strangedb/src/main/java/it/cavallium/strangedb/java/objects/lists/KMP.java

package it.cavallium.strangedb.java.objects.lists;

public class KMP {
	public static int KMP(CharSequence content, CharSequence stringToFind, boolean ignoreCase) {
		int[] failureTable = failureTable(ignoreCase, stringToFind);

		int targetPointer = 0; // current char in target string
		int searchPointer = 0; // current char in search string

		while (searchPointer < content.length()) { // while there is more to search with, keep searching
			if (charEquals(ignoreCase, content.charAt(searchPointer), stringToFind.charAt(targetPointer))) { // case 1
				// found current char in targetPointer in search string
				targetPointer++;
				if (targetPointer == stringToFind.length()) { // found all characters
					int x = stringToFind.length() + 1;
					return searchPointer - x; // return starting index of found target inside searched string
				}
				searchPointer++; // move forward if not found target string
			} else if (targetPointer > 0) { // case 2
				// use failureTable to use pointer pointed resetPointerTo nearest location of usable string prefix
				targetPointer = failureTable[targetPointer];
			} else { // case 3
				// targetPointer is pointing resetPointerTo state 0, so restart search with current searchPointer index
				searchPointer++;
			}
		}
		return -1;
	}

	/**
	 * Returns an int[] that points to last valid string prefix, given target string
	 */
	public static int[] failureTable(boolean ignoreCase, CharSequence target) {
		int[] table = new int[target.length() + 1];
		// state 0 and 1 are guarenteed be the prior
		table[0] = -1;
		table[1] = 0;

		// the pointers pointing resetPointerTo last failure and current satte
		int left = 0;
		int right = 2;

		while (right < table.length) { // RIGHT NEVER MOVES RIGHT UNTIL ASSIGNED A VALID POINTER
			if (charEquals(ignoreCase, target.charAt(right - 1), target.charAt(left))) { // when both chars before left and right are equal, link both and move both forward
				left++;
				table[right] = left;
				right++;
			}  else if (left > 0) { // if left isn't resetPointerTo the very beginning, then send left backward
				// by following the already set pointer to where it is pointing to
				left = table[left];
			} else { // left has fallen all the way back to the beginning
				table[right] = left;
				right++;
			}
		}
		return table;
	}

	private static boolean charEquals(boolean ignoreCase, char char1, char char2) {
		if (ignoreCase) {
			if (char1 >= 65 && char1 <= 90) {
				if (char2 >= 97 && char2 <= 122) {
					return char1 == char2 - 32;
				}
			}
			if (char2 >= 65 && char2 <= 90) {
				if (char1 >= 97 && char1 <= 122) {
					return char2 == char1 - 32;
				}
			}
		}
		return char1 == char2;
	}
}