1  // Copyright (c) The Bitcoin Core 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(std::span<const FeeFrac> chunks0, std::span<const FeeFrac> chunks1)
11  {
12      /** Array to allow indexed access to input diagrams. */
13      const std::array<std::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  }