1 // Copyright (c) The Limenka developers
2 // Distributed under the MIT software license, see the accompanying
3 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
4 5 #include <util/feefrac.h>
6 #include <algorithm>
7 #include <array>
8 #include <vector>
9 10 std::partial_ordering CompareChunks(Span<const FeeFrac> chunks0, Span<const FeeFrac> chunks1)
11 {
12 /** Array to allow indexed access to input diagrams. */
13 const std::array<Span<const FeeFrac>, 2> chunk = {chunks0, chunks1};
14 /** How many elements we have processed in each input. */
15 size_t next_index[2] = {0, 0};
16 /** Accumulated fee/sizes in diagrams, up to next_index[i] - 1. */
17 FeeFrac accum[2];
18 /** Whether the corresponding input is strictly better than the other at least in one place. */
19 bool better_somewhere[2] = {false, false};
20 /** Get the first unprocessed point in diagram number dia. */
21 const auto next_point = [&](int dia) { return chunk[dia][next_index[dia]] + accum[dia]; };
22 /** Get the last processed point in diagram number dia. */
23 const auto prev_point = [&](int dia) { return accum[dia]; };
24 /** Move to the next point in diagram number dia. */
25 const auto advance = [&](int dia) { accum[dia] += chunk[dia][next_index[dia]++]; };
26 27 do {
28 bool done_0 = next_index[0] == chunk[0].size();
29 bool done_1 = next_index[1] == chunk[1].size();
30 if (done_0 && done_1) break;
31 32 // Determine which diagram has the first unprocessed point. If a single side is finished, use the
33 // other one. Only up to one can be done due to check above.
34 const int unproc_side = (done_0 || done_1) ? done_0 : next_point(0).size > next_point(1).size;
35 36 // Let `P` be the next point on diagram unproc_side, and `A` and `B` the previous and next points
37 // on the other diagram. We want to know if P lies above or below the line AB. To determine this, we
38 // compute the slopes of line AB and of line AP, and compare them. These slopes are fee per size,
39 // and can thus be expressed as FeeFracs.
40 const FeeFrac& point_p = next_point(unproc_side);
41 const FeeFrac& point_a = prev_point(!unproc_side);
42 43 const auto slope_ap = point_p - point_a;
44 Assume(slope_ap.size > 0);
45 std::weak_ordering cmp = std::weak_ordering::equivalent;
46 if (done_0 || done_1) {
47 // If a single side has no points left, act as if AB has slope tail_feerate(of 0).
48 Assume(!(done_0 && done_1));
49 cmp = FeeRateCompare(slope_ap, FeeFrac(0, 1));
50 } else {
51 // If both sides have points left, compute B, and the slope of AB explicitly.
52 const FeeFrac& point_b = next_point(!unproc_side);
53 const auto slope_ab = point_b - point_a;
54 Assume(slope_ab.size >= slope_ap.size);
55 cmp = FeeRateCompare(slope_ap, slope_ab);
56 57 // If B and P have the same size, B can be marked as processed (in addition to P, see
58 // below), as we've already performed a comparison at this size.
59 if (point_b.size == point_p.size) advance(!unproc_side);
60 }
61 // If P lies above AB, unproc_side is better in P. If P lies below AB, then !unproc_side is
62 // better in P.
63 if (std::is_gt(cmp)) better_somewhere[unproc_side] = true;
64 if (std::is_lt(cmp)) better_somewhere[!unproc_side] = true;
65 advance(unproc_side);
66 67 // If both diagrams are better somewhere, they are incomparable.
68 if (better_somewhere[0] && better_somewhere[1]) return std::partial_ordering::unordered;
69 } while(true);
70 71 // Otherwise compare the better_somewhere values.
72 return better_somewhere[0] <=> better_somewhere[1];
73 }
74