feat: Add KMP Substring Search Algorithm (#231)

README.md

@@ -2050,6 +2050,78 @@ public class StringToDateSnippet {
 }
 ```
 
+
+### KMP Substring Search Algorithm
+
+```java
+public class KMPSubstringSearchSnippet {
+
+    /**
+     * Implements the Knuth-Morris-Pratt (KMP) algorithm to find the index of the first occurrence of a substring in a given text.
+     *
+     * @param text The text in which the substring is to be searched.
+     * @param pattern The substring pattern to search for.
+     * @return The index of the first occurrence of the pattern in the text, or -1 if the pattern is not found.
+     */
+    public static int kmpSearch(String text, String pattern) {
+        if (pattern == null || pattern.length() == 0) {
+            return 0; // Trivial case: empty pattern
+        }
+
+        int[] lps = computeLPSArray(pattern);
+        int i = 0; // index for text
+        int j = 0; // index for pattern
+
+        while (i < text.length()) {
+            if (pattern.charAt(j) == text.charAt(i)) {
+                i++;
+                j++;
+            }
+
+            if (j == pattern.length()) {
+                return i - j; // Found pattern at index (i - j)
+            } else if (i < text.length() && pattern.charAt(j) != text.charAt(i)) {
+                if (j != 0) {
+                    j = lps[j - 1]; // Use the LPS array to skip characters
+                } else {
+                    i++; // If no match and j is 0, move to the next character in text
+                }
+            }
+        }
+        return -1; // Pattern not found
+    }
+
+    /**
+     * Computes the LPS (Longest Prefix Suffix) array for the pattern, which indicates the longest proper prefix which is also a suffix.
+     *
+     * @param pattern The pattern for which the LPS array is to be computed.
+     * @return The LPS array.
+     */
+    private static int[] computeLPSArray(String pattern) {
+        int length = 0;
+        int i = 1;
+        int[] lps = new int[pattern.length()];
+        lps[0] = 0; // LPS for the first character is always 0
+
+        while (i < pattern.length()) {
+            if (pattern.charAt(i) == pattern.charAt(length)) {
+                length++;
+                lps[i] = length;
+                i++;
+            } else {
+                if (length != 0) {
+                    length = lps[length - 1]; // Fall back to the previous LPS value
+                } else {
+                    lps[i] = 0;
+                    i++;
+                }
+            }
+        }
+        return lps;
+    }
+}
+```
+
 ## Thread
 
 ### Thread Pool

src/main/java/string/KmpSubstringSearchSnippet.java

@@ -0,0 +1,96 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2017-2024 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package string;
+
+/**
+ * KmpSubstringSearchSnippet.
+ */
+
+public class KmpSubstringSearchSnippet {
+
+  /**
+   * Implements the Knuth-Morris-Pratt (KMP) algorithm to find the of a substring.
+   *
+   * @param text The text in which the substring is to be searched.
+   * @param pattern The substring pattern to search for.
+   * @return The index of the first occurrence, or -1 if the pattern is not found.
+   */
+  public static int kmpSearch(String text, String pattern) {
+    if (pattern == null || pattern.length() == 0) {
+      return 0; // Trivial case: empty pattern
+    }
+
+    int[] lps = computeLpsArray(pattern);
+    int i = 0; // index for text
+    int j = 0; // index for pattern
+
+    while (i < text.length()) {
+      if (pattern.charAt(j) == text.charAt(i)) {
+        i++;
+        j++;
+      }
+
+      if (j == pattern.length()) {
+        return i - j; // Found pattern at index (i - j)
+      } else if (i < text.length() && pattern.charAt(j) != text.charAt(i)) {
+        if (j != 0) {
+          j = lps[j - 1]; // Use the LPS array to skip characters
+        } else {
+          i++; // If no match and j is 0, move to the next character in text
+        }
+      }
+    }
+    return -1; // Pattern not found
+  }
+
+  /**
+  * Computes the LPS (Longest Prefix Suffix) array, which indicates the longest proper prefix.
+  *
+  * @param pattern The pattern for which the LPS array is to be computed.
+  * @return The LPS array.
+  */
+  private static int[] computeLpsArray(String pattern) {
+    int length = 0;
+    int i = 1;
+    int[] lps = new int[pattern.length()];
+    lps[0] = 0; // LPS for the first character is always 0
+
+    while (i < pattern.length()) {
+      if (pattern.charAt(i) == pattern.charAt(length)) {
+        length++;
+        lps[i] = length;
+        i++;
+      } else {
+        if (length != 0) {
+          length = lps[length - 1]; // Fall back to the previous LPS value
+        } else {
+          lps[i] = 0;
+          i++;
+        }
+      }
+    }
+    return lps;
+  }
+} 

src/test/java/string/KmpSubstringSearchSnippetTest.java

@@ -0,0 +1,49 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2017-2024 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package string;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+/*
+* Tests for 30 Seconds of Java code library
+*
+*/
+class KmpSubstringSearchSnippetTest {
+
+  /**
+  * Tests for {@link KmpSubstringSearchSnippet#kmpSearch(String, String)}.
+  */
+  @Test
+  void testKmpSearch() {
+    // Test cases for KMP substring search
+    assertEquals(6, KmpSubstringSearchSnippet.kmpSearch("abxabcabcaby", "abcaby"));
+    assertEquals(7, KmpSubstringSearchSnippet.kmpSearch("subash pandey", "pandey"));
+    assertEquals(-1, KmpSubstringSearchSnippet.kmpSearch("abcd", "e"));
+    assertEquals(0, KmpSubstringSearchSnippet.kmpSearch("aaaaa", "a"));
+    assertEquals(2, KmpSubstringSearchSnippet.kmpSearch("abcdabcd", "cdab"));
+  }
+}