// SPDX-FileCopyrightText: 2026 Filip Leonarski, Paul Scherrer Institute <filip.leonarski@psi.ch>
// SPDX-License-Identifier: GPL-3.0-only

#include "RingBackground.h"

#include <algorithm>
#include <cmath>

RingBackground::RingBackground(const std::vector<float> &ring_bkg, const std::vector<double> &q_centre) {
    const size_t q_bins = q_centre.size();
    if (q_bins == 0)
        return;
    std::vector<double> sum(q_bins, 0.0);
    std::vector<int> cnt(q_bins, 0);
    for (size_t i = 0; i < ring_bkg.size(); ++i)
        if (std::isfinite(ring_bkg[i])) {
            sum[i % q_bins] += ring_bkg[i];
            ++cnt[i % q_bins];
        }
    for (size_t i = 0; i < q_bins; ++i)
        if (cnt[i] > 0) {
            q.push_back(q_centre[i]);
            b.push_back(sum[i] / cnt[i]);
        }
}

double RingBackground::At(double q_recipA) const {
    if (q_recipA <= q.front())
        return b.front();
    if (q_recipA >= q.back())
        return b.back();
    const size_t j = std::upper_bound(q.begin(), q.end(), q_recipA) - q.begin() - 1;
    return b[j] + (b[j + 1] - b[j]) * (q_recipA - q[j]) / (q[j + 1] - q[j]);
}