sparsehistogram.h

#pragma once

#include <format>
#include <map>
#include <fstream>
#if __cplusplus >= 202002L
#include <concepts>
#endif

namespace Faunus {

template <std::floating_point T = double> class SparseHistogram
{
    const T resolution;
    using map_type = std::map<int, unsigned int>;
    map_type data;

  public:
    explicit SparseHistogram(T resolution)
        : resolution(resolution)
    {
    }

    const T getResolution() const { return resolution; }

    void add(const T value)
    {
        if (std::isfinite(value)) {
            const auto index = static_cast<map_type::key_type>(std::round(value / resolution));
            data[index]++;
        }
        else {
            faunus_logger->warn("histogram: skipping inf/nan number");
        }
    }

    friend auto& operator<<(std::ostream& stream, const SparseHistogram& histogram)
    {
        std::for_each(histogram.data.begin(), histogram.data.end(), [&](const auto& sample) {
            stream << std::format(
                "{:.6E} {}\n", static_cast<T>(sample.first) * histogram.resolution, sample.second);
        });
        return stream;
    }
};
} // namespace Faunus