如何在 Scala 中进行快速前缀字符串匹配

Question

我正在使用一些 Java 代码来进行快速前缀查找，使用 java.util.TreeSet，我可以使用 scala 的 TreeSet 来代替吗？或者不同的解决方案？

/** A class that uses a TreeSet to do fast prefix matching
 */
class PrefixMatcher {
  private val _set = new java.util.TreeSet[String]

  def add(s: String) = _set.add(s)

  def findMatches(prefix: String): List[String] = {
    val matches = new ListBuffer[String]
    val tailSet = _set.tailSet(prefix)
    for ( tail <- tailSet.toArray ) {
      val tailString = tail.asInstanceOf[String]
      if ( tailString.startsWith(prefix) ) 
        matches += tailString
      else
        return matches.toList    
    }

    matches.toList
  }
}

Answer 1

使用特里树。尽管事实上有些人已经发布了排序的 TreeMap 数据结构，但他们将其错误命名为 trie，但实际上还没有人在这里发布 Trie。 这里是一个相当有代表性的样本Java 中的 Trie 实现。我不知道 Scala 有什么。另请参阅 Wikipedia 上对 Tries 的解释。

Answer 2

来自 & takeWhile 方法：

class PrefixMatcher {
    private val _set = new TreeSet[String]
    def add(s: String) = _set.add(s)
    def findMatches(prefix: String): Iterable[String] =
        _set from prefix takeWhile(_ startsWith prefix)
}

另一种方法是从 prefix 到 prefix++（前缀后的最小字符串）选择一个子集。这仅选择实际以给定前缀开头的树的范围。不需要过滤条目。 subSet 方法将创建基础集的视图。

在增量方法中仍然有一些工作（溢出和空字符串不起作用），但意图应该很明确。

class PrefixMatcher {
  private val _set = new java.util.TreeSet[String]

  def add(s: String) = _set.add(s)

  def findMatches(prefix: String) : Set[String] = {
    def inc(x : String) = { //ignores overflow
       assert(x.length > 0) 
       val last = x.length - 1
       (x take last) + (x(last) + 1).asInstanceOf[Char]
    }
   _set.subSet(prefix, inc(prefix))
  }
}

同样适用于 scala jcl.TreeSet#range实施。

Answer 3

据我了解，Scala TreeSet 由 Java TreeSet 支持，但使用 Scala 变体将允许您使用序列理解来缩短代码（http://www.scala-lang.org/node/111) 给你一个看起来像这样的实现（对于 Scala 2.7）：

import scala.collection.jcl.TreeSet;

class PrefixMatcher 
{
    private val _set = new TreeSet[String]

    def add(s: String) = _set.add(s)

    def findMatches(prefix: String): Iterable[String] =
        for (s <- _set.from(prefix) if s.startsWith(prefix)) yield s
}

object Main
{
    def main(args: Array[String]): Unit =
    {
        val pm = new PrefixMatcher()

        pm.add("fooBar")
        pm.add("fooCow")
        pm.add("barFoo")

        pm.findMatches("foo").foreach(println)
    }
}

对于任何不好的 Scala 表示歉意就我而言，我自己正在习惯这种语言。

Answer 4

不久前，我在博客中介绍了如何查找前缀组合的匹配项。这是一个更难的问题，因为您不知道一个前缀何时结束而另一个前缀何时开始。您可能会感兴趣。我什至会在我没有写博客的代码下面发布（但希望是:），尽管它被剥夺了所有评论，其中没有一个是用英语写的：

package com.blogspot.dcsobral.matcher.DFA

object DFA {
  type Matched = List[(String, String)]
  def words(s : String) = s.split("\\W").filter(! _.isEmpty).toList
}

import DFA._
import scala.runtime.RichString

class DFA {
  private val initialState : State = new State(None, "")
  private var currState : State = initialState
  private var _input : RichString = ""
  private var _badInput : RichString = ""
  private var _accepted : Boolean = true

  def accepted : Boolean = _accepted
  def input : String = _input.reverse + _badInput.reverse 

  def transition(c : Char) : List[(String, Matched)] = {
    if (c == '\b') backtrack
    else {
      if (accepted) {
        val newState = currState(c)
        newState match {
          case Some(s) => _input = c + _input; currState = s
          case None => _badInput = c + _badInput; _accepted = false
        }
      } else {
        _badInput = c + _badInput
      }
      optionList
    }
  }

  def transition(s : String) : List[(String, Matched)] = {
    s foreach (c => transition(c))
    optionList
  }

  def apply(c : Char) : List[(String, Matched)] = transition(c)
  def apply(s : String) : List[(String,Matched)] = transition(s)

  def backtrack : List[(String, Matched)] = {
    if(_badInput isEmpty) {
      _input = _input drop 1
      currState.backtrack match {
        case Some(s) => currState = s
        case None =>
      }
    } else {
      _badInput = _badInput drop 1
      if (_badInput isEmpty) _accepted = true
    }
    optionList
  }

  def optionList : List[(String, Matched)] = if (accepted) currState.optionList else Nil

  def possibleTransitions : Set[Char] = if (accepted) (currState possibleTransitions) else Set.empty

  def reset : Unit = {
    currState = initialState
    _input = ""
    _badInput = ""
    _accepted = true
  }

  def addOption(s : String) : Unit = {
    initialState addOption s
    val saveInput = input
    reset
    transition(saveInput)
  }
  def removeOption(s : String) : Unit = {
    initialState removeOption s
    val saveInput = input
    reset
    transition(saveInput)
  }
}

class State (val backtrack : Option[State],
             val input : String) {
  private var _options : List[PossibleMatch] = Nil
  private val transitions : scala.collection.mutable.Map[Char, State] = scala.collection.mutable.Map.empty
  private var _possibleTransitions : Set[Char] = Set.empty

  private def computePossibleTransitions = {
    if (! options.isEmpty)
      _possibleTransitions = options map (_.possibleTransitions) reduceLeft (_++_)
    else
      _possibleTransitions = Set.empty
  }

  private def computeTransition(c : Char) : State = {
    val newState = new State(Some(this), input + c)
    options foreach (o => if (o.possibleTransitions contains c) (o computeTransition (newState, c)))
    newState
  }

  def options : List[PossibleMatch] = _options
  def optionList : List[(String, Matched)] = options map (pm => (pm.option, pm.bestMatch))
  def possibleTransitions : Set[Char] = _possibleTransitions

  def transition(c : Char) : Option[State] = {
    val t = c.toLowerCase
    if (possibleTransitions contains t) Some(transitions getOrElseUpdate (t, computeTransition(t))) else None
  }

  def apply(c : Char) : Option[State] = transition(c)

  def addOption(option : String) : Unit = {
    val w = words(option)
    addOption(option, w.size, List(("", w.head)), w)
  }

  def addOption(option : String, priority : Int, matched : Matched, remaining : List[String]) : Unit = {
    options find (_.option == option) match {
      case Some(pM) => 
        if (!pM.hasMatchOption(matched)) {
          pM.addMatchOption(priority, matched, remaining)
          if (priority < pM.priority) {
            val (before, _ :: after) = options span (_ != pM)
            val (highPriority, lowPriority) = before span (p => p.priority < priority || 
              (p.priority == priority && p.option < option))
            _options = highPriority ::: (pM :: lowPriority) ::: after
          }
          transitions foreach (t => pM computeTransition (t._2, t._1))
        }
      case None => 
        val (highPriority, lowPriority) = options span (p => p.priority < priority || 
              (p.priority == priority && p.option < option))
        val newPM = new PossibleMatch(option, priority, matched, remaining)
        _options = highPriority ::: (newPM :: lowPriority)
        transitions foreach (t => newPM computeTransition (t._2, t._1))
    }
    computePossibleTransitions
  }

  def removeOption(option : String) : Unit = {
    options find (_.option == option) match {
      case Some(possibleMatch) =>
        val (before, _ :: after) = options span (_ != possibleMatch)
        (possibleMatch.possibleTransitions ** Set(transitions.keys.toList : _*)).toList foreach (t => 
          transition(t).get.removeOption(option))
        _options = before ::: after
        computePossibleTransitions
      case None =>
    }
  }
}

class PossibleMatch (val option : String, 
                     thisPriority : Int, 
                     matched : Matched, 
                     remaining : List[String]) {
  private var _priority = thisPriority
  private var matchOptions = List(new MatchOption(priority, matched, remaining))
  private var _possibleTransitions = matchOptions map (_.possibleTransitions) reduceLeft (_++_)
  private def computePossibleTransitions = {
    _possibleTransitions = matchOptions map (_.possibleTransitions) reduceLeft (_++_)
  }

  def priority : Int = _priority
  def hasMatchOption(matched : Matched) : Boolean = matchOptions exists (_.matched == matched)
  def addMatchOption(priority : Int, matched : Matched, remaining : List[String]) : Unit = {
    if (priority < _priority) _priority = priority
    val (highPriority, lowPriority) = matchOptions span (_.priority < priority)
    val newMO = new MatchOption(priority, matched, remaining)
    matchOptions = highPriority ::: (newMO :: lowPriority)
    computePossibleTransitions
  }
  def bestMatch : Matched = matchOptions.head.matched.reverse.map(p => (p._1.reverse.toString, p._2)) ::: 
    remaining.tail.map(w => ("", w))
  def possibleTransitions : Set[Char] = _possibleTransitions

  def computeTransition(s: State, c : Char) : Unit = {
    def computeOptions(state : State,
                       c : Char, 
                       priority : Int, 
                       matched : Matched, 
                       remaining : List[String]) : Unit = {
      remaining match {
        case w :: ws => 
          if (!w.isEmpty && w(0).toLowerCase == c.toLowerCase) {
            val newMatched = (w(0) + matched.head._1, matched.head._2.substring(1)) :: matched.tail
            val newPriority = if (matched.head._1 isEmpty) (priority - 1) else priority

            if (w.drop(1) isEmpty)
              s.addOption(option, newPriority - 1, ("", ws.head) :: newMatched , ws)
            else
              s.addOption(option, newPriority, newMatched, w.substring(1) :: ws)
          }
          if (ws != Nil) computeOptions(s, c, priority, ("", ws.head) :: matched, ws)
        case Nil =>
      }
    }

    if(possibleTransitions contains c)
      matchOptions foreach (mO => computeOptions(s, c, mO.priority, mO.matched, mO.remaining))
  }
}

class MatchOption (val priority : Int,
                   val matched : Matched,
                   val remaining : List[String]) {
  lazy val possibleTransitions : Set[Char] = Set( remaining map (_(0) toLowerCase) : _* )
}

不过，它确实需要一些重构。当我开始为博客解释它时，我总是这样做。

Answer 5

好吧，我刚刚意识到你想要的几乎就是我的一个朋友针对另一个问题所建议的。所以，这是他的答案，根据您的需求进行了简化。

class PrefixMatcher {
  // import scala.collection.Set // Scala 2.7 needs this -- and returns a gimped Set
  private var set = new scala.collection.immutable.TreeSet[String]()
  private def succ(s : String) = s.take(s.length - 1) + ((s.charAt(s.length - 1) + 1)).toChar

  def add(s: String) = set += s

  def findMatches(prefix: String): Set[String] = 
    if (prefix.isEmpty) set else set.range(prefix, succ(prefix))
}