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// 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 <algorithm>
#include <array>
#include <vector>
std::partial_ordering CompareChunks(Span<const FeeFrac> chunks0, Span<const FeeFrac> chunks1)
{
/** Array to allow indexed access to input diagrams. */
const std::array<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];
}
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