From 6b52d4d9cdfa8e3ff10a354d05966174c61d4e8e Mon Sep 17 00:00:00 2001
From: Camden Dixie O'Brien <cdo@wip.sh>
Date: Tue, 29 Oct 2024 16:02:59 +0000
Subject: [PATCH] Implement min heap

---
 lib/CMakeLists.txt     |  1 +
 lib/include/min_heap.h | 12 ++++++++++
 lib/min_heap.c         | 53 ++++++++++++++++++++++++++++++++++++++++++
 tests/CMakeLists.txt   |  1 +
 tests/min_heap_tests.c | 49 ++++++++++++++++++++++++++++++++++++++
 5 files changed, 116 insertions(+)
 create mode 100644 lib/include/min_heap.h
 create mode 100644 lib/min_heap.c
 create mode 100644 tests/min_heap_tests.c

diff --git a/lib/CMakeLists.txt b/lib/CMakeLists.txt
index d83b1e2..1707e23 100644
--- a/lib/CMakeLists.txt
+++ b/lib/CMakeLists.txt
@@ -2,6 +2,7 @@ add_library(lib
   construct.c
   desugar.c
   fsa.c
+  min_heap.c
   parse.c
   regex.c
 )
diff --git a/lib/include/min_heap.h b/lib/include/min_heap.h
new file mode 100644
index 0000000..c7f94e6
--- /dev/null
+++ b/lib/include/min_heap.h
@@ -0,0 +1,12 @@
+/*
+ * Copyright (c) Camden Dixie O'Brien
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+#ifndef MIN_HEAP_H
+#define MIN_HEAP_H
+
+void min_heap_heapify(int *xs, int count);
+int min_heap_pop(int *xs, int *count);
+
+#endif
diff --git a/lib/min_heap.c b/lib/min_heap.c
new file mode 100644
index 0000000..7e4c4f9
--- /dev/null
+++ b/lib/min_heap.c
@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) Camden Dixie O'Brien
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+#include "min_heap.h"
+
+static inline int left(int i)
+{
+	return 2 * i + 1;
+}
+
+static inline int parent(int i)
+{
+	return (i - 1) / 2;
+}
+
+static inline void swap(int *xs, int a, int b)
+{
+	int tmp = xs[a];
+	xs[a] = xs[b];
+	xs[b] = tmp;
+}
+
+static void sift_down(int *xs, int root, int count)
+{
+	int child;
+	while ((child = left(root)) < count) {
+		if (child + 1 < count && xs[child] > xs[child + 1])
+			++child;
+		if (xs[root] > xs[child]) {
+			swap(xs, root, child);
+			root = child;
+		} else {
+			return;
+		}
+	}
+}
+
+void min_heap_heapify(int *xs, int count)
+{
+	for (int i = parent(count - 1); i >= 0; --i)
+		sift_down(xs, i, count);
+}
+
+int min_heap_pop(int *xs, int *count)
+{
+	int min = xs[0];
+	--(*count);
+	xs[0] = xs[*count];
+	sift_down(xs, 0, *count);
+	return min;
+}
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 946ba70..7751e97 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -20,5 +20,6 @@ add_test_suites(
   construct_tests.c
   desugar_tests.c
   fsa_tests.c
+  min_heap_tests.c
   parse_tests.c
 )
diff --git a/tests/min_heap_tests.c b/tests/min_heap_tests.c
new file mode 100644
index 0000000..127a4fb
--- /dev/null
+++ b/tests/min_heap_tests.c
@@ -0,0 +1,49 @@
+/*
+ * Copyright (c) Camden Dixie O'Brien
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+#include "min_heap.h"
+#include "testing.h"
+
+#include <stdbool.h>
+
+static bool is_min_heap(int *xs, int count)
+{
+	for (int i = 0; i < count; ++i) {
+		const int left = 2 * i + 1;
+		const int right = 2 * i + 2;
+		if (left < count && xs[left] < xs[i])
+			return false;
+		if (right < count && xs[right] < xs[i])
+			return false;
+	}
+	return true;
+}
+
+static void array_is_min_heap_after_heapify(void)
+{
+	int xs[] = { 54, 12, 35, 43, 21, 12, 34, 52, 34, 23 };
+	const int len = sizeof(xs) / sizeof(int);
+	min_heap_heapify(xs, len);
+	ASSERT_TRUE(is_min_heap(xs, len));
+}
+
+static void extract_root_yields_min(void)
+{
+	int xs[] = { 71, 31, 12, 21, 65, 53, 54, 10 };
+	int len = 8;
+	min_heap_heapify(xs, len);
+	ASSERT_EQ(10, min_heap_pop(xs, &len));
+	ASSERT_EQ(12, min_heap_pop(xs, &len));
+	ASSERT_EQ(21, min_heap_pop(xs, &len));
+	ASSERT_EQ(5, len);
+}
+
+int main(void)
+{
+	TESTING_BEGIN();
+	array_is_min_heap_after_heapify();
+	extract_root_yields_min();
+	return TESTING_END();
+}