AK: Testing for AK::quickselect_inline

The testing code found here is mainly derived from the Tests for
`AK::quick_sort`.

Tests/AK/CMakeLists.txt

@@ -58,6 +58,7 @@ set(AK_TEST_SOURCES
     TestOwnPtr.cpp
     TestPrint.cpp
     TestQueue.cpp
+    TestQuickSelect.cpp
     TestQuickSort.cpp
     TestRedBlackTree.cpp
     TestRefPtr.cpp

Tests/AK/TestQuickSelect.cpp

@@ -0,0 +1,66 @@
+/*
+ * Copyright (c) 2023, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibTest/TestCase.h>
+
+#include <AK/QuickSelect.h>
+#include <AK/QuickSort.h>
+
+template<typename Collection>
+int naive_select(Collection& a, int k)
+{
+    quick_sort(a);
+    return k;
+}
+
+TEST_CASE(quickselect_inplace)
+{
+
+    // Test the various Quickselect Pivot methods against the naive select
+    Array<int, 64> array;
+    Array<int, 64> naive_results;
+
+    auto reset_array = [&]() {
+        for (size_t i = 0; i < 64; ++i)
+            array[i] = (64 - i) % 32 + 32;
+    };
+
+    // Populate naive results
+    reset_array();
+    for (size_t k = 0; k < 64; ++k) {
+        naive_results[k] = array.at(naive_select(array, k));
+    }
+
+    // Default configuration of `quick_select`
+    reset_array();
+    for (size_t k = 0; k < 64; ++k) {
+        EXPECT(naive_results[k] == array.at(AK::quickselect_inplace(array, k)));
+    }
+
+    // first_element pivot function
+    reset_array();
+    for (size_t k = 0; k < 64; ++k) {
+        EXPECT(naive_results[k] == array.at(AK::quickselect_inplace(array, k, [](auto collection, size_t left, size_t right, auto less_than) { return AK::PivotFunctions::first_element(collection, left, right, less_than); })));
+    }
+
+    // middle_element pivot function
+    reset_array();
+    for (size_t k = 0; k < 64; ++k) {
+        EXPECT(naive_results[k] == array.at(AK::quickselect_inplace(array, k, [](auto collection, size_t left, size_t right, auto less_than) { return AK::PivotFunctions::middle_element(collection, left, right, less_than); })));
+    }
+
+    // random_element pivot function
+    reset_array();
+    for (size_t k = 0; k < 64; ++k) {
+        EXPECT(naive_results[k] == array.at(AK::quickselect_inplace(array, k, [](auto collection, size_t left, size_t right, auto less_than) { return AK::PivotFunctions::random_element(collection, left, right, less_than); })));
+    }
+
+    // median_of_medians pivot function
+    reset_array();
+    for (size_t k = 0; k < 64; ++k) {
+        EXPECT(naive_results[k] == array.at(AK::quickselect_inplace(array, k, [](auto collection, size_t left, size_t right, auto less_than) { return AK::PivotFunctions::median_of_medians(collection, left, right, less_than); })));
+    }
+}