Java Program to Implement Commentz-Walter Algorithm
import java.io.*;
public class CommentzWalter {
public static int search(String text, String pattern)
{
int[] failure = createFailureFunction(pattern);
int i = 0;
int j = 0;
while (i < text.length()) {
if (text.charAt(i) == pattern.charAt(j)) {
i++;
j++;
}
if (j == pattern.length()) {
return i - j;
}
if (i < text.length()
&& text.charAt(i) != pattern.charAt(j)) {
if (j > 0) {
j = failure[j - 1];
}
else {
i++;
}
}
}
return -1;
}
public static int[] createFailureFunction(
String pattern)
{
int[] failure = new int[pattern.length()];
int i = 0;
int j = 1;
while (j < pattern.length()) {
if (pattern.charAt(i) == pattern.charAt(j)) {
failure[j] = i + 1;
i++;
j++;
}
else {
if (i > 0) {
i = failure[i - 1];
}
else {
failure[j] = 0;
j++;
}
}
}
return failure;
}
public static void main(String[] args)
{
String text = "Hello, World! How are you?";
String pattern = "World";
int index = search(text, pattern);
if (index >= 0) {
System.out.println("Pattern found at index "
+ index);
}
else {
System.out.println(
"Pattern not found in the text string");
}
}
}
import java.io.*;
public class CommentzWalter {
public static int search(String text, String pattern)
{
int[] failure = createFailureFunction(pattern);
int i = 0;
int j = 0;
while (i < text.length()) {
if (text.charAt(i) == pattern.charAt(j)) {
i++;
j++;
}
if (j == pattern.length()) {
return i - j;
}
if (i < text.length()
&& text.charAt(i) != pattern.charAt(j)) {
if (j > 0) {
j = failure[j - 1];
}
else {
i++;
}
}
}
return -1;
}
public static int[] createFailureFunction(
String pattern)
{
int[] failure = new int[pattern.length()];
int i = 0;
int j = 1;
while (j < pattern.length()) {
if (pattern.charAt(i) == pattern.charAt(j)) {
failure[j] = i + 1;
i++;
j++;
}
else {
if (i > 0) {
i = failure[i - 1];
}
else {
failure[j] = 0;
j++;
}
}
}
return failure;
}
public static void main(String[] args)
{
String text = "Hello, World! How are you?";
String pattern = "World";
int index = search(text, pattern);
if (index >= 0) {
System.out.println("Pattern found at index "
+ index);
}
else {
System.out.println(
"Pattern not found in the text string");
}
}
}
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