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clusterlin: permit passing in existing linearization to Linearize

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This implements the LIMO algorithm for linearizing by improving an existing
linearization. See
https://delvingbitcoin.org/t/limo-combining-the-best-parts-of-linearization-search-and-merging
for details.
This commit is contained in:
Pieter Wuille
2024-05-09 09:02:18 -04:00
parent 97d98718b0
commit 28549791b3
3 changed files with 53 additions and 9 deletions
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29
src/cluster_linearize.h
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@@ -663,23 +663,27 @@ public:
}
};
/** Find a linearization for a cluster.
/** Find or improve a linearization for a cluster.
*
* @param[in] depgraph Dependency graph of the cluster to be linearized.
* @param[in] max_iterations Upper bound on the number of optimization steps that will be done.
* @param[in] rng_seed A random number seed to control search order. This prevents peers
* from predicting exactly which clusters would be hard for us to
* linearize.
* @param[in] old_linearization An existing linearization for the cluster (which must be
* topologically valid), or empty.
* @return A pair of:
* - The resulting linearization.
* - The resulting linearization. It is guaranteed to be at least as
* good (in the feerate diagram sense) as old_linearization.
* - A boolean indicating whether the result is guaranteed to be
* optimal.
*
* Complexity: O(N * min(max_iterations + N, 2^N)) where N=depgraph.TxCount().
*/
template<typename SetType>
std::pair<std::vector<ClusterIndex>, bool> Linearize(const DepGraph<SetType>& depgraph, uint64_t max_iterations, uint64_t rng_seed) noexcept
std::pair<std::vector<ClusterIndex>, bool> Linearize(const DepGraph<SetType>& depgraph, uint64_t max_iterations, uint64_t rng_seed, Span<const ClusterIndex> old_linearization = {}) noexcept
{
Assume(old_linearization.empty() || old_linearization.size() == depgraph.TxCount());
if (depgraph.TxCount() == 0) return {{}, true};
uint64_t iterations_left = max_iterations;
@@ -690,9 +694,17 @@ std::pair<std::vector<ClusterIndex>, bool> Linearize(const DepGraph<SetType>& de
linearization.reserve(depgraph.TxCount());
bool optimal = true;
/** Chunking of what remains of the old linearization. */
LinearizationChunking old_chunking(depgraph, old_linearization);
while (true) {
// Initialize best as the best remaining ancestor set.
// Find the highest-feerate prefix of the remainder of old_linearization.
SetInfo<SetType> best_prefix;
if (old_chunking.NumChunksLeft()) best_prefix = old_chunking.GetChunk(0);
// Then initialize best to be either the best remaining ancestor set, or the first chunk.
auto best = anc_finder.FindCandidateSet();
if (!best_prefix.feerate.IsEmpty() && best_prefix.feerate >= best.feerate) best = best_prefix;
// Invoke bounded search to update best, with up to half of our remaining iterations as
// limit.
@@ -703,6 +715,12 @@ std::pair<std::vector<ClusterIndex>, bool> Linearize(const DepGraph<SetType>& de
if (iterations_done_now == max_iterations_now) {
optimal = false;
// If the search result is not (guaranteed to be) optimal, run intersections to make
// sure we don't pick something that makes us unable to reach further diagram points
// of the old linearization.
if (old_chunking.NumChunksLeft() > 0) {
best = old_chunking.Intersect(best);
}
}
// Add to output in topological order.
@@ -712,6 +730,9 @@ std::pair<std::vector<ClusterIndex>, bool> Linearize(const DepGraph<SetType>& de
anc_finder.MarkDone(best.transactions);
if (anc_finder.AllDone()) break;
src_finder.MarkDone(best.transactions);
if (old_chunking.NumChunksLeft() > 0) {
old_chunking.MarkDone(best.transactions);
}
}
return {std::move(linearization), optimal};