Jungfraujoch/image_analysis/indexing/FFTIndexerCPU.cpp

// SPDX-FileCopyrightText: 2025 Filip Leonarski, Paul Scherrer Institute <filip.leonarski@psi.ch>
// SPDX-License-Identifier: GPL-3.0-only

#include "FFTIndexerCPU.h"

#include <cmath>
#include <algorithm>
#include <stdexcept>
#include <cassert>
#include <mutex>
#include <vector>

static std::mutex fftw_plan_mutex;

static inline double dot_abs(const Coord& a, const Coord& b) {
    return std::fabs(a.x * b.x + a.y * b.y + a.z * b.z);
}

FFTIndexerCPU::FFTIndexerCPU(const IndexingSettings& settings)
        : FFTIndexer(settings) {

    // Allocate host buffers
    h_input_fft.resize(input_size, 0.0);
    h_output_fft.resize(output_size);

    // Validate allocations vs. expected sizes
    if (h_input_fft.size() != input_size)
        throw std::runtime_error("FFTWIndexer: input buffer size mismatch");
    if (h_output_fft.size() != output_size)
        throw std::runtime_error("FFTWIndexer: output buffer size mismatch");

    const int H = static_cast<int>(histogram_size);
    const int out_len = (H / 2) + 1;

    int n[1] = { H };

    {
        std::unique_lock ul(fftw_plan_mutex);
        plan = fftwf_plan_many_dft_r2c(
                1, n, nDirections,
                h_input_fft.data(), nullptr, 1, H,
                reinterpret_cast<fftwf_complex*>(h_output_fft.data()), nullptr, 1, out_len,
                FFTW_ESTIMATE);
    }

    if (!plan)
        throw std::runtime_error("fftw_plan_many_dft_r2c failed");
}


FFTIndexerCPU::~FFTIndexerCPU() {
    std::unique_lock ul(fftw_plan_mutex);
    fftwf_destroy_plan(plan);
}

void FFTIndexerCPU::ExecuteFFT(const std::vector<Coord> &coord, size_t nspots) {
    // Build histograms: one per direction
    const int H = static_cast<int>(histogram_size);
    const int D = nDirections;
    const int out_len = (H / 2) + 1;

    std::fill(h_input_fft.begin(), h_input_fft.end(), 0.0);

    for (int d = 0; d < D; ++d) {
        float* hist = h_input_fft.data() + static_cast<size_t>(d) * H;

        for (size_t i = 0; i < nspots; i++) {
            const auto& r = coord[i];
            double dot = dot_abs(direction_vectors[d], r);
            long long bin = static_cast<long long>(dot / histogram_spacing);
            if (bin >= 0 && bin < H) {
                hist[bin] += 1.0;
            }
        }
    }

    // Plan and execute batched R2C FFT with FFTW
    fftwf_execute(plan);

    // Post-process: pick the peak past min_length_A by PROMINENCE above a local background.
    // The projected histogram has a broad low-frequency ENVELOPE (spots cluster near the
    // origin) whose magnitude can exceed the true lattice peaks; a plain argmax|spec| then
    // returns a short envelope vector (~10A) on weak/pink-beam frames and the real axes are
    // lost. Subtracting a running-mean background of half-width BG_HALF bins removes that
    // smooth envelope (it cancels to ~0) while sharp lattice peaks - fundamentals AND
    // harmonics - keep their height. The prominence is also reported as the magnitude so
    // FilterFFTResults ranks directions by real-peak strength, not by envelope.
    const double len_coeff = 2.0 * static_cast<double>(max_length_A) / static_cast<double>(H);
    // Background half-window ~15 A (in length, so it is independent of the histogram sizing);
    // wide enough to span the envelope yet narrow enough not to smooth real peaks. Validated
    // optimum on serial-still jet data (de-novo indexing improved markedly vs FFBIDX); a second dataset unchanged.
    constexpr double BG_HALF_WIDTH_A = 15.0;
    const int bg_half = std::max(1, static_cast<int>(std::lround(BG_HALF_WIDTH_A / len_coeff)));

    std::vector<double> mag(out_len), pref(out_len + 1);

    for (int d = 0; d < D; ++d) {
        const auto* spec = h_output_fft.data() + static_cast<size_t>(d) * out_len;

        pref[0] = 0.0;
        for (int j = 0; j < out_len; ++j) {
            mag[j] = std::hypot(spec[j][0], spec[j][1]);
            pref[j + 1] = pref[j] + mag[j];
        }

        double best_prom = 0.0;
        double best_len = -1.0;

        for (int j = 0; j < out_len; ++j) {
            double len = len_coeff * static_cast<double>(j);
            if (len <= static_cast<double>(min_length_A)) continue;

            // Slide the background window inward at the ends rather than truncating it. A peak within
            // bg_half bins of either end - which is where the LONGEST cells sit, the last usable bin
            // being max_length_A itself - otherwise gets its background from a one-sided window, and
            // the prominence it is judged on is biased by however much the spectrum slopes there.
            int lo = j - bg_half, hi = j + bg_half + 1;
            if (lo < 0) { hi = std::min(out_len, hi - lo); lo = 0; }
            if (hi > out_len) { lo = std::max(0, lo - (hi - out_len)); hi = out_len; }
            const double background = (pref[hi] - pref[lo]) / static_cast<double>(hi - lo);
            const double prominence = mag[j] - background;

            if (prominence > best_prom) {
                best_prom = prominence;
                best_len = len;
            }
        }

        result_fft[d] = FFTResult{
                .magnitude = static_cast<float>(best_prom),
                .direction = d,
                .length    = static_cast<float>(best_len)
        };
    }
}