From 1427328c7e450cd534608c32a12858b6b651ca16 Mon Sep 17 00:00:00 2001
From: Rosander0 <astereen.0107@gmail.com>
Date: Tue, 16 Jun 2026 11:11:35 +0530
Subject: [PATCH] feat: add LibrarySort implementation

---
 .../com/thealgorithms/sorts/LibrarySort.java  | 79 +++++++++++++++++++
 .../thealgorithms/sorts/LibrarySortTest.java  | 45 +++++++++++
 2 files changed, 124 insertions(+)
 create mode 100644 src/main/java/com/thealgorithms/sorts/LibrarySort.java
 create mode 100644 src/test/java/com/thealgorithms/sorts/LibrarySortTest.java

diff --git a/src/main/java/com/thealgorithms/sorts/LibrarySort.java b/src/main/java/com/thealgorithms/sorts/LibrarySort.java
new file mode 100644
index 000000000000..1a37aed52bd2
--- /dev/null
+++ b/src/main/java/com/thealgorithms/sorts/LibrarySort.java
@@ -0,0 +1,79 @@
+package com.thealgorithms.sorts;
+// author: Vraj Prajapati @Rosander0
+
+/**
+ * Library Sort (also known as Gapped Insertion Sort) is a sorting algorithm
+ * that works like insertion sort but leaves gaps between elements to make
+ * future insertions faster.
+ * Time Complexity: O(n log n) average case
+ * Space Complexity: O(n)
+ *
+ * @see <a href="https://en.wikipedia.org/wiki/Library_sort">
+ *     Wikipedia: Library Sort</a>
+ */
+public final class LibrarySort {
+
+    private LibrarySort() {
+        // Utility class
+    }
+
+    /**
+     * Sorts an array using the Library Sort algorithm.
+     *
+     * @param array the array to sort (must not be null)
+     * @return the sorted array
+     */
+    public static int[] sort(final int[] array) {
+        if (array == null) {
+            throw new IllegalArgumentException("Input array must not be null!");
+        }
+        if (array.length <= 1) {
+            return array;
+        }
+
+        int n = array.length;
+        Integer[] spaced = new Integer[2 * n];
+
+        spaced[0] = array[0];
+        int inserted = 1;
+
+        for (int i = 1; i < n; i++) {
+            int pos = binarySearch(spaced, inserted, array[i]);
+            for (int j = inserted; j > pos; j--) {
+                spaced[j] = spaced[j - 1];
+            }
+            spaced[pos] = array[i];
+            inserted++;
+        }
+
+        int idx = 0;
+        for (int i = 0; i < 2 * n; i++) {
+            if (spaced[i] != null) {
+                array[idx++] = spaced[i];
+            }
+        }
+        return array;
+    }
+
+    /**
+     * Binary search to find insertion position among inserted elements.
+     *
+     * @param spaced the spaced array
+     * @param inserted number of elements inserted so far
+     * @param target the value to find position for
+     * @return the correct insertion index
+     */
+    private static int binarySearch(final Integer[] spaced, final int inserted, final int target) {
+        int lo = 0;
+        int hi = inserted;
+        while (lo < hi) {
+            int mid = lo + (hi - lo) / 2;
+            if (spaced[mid] <= target) {
+                lo = mid + 1;
+            } else {
+                hi = mid;
+            }
+        }
+        return lo;
+    }
+}
diff --git a/src/test/java/com/thealgorithms/sorts/LibrarySortTest.java b/src/test/java/com/thealgorithms/sorts/LibrarySortTest.java
new file mode 100644
index 000000000000..87df9ef09b5f
--- /dev/null
+++ b/src/test/java/com/thealgorithms/sorts/LibrarySortTest.java
@@ -0,0 +1,45 @@
+package com.thealgorithms.sorts;
+// author: Vraj Prajapati @Rosander0
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.Test;
+
+public class LibrarySortTest {
+
+    @Test
+    public void testBasicSort() {
+        assertArrayEquals(new int[] {1, 2, 3, 4, 5}, LibrarySort.sort(new int[] {5, 3, 1, 4, 2}));
+    }
+
+    @Test
+    public void testAlreadySorted() {
+        assertArrayEquals(new int[] {1, 2, 3, 4, 5}, LibrarySort.sort(new int[] {1, 2, 3, 4, 5}));
+    }
+
+    @Test
+    public void testReverseSorted() {
+        assertArrayEquals(new int[] {1, 2, 3, 4, 5}, LibrarySort.sort(new int[] {5, 4, 3, 2, 1}));
+    }
+
+    @Test
+    public void testDuplicates() {
+        assertArrayEquals(new int[] {1, 2, 2, 3, 3}, LibrarySort.sort(new int[] {3, 2, 1, 3, 2}));
+    }
+
+    @Test
+    public void testSingleElement() {
+        assertArrayEquals(new int[] {1}, LibrarySort.sort(new int[] {1}));
+    }
+
+    @Test
+    public void testEmptyArray() {
+        assertArrayEquals(new int[] {}, LibrarySort.sort(new int[] {}));
+    }
+
+    @Test
+    public void testNullArray() {
+        assertThrows(IllegalArgumentException.class, () -> LibrarySort.sort(null));
+    }
+}