逆向最大匹配演算法，中文分詞機械化分詞中最基本的演算法，也是入門級別的演算法。但是，在機械化分詞方面的效果，表現卻很好。尤其是在大文字的時候，一次取較多詞語進行匹配，因為大文字匹配成詞的概率遠遠高於小文字，所以會有很好的表現。下面的程式碼，來自IK分詞的一部分原始碼包，2016年本人進行了逆向最大匹配演算法的改造，閒著沒事幹，算是入門級別的分詞。

package org.wltea.analyzer.core;

import java.io.IOException;
import java.io.Reader;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.Map;
import java.util.Set;

import org.wltea.analyzer.cfg.Configuration;
import org.wltea.analyzer.dic.Dictionary;
/**
* 中分分詞上下文環境
* @author TongXueQiang
* @date 2016/01/22
* @since 1.7
*/
class AnalyzeContext {
private char[] segmentBuff;
private int[] charTypes;
private int buffOffset;
private int cursor;
private int available;
private Set<String> buffLocker;
private QuickSortSet orgLexemes;
private Map<Integer, LexemePath> pathMap;
private LinkedList<Lexeme> results;
private Configuration cfg;
private Integer moveIndex;

public AnalyzeContext(Configuration cfg) {
this.cfg = cfg;
this.segmentBuff = new char[4096];
this.charTypes = new int[4096];
this.buffLocker = new HashSet<String>();
this.orgLexemes = new QuickSortSet();
this.pathMap = new HashMap<Integer, LexemePath>();
this.results = new LinkedList<Lexeme>();
}

int getCursor() {
return this.cursor;
}

char[] getSegmentBuff() {
return this.segmentBuff;
}

char getCurrentChar() {
return this.segmentBuff[this.cursor];
}

int getCurrentCharType() {
return this.charTypes[this.cursor];
}

int getBufferOffset() {
return this.buffOffset;
}
/**
* 向緩衝區填充字元
* @param reader
* @return
* @throws IOException
*/
int fillBuffer(Reader reader) throws IOException {
int readCount = 0;
if (this.buffOffset == 0) {
readCount = reader.read(this.segmentBuff);
} else {
int offset = this.available - this.cursor;
if (offset > 0) {
System.arraycopy(this.segmentBuff, this.cursor,
this.segmentBuff, 0, offset);
readCount = offset;
}

readCount += reader.read(this.segmentBuff, offset, -offset);
}

this.available = readCount;
this.cursor = 0;
return readCount;
}

void initCursor() {
this.cursor = this.available-1;
//規範會字元
this.segmentBuff[this.cursor] = CharacterUtil
.regularize(this.segmentBuff[this.cursor]);
//為字元指定型別，比如阿拉伯數字型別，英文字母型別等等
this.charTypes[this.cursor] = CharacterUtil
.identifyCharType(this.segmentBuff[this.cursor]);
}

boolean moveCursor() {
if ((this.cursor-moveIndex) > 0) {
this.cursor -= (moveIndex+1);
//System.out.println("移動指標後的cursor位置:"+cursor);
//移動指標後還要進行規範化當前字元
this.segmentBuff[this.cursor] = CharacterUtil
.regularize(this.segmentBuff[this.cursor]);
//指定當前字元的型別
this.charTypes[this.cursor] = CharacterUtil
.identifyCharType(this.segmentBuff[this.cursor]);
return true;
}
return false;
}

void lockBuffer(String segmenterName) {
this.buffLocker.add(segmenterName);
}

void unlockBuffer(String segmenterName) {
this.buffLocker.remove(segmenterName);
}

boolean isBufferLocked() {
return (this.buffLocker.size() > 0);
}

boolean isBufferConsumed() {
return (this.cursor == this.available - 1);
}

boolean needRefillBuffer() {
return ((this.available == 4096) && (this.cursor < this.available - 1)
&& (this.cursor > this.available - 100) && (!(isBufferLocked())));
}

void markBufferOffset() {
this.buffOffset += this.cursor;
}

void addLexeme(Lexeme lexeme) {
this.orgLexemes.addLexeme(lexeme);
}

void addLexemePath(LexemePath path) {
if (path != null)
this.pathMap.put(Integer.valueOf(path.getPathBegin()), path);
}

QuickSortSet getOrgLexemes() {
return this.orgLexemes;
}
/**
* 輸出結果集
*/
void outputToResult() {
int index = 0;
while (index <= this.cursor) {
LexemePath path = (LexemePath) this.pathMap.get(Integer
.valueOf(index));

if (path != null) {
Lexeme l = path.pollFirst();

if (l != null) {
this.results.add(l);
index = l.getBegin() + l.getLength();
this.cursor = index;
}
} else {
outputSingleCJK(index);
++index;
}

}
this.pathMap.clear();
}

private void outputSingleCJK(int index) {
Lexeme singleCharLexeme;
if (4 == this.charTypes[index]) {
singleCharLexeme = new Lexeme(this.buffOffset, index, 1, 64);
this.results.add(singleCharLexeme);
} else if (8 == this.charTypes[index]) {
singleCharLexeme = new Lexeme(this.buffOffset, index, 1, 8);
this.results.add(singleCharLexeme);
}
}
/**
* 取出詞元，為詞元賦值
* @return
*/
Lexeme getNextLexeme() {
Lexeme result = (Lexeme) this.results.pollFirst();
while (result != null) {
compound(result);//數量詞合併
//過濾掉停用詞
if (Dictionary.getSingleton().isStopWord(this.segmentBuff,
result.getBegin(), result.getLength())) {
//System.out.println(Dictionary.getSingleton().isStopWord(this.segmentBuff,
//result.getBegin(), result.getLength()));
result = (Lexeme) this.results.pollFirst();
} else {
//為Lexeme賦值
result.setLexemeText(String.valueOf(this.segmentBuff,
result.getBegin(), result.getLength()));
break;
}
}
return result;
}

void reset() {
this.buffLocker.clear();
this.orgLexemes = new QuickSortSet();
this.available = 0;
this.buffOffset = 0;
this.charTypes = new int[4096];
this.cursor = 0;
this.results.clear();
this.segmentBuff = new char[4096];
this.pathMap.clear();
}
/**
* 數量詞合併
* @param result
*/
private void compound(Lexeme result) {
if (!(this.cfg.useSmart())) {
return;
}

if (this.results.isEmpty())
return;
Lexeme nextLexeme;
boolean appendOk;
if (2 == result.getLexemeType()) {
nextLexeme = (Lexeme) this.results.peekFirst();
appendOk = false;
if (16 == nextLexeme.getLexemeType()) {
appendOk = result.append(nextLexeme, 16);
} else if (32 == nextLexeme.getLexemeType()) {
appendOk = result.append(nextLexeme, 48);
}
if (appendOk) {
this.results.pollFirst();
}

}

if ((16 == result.getLexemeType()) && (!(this.results.isEmpty()))) {
nextLexeme = (Lexeme) this.results.peekFirst();
appendOk = false;
if (32 == nextLexeme.getLexemeType()) {
appendOk = result.append(nextLexeme, 48);
}
if (!(appendOk))
return;
this.results.pollFirst();
}
}

public void setMoveIndex(Integer moveIndex) {
this.moveIndex = moveIndex;

}

}

以下是CJK逆向最大匹配演算法：

package org.wltea.analyzer.core;

import org.wltea.analyzer.dic.Dictionary;
import org.wltea.analyzer.dic.Hit;

/**
* 中日韓分詞器，逆向最大匹配演算法
*
* @author TongXueQiang
* @date 2016/01/20
* @since 1.7
*/
class CJKSegmenter implements ISegmenter {
static final String SEGMENTER_NAME = "CJK_SEGMENTER";
static Integer MATCH_LEN = 7;
static Integer moveIndex = MATCH_LEN - 1;

CJKSegmenter() {

}

/*
* 逆向最大匹配演算法
*
* @see org.wltea.analyzer.core.ISegmenter#analyze(org.wltea.analyzer.core.
* AnalyzeContext)
*/
public void analyze(AnalyzeContext context) {
if (context.getCursor() < moveIndex) {
moveIndex = context.getCursor();
MATCH_LEN = context.getCursor() + 1;
}
Hit singleCharHit = Dictionary.getSingleton().matchInMainDict(
context.getSegmentBuff(), context.getCursor() - moveIndex,
MATCH_LEN);
if (singleCharHit.isMatch() || MATCH_LEN == 1) {
Lexeme newLexeme = new Lexeme(context.getBufferOffset(),
context.getCursor() - moveIndex, MATCH_LEN, 4);
context.addLexeme(newLexeme);
context.setMoveIndex(moveIndex);
init();
} else {
if (!singleCharHit.isUnmatch() || singleCharHit.isUnmatch()) {
--moveIndex;
--MATCH_LEN;
analyze(context);
}
}

}

private void init() {
moveIndex = 6;
MATCH_LEN = 7;
}

@Override
public void reset() {

}
}