// Copyright (c) The Bitcoin Core developers

// Distributed under the MIT software license, see the accompanying

// file COPYING or http://www.opensource.org/licenses/mit-license.php.

#include <util/feefrac.h>

#include <util/check.h>

#include <array>

#include <cstddef>

std::partial_ordering CompareChunks(std::span<const FeeFrac> chunks0, std::span<const FeeFrac> chunks1)

{

    /** Array to allow indexed access to input diagrams. */

    const std::array<std::span<const FeeFrac>, 2> chunk = {chunks0, chunks1};

    /** How many elements we have processed in each input. */

    size_t next_index[2] = {0, 0};

    /** Accumulated fee/sizes in diagrams, up to next_index[i] - 1. */

    FeeFrac accum[2];

    /** Whether the corresponding input is strictly better than the other at least in one place. */

    bool better_somewhere[2] = {false, false};

    /** Get the first unprocessed point in diagram number dia. */

    const auto next_point = [&](int dia) { return chunk[dia][next_index[dia]] + accum[dia]; };

    /** Get the last processed point in diagram number dia. */

    const auto prev_point = [&](int dia) { return accum[dia]; };

    /** Move to the next point in diagram number dia. */

    const auto advance = [&](int dia) { accum[dia] += chunk[dia][next_index[dia]++]; };

    do {

        bool done_0 = next_index[0] == chunk[0].size();

        bool done_1 = next_index[1] == chunk[1].size();

        if (done_0 && done_1) break;

        // Determine which diagram has the first unprocessed point. If a single side is finished, use the

        // other one. Only up to one can be done due to check above.

        const int unproc_side = (done_0 || done_1) ? done_0 : next_point(0).size > next_point(1).size;

        // Let `P` be the next point on diagram unproc_side, and `A` and `B` the previous and next points

        // on the other diagram. We want to know if P lies above or below the line AB. To determine this, we

        // compute the slopes of line AB and of line AP, and compare them. These slopes are fee per size,

        // and can thus be expressed as FeeFracs.

        const FeeFrac& point_p = next_point(unproc_side);

        const FeeFrac& point_a = prev_point(!unproc_side);

        const auto slope_ap = point_p - point_a;

        Assume(slope_ap.size > 0);

        std::weak_ordering cmp = std::weak_ordering::equivalent;

        if (done_0 || done_1) {

            // If a single side has no points left, act as if AB has slope tail_feerate(of 0).

            Assume(!(done_0 && done_1));

            cmp = FeeRateCompare(slope_ap, FeeFrac(0, 1));

        } else {

            // If both sides have points left, compute B, and the slope of AB explicitly.

            const FeeFrac& point_b = next_point(!unproc_side);

            const auto slope_ab = point_b - point_a;

            Assume(slope_ab.size >= slope_ap.size);

            cmp = FeeRateCompare(slope_ap, slope_ab);

            // If B and P have the same size, B can be marked as processed (in addition to P, see

            // below), as we've already performed a comparison at this size.

            if (point_b.size == point_p.size) advance(!unproc_side);

        }

        // If P lies above AB, unproc_side is better in P. If P lies below AB, then !unproc_side is

        // better in P.

        if (std::is_gt(cmp)) better_somewhere[unproc_side] = true;

        if (std::is_lt(cmp)) better_somewhere[!unproc_side] = true;

        advance(unproc_side);

        // If both diagrams are better somewhere, they are incomparable.

        if (better_somewhere[0] && better_somewhere[1]) return std::partial_ordering::unordered;

    } while(true);

    // Otherwise compare the better_somewhere values.

    return better_somewhere[0] <=> better_somewhere[1];

}