From d4cbad40c315fb664e85d58dd36b2eb4b255eeb2 Mon Sep 17 00:00:00 2001
From: Gregory <neogen570@gmail.com>
Date: Mon, 3 Jan 2022 23:07:11 +0200
Subject: [PATCH] heap sort

---
 sorting/heap.go                               | 55 +++++++++++++++++++
 .../index_priority_queue_test.go              |  2 -
 sorting/sort.go                               |  2 +
 sorting/sort_test.go                          | 10 ++++
 4 files changed, 67 insertions(+), 2 deletions(-)
 create mode 100644 sorting/heap.go

diff --git a/sorting/heap.go b/sorting/heap.go
new file mode 100644
index 0000000..de7c9d3
--- /dev/null
+++ b/sorting/heap.go
@@ -0,0 +1,55 @@
+package sorting
+
+func swap[T any](i, j int, items []T) {
+	items[i], items[j] = items[j], items[i]
+}
+
+func swim[T any](child int, depth int, items []T, less func(T, T) bool) {
+	for {
+		parent := (child - 1) / 2
+
+		if child <= 0 || less(items[child], items[parent]) {
+			break
+		}
+
+		swap(parent, child, items)
+
+		child = parent
+	}
+}
+
+func sink[T any](parent int, depth int, items []T, less func(T, T) bool) {
+	for {
+		child := parent*2 + 1
+
+		if child >= depth {
+			break
+		}
+
+		if child+1 < depth && less(items[child], items[child+1]) {
+			child++
+		}
+
+		if !less(items[parent], items[child]) {
+			break
+		}
+
+		swap(parent, child, items)
+
+		parent = child
+	}
+}
+
+func Heap[T any](items []T, less func(T, T) bool) {
+	len := len(items)
+
+	for k := len / 2; k >= 0; k-- {
+		sink(k, len, items, less)
+	}
+
+	for len > 0 {
+		len--
+		swap(0, len, items)
+		sink(0, len, items, less)
+	}
+}
diff --git a/sorting/priority_queue/index_priority_queue_test.go b/sorting/priority_queue/index_priority_queue_test.go
index 92a3650..0cc2f73 100644
--- a/sorting/priority_queue/index_priority_queue_test.go
+++ b/sorting/priority_queue/index_priority_queue_test.go
@@ -15,8 +15,6 @@ func TestNewIPQ(t *testing.T) {
 	assert.Equal(t, 0, q.Size())
 	assert.Equal(t, true, q.IsEmpty())
 	assert.Equal(t, -1, q.TopIndex())
-	// TODO: maybe should return nil?
-	// assert.Equal(t, 0, q.Top())
 }
 
 func TestIPQInsert(t *testing.T) {
diff --git a/sorting/sort.go b/sorting/sort.go
index 71b3e91..07c77a6 100644
--- a/sorting/sort.go
+++ b/sorting/sort.go
@@ -1,3 +1,5 @@
 package sorting
 
+// TODO: compare function should receive pointers to slice elements
+// to prevent unnecessary coping
 type SliceSorter[T any] func([]T, func(T, T) bool)
diff --git a/sorting/sort_test.go b/sorting/sort_test.go
index 5c85147..52e2cdf 100644
--- a/sorting/sort_test.go
+++ b/sorting/sort_test.go
@@ -73,3 +73,13 @@ func BenchmarkQuick(b *testing.B) {
 		BenchmarkSort(10000, Quick[int])
 	}
 }
+
+func TestHeap(t *testing.T) {
+	CheckSliceSorter(Heap[int])
+}
+
+func BenchmarkHeap(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		BenchmarkSort(10000, Heap[int])
+	}
+}