Z Algorithm

src/main/java/com/thealgorithms/strings/Zalgorithm.java

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+package com.thealgorithms.strings;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Z-algorithm implementation to find all occurrences of a pattern in a text.
+ */
+public final class Zalgorithm {
+    private Zalgorithm() {
+    }
+
+    /**
+     * Finds occurrences of a pattern in a text using Z-algorithm.
+     *
+     * @param text    the input text in which we are searching the pattern
+     * @param pattern the pattern to search for
+     * @return a list of indices where the pattern occurs in the text
+     */
+    public static List<Integer> findPatternOccurrences(String text, String pattern) {
+        String combined = pattern + "$" + text;
+        int[] zArray = calculateZ(combined);
+        List<Integer> occurrences = new ArrayList<>();
+        int patternLength = pattern.length();
+
+        for (int i = patternLength + 1; i < zArray.length; i++) {
+            if (zArray[i] >= patternLength) {
+                occurrences.add(i - patternLength - 1);
+            }
+        }
+        return occurrences;
+    }
+
+    /**
+     * Helper method to calculate Z-array for a given string.
+     *
+     * @param s the input string
+     * @return the Z-array where Z[i] is the length of the longest substring
+     * starting from i that is also a prefix of s
+     */
+    private static int[] calculateZ(String s) {
+        int n = s.length();
+        int[] z = new int[n];
+        int l = 0, r = 0;
+
+        for (int i = 1; i < n; i++) {
+            if (i <= r) {
+                z[i] = Math.min(r - i + 1, z[i - l]);
+            }
+            while (i + z[i] < n && s.charAt(z[i]) == s.charAt(i + z[i])) {
+                z[i]++;
+            }
+            if (i + z[i] - 1 > r) {
+                l = i;
+                r = i + z[i] - 1;
+            }
+        }
+        return z;
+    }
+}

src/test/java/com/thealgorithms/strings/ZalgorithmTest.java

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+package com.thealgorithms.strings;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.Arrays;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+public class ZalgorithmTest {
+
+    /**
+     * Test 1:
+     * Pattern: "abc", Text: "ababcabcabc"
+     * Expected: [2, 5, 8]
+     */
+    @Test
+    public void testFindPatternOccurrences() {
+        String text = "ababcabcabc";
+        String pattern = "abc";
+        List<Integer> expected = Arrays.asList(2, 5, 8);
+        assertEquals(expected, Zalgorithm.findPatternOccurrences(text, pattern));
+    }
+
+    /**
+     * Test 2:
+     * Pattern: "aa", Text: "aaaaaa"
+     * Expected: [0, 1, 2, 3, 4]
+     */
+    @Test
+    public void testFindPatternOccurrencesRepeated() {
+        String text = "aaaaaa";
+        String pattern = "aa";
+        List<Integer> expected = Arrays.asList(0, 1, 2, 3, 4);
+        assertEquals(expected, Zalgorithm.findPatternOccurrences(text, pattern));
+    }
+}