Fulcrum/Mempool.cpp

#include "Mempool.h"

#include <functional>
#include <map>

auto Mempool::calcCompactFeeHistogram(double binSize) const -> FeeHistogramVec
{
    // this algorithm is taken from:
    // https://github.com/Electron-Cash/electrumx/blob/fbd00416d804c286eb7de856e9399efb07a2ceaf/electrumx/server/mempool.py#L139
    FeeHistogramVec ret;
    std::map<unsigned, unsigned, std::greater<unsigned>> histogram; // sorted map, descending order by key

    for (const auto & [txid, tx] : txs) {
        const auto feeRate = unsigned(tx->fee / bitcoin::Amount::satoshi()) // sats
                             /  std::max(tx->sizeBytes, 1u); // per byte
        histogram[feeRate] += tx->sizeBytes; // accumulate size by feeRate
    }

    // now, compact the bins
    ret.reserve(8);
    unsigned cumSize = 0;
    double r = 0.;

    for (const auto & [feeRate, size] : histogram) {
        cumSize += size;
        if (cumSize + r > binSize) {
            ret.emplace_back(FeeHistogramItem{feeRate, cumSize});
            r += double(cumSize) - binSize;
            cumSize = 0;
            binSize *= 1.1;
        }
    }
    ret.shrink_to_fit(); // save memory
    return ret;
}