From f6cc3e2878686ae694b913fabff92b33bbf72d49 Mon Sep 17 00:00:00 2001
From: Billy Brawner <billybrawner@gmail.com>
Date: Thu, 11 Oct 2018 16:35:54 -0500
Subject: [PATCH] Add Heapsort algorithm

---
 .../wbrawner/algorithms/sorting/HeapSort.java | 83 +++++++++++++++++++
 .../sorting/ParameterizedHeapSortTest.java    | 31 +++++++
 2 files changed, 114 insertions(+)
 create mode 100644 sorting/src/main/java/com/wbrawner/algorithms/sorting/HeapSort.java
 create mode 100644 sorting/src/test/java/com/wbrawner/algorithms/sorting/ParameterizedHeapSortTest.java

diff --git a/sorting/src/main/java/com/wbrawner/algorithms/sorting/HeapSort.java b/sorting/src/main/java/com/wbrawner/algorithms/sorting/HeapSort.java
new file mode 100644
index 0000000..ea139f1
--- /dev/null
+++ b/sorting/src/main/java/com/wbrawner/algorithms/sorting/HeapSort.java
@@ -0,0 +1,83 @@
+package com.wbrawner.algorithms.sorting;
+
+import static com.wbrawner.algorithms.sorting.SortUtils.swap;
+
+/**
+ * The Heap Sort algorithm works by first organizing the data in a heap and
+ * then sorting it.
+ */
+public class HeapSort {
+    /**
+     * Sort a given array of ints using the Heap Sort algorithm
+     *
+     * @param unsorted an unsorted array of ints
+     * @return a sorted array of ints
+     */
+    public static int[] sort(int[] unsorted) {
+        // An empty array or array with only a single value can't be sorted
+        if (unsorted.length < 2) {
+            return unsorted;
+        }
+
+        buildMaxHeap(unsorted);
+
+        int end = unsorted.length - 1;
+        while (end > 0) {
+            swap(unsorted, end, 0);
+            end--;
+            siftDown(unsorted, 0, end);
+        }
+
+        return unsorted;
+    }
+
+
+    private static void buildMaxHeap(int[] unsorted) {
+        int startIndex = getParentIndex(unsorted.length - 1);
+
+        while (startIndex >= 0) {
+            siftDown(unsorted, startIndex, unsorted.length - 1);
+            startIndex--;
+        }
+    }
+
+
+    private static void siftDown(int[] array, int start, int end) {
+        int root = start;
+        int child;
+        while ((child = getLeftChild(root)) <= end) {
+            int swap = root;
+
+            if (array[swap] < array[child]) {
+                // If the left child is greater than the parent, they need to
+                // be swapped
+                swap = child;
+            }
+
+            if (child + 1 <= end && array[swap] < array[child + 1]) {
+                // If there is a right child though and it is greater than
+                // the left child, then it should be the one to be swapped
+                swap = child + 1;
+            }
+
+            if (swap == root) {
+                // If the children are absent or less than the parent, then
+                // this node is fine and no swaps need to be made
+                return;
+            }
+
+            swap(array, root, swap);
+            root = swap;
+        }
+
+    }
+
+    private static int getParentIndex(int index) {
+        return (int) Math.floor((index - 1) / 2);
+    }
+
+
+    private static int getLeftChild(int index) {
+        return index * 2 + 1;
+    }
+}
diff --git a/sorting/src/test/java/com/wbrawner/algorithms/sorting/ParameterizedHeapSortTest.java b/sorting/src/test/java/com/wbrawner/algorithms/sorting/ParameterizedHeapSortTest.java
new file mode 100644
index 0000000..11c82de
--- /dev/null
+++ b/sorting/src/test/java/com/wbrawner/algorithms/sorting/ParameterizedHeapSortTest.java
@@ -0,0 +1,31 @@
+package com.wbrawner.algorithms.sorting;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.Parameterized;
+
+import static org.junit.Assert.assertArrayEquals;
+
+@RunWith(Parameterized.class)
+public class ParameterizedHeapSortTest {
+
+
+    @Parameterized.Parameter(1)
+    public int[] sorted;
+
+    @Parameterized.Parameter()
+    public int[] unsorted;
+
+    @Test
+    public void sortTest() {
+        assertArrayEquals(
+                sorted,
+                HeapSort.sort(unsorted)
+        );
+    }
+
+    @Parameterized.Parameters
+    public static int[][][] getData() {
+        return SortData.get();
+    }
+}
\ No newline at end of file