package algorithm;

public class KmpSearch {
	public static void main(String[] args) {
		String s1 = "ABABCABAA";
		char[] pattern = s1.toCharArray();
		
		String s2 = "ABABABCABAACDEFSA";
		char[] text = s2.toCharArray();
		
		System.out.println("----------------------------------");
		int[] prefix = new int[pattern.length];
		int n = pattern.length;
		prefix_table(pattern, prefix, n);
		//輸出prefix檢視結果
		System.out.println("往右移動之前");
		for (int i = 0; i < prefix.length; i++) {
			System.out.println(prefix[i]);
		}
		
		move_prefix_table(prefix, n);
		System.out.println("往右移動之後");
		for (int i = 0; i < prefix.length; i++) {
			System.out.println(prefix[i]);
		}
		System.out.println("----------------------------------");
		
		kmp_search(text, pattern);
		
	}
	/**
	 * 往右移動一位的prefix陣列內容
	 * @param prefix   prefix陣列
	 * @param n  子串字元陣列長度
	 */
	private static void move_prefix_table(int[] prefix, int n){
		for (int i = n-1; i > 0; i--) {
			prefix[i] = prefix[i-1];
		}
		prefix[0] = -1;
	}
	/**
	 * 
	 * @param pattern  子串字元陣列
	 * @param prefix    prefix陣列
	 * @param n    子串字元陣列長度
	 */
	private static void prefix_table(char[] pattern, int[] prefix, int n){
		prefix[0] = 0;
		int len = 0;//最長公共長度
		int i = 1;
		while(i < n){
			if(pattern[i] == pattern[len]){
				len++;
				prefix[i] = len;
				i++;
			}else{
				if(len > 0){
					len = prefix[len -1];
				}else{
					prefix[i] = len;
					i++;
				}
			}
		}
	}
	
	/**
	 * 在text長串中查詢pattern子串
	 * @param text
	 * @param pattern
	 */
	private static void kmp_search(char[] text, char[] pattern){
		//子串的長度
		int n = pattern.length;
		int[] prefix = new int[n];
		//求出prefix
		prefix_table(pattern, prefix, n);
		move_prefix_table(prefix, n);
		//text[i]  len(text) = m;
		//pattern[j]   len(pattern) = n;
		int i = 0,j = 0;
		int m = text.length;
		while(i < m){
			if(j == n-1 && text[i] == pattern[j]){
				System.out.println("Found pattern at "+(i - j));
				j = prefix[j];
			}
			if(text[i] == pattern[j]){
				i++;
				j++;
			}else{
				j = prefix[j];
				if(j == -1){
					i++;
					j++;
				}
			}
		}
		
	}

}