libcruft-util/list/sort.hpp

/*
 * 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 {
    /// A simple tuple-style linked list node.
    template <typename ValueT>
    struct node {
        node *next;
        ValueT data;
    };


    /// Find the midpoint of the list specified by the sub-list [first, last].
    ///
    /// On even length lists the earlier middle node will be returned.
    ///
    /// Uses the fast/slow walking method. Double advancing the fast iterator,
    /// and singly advancing the slow iterator.
    ///
    /// This necessarily walks the entire list, so the performance may be
    /// quite low.
    ///
    /// NOTE: `last` is a valid node and forms part of the sub-list.
    template <typename ValueT>
    node<ValueT>*
    midpoint [[gnu::nonnull]] (
        node<ValueT> *first,
        node<ValueT> *last
    ) {
        // Pre-increment the fast node to trigger a preference for the earlier
        // of the middle nodes.
        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;
    }

    /// Merges a list that has been divided into two sorted halves.
    ///
    /// This operation will mutate the `next` pointers but will not mutate
    /// the inner data objects.
    ///
    /// NOTE: The list is specified inclusively, and the `last` node is a
    /// valid piece of data.
    ///
    /// Returns the first node of the newly sorted list. The caller _must_
    /// store this value otherwise a portion of the list will be lost.
    template <typename ValueT>
    node<ValueT>*
    merge [[nodiscard]] [[using gnu: nonnull, returns_nonnull]] (
        node<ValueT> *first,
        node<ValueT> *middle,
        node<ValueT> *last
    ) {
        // clang#xxxx clang mistakenly warns that the values of 'first' and
        // 'middle' will be unconditionally true on the first iteration of the
        // while loop.
        #if defined(COMPILER_CLANG)
        #pragma GCC diagnostic push
        #pragma GCC diagnostic ignored "-Wpointer-bool-conversion"
        #endif

        CHECK (first);
        CHECK (middle);
        CHECK (last);

        if (first == last)
            return first;

        // Return a reference to the lower of 'first' and 'middle', and
        // advance the cursor.
        auto extract = [&first, &middle] () {
            auto &target = first->data < middle->data ? first : middle;
            auto res = target;
            target = target->next;
            return res;
        };

        node<ValueT> *cursor = extract ();
        auto head = cursor;

        // Keep adding the lowest node to the new list until we exhaust one
        // or both of our sources.
        while (first && middle && first != middle && middle != last->next) {
            auto target = extract ();
            cursor->next = target;
            cursor = target;
        }

        // Append the remaining data to the new list.
        if (first == middle || !first)
            cursor->next = middle;
        else if (middle == last->next || !middle)
            cursor->next = first;

        #if defined(COMPILER_CLANG)
        #pragma GCC diagnostic pop
        #endif

        return head;
    }


    /// An in-place merge sort for singly linked lists.
    template <typename ValueT>
    node<ValueT>*
    sort [[nodiscard]] [[using gnu: nonnull, returns_nonnull]] (
        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);
    }


    /// Tests if a singly linked list is sorted.
    template <typename ValueT>
    bool
    is_sorted [[gnu::nonnull]] (
        node<ValueT> const *head
    ) {
        if (!head->next)
            return true;

        auto a = head;
        auto b = head->next;

        while (b) {
            if (b->data < a->data)
                return false;

            a = a->next;
            b = b->next;
        }

        return true;
    }
}