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 "node.hpp"

#include "../debug/assert.hpp"

#include <iterator>
#include <utility>

namespace cruft::list {
    /// Return the total number of nodes in the list
    ///
    /// It is safe to call this function with a null node pointer.
    template <typename NodeT>
    std::size_t
    size (NodeT const *head)
    {
        std::size_t count = 0;

        while (head) {
            ++count;
            head = next (head);
        }

        return count;
    }


    /// Returns the last valid node of a linked list. ie, the node where the
    /// successor is null.
    ///
    /// The results are undefined if the head node is null (as it is
    /// impossible to return a valid node if none are valid).
    template <typename NodeT>
    NodeT*
    end (NodeT *head)
    {
        CHECK (head);

        while (next (head))
            head = next (head);
        return head;
    }


    /// 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 NodeT>
    NodeT*
    midpoint [[gnu::nonnull]] (
        NodeT *first,
        NodeT *last
    ) {
        // Pre-increment the fast node to trigger a preference for the earlier
        // of the middle nodes.
        auto slow = first;
        auto fast = next (first);

        while (fast != next (last)) {
            fast = next (fast);
            if (fast == next (last))
                return slow;

            fast = next (fast);
            slow = next (slow);
        }

        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 NodeT, typename ComparatorT>
    NodeT*
    merge [[nodiscard]] [[using gnu: nonnull, returns_nonnull]] (
        NodeT *first,
        NodeT *middle,
        NodeT *last,
        ComparatorT &&cmp
    ) {
        // 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"
        #elif defined(COMPILER_GCC)
        // gcc#11 If we rely on the _Pragma embedded in `CHECK` then GCC 11
        // will still issue a warning for the null assert we're about to do
        // (but oddly only for some uses of this file...).
        //
        // Issuing the _Pragma locally is an effective workaround.
        // It's unclear how we can produce a repro for this.
        #pragma GCC diagnostic push
        #pragma GCC diagnostic ignored "-Wnonnull-compare"
        #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, &cmp] () {
            auto &target = cmp (first, middle) ? first : middle;
            auto res = target;
            target = next (target);
            return res;
        };

        NodeT *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 != next (last)) {
            auto target = extract ();
            next (cursor, target);
            cursor = target;
        }

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

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

        return head;
    }


    /// Sort a linked list using a supplied comparator.
    ///
    /// The current implementation uses an in-place merge sort.
    ///
    /// Each nodes successor may be modified, but the data will not be.
    ///
    /// Returns the head of the newly sorted list. The caller _must_ reset
    /// their stored head node with this value otherwise data may be lost.
    template <typename NodeT, typename ComparatorT>
    NodeT*
    sort [[nodiscard]] [[using gnu: nonnull, returns_nonnull]] (
        NodeT *first,
        NodeT *last,
        ComparatorT &&cmp
    ) {
        if (first == last)
            return first;

        auto a_end = ::cruft::list::midpoint (first, last);
        auto middle = next (a_end);
        next (a_end, nullptr);

        auto a = ::cruft::list::sort (first, a_end, cmp);
        auto b = ::cruft::list::sort (middle, last, cmp);
        return ::cruft::list::merge (a, b, last, cmp);
    }


    /// Sort a linked list using the default (value) comparator.
    template <typename NodeT>
    NodeT*
    sort [[nodiscard]] [[using gnu: nonnull, returns_nonnull]] (
        NodeT *head,
        NodeT *tail
    ) {
        return sort (
            head,
            tail,
            node::value_comparator (std::less<> ())
        );
    }


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

        auto a = head;
        auto b = next (head);

        while (b) {
            if (cmp (b, a))
                return false;

            a = b;
            b = next (b);
        }

        return true;
    }


    template <typename NodeT>
    bool
    is_sorted [[gnu::nonnull]] (
        NodeT const *head
    ) {
        return is_sorted (head, node::value_comparator {std::less<> {}});
    };
}