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public class KMP{

public static void main(String args []){
testRollBackArray();
testKMPSearchMatch();
testKMPSearchNoMatch();
}

/**
* each value of array rollback means: when source[i] mismatch pattern[i],
* KMP will restart match process form rollback[j] of pattern with
* source[i]. And if rollback[i] == -1, it means the current source[i] will
* never match pattern. then i should be added by 1 and j should be set to
* 0, which means restart match process from source[i+1] with pattern from
* pattern[0].
*
* @param pattern
* @return
*/
private static int[] getRollbackArray(char[] pattern) {
int[] rollback = new int[pattern.length];
for (int i = 0; i < pattern.length; i++) {
rollback[i] = 0;
}
rollback[0] = -1;
for (int i = 1; i < rollback.length; i++) {
char prevChar = pattern[i - 1];
int prevRollback = i - 1;
while (prevRollback >= 0) {
int previousRollBackIdx = rollback[prevRollback];
if ((previousRollBackIdx == -1)
|| (prevChar == pattern[previousRollBackIdx])) {
rollback[i] = previousRollBackIdx + 1;
break;
} else {
prevRollback = rollback[prevRollback];
}
}
}
return rollback;
}

/**
* search pattern chars in source chars.
*
* @param source
* @param pattern
* @return
*/
public static int searchKMP(char[] source, char[] pattern) {
// validation
if (source == null || source.length == 0 || pattern == null
|| pattern.length == 0) {
return -1;
}

// get the rollback array.
int[] rollback = getRollbackArray(pattern);

// incremental index of pattern. pointing the char to compare with.
int currMatch = 0;
int len = pattern.length;
// i point the char to compare with
for (int i = 0; i < source.length;) {
// if current char match
if ((currMatch == -1) || (source[i] == pattern[currMatch])) {
/*
* then each of the indexes adding by one, moving to the next
* char for comparation. notice that if currMatch is -1, it
* means the first char in pattern can not be matched. so i add
* by one to move on. and currMatch add by one so its value is
* 0.
*/
i++;
currMatch++;
/*
* if reaches the end of pattern, then match success, return the
* index of first matched char.
*/
if (currMatch == len) {
return i - len;
}
} else {
/*
* if current char mismatch, then rollback the next char to
* compare in pattern.
*/
currMatch = rollback[currMatch];
}
}
return -1;
}

public void testRollBackArray() {
int[] expectedRollback = new int[] { -1, 0, 0, 0, 0, 0, 0, 0, 1, 2, 0,
0, 0, 0, 0, 0, 1, 2, 3, 0, 0, 0, 0, 0 };
int[] rollback = getRollbackArray("PARTICIPATE IN PARACHUTE"
.toCharArray());
Assert.assertArrayEquals("Rollback array compare failed to match!",
expectedRollback, rollback);
}

public void testKMPSearchMatch() {
int matchIndex = searchKMP(
"aaaaaababacbaslierjalsdzmflkasjf".toCharArray(),
"ababacb".toCharArray());
Assert.assertEquals(5, matchIndex);

matchIndex = searchKMP(
"aaaaaababacbaslierjalsdzmflkasjf".toCharArray(),
"aaaaaababacbaslierjalsdzmflkasjf".toCharArray());
Assert.assertEquals(0, matchIndex);
}

public void testKMPSearchNoMatch() {
int matchIndex = searchKMP("ABCABCDABABCDABCDABDE".toCharArray(),
"hjABCDABD".toCharArray());
Assert.assertEquals(-1, matchIndex);

}

}