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

#pragma once

#include <array>
#include <cstddef>
#include <cstdint>
#include <optional>
#include <vector>

// The background noise of a detector that reports ADU rather than counted photons. For a photon
// counter a background pixel's variance equals its mean; a CCD's pixel value is a gain times the
// photons plus read noise, so its variance is g*mean (+ a constant). The gain is not in the headers
// (an SMV file states none, every marCCD file states the writer's default of 1 ADU/photon), so it is
// measured: the variance per unit mean of the difference between two pixels two columns apart, which
// share the background, but are far enough apart that the detector's point spread does not correlate
// them. Bragg peaks and zingers are clipped per level. Taken per level (bins of the pair's mean, a
// factor 1.25 wide) and summarised as the median ratio over all pairs, i.e. over the background that
// holds most of the pixels.
//
// The variance/mean of pixels in a neighbourhood is how XDS fills its GAIN table (INIT step,
// Kabsch (2010) Acta Cryst. D66, 125-132).
namespace background_gain {

constexpr int N_BINS = 64;              // levels 1.25^0 .. 1.25^63, i.e. up to ~1.3e6 ADU
constexpr size_t MIN_PAIRS_PER_BIN = 1000;
constexpr size_t MIN_BINS = 3;
// Frames of the pre-scan sample it is measured on, spread over the sample. The answer moves by under
// 1 % between frames of a sweep, so more only cost time.
constexpr size_t GAIN_MAX_FRAMES = 16;

// Per level, over the pairs that survived the clip: the count, the sum of the pair means and the sum
// of the squared differences.
struct Levels {
    std::array<uint64_t, N_BINS> n{};
    std::array<double, N_BINS> sum_mean{};
    std::array<double, N_BINS> sum_diff2{};
};

// One frame. A pixel takes part if the mask is zero there and 1 <= value < saturation (a CCD reads
// its bias in the dark, so a zero is a pixel outside the active area).
Levels MeasureFrame(const std::vector<int32_t> &image, size_t width, size_t height,
                    const std::vector<uint32_t> &mask, int64_t saturation);

// The gain over frames: the pair-weighted median of the per-level variance/mean, or nothing with
// fewer than MIN_BINS levels holding MIN_PAIRS_PER_BIN pairs.
std::optional<float> Gain(const std::vector<Levels> &frames);

// The gain of a detector whose background resolves single photons (a JUNGFRAU with its pedestal
// subtracted): the histogram of pixel values has a peak at zero photons, one at a single photon and
// another at two, a gain apart. The variance per unit mean above does not see it on a sparse background -
// the empty pixels it leaves out are most of it - so the quantum is read off the histogram instead. The
// gain is the spacing of the peaks, not the single photon's value, so a frame whose baseline was never
// subtracted - its zero-photon peak well above zero - reads the same gain, and its baseline besides.
constexpr size_t PEAK_HISTOGRAM_BINS = 16384;

// Adds the frame's unmasked pixels with 0 <= value < min(saturation, PEAK_HISTOGRAM_BINS) to histogram
// (PEAK_HISTOGRAM_BINS long, one bin per value).
void AddToHistogram(const std::vector<int32_t> &image, const std::vector<uint32_t> &mask, int64_t saturation,
                    std::vector<uint64_t> &histogram);

struct GainEstimate {
    float gain;       // detector units per photon (photon peaks: from the zero-photon to the single-photon peak)
    float pedestal;   // where the zero-photon peak sits; 0 where the baseline was subtracted or no peak shows
};

// The zero-photon peak is the highest bin; the single-photon peak the highest past its descent, standing out
// of the valley before it, falling to half its height before the midpoint to the next, and repeated by a
// two-photon peak near one more gain on. Nothing where the histogram shows no such pair - integer photon
// counts, whose single photon is the bin next to the zero peak, or a detector whose noise hides the photons.
std::optional<GainEstimate> PhotonPeak(const std::vector<uint64_t> &histogram);

// The gain of any input that reports detector units, rotation or stills, from the same frames: the photon
// peaks where the histogram shows them, else the variance per unit mean of the levels.
std::optional<GainEstimate> Estimate(const std::vector<Levels> &frames, const std::vector<uint64_t> &histogram);

}  // namespace background_gain