关键字过虑实现的思路及Aho–Corasick高效字符串匹配算法应用

在本人昨晚发的强大灵活的脏字过虑：1万字文章过虑1万关键词用时只要1毫秒（包括扩展的高亮功能）　文章中，只是介绍过虑的功能和性能，这个文章主要讲一下实现的思路，另外给大家看一下Aho–Corasick算法的C#实现。

既然是要过虑，那就要先查找，如果是直接的一个字符一个字符的匹配，那是很耗时的，因为时间花在不需要匹配的工作，有不少人会用正则去解决过虑，我09年的时候也这样，但后来发现大量关键词下性能确实极低下，所以才会另想它法。上一文中的过虑主要思想是这样的，开始会先用一个字典保存保存所有关键词，同一个字母开头的会另放在一个子字典里，这样一来，扫描的范围就大大的缩小了，然后再考虑到脏字一般是2个字的占了很大的比例，所以再在第二个字母做判断，如果不存在就不需要再扫描下去了，至于可跳字符，就是在直接需要扫描的时候一一判断的，没技巧可讲，另外一点值得注意的是，大小写敏感的情况下，在判断时需要转换大小写，大量关键词影响不小，所以就初始化时再保存了一分小写的，所以在扫描的时候就不需要转换了，所以是否大小写的两个情况性能上不会有什么变化，基本的思路就这样了。如果说，你想要很准确的过虑，那就要用到分词了（判断可以人性化），我的方法只能处理比较简单匹配与过虑。实现过程并没有使用Aho–Corasick算法。

在查找资料的时候还了解到Aho–Corasick算法，它可以帮助我们快速的找出多个子字符串。可以到这里了解算法：http://en.wikipedia.org/wiki/Aho%E2%80%93Corasick_string_matching_algorithm

５点有约会，先直接上实现代码了（从一老外的例子里小改的），本人测试过我上面的方法和使用Aho–Corasick过虑用的时候差不了多少：

/// <summary>
/// 表示一个查找结果
/// </summary>
public struct KeywordSearchResult
{
    private int index;
    private string keyword;
    public static readonly KeywordSearchResult Empty = new KeywordSearchResult(-1, string.Empty);
 
    public KeywordSearchResult(int index, string keyword)
    {
        this.index = index; 
        this.keyword = keyword;
    }
 
    /// <summary>
    /// 位置
    /// </summary>
    public int Index
    {
        get { return index; }
    }
 
    /// <summary>
    /// 关键词
    /// </summary>
    public string Keyword
    {
        get { return keyword; }
    }
}
 
 
/// <summary>
/// Aho-Corasick算法实现
/// </summary>
public class KeywordSearch
{
    /// <summary>
    /// 构造节点
    /// </summary>
    private class Node
    {
        private Dictionary<char, Node> transDict;
 
        public Node(char c, Node parent)
        {
            this.Char = c;
            this.Parent = parent;
            this.Transitions = new List<Node>();
            this.Results = new List<string>();
 
            this.transDict = new Dictionary<char, Node>();
        }
 
        public char Char
        {
            get;
            private set;
        }
 
        public Node Parent
        {
            get;
            private set;
        }
 
        public Node Failure
        {
            get;
            set;
        }
 
        public List<Node> Transitions
        {
            get;
            private set;
        }
 
        public List<string> Results
        {
            get;
            private set;
        }
 
        public void AddResult(string result)
        {
            if (!Results.Contains(result))
            {
                Results.Add(result);
            }
        }
 
        public void AddTransition(Node node)
        {
            this.transDict.Add(node.Char, node);
            this.Transitions = this.transDict.Values.ToList();
        }
 
        public Node GetTransition(char c)
        {
            Node node;
            if (this.transDict.TryGetValue(c, out node))
            {
                return node;
            }
 
            return null;
        }
 
        public bool ContainsTransition(char c)
        {
            return GetTransition(c) != null;
        }
    }
 
    private Node root; // 根节点
    private string[] keywords; // 所有关键词
 
    public KeywordSearch(IEnumerable<string> keywords)
    {
        this.keywords = keywords.ToArray();
        this.Initialize();
    }
 
    /// <summary>
    /// 根据关键词来初始化所有节点
    /// </summary>
    private void Initialize()
    {
        this.root = new Node(' ', null);
 
        // 添加模式
        foreach (string k in this.keywords)
        {
            Node n = this.root;
            foreach (char c in k)
            {
                Node temp = null;
                foreach (Node tnode in n.Transitions)
                {
                    if (tnode.Char == c)
                    {
                        temp = tnode; break; 
                    }
                }
 
                if (temp == null)
                {
                    temp = new Node(c, n);
                    n.AddTransition(temp);
                }
                n = temp;
            }
            n.AddResult(k);
        }
 
        // 第一层失败指向根节点
        List<Node> nodes = new List<Node>();
        foreach (Node node in this.root.Transitions)
        {
            // 失败指向root
            node.Failure = this.root;
            foreach (Node trans in node.Transitions)
            {
                nodes.Add(trans);
            }
        }
        // 其它节点 BFS
        while (nodes.Count != 0)
        {
            List<Node> newNodes = new List<Node>();
            foreach (Node nd in nodes)
            {
                Node r = nd.Parent.Failure;
                char c = nd.Char;
 
                while (r != null && !r.ContainsTransition(c))
                {
                    r = r.Failure;
                }
 
                if (r == null)
                {
                    // 失败指向root
                    nd.Failure = this.root;
                }
                else
                {
                    nd.Failure = r.GetTransition(c);
                    foreach (string result in nd.Failure.Results)
                    {
                        nd.AddResult(result);
                    }
                }
 
                foreach (Node child in nd.Transitions)
                {
                    newNodes.Add(child);
                }
            }
            nodes = newNodes;
        }
        // 根节点的失败指向自己
        this.root.Failure = this.root;
    }
 
    /// <summary>
    /// 找出所有出现过的关键词
    /// </summary>
    /// <param name="text"></param>
    /// <returns></returns>
    public List<KeywordSearchResult> FindAllKeywords(string text)
    {
        List<KeywordSearchResult> list = new List<KeywordSearchResult>();
 
        Node current = this.root;
        for (int index = 0; index < text.Length; ++index)
        {
            Node trans;
            do
            {
                trans = current.GetTransition(text[index]);
 
                if (current == this.root)
                    break;
 
                if (trans == null)
                {
                    current = current.Failure;
                }
            } while (trans == null);
 
            if (trans != null)
            {
                current = trans;
            }
 
            foreach (string s in current.Results)
            {
                list.Add(new KeywordSearchResult(index - s.Length + 1, s));
            }
        }
 
        return list;
    }
 
    /// <summary>
    /// 简单地过虑关键词
    /// </summary>
    /// <param name="text"></param>
    /// <returns></returns>
    public string FilterKeywords(string text)
    {
        StringBuilder sb = new StringBuilder();
 
        Node current = this.root;
        for (int index = 0; index < text.Length; index++)
        {
            Node trans;
            do
            {
                trans = current.GetTransition(text[index]);
 
                if (current == this.root)
                    break;
 
                if (trans == null)
                {
                    current = current.Failure;
                }
 
            } while (trans == null);
 
            if (trans != null)
            {
                current = trans;
            }
 
            // 处理字符
            if (current.Results.Count > 0)
            {
                string first = current.Results[0];
                sb.Remove(sb.Length - first.Length + 1, first.Length -1);// 把匹配到的替换为**
                sb.Append(new string('*', current.Results[0].Length));
 
            }
            else
            {
                sb.Append(text[index]);
            }
        }
 
        return sb.ToString();
    }
}