suffix array implementation update

Author: NithinU2802

DIRECTORY.md

@@ -744,6 +744,7 @@
           - 📄 [Rotation](src/main/java/com/thealgorithms/strings/Rotation.java)
           - 📄 [StringCompression](src/main/java/com/thealgorithms/strings/StringCompression.java)
           - 📄 [StringMatchFiniteAutomata](src/main/java/com/thealgorithms/strings/StringMatchFiniteAutomata.java)
+          - 📄 [SuffixArray](src/main/java/com/thealgorithms/strings/SuffixArray.java)
           - 📄 [Upper](src/main/java/com/thealgorithms/strings/Upper.java)
           - 📄 [ValidParentheses](src/main/java/com/thealgorithms/strings/ValidParentheses.java)
           - 📄 [WordLadder](src/main/java/com/thealgorithms/strings/WordLadder.java)
@@ -1413,6 +1414,7 @@
           - 📄 [RotationTest](src/test/java/com/thealgorithms/strings/RotationTest.java)
           - 📄 [StringCompressionTest](src/test/java/com/thealgorithms/strings/StringCompressionTest.java)
           - 📄 [StringMatchFiniteAutomataTest](src/test/java/com/thealgorithms/strings/StringMatchFiniteAutomataTest.java)
+          - 📄 [SuffixArray](src/test/java/com/thealgorithms/strings/SuffixArrayTest.java)
           - 📄 [UpperTest](src/test/java/com/thealgorithms/strings/UpperTest.java)
           - 📄 [ValidParenthesesTest](src/test/java/com/thealgorithms/strings/ValidParenthesesTest.java)
           - 📄 [WordLadderTest](src/test/java/com/thealgorithms/strings/WordLadderTest.java)

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

@@ -0,0 +1,55 @@
+import java.util.Arrays;
+
+/**
+ * Suffix Array implementation in Java.
+ * Builds an array of indices that represent all suffixes of a string in sorted order.
+ * Wikipedia Reference: https://en.wikipedia.org/wiki/Suffix_array
+ * Author: Nithin U.
+ * Github: https://github.com/NithinU2802
+ */
+
+public class SuffixArray {
+
+    public static int[] buildSuffixArray(String text) {
+        int n = text.length();
+        Integer[] suffixArray = new Integer[n];
+        int[] rank = new int[n];
+        int[] tempRank = new int[n];
+
+        // Initial ranking based on characters
+        for (int i = 0; i < n; i++) {
+            suffixArray[i] = i;
+            rank[i] = text.charAt(i);
+        }
+
+        for (int k = 1; k < n; k *= 2) {
+            final int step = k;
+
+            // Comparator: first by rank, then by rank + step
+            Arrays.sort(suffixArray, (a, b) -> {
+                if (rank[a] != rank[b])
+                    return Integer.compare(rank[a], rank[b]);
+                int ra = (a + step < n) ? rank[a + step] : -1;
+                int rb = (b + step < n) ? rank[b + step] : -1;
+                return Integer.compare(ra, rb);
+            });
+
+            // Re-rank
+            tempRank[suffixArray[0]] = 0;
+            for (int i = 1; i < n; i++) {
+                int prev = suffixArray[i - 1];
+                int curr = suffixArray[i];
+                boolean sameRank = rank[prev] == rank[curr] &&
+                        ((prev + step < n ? rank[prev + step] : -1) == (curr + step < n ? rank[curr + step] : -1));
+                tempRank[curr] = sameRank ? tempRank[prev] : tempRank[prev] + 1;
+            }
+
+            System.arraycopy(tempRank, 0, rank, 0, n);
+
+            if (rank[suffixArray[n - 1]] == n - 1)
+                break;
+        }
+        return Arrays.stream(suffixArray).mapToInt(Integer::intValue).toArray();
+    }
+
+}

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

@@ -0,0 +1,48 @@
+package com.thealgorithms.strings;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+
+import org.junit.jupiter.api.Test;
+
+class SuffixArrayTest {
+
+    @Test
+    void testEmptyString() {
+        int[] result = SuffixArray.buildSuffixArray("");
+        assertArrayEquals(new int[] {}, result, "Empty string should return empty suffix array");
+    }
+
+    @Test
+    void testSingleCharacter() {
+        int[] result = SuffixArray.buildSuffixArray("a");
+        assertArrayEquals(new int[] {0}, result, "Single char string should return [0]");
+    }
+
+    @Test
+    void testDistinctCharacters() {
+        int[] result = SuffixArray.buildSuffixArray("abc");
+        assertArrayEquals(new int[] {0, 1, 2}, result, "Suffixes already in order for distinct chars");
+    }
+
+    @Test
+    void testBananaExample() {
+        int[] result = SuffixArray.buildSuffixArray("banana");
+        assertArrayEquals(new int[] {5, 3, 1, 0, 4, 2}, result, "Suffix array of 'banana' should be [5,3,1,0,4,2]");
+    }
+
+    @Test
+    void testStringWithDuplicates() {
+        int[] result = SuffixArray.buildSuffixArray("aaaa");
+        assertArrayEquals(new int[] {3, 2, 1, 0}, result, "Suffix array should be descending indices for 'aaaa'");
+    }
+
+    @Test
+    void testRandomString() {
+        int[] result = SuffixArray.buildSuffixArray("mississippi");
+        assertArrayEquals(
+            new int[] {10, 7, 4, 1, 0, 9, 8, 6, 3, 5, 2},
+            result,
+            "Suffix array for 'mississippi' should match expected"
+        );
+    }
+}