From ce03a24f88aceb1e2c26d8f5c0f59229b12f1d29 Mon Sep 17 00:00:00 2001
From: Danny Robson <danny@nerdcruft.net>
Date: Mon, 9 Sep 2019 10:22:36 +1000
Subject: [PATCH] list/sort: add initial linked list merge sort

---
 CMakeLists.txt     |   2 +
 list/sort.hpp      | 125 +++++++++++++++++++++++++++++++++++++++++++++
 test/list/sort.cpp |  90 ++++++++++++++++++++++++++++++++
 3 files changed, 217 insertions(+)
 create mode 100644 list/sort.hpp
 create mode 100644 test/list/sort.cpp

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 718a91b0..addd9270 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -419,6 +419,7 @@ list (
     job/queue.hpp
     kmeans.hpp
     library.hpp
+    list/sort.hpp
     log.cpp
     log.hpp
     map/fixed.cpp
@@ -673,6 +674,7 @@ if (TESTS)
         job/dispatch
         job/queue
         kmeans
+        list/sort
         map/fixed
         maths
         maths/fast
diff --git a/list/sort.hpp b/list/sort.hpp
new file mode 100644
index 00000000..034f65f8
--- /dev/null
+++ b/list/sort.hpp
@@ -0,0 +1,125 @@
+/*
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * Copyright 2019 Danny Robson <danny@nerdcruft.net>
+ */
+
+#pragma once
+
+#include "../debug/assert.hpp"
+
+#include <iterator>
+#include <utility>
+
+namespace cruft::list {
+    template <typename ValueT>
+    struct node {
+        node *next;
+        ValueT data;
+
+        bool operator< (node const &rhs) const
+        {
+            return data < rhs.data;
+        }
+    };
+
+
+    template <typename ValueT>
+    node<ValueT>*
+    midpoint (node<ValueT> *first, node<ValueT> *last)
+    {
+        auto slow = first;
+        auto fast = first->next;
+
+        while (fast != last->next) {
+            fast = fast->next;
+            if (fast == last->next)
+                return slow;
+
+            fast = fast->next;
+            slow = slow->next;
+        }
+
+        return slow;
+    }
+
+
+    template <typename ValueT>
+    node<ValueT>*
+    merge (
+        node<ValueT> *first,
+        node<ValueT> *middle,
+        node<ValueT> *last
+    ) {
+        CHECK (first);
+        CHECK (middle);
+        CHECK (last);
+
+        if (first == last)
+            return first;
+
+        auto extract = [&] () {
+            auto &target = *first < *middle ? first : middle;
+            auto res = target;
+            target = target->next;
+            return res;
+        };
+
+        node<ValueT> *cursor = extract ();
+        auto head = cursor;
+
+        while (first && middle && first != middle && middle != last->next) {
+            auto target = extract ();
+            cursor->next = target;
+            cursor = target;
+        }
+
+        if (first == middle || !first)
+            cursor->next = middle;
+        else if (middle == last->next || !middle)
+            cursor->next = first;
+
+        return head;
+    }
+
+
+    // Merge sort for singly-linked lists
+    template <typename ValueT>
+    node<ValueT>*
+    sort (node<ValueT> *first, node<ValueT> *last)
+    {
+        if (first == last)
+            return first;
+
+        auto a_end = ::cruft::list::midpoint (first, last);
+        auto middle = a_end->next;
+        a_end->next = nullptr;
+
+        auto a = ::cruft::list::sort (first, a_end);
+        auto b = ::cruft::list::sort (middle, last);
+        return ::cruft::list::merge (a, b, last);
+    }
+
+
+    template <typename ValueT>
+    bool is_sorted (node<ValueT> const *head)
+    {
+        if (!head->next)
+            return true;
+
+        auto a = head;
+        auto b = head->next;
+
+        while (b) {
+            if (*b < *a)
+                return false;
+
+            a = a->next;
+            b = b->next;
+        }
+
+        return true;
+    }
+}
diff --git a/test/list/sort.cpp b/test/list/sort.cpp
new file mode 100644
index 00000000..3524cbf6
--- /dev/null
+++ b/test/list/sort.cpp
@@ -0,0 +1,90 @@
+/*
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * Copyright 2019 Danny Robson <danny@nerdcruft.net>
+ */
+
+#include "list/sort.hpp"
+#include "random.hpp"
+#include "tap.hpp"
+#include "iterator/zip.hpp"
+
+#include <list>
+#include <vector>
+#include <algorithm>
+
+
+///////////////////////////////////////////////////////////////////////////////
+template <std::size_t N, typename ...ArgsT>
+static void
+test_sort (
+    cruft::TAP::logger &tap,
+    std::vector<int> &&data,
+    char const (&fmt)[N],
+    ArgsT&&...args
+) {
+    // Pre-allocate the array so that we don't get caught by pointer
+    // invalidation as we build the nodes.
+    std::vector<
+        cruft::list::node<int>
+    > nodes (data.size ());
+
+    // Build the list and fold in the supplied data.
+    for (auto [node, value]: cruft::iterator::zip (nodes, data)) {
+        node.next = &node + 1;
+        node.data = value;
+    }
+
+    nodes.back ().next = nullptr;
+
+
+    // Actually do the sort and test the result.
+    auto head = cruft::list::sort (
+        &nodes.front (),
+        &nodes.back  ()
+    );
+
+    tap.expect (
+        is_sorted (head),
+        fmt,
+        std::forward<ArgsT> (args)...
+    );
+}
+
+
+//-----------------------------------------------------------------------------
+int main ()
+{
+    cruft::TAP::logger tap;
+
+    // Try sorting a 'large' array of unique, but shuffled, contiguous integers.
+    {
+        // Don't use a power of two here. It won't exercise any edge cases in
+        // the splitting logic of some sorts (like merge).
+        static constexpr int COUNT = 500'000;
+
+        // Create a list of randomly distributed integers. Use a constant seed so
+        // we don't get randomised test results.
+        std::vector<int> data (COUNT);
+        std::iota (std::begin (data), std::end (data), 0);
+        std::shuffle (
+            std::begin (data),
+            std::end   (data),
+            std::mt19937 {COUNT}
+        );
+
+        test_sort (tap, std::move (data), "sorting large unique and shuffled array");
+    }
+
+    // Try sorting a small array with identical elements
+    {
+        static constexpr int COUNT = 15;
+        std::vector<int> data (COUNT, COUNT);
+        test_sort (tap, std::move (data), "sorting small identical array");
+    }
+
+
+    return tap.status ();
+}
\ No newline at end of file