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..85762dc3e46b --- /dev/null +++ b/src/main/java/com/thealgorithms/sorts/LibrarySort.java @@ -0,0 +1,199 @@ +package com.thealgorithms.sorts; + +/** + * Library Sort (also known as Gapped Insertion Sort) maintains a sparse + * working array with gaps distributed between elements, so that most + * insertions land directly in an empty gap without shifting anything. + * Elements are inserted in rounds that double in size (1, 2, 4, 8, ...); + * after each round the array is rebalanced so gaps are spread out evenly + * again for the next round. + * Time Complexity: O(n log n) expected, O(n^2) worst case if gaps collapse + * Space Complexity: O(n) + * + * @see + * Wikipedia: Library Sort + * @author Vraj Prajapati (@Rosander0) + */ +public final class LibrarySort { + + private static final int GAP_FACTOR = 2; + + 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 + * @throws IllegalArgumentException if {@code array} is {@code null} + */ + 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; + } + + final int n = array.length; + final int capacity = GAP_FACTOR * n; + final int[] data = new int[capacity]; + final boolean[] occupied = new boolean[capacity]; + + final int mid = capacity / 2; + data[mid] = array[0]; + occupied[mid] = true; + + int filled = 1; + int nextToInsert = 1; + int round = 0; + while (nextToInsert < n) { + final int roundSize = Math.min(1 << round, n - nextToInsert); + for (int i = 0; i < roundSize; i++) { + insert(data, occupied, array[nextToInsert + i]); + filled++; + } + nextToInsert += roundSize; + round++; + if (nextToInsert < n) { + rebalance(data, occupied, filled); + } + } + + int idx = 0; + for (int i = 0; i < capacity; i++) { + if (occupied[i]) { + array[idx++] = data[i]; + } + } + return array; + } + + /** + * Inserts {@code value} into the gapped array, placing it directly in an + * empty gap when possible, otherwise shifting toward the nearest gap. + */ + private static void insert(final int[] data, final boolean[] occupied, final int value) { + final int pos = findInsertionIndex(data, occupied, value); + if (pos >= data.length) { + insertAtEnd(data, occupied, value); + return; + } + + if (!occupied[pos]) { + data[pos] = value; + occupied[pos] = true; + return; + } + + int right = pos; + while (right < data.length && occupied[right]) { + right++; + } + int left = pos - 1; + while (left >= 0 && occupied[left]) { + left--; + } + + final boolean canGoRight = right < data.length; + final boolean canGoLeft = left >= 0; + + if (canGoRight && (!canGoLeft || (right - pos) <= (pos - left))) { + for (int j = right; j > pos; j--) { + data[j] = data[j - 1]; + occupied[j] = true; + } + data[pos] = value; + occupied[pos] = true; + } else if (canGoLeft) { + for (int j = left; j < pos - 1; j++) { + data[j] = data[j + 1]; + } + occupied[left] = true; + data[pos - 1] = value; + occupied[pos - 1] = true; + } else { + throw new IllegalStateException("No gap available for insertion; rebalance too infrequent."); + } + } + + /** + * Handles insertion of a new global maximum, which must land after every + * currently occupied slot. Since there is no room to its right, this + * shifts occupied slots left into the nearest gap instead. + */ + private static void insertAtEnd(final int[] data, final boolean[] occupied, final int value) { + final int last = data.length - 1; + if (!occupied[last]) { + data[last] = value; + occupied[last] = true; + return; + } + int left = last - 1; + while (left >= 0 && occupied[left]) { + left--; + } + if (left < 0) { + throw new IllegalStateException("No gap available for insertion; rebalance too infrequent."); + } + for (int j = left; j < last; j++) { + data[j] = data[j + 1]; + } + occupied[left] = true; + data[last] = value; + occupied[last] = true; + } + + /** + * Finds the leftmost index at which {@code value} can be inserted so + * that occupied slots remain sorted. Empty slots are compared using the + * value of the nearest occupied slot at or after them, which is a + * monotonic function of index and therefore safe to binary search over. + */ + private static int findInsertionIndex(final int[] data, final boolean[] occupied, final int value) { + int lo = 0; + int hi = data.length; + while (lo < hi) { + final int mid = lo + (hi - lo) / 2; + final int probe = nearestOccupiedValueAtOrAfter(data, occupied, mid); + if (probe != Integer.MAX_VALUE && probe <= value) { + lo = mid + 1; + } else { + hi = mid; + } + } + return lo; + } + + private static int nearestOccupiedValueAtOrAfter(final int[] data, final boolean[] occupied, final int index) { + for (int i = index; i < data.length; i++) { + if (occupied[i]) { + return data[i]; + } + } + return Integer.MAX_VALUE; + } + + /** + * Redistributes the {@code filled} occupied elements evenly across the + * full capacity of {@code data}, restoring uniform gaps between them. + */ + private static void rebalance(final int[] data, final boolean[] occupied, final int filled) { + final int capacity = data.length; + final int[] temp = new int[filled]; + int idx = 0; + for (int i = 0; i < capacity; i++) { + if (occupied[i]) { + temp[idx++] = data[i]; + occupied[i] = false; + } + } + for (int k = 0; k < filled; k++) { + final int pos = (int) ((long) k * capacity / filled); + data[pos] = temp[k]; + occupied[pos] = true; + } + } +} 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..bb14db36fe95 --- /dev/null +++ b/src/test/java/com/thealgorithms/sorts/LibrarySortTest.java @@ -0,0 +1,44 @@ +package com.thealgorithms.sorts; + +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)); + } +}