java Java实现的KMP算法

Posted 2021-05-07

public class KMP {

	private final String pattern;
	private final int[] next;

	public KMP(String _pattern) {
		if (_pattern == null) {
			throw new IllegalArgumentException("pattern cannot be null");
		}
		else if (_pattern.isEmpty()) {
			throw new IllegalArgumentException("pattern cannot be empty");
		}

		pattern = _pattern;
		next = computeNext(pattern);
	}

	public String getPattern() {
		return pattern;
	}

	public int firstOccurenceAt(String str) {
		return firstOccurenceAt(str, 0);
	}

	public int firstOccurenceAt(String str, int initPos) {
		if (str == null
		 || str.isEmpty()) {
			return -1;
		}

		int iStr = initPos;
		int iPattern = 0;
		while (iStr < str.length() && iPattern < pattern.length()) {
			if (iPattern == -1
			 || str.charAt(iStr) == pattern.charAt(iPattern)) {
				iStr++;
				iPattern++;
			}
			else {
				iPattern = next[iPattern];
			}
		}

		if (iPattern == pattern.length()) {
			return iStr - pattern.length();
		}
		else {
			return -1;
		}
	}

	private int[] computeNext(String pattern) {
		int patternLength = pattern.length();
		int[] next = new int[patternLength];

		next[0] = -1;
		for (int i = 1; i < patternLength; i++) {
			// next[0..i-1] has been set properly
			// set next[i] properly
			char prevChar = pattern.charAt(i - 1);
			int potentialMatchPosition = next[i - 1];

			while (potentialMatchPosition != -1
			    && prevChar != pattern.charAt(potentialMatchPosition)) {
					potentialMatchPosition = next[potentialMatchPosition];
			}

			next[i] = potentialMatchPosition + 1;
		}
		return next;
	}

	public static void main(String[] args) {
		KMP kmp = new KMP("abcdabd");
		System.out.println(kmp.firstOccurenceAt("bbc abcdab abcdabcdabde"));
	}

}