EditDistance.java

package edu.fudan.nlp.similarity;

/**
 * 计算编辑距离
 * @author xpqiu
 * @version 1.0
 * @since 1.0
 */
public class EditDistance {
	
	
	public float calcNormalise(String word, String word2) {
		float distance = calc(word, word2);
		int len = word.length() > word2.length() ? word.length() : word2.length();
		return	(len - distance) / len;
	}
	
	/**
	 * 将x转换到y的编辑距离，可以自定义一些代价
	 * @param cSeq1
	 * @param cSeq2
	 * @return 距离
	 */
	public  float calc(String cSeq1,
			String cSeq2) {
		//+1 : 下标为0节点为动态规划的起点
		// cSeq1.length >= cSeq2.length > 1
		int xsLength = cSeq1.length() + 1; // > ysLength
		int ysLength = cSeq2.length() + 1; // > 2

		float[] lastSlice = new float[ysLength];
		float[] currentSlice = new float[ysLength];

		// first slice is just inserts
		currentSlice[0]=0;
		for (int y = 1; y < ysLength; ++y)
			currentSlice[y] = currentSlice[y-1] + costIns(cSeq2.charAt(y-1)); // y inserts down first column of lattice

		for (int x = 1; x < xsLength; ++x) {
			char cX = cSeq1.charAt(x-1);
			///////////exchange between lastSlice and currentSlice////////////
			float[] lastSliceTmp = lastSlice;
			lastSlice = currentSlice;
			currentSlice = lastSliceTmp;
			/////////////////////////////
			currentSlice[0] = lastSlice[0]+costDel(cSeq1.charAt(x-1)); // x deletes across first row of lattice
			for (int y = 1; y < ysLength; ++y) {
				int yMinus1 = y - 1;
				char cY = cSeq2.charAt(yMinus1);
				// unfold this one step further to put 1 + outside all mins on match
				currentSlice[y] = Math.min(cX == cY
						? lastSlice[yMinus1] // match
						            : costReplace(cX,cY) + lastSlice[yMinus1], // 替换代价
						             Math.min(costDel(cX)+lastSlice[y], // 删除代价
						            		costIns(cY)+currentSlice[yMinus1])); // 插入代价
			}
		}
		return currentSlice[currentSlice.length-1];
	}
	
	static String noCostChars = "的 最和";
	static String maxCostChars = "不";
	
	/**
	 * @param c
	 * @return 插入代价
	 */
	protected static float costIns(char c) {
		if(noCostChars.indexOf(c)!=-1)
			return 0;
		if(maxCostChars.indexOf(c)!=-1)
			return 5;
		return 1;
		
	}


	/**
	 * 删除
	 * @param c
	 * @return 删除代价
	 */
	protected static float costDel(char c) {
		if(noCostChars.indexOf(c)!=-1)
			return 0;
		if(maxCostChars.indexOf(c)!=-1)
			return 5;
		return 1;
	}

	static char[][] repCostChars = new char[][]{{'C','G'}};
	/**
	 * x和y肯定不同的
	 * @param x
	 * @param y
	 * @return 代价
	 */
	protected static float costReplace(char x, char y) {
		int cost = 1;
		for(char[] xy: repCostChars){
			if(xy[0]==x&&xy[1]==y){
				cost =2;
				break;
			}else if(xy[0]==y&&xy[1]==x){
				cost =2;
				break;
			}
		}
		return cost;//noCostChars.indexOf(c)!=-1?1:0;
	}
	
	public float sim(String str1, String str2) {   
		float ld = calc(str1, str2);   
		return 1 - ld / Math.max(str1.length(), str2.length());    
	}   

}