Java实现AC自动机全文检索示例

第一步，构建Trie树，定义Node类型：

/**
 * Created by zhaoyy on 2017/2/7.
 */
interface Node {

  char value();

  boolean exists();

  boolean isRoot();

  Node parent();

  Node childOf(char c);

  Node fail();

  void setFail(Node node);

  void setExists(boolean exists);

  void add(Node child);

  List<Node> children();
}

第二步，实现两种Node，如果词汇全是可打印的ASCII字符，就采用AsciiNode，否则（比如包含汉字），使用基于hash表的MapNode；这两种Node均集成自AbstractNode:

/**
 * Created by zhaoyy on 2017/2/8.
 */
abstract class AbstractNode implements Node {

  private static final char EMPTY = '\0';
  private final char value;
  private final Node parent;
  private boolean exists;
  private Node fail;

  public AbstractNode(Node parent, char value) {
    this.parent = parent;
    this.value = value;
    this.exists = false;
    this.fail = null;
  }

  public AbstractNode() {
    this(null, EMPTY);
  }

  private static String tab(int n) {
    StringBuilder builder = new StringBuilder();
    for (int i = 0; i < n; i++) {
      builder.append('\t');
    }
    return builder.toString();
  }

  private static String toString(Node node, int depth) {
    StringBuilder builder = new StringBuilder();
    String tab = tab(depth);
    Node fail = node.fail();
    Node parent = node.parent();
    builder
        .append(tab)
        .append('<')
        .append(node.value())
        .append(" exists=\"")
        .append(node.exists())
        .append('"')
        .append(" parent=\"")
        .append(parent == null ? "null" : parent.value())
        .append('"')
        .append(" fail=\"")
        .append(fail == null ? "null" : fail.value())
        .append("\">\r\n");
    for (Node child : node.children())
      builder.append(toString(child, depth + 1));
    builder
        .append(tab)
        .append("</")
        .append(node.value())
        .append(">\r\n");

    return builder.toString();
  }

  @Override
  public char value() {
    return value;
  }

  @Override
  public boolean exists() {
    return exists;
  }

  @Override
  public boolean isRoot() {
    return value == EMPTY;
  }

  @Override
  public Node parent() {
    return parent;
  }

  @Override
  public Node fail() {
    return fail;
  }

  @Override
  public void setFail(Node node) {
    this.fail = node;
  }

  @Override
  public void setExists(boolean exists) {
    this.exists = exists;
  }

  @Override
  public String toString() {
    return toString(this, 0);
  }
}

/////////////////////////////////////////////////////////////////////////////////////////////

/**
 * Created by zhaoyy on 2017/2/8.
 */
final class AsciiNode extends AbstractNode implements Node {

  private static final char FROM = 32;
  private static final char TO = 126;
  private final Node[] children;

  public AsciiNode() {
    super();
    this.children = new Node[TO - FROM + 1];
  }

  public AsciiNode(Node parent, char value) {
    super(parent, value);
    this.children = new Node[TO - FROM + 1];
  }

  @Override
  public Node childOf(char c) {
    if (c >= FROM && c <= TO)
      return children[(int) c - FROM];
    else return null;
  }

  @Override
  public void add(Node child) {
    int index = (int) child.value();
    children[index - FROM] = child;
  }

  @Override
  public List<Node> children() {
    List<Node> nodes = new ArrayList<Node>();
    for (Node child : children)
      if (child != null)
        nodes.add(child);
    return nodes;
  }
}

//////////////////////////////////////////////////////////////////////////////////////////////

/**
 * Created by zhaoyy on 2017/2/8.
 */
final class MapNode extends AbstractNode implements Node {

  private final Map<Character, Node> children;

  public MapNode() {
    super();
    this.children = new HashMap<Character, Node>();
  }

  public MapNode(Node parent, char value) {
    super(parent, value);
    this.children = new HashMap<Character, Node>();
  }

  @Override
  public Node childOf(char c) {
    return children.get(c);
  }

  @Override
  public void add(Node child) {
    children.put(child.value(), child);
  }

  @Override
  public List<Node> children() {
    List<Node> nodes = new ArrayList<Node>();
    nodes.addAll(children.values());
    return nodes;
  }
}

第三步，

首先定义一个Node构造器：

/**
 * Created by zhaoyy on 2017/2/8.
 */
public interface NodeMaker {

  Node make(Node parent, char value);

  Node makeRoot();
}

然后构建AC自动机，实现创建及查找方法：

/**
 * Created by zhaoyy on 2017/2/7.
 */
public final class WordTable {

  private final Node root;

  private WordTable(Collection<? extends CharSequence> words, NodeMaker maker) {
    Node root = buildTrie(words, maker);
    setFailNode(root);
    this.root = root;
  }

  public static WordTable compile(Collection<? extends CharSequence> words) {
    if (words == null || words.isEmpty())
      throw new IllegalArgumentException();
    final NodeMaker maker;
    if (isAscii(words))
      maker = new NodeMaker() {
        @Override
        public Node make(Node parent, char value) {
          return new AsciiNode(parent, value);
        }

        @Override
        public Node makeRoot() {
          return new AsciiNode();
        }
      };
    else maker = new NodeMaker() {
      @Override
      public Node make(Node parent, char value) {
        return new MapNode(parent, value);
      }

      @Override
      public Node makeRoot() {
        return new MapNode();
      }
    };
    return new WordTable(words, maker);
  }

  private static boolean isAscii(Collection<? extends CharSequence> words) {
    for (CharSequence word : words) {
      int len = word.length();
      for (int i = 0; i < len; i++) {
        int c = (int) word.charAt(i);
        if (c < 32 || c > 126)
          return false;
      }
    }
    return true;
  }

  private static Node buildTrie(Collection<? extends CharSequence> sequences, NodeMaker maker) {
    Node root = maker.makeRoot();
    for (CharSequence sequence : sequences) {
      int len = sequence.length();
      Node current = root;
      for (int i = 0; i < len; i++) {
        char c = sequence.charAt(i);
        Node node = current.childOf(c);
        if (node == null) {
          node = maker.make(current, c);
          current.add(node);
        }
        current = node;
        if (i == len - 1)
          node.setExists(true);
      }
    }
    return root;
  }

  private static void setFailNode(final Node root) {
    root.setFail(null);
    Queue<Node> queue = new LinkedList<Node>();
    queue.add(root);
    while (!queue.isEmpty()) {
      Node parent = queue.poll();
      Node temp;
      for (Node child : parent.children()) {
        if (parent.isRoot())
          child.setFail(root);
        else {
          temp = parent.fail();
          while (temp != null) {
            Node node = temp.childOf(child.value());
            if (node != null) {
              child.setFail(node);
              break;
            }
            temp = temp.fail();
          }
          if (temp == null)
            child.setFail(root);
        }
        queue.add(child);
      }
    }
  }

  public boolean findAnyIn(CharSequence cs) {
    int len = cs.length();
    Node node = root;
    for (int i = 0; i < len; i++) {
      Node next = node.childOf(cs.charAt(i));
      if (next == null) {
        next = node.fail();
        if (next == null) {
          node = root;
          continue;
        }
      }
      if (next.exists())
        return true;
    }

    return false;
  }

  public List<MatchInfo> search(CharSequence cs) {
    if (cs == null || cs.length() == 0)
      return Collections.emptyList();
    List<MatchInfo> result = new ArrayList<MatchInfo>();
    int len = cs.length();
    Node node = root;
    for (int i = 0; i < len; i++) {
      Node next = node.childOf(cs.charAt(i));
      if (next == null) {
        next = node.fail();
        if (next == null) {
          node = root;
          continue;
        }
      }
      if (next.exists()) {
        MatchInfo info = new MatchInfo(i, next);
        result.add(info);
        node = root;
        continue;
      }
      node = next;
    }
    return result;
  }

  @Override
  public String toString() {
    return root.toString();
  }
}

定义一个保存查找结果的实体：

/**
 * Created by zhaoyy on 2017/2/7.
 */
public final class MatchInfo {

  private final int index;
  private final String word;

  public MatchInfo(int index, String word) {
    this.index = index;
    this.word = word;
  }

  public MatchInfo(int index, Node node) {
    StringBuilder builder = new StringBuilder();
    while (node != null) {
      if (!node.isRoot())
        builder.append(node.value());
      node = node.parent();
    }
    String word = builder.reverse().toString();
    this.index = index + 1 - word.length();
    this.word = word;
  }

  public int getIndex() {
    return index;
  }

  public String getWord() {
    return word;
  }

  @Override
  public String toString() {
    return index + ":" + word;
  }
}

第四步，调用Demo：

public static void main(String[] args) {
    List<String> list = Arrays.asList("say", "her", "he", "she", "shr", "alone");
    WordTable table = WordTable.compile(list);
    System.out.println(table);
    System.out.println(table.search("1shesaynothingabouthislivinghimalone"));
  }

以下是输出结果：

< exists="false" parent="null" fail="null">
 <s exists="false" parent=" " fail=" ">
 <a exists="false" parent="s" fail="a">
  <y exists="true" parent="a" fail=" ">
  </y>
 </a>
 <h exists="false" parent="s" fail="h">
  <e exists="true" parent="h" fail="e">
  </e>
  <r exists="true" parent="h" fail=" ">
  </r>
 </h>
 </s>
 <h exists="false" parent=" " fail=" ">
 <e exists="true" parent="h" fail=" ">
  <r exists="true" parent="e" fail=" ">
  </r>
 </e>
 </h>
 <a exists="false" parent=" " fail=" ">
 <l exists="false" parent="a" fail=" ">
  <o exists="false" parent="l" fail=" ">
  <n exists="false" parent="o" fail=" ">
   <e exists="true" parent="n" fail=" ">
   </e>
  </n>
  </o>
 </l>
 </a>
</ >

[1:she, 4:say, 31:alone]

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