignore case of letters</param>
            /// <returns>Returns TreeNode or null</returns>
            public TreeNode GetTransition(char c, bool ignoreCase)
            {
                if (ignoreCase)
                    return (TreeNode)_transHash[char.ToLower(c)];
                return (TreeNode)_transHash[c];
            }

            /// <summary>
            /// Returns true if node contains transition to specified character
            /// </summary>
            /// <param name="c">Character</param>
            /// <param name="ignoreCase">Ignore case of letters</param>
            /// <returns>True if transition exists</returns>
            public bool ContainsTransition(char c, bool ignoreCase)
            {
                return GetTransition(c, ignoreCase) != null;
            }
            #endregion
            #region Properties
            private char _char;
            private TreeNode _parent;
            private TreeNode _failure;
            private ArrayList _results;
            private TreeNode[] _transitionsAr;
            private string[] _resultsAr;
            private Hashtable _transHash;
            /// <summary>
            /// Character
            /// </summary>
            public char Char
            {
                get { return _char; }
            }

            /// <summary>
            /// Parent tree node
            /// </summary>
            public TreeNode Parent
            {
                get { return _parent; }
            }

            /// <summary>
            /// Failure function - descendant node
            /// </summary>
            public TreeNode Failure
            {
                get { return _failure; }
                set { _failure = value; }
            }

            /// <summary>
            /// Transition function - list of descendant nodes
            /// </summary>
            public TreeNode[] Transitions
            {
                get { return _transitionsAr; }
            }

            /// <summary>
            /// Returns list of patterns ending by this letter
            /// </summary>
            public string[] Results
            {
                get { return _resultsAr; }
            }
            #endregion
        }
        #endregion
        #region Local fields
        /// <summary>
        /// Root of keyword tree
        /// </summary>
        private TreeNode _root;
        /// <summary>
        /// Keywords to search for
        /// </summary>
        private string[] _keywords;
        #endregion
        #region Initialization
        /// <summary>
        /// Initialize search algorithm (Build keyword tree)
        /// </summary>
        /// <param name="keywords">Keywords to search for</param>
        /// <param name="ignoreCase">Ignore case of letters (the default is false)</param>
        public StringSearch(string[] keywords, bool ignoreCase)
            : this(keywords)
        {
            IgnoreCase = ignoreCase;
        }
        /// <summary>
        /// Initialize search algorithm (Build keyword tree)
        /// </summary>
        /// <param name="keywords">Keywords to search for</param>
        public StringSearch(string[] keywords)
        {
            Keywords = keywords;
        }

        /// <summary>
        /// Initialize search algorithm with no keywords
        /// (Use Keywords property)
        /// </summary>
        public StringSearch()
        { }
        #endregion
        #region Implementation
        /// <summary>
        /// Build tree from specified keywords
        /// </summary>
        void BuildTree()
        {
            // Build keyword tree and transition function
            _root = new TreeNode(null, ' ');
            foreach (string p in _keywords)
            {
                // add pattern to tree
                TreeNode nd = _root;
                foreach (char c in p)
                {
                    TreeNode ndNew = null;
                    foreach (TreeNode trans in nd.Transitions)
                    {
                        if (this.IgnoreCase)
                        {
                            if (char.ToLower(trans.Char) == char.ToLower(c)) { ndNew = trans; break; }
                        }
                        else
                        {
                            if (trans.Char == c) { ndNew = trans; break; }
                        }
                    }
                    if (ndNew == null)
                    {
                        ndNew = new TreeNode(nd, c);
                        nd.AddTransition(ndNew, this.IgnoreCase);
                    }
                    nd = ndNew;
                }
                nd.AddResult(p);
            }
            // Find failure functions
            ArrayList nodes = new ArrayList();
            // level 1 nodes - fail to root node
            foreach (TreeNode nd in _root.Transitions)
            {
                nd.Failure = _root;
                foreach (TreeNode trans in nd.Transitions) nodes.Add(trans);
            }
            // other nodes - using BFS
            while (nodes.Count != 0)
            {
                ArrayList newNodes = new ArrayList();
                foreach (TreeNode nd in nodes)
                {
                    TreeNode r = nd.Parent.Failure;
                    char c = nd.Char;
                    while (r != null && !r.ContainsTransition(c, this.IgnoreCase)) r = r.Failure;
                    if (r == null)
                        nd.Failure = _root;
                    else
                    {
                        nd.Failure = r.GetTransition(c, this.IgnoreCase);
                        foreach (string result in nd.Failure.Results)
                            nd.AddResult(result);
                    }
                    // add child nodes to BFS list
                    foreach (TreeNode child in nd.Transitions)
                        newNodes.Add(child);
                }
                nodes = newNodes;
            }
            _root.Failure = _root;
        }

        #endregion
        #region Methods & Properties
        /// <summary>
        /// Ignore case of letters
        /// </summary>
        public bool IgnoreCase
        {
            get;
            set;
        }
        /// <summary>
        /// Keywords to search for (setting this property is slow, because
        /// it requieres rebuilding of keyword tree)
        /// </summary>
        public string[] Keywords
        {
            get { return _keywords; }
            set
            {
                _keywords = value;
                BuildTree();
            }
        }

        /// <summary>
        /// Searches passed text and returns all occurrences of any keyword
        /// </summary>
        /// <param name="text">Text to search</param>
        /// <returns>Array of occurrences</returns>
        public StringSearchResult[] FindAll(string text)
        {
            ArrayList ret = new ArrayList();
            TreeNode ptr = _root;
            int index = 0;
            while (index < text.Length)
            {
                TreeNode trans = null;
                while (trans == null)
                {
                    trans = ptr.GetTransition(text[index], this.IgnoreCase);
                    if (ptr == _root) break;
                    if (trans == null) ptr = ptr.Failure;
                }
                if (trans != null) ptr = trans;
                foreach (string found in ptr.Results)
                    ret.Add(new StringSearchResult(index - found.Length + 1, found));
                index++;
            }
            return (StringSearchResult[])ret.ToArray(typeof(StringSearchResult));
        }

        /// <summary>
        /// Searches passed text and returns first occurrence of any keyword
        /// </summary>
        /// <param name="text">Text to search</param>
        /// <returns>First occurrence of any keyword (or StringSearchResult.Empty if text doesn't contain any keyword)</returns>
        public StringSearchResult FindFirst(string text)
        {
            ArrayList ret = new ArrayList();
            TreeNode ptr = _root;
            int index = 0;
            while (index < text.Length)
            {
                TreeNode trans = null;
                while (trans == null)
                {
                    trans = ptr.GetTransition(text[index], this.IgnoreCase);
                    if (ptr == _root) break;
                    if (trans == null) ptr = ptr.Failure;
                }
                if (trans != null) ptr = trans;
                foreach (string found in ptr.Results)
                    return new StringSearchResult(index - found.Length + 1, found);
                index++;
            }
            return StringSearchResult.Empty;
        }

        /// <summary>
        /// Searches passed text and returns true if text contains any keyword
        /// </summary>
        /// <param name="text">Text to search</param>
        /// <returns>True when text contains any keyword</returns>
        public bool ContainsAny(string text)
        {
            TreeNode ptr = _root;
            int index = 0;
            while (index < text.Length)
            {
                TreeNode trans = null;
                while (trans == null)
                {
                    trans = ptr.GetTransition(text[index], this.IgnoreCase);
                    if (ptr == _root) break;
                    if (trans == null) ptr = ptr.Failure;
                }
                if (trans != null) ptr = trans;
                if (ptr.Results.Length > 0) return true;
                index++;
            }
            return false;
        }
        #endregion
    }
}

示例下载页面：http://www.uushare.com/user/m2nlight/file/2722093