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

#pragma once

#include <cstdint>
#include <memory>
#include <optional>
#include <vector>

// GPU engine for the RotationScaleMerge hot loops. The class keeps the per-observation data resident on
// the device as a structure-of-arrays (coalesced) and runs the scaling loop there. The host keeps the
// one-time raw-hkl sort and the per-space-group ASU keying (gemmi); it hands the GPU the dense group ids
// plus a group-ordered permutation so the per-group reduction is a deterministic segmented reduction
// (one block per group, fixed order, no atomics) - matching the run-to-run determinism of the CPU path.
//
// Only compiled when CUDA is available; the header is safe to include unconditionally (the impl behind
// the pimpl is null without CUDA, and RotationScaleMerge falls back to the CPU loops).
class RotationScaleMergeGPU {
public:
    RotationScaleMergeGPU();
    ~RotationScaleMergeGPU();
    RotationScaleMergeGPU(const RotationScaleMergeGPU &) = delete;
    RotationScaleMergeGPU &operator=(const RotationScaleMergeGPU &) = delete;

    // True if a GPU was found and the engine is usable.
    [[nodiscard]] bool Available() const;

    // The observation count and the frame CSR (frame_start/frame_count, length n_frames, indexing
    // the obs arrays in frame order), plus every device-side observation buffer, allocated here so
    // the chunked uploads below have somewhere to land. Call once, before them.
    void SetPartialsLayout(int n_obs, int n_frames,
                           const int32_t *frame_start, const int32_t *frame_count);

    // One slice ([offset, offset+count) of n_obs) of one immutable per-observation field, so the
    // host can stage a bounded chunk at a time out of its AoS record: staged whole and all at once,
    // the fourteen arrays are more than half the entire observation payload held live again.
    // Corr0 seeds the resident, mutable corr (SetCorr refreshes it). Bkg/VarBkg/ImageNumber/D/Px/Py
    // are the combine inputs (image-local background, fractional frame position for event
    // contiguity, resolution, and the predicted detector position carried to the full for the
    // absorption surface).
    enum class ObsField { I, Sigma, PrescalingCorr, Partiality, Zeta, Corr0,
                          Bkg, VarBkg, ImageNumber, D, Px, Py };
    void SetObsField(ObsField f, int offset, int count, const float *v);
    void SetObsFrame(int offset, int count, const int32_t *frame);
    void SetObsOnIce(int offset, int count, const uint8_t *on_ice);
    void SetObsClipped(int offset, int count, const uint8_t *clipped);

    // Per space group: the dense ASU-group id per obs, and a group-ordered permutation of the obs whose
    // group >= 0 (group_perm), with its CSR (group_start/group_count, length n_groups) - so each group's
    // observations are a contiguous, fixed-order segment for the reduction.
    void SetGroups(int n_groups, const int32_t *group,
                   const int32_t *group_perm, int n_group_perm,
                   const int32_t *group_start, const int32_t *group_count);

    // Re-upload the working corr (length n_obs) before a scaling pass (the host mutates it via smooth-G
    // between passes). SetObsField(Corr0) uploads the initial corr; this refreshes it.
    void SetCorr(const float *corr);

    // Run `iters` of {reduce group means -> per-frame weighted-LS G -> update corr} on the device,
    // in place on the resident corr. Rotation model (partiality folded via the stored partiality).
    void ScalePartials(int iters, double min_partiality, bool has_d_min);

    // Copy the updated corr back to the host (length n_obs), and the fitted per-frame G (length
    // n_frames, as double) plus the per-frame "was fitted" flag.
    void GetCorr(float *corr_out) const;
    void GetG(double *g_out, uint8_t *scaled_out) const;

    // Apply the smooth-G correction to the resident corr: corr[i] *= ratio[frame[i]] for frames with
    // apply[f] (both length n_frames), matching CPU SmoothG. Keeps corr resident (no round-trip).
    void SmoothCorr(const uint8_t *apply, const double *ratio);

    // The two pass filters, on the resident corr (both mirror the host loops in Run and keep corr on the
    // device). By zeta: zero corr wherever the rocking geometry fails the de-novo search threshold,
    // returning how many observations that removed from the merge, i.e. how many had a finite, positive
    // corr. By frame: zero corr on the frames flagged in `reject` (length n_frames).
    int64_t FilterCorrByZeta(double min_zeta);
    void FilterCorrByFrame(const uint8_t *reject);

    // --- merge + error-model reductions over the resident, scaled fulls (reuse the fulls group CSR) ---

    // The per-frame cell-consistency mask (length n_frames) used by the merge filter. Uploaded once.
    void SetFrameCellOk(const uint8_t *frame_cell_ok);

    // Per-group inv-var mean (em_mean, length n_groups) + per-full leverage-corrected error-model samples
    // (s2/I2/dev2 + valid flag, length n_fulls), mirroring MergeAndStats' first two error-model loops.
    // Stashes (for_search, min_partiality) for the MergeAccum/MergeRmeas calls that follow.
    // hand / has_hands are the per-full Bijvoet hand and the per-group "this group pools two hands",
    // which the error-model samples are formed on; both null leaves the fit on the pooled group.
    void MergeEmSamples(bool for_search, double min_partiality,
                        const uint8_t *hand, const uint8_t *has_hands,
                        double *em_mean_out, int32_t *cnt_out, double *s2_out, double *I2_out,
                        double *dev2_out, uint8_t *valid_out);

    // Per-group merge accumulators (inv-var sums + deterministic half-sets, error-model-corrected sigma
    // from a/b). `half` is the per-full CC1/2 half-set (length n_fulls) assigned on the host, so this
    // kernel and the host merge loop cannot disagree about it.
    // The per-group results stay on the device for MergeAccumRange to download; reject_median is
    // uploaded (NAN where none). rejected_obs is the per-full flag (length n_fulls): on entry the
    // observations the host already rejected (the Wilson test), on return those plus the median test's;
    // the host needs it for the reductions it still does itself, above all the anomalous I(+)/I(-) split.
    // frame_cc_factor (length n_frames) is each frame's (G_ref/G)^2 floored at 1; swh_typ0/1 are the
    // half-set weights multiplied by it. Requires MergeEmSamples first (em_mean resident).
    // reject_var_add (n_groups) widens the pooled cut by the shell's own measured Bijvoet
    // variance; null leaves the plain n-sigma test.
    void MergeAccum(double error_model_a, double error_model_b, bool error_model_active,
                    bool reject_outliers, double reject_nsigma, const float *reject_median,
                    const float *reject_var_add,
                    const uint8_t *half, const double *frame_cc_factor, uint8_t *rejected_obs);
    // Download groups [g0, g0 + n) of what MergeAccum left on the device; every output has length n.
    // rejected[g] counts the outliers dropped from the group.
    void MergeAccumRange(int g0, int n, double *swI, double *sw, double *swIh0, double *swIh1,
                         double *swh0, double *swh1, double *swh_typ0, double *swh_typ1,
                         int32_t *nh0, int32_t *nh1, double *d_out,
                         int32_t *rejected, uint8_t *on_ice_out);

    // Per-group R_meas accumulators: sum|I_corr-merged_I| and sum I, the same with each observation
    // weighted by its merge weight v = 1/sigma^2 under the error model of the last MergeAccum,
    // sum v, sum v^2, n, and the count this looser walk accepted - which the host uses only to skip
    // empty groups, NOT as the per-shell total_observations; merged_I is uploaded. All arrays length
    // n_groups.
    void MergeRmeas(const double *merged_I, double *absdev, double *sumI, double *wabsdev, double *wsumI,
                    double *sumv, double *sumv2, int32_t *n, int32_t *nusable);

    // Post-smooth per-frame diagnostic CC: recompute the group means from the resident (smoothed) corr
    // and the Pearson CC of each frame's I*corr vs its group mean, downloading only the per-frame cc /
    // cc_n (length n_frames). Mirrors ReduceGroupMeans(partials) + FinalizePerFrameScale's CC loop.
    void ComputePartialCC(double min_partiality, double *cc_out, int64_t *cc_n_out);

    // --- 3D combine (partials -> fulls), all on the device ---

    // The one-time raw-hkl run layout (space-group-independent): the (raw h,k,l, image_number)-sorted
    // permutation of the obs, split into contiguous per-raw-hkl runs. Uploaded once in Ingest.
    void SetRawRuns(int n_runs, int n_perm, const int32_t *perm,
                    const int32_t *rawrun_start, const int32_t *rawrun_count,
                    const int32_t *rawrun_h, const int32_t *rawrun_k, const int32_t *rawrun_l);

    // Combine the resident partials (reading the current resident corr) into fulls on the device,
    // mirroring RotationScaleMerge::Combine: one thread per raw-hkl run splits its usable partials into
    // rocking events (frame gap <= max_frame_gap, the host's RockingEventFrameGap), pools background, seeds F, does 3 de-biased Poisson reweights and
    // adds the capture-uncertainty term. rawrun_group (length n_runs) is the current space group's ASU
    // id per raw hkl (it becomes the full's group). Deterministic: fulls are emitted in raw-run-major,
    // event order (a count pass -> host prefix sum -> emit-at-offset), matching the CPU path. Returns the
    // number of fulls (call GetFulls with buffers of that length).
    int Combine(const int32_t *rawrun_group, double min_partiality, double capture_uncertainty_coeff,
                double min_captured_fraction, float max_frame_gap);

    // Download the combined fulls SoA (length = Combine()'s return). The working corr is downloaded
    // separately by GetFullsCorr (it is only meaningful after ScaleFulls; otherwise the caller sets it).
    void GetFulls(int32_t *h, int32_t *k, int32_t *l, float *I, float *sigma, float *d,
                  float *image_number, int32_t *frame, uint8_t *on_ice, uint8_t *clipped,
                  int32_t *group) const;

    // Download the fulls' predicted detector position (peak partial's px/py), for the host absorption
    // surface. Length = n_fulls.
    void GetFullsPxPy(float *px, float *py) const;

    // Download the fulls' variance model, var(I) = var_bkg + var_per_I * I. Length = n_fulls.
    void GetFullsVariance(float *var_bkg, float *var_per_I) const;

    // Re-upload the fulls' working corr (length n_fulls) after the host correction surfaces (decay /
    // absorption) mutate it, so the resident merge reads the corrected scale.
    void SetFullsCorr(const float *corr);

    // --- scale the resident fulls on the device (Unity model), no round-trip ---

    // Download the fulls' frame and ASU-group keys (emit order) so the host can build the frame/group CSRs
    // with a counting sort (deterministic, no GPU stable-sort) and hand them back below.
    void GetFullsKeys(int32_t *frame, int32_t *group) const;

    // The fulls' per-frame CSR: frame_perm groups the emit-ordered fulls by frame (frame_start/count length
    // n_frames index it), so FitPerFrameG can scale the fulls without physically reordering them.
    void SetFullsFrameCSR(const int32_t *frame_perm, int n_perm,
                          const int32_t *frame_start, const int32_t *frame_count);

    // The fulls' per-ASU-group CSR (group-ordered permutation of the fulls with group>=0, + its CSR).
    void SetFullsGroups(const int32_t *gperm, int n_gperm,
                        const int32_t *gstart, const int32_t *gcount);

    // Start the Unity scaling of the resident fulls: corr, partiality, prescaling and zeta all 1, no
    // frame fitted yet. Once before the ScaleFulls iterations.
    void ResetFullsScale();

    // Run `iters` of the Unity scaling loop on the resident fulls (reduce group means -> per-frame LS G
    // -> update corr), in place on the fulls' working corr. Requires SetFullsFrameCSR + SetFullsGroups
    // and ResetFullsScale.
    void ScaleFulls(int iters, double min_partiality);

    // The fulls' counterpart of SmoothCorr: f_corr[i] *= ratio[f_frame[i]] where apply[f].
    void SmoothFullsCorr(const uint8_t *apply, const double *ratio);

    // Download the fulls' working corr (length = n_fulls), valid after ScaleFulls.
    void GetFullsCorr(float *corr) const;

private:
    struct Impl;
    std::unique_ptr<Impl> impl_;
};