//
// This file was written by André Offringa and is published
// under the GPL 3 license.
//

#ifndef WSTACKING_GRIDDER_H
#define WSTACKING_GRIDDER_H

#ifndef AVOID_CASACORE
#include <aocommon/multibanddata.h>
#endif

#include "gridmode.h"

#include <aocommon/image.h>

#include <cmath>
#include <cstring>
#include <complex>
#include <mutex>
#include <vector>
#include <stack>
#include <thread>

namespace wsclean {

/**
 * This class grids and/or samples visibilities to/from UV space.
 * It also executes the FFT(s) required and performs the w-term correction.
 * It does this with the 'w-stacking' method (see
 * [Offringa et al., 2014](http://arxiv.org/abs/1407.1943)).
 *
 * This class is quite generic in that it is agnostic to the underlying data
 format
 * of the visibilities, and it is written to be of high performance. It is used
 * by the @ref WSMSGridder class to perform inversion and prediction. There,
 large
 * imaging operations are split up in multiple 'passes' over the data if
 necessary,
 * and in each pass a few w-layers are gridded/sampled. However, simpler
 applications
 * might chose not to support this when very large images/large w-terms are not
 a
 * concern, in which case things become easier.
 *
 * This class can do both the calculation from visibilities to image, as well as
 going
 * from (model) image to visibilities. The first is referred to as "Inversion"
 in this
 * class, and the latter is called "Prediction".
 *
 * The general call sequence for inversion (/imaging) is as follows:
 *
 * - Construct an instance with @ref WStackingGridder()
 * - Set settings if necessary (grid mode, denormal phase centre, complex...)
 * - Call @ref PrepareWLayers();
 * - For each pass if multiple passes are necessary (or once otherwise) :
 *   - Call @ref StartInversionPass();
 *   - Add all samples with @ref AddDataSample();
 *   - Call @ref FinishInversionPass();
 * - Finally, call @ref FinalizeImage();
 * - Now, @ref RealImage() and optionally @ref ImaginaryImage() will return the
 *   image(s).
 *
 * Alternatively, @ref AddData() can be used instead of @ref AddDataSample, to
 grid
 * several samples that only differ in frequency. To use @ref AddData(), it is
 necessary to
 * call @ref PrepareBands() first.
 *
 * For prediction, the sequence is similar:
 *
 * - Construct an instance with @ref WStackingGridder()
 * - Set settings if necessary
 * - Call @ref PrepareWLayers();
   - Call @ref InitializePrediction();
 * - For each pass if multiple passes are necessary (or once otherwise) :
 *   - Call @ref StartPredictionPass();
 *   - Get the predicted visibilities with @ref SampleDataSample();
 *
 * Similar to Inversion, all values for a band can be predicted at once by
 calling
 * @ref SampleData() instead of @ref SampleDataSample() when the band has been
 * set beforehand with @ref PrepareBands().
 *
 * Prediction does not require any finalisation calls.
 *
 * @author André Offringa
 * @date 2013 (first version)
 * @sa [WSClean: an implementation of a fast, generic wide-field imager for
 radio astronomy](http://arxiv.org/abs/1407.1943)
 */
template <typename T>
class WStackingGridder {
  /**
   * @example wspredictionexample.cpp
   * This example demonstrates how the WStackingGridder can be used in a
   * minimalistic way to predict visibilities.
   */
 public:
  /**
   * Numeric type used for the uv grid and images.
   */
  typedef T num_t;

  /** Construct a new gridder with given settings.
   * @param width The width of the image in pixels
   * @param height The height of the image in pixels.
   * @param pixelSizeX The angular width of a pixel in radians.
   * @param pixelSizeY The angular height of a pixel in radians.
   * @param fftThreadCount The number of threads used for FFTs.
   *   See @ref NFFTThreads() for more info.
   * @param kernelSize The full width and height in pixels of the kernel. Should
   * be odd. Large kernels cause smaller aliasing artefacts but are more
   * expensive. The default of 7 is a reasonable compromise between
   * effectiveness and speed.
   * @param overSamplingFactor The number of different horizontal and vertical
   * kernels that are precalculated at different rational positions. Larger is
   * more accurate but requires more memory and becomes slower, probably mainly
   * due to cache misses.
   */
  WStackingGridder(size_t width, size_t height, double pixelSizeX,
                   double pixelSizeY, size_t fftThreadCount,
                   size_t kernelSize = 7, size_t overSamplingFactor = 1023);

  /** De-allocate imagebuffers and perform other clean-up. */
  ~WStackingGridder() noexcept;

  /** Initialize the inversion/prediction stage with the given number of
   * w-layers. The number of w-layers sets the accuracy of w-term correction and
   * can normally be calculated from the image dimensions and maximum w-term.
   *
   * After this call, @ref NPasses() can be called to acquire the number of
   * required passes.
   *
   * Note that for non-complex images (the default), @p minW and @p maxW can be
   * set to the minimum absolute w-value, which results
   * in finer w-gridding and thus better w-term correction.
   *
   * The @p minW and @p maxW values need to be
   * calculated before gridding, and this normally requires an extra iteration
   * through the w-values of the data before the data can be gridded.
   *
   * For imaging without w-term correction, the number of layers can be set to
   * one (note that one w-layer disables w-term correcting -- do not set it to
   * zero, as one uv-grid is always required). In that case, it is best to set
   * @p minW to
   * -@p maxW.
   *
   * In cases where nwlayer > 1, the w-value @c w of all values should satisfy
   * @p minW < abs(@c w) < @p maxW. When @p nWLayers == 1, this is not required.
   *
   * @param nWLayers Number of uv grids at different w-values, should be >= 1.
   * @param maxMem Allowed memory in bytes. The gridder will try to set the
   * number of passes such that this value is not exceeded. Note that this is
   * approximate.
   * @param minW The smallest w-value to be inverted/predicted.
   * @param maxW The largest w-value to be inverted/predicted.
   */
  void PrepareWLayers(size_t nWLayers, double maxMem, double minW, double maxW);

#ifndef AVOID_CASACORE
  /**
   * Initialize the inversion/prediction stage with a given band. This is
   * required for calling methods that take a dataDescId, specifically @ref
   * AddData() and
   * @ref SampleData(). This call can be avoided
   * by using the @ref AddDataSample() and @ref SampleDataSample methods
   * instead.
   * @param bandData The information about the bands. A MultiBandData links a
   * dataDescId to a set of contiguous frequencies. This corresponds with the
   * DATA_DESC_ID field in meaurement sets.
   */
  void PrepareBands(const aocommon::MultiBandData &bands) { _bands = bands; }
#endif  // AVOID_CASACORE

  /**
   * Calculate on which layer this w-value belongs. Only valid once
   * @ref PrepareWLayers() has been called.
   * @param wInLambda The w-value, in units of number of wavelengths.
   * @returns W-layer index.
   * @see @ref LayerToW()
   */
  size_t WToLayer(double wInLambda) const {
    if (_nWLayers == 1)
      return 0;
    else {
      if (_isComplex)
        return size_t(
            round((wInLambda + _maxW) * (_nWLayers - 1) / (_maxW + _maxW)));
      else
        return size_t(round((fabs(wInLambda) - _minW) * (_nWLayers - 1) /
                            (_maxW - _minW)));
    }
  }

  /**
   * Calculate the central w-value for the given w-layer index. This is the
   * inverse of @ref WToLayer().
   * @param layer W-layer index
   * @returns Central w-value for specified w-layer in units of number of
   * wavelengths.
   */
  double LayerToW(size_t layer) const {
    if (_nWLayers == 1)
      return 0.0;
    else {
      if (_isComplex)
        // In this case, replace minW with -maxW :
        return layer * (_maxW + _maxW) / (_nWLayers - 1) - _maxW;
      else
        return layer * (_maxW - _minW) / (_nWLayers - 1) + _minW;
    }
  }

  /**
   * Number of w-layers, as specified in @ref PrepareWLayers().
   * @returns Number of w-layers.
   */
  size_t NWLayers() const { return _nWLayers; }

  /**
   * Determine whether specified w-value should be gridded in this pass.
   * This method can only be called after calling @ref StartInversionPass()
   * or @ref StartPredictionPass().
   * @param wInLambda W-value in units of number of wavelengths.
   * @returns true if the w-value is gridded on one of the w-layers that are
   * processed in this pass.
   */
  bool IsInLayerRange(double wInLambda) const {
    size_t layer = WToLayer(wInLambda);
    return layer >= layerRangeStart(_curLayerRangeIndex) &&
           layer < layerRangeStart(_curLayerRangeIndex + 1);
  }

  /**
   * Determine whether any samples within the specified w-value range
   * should be gridded in this pass. This can for example be used to
   * optimize which samples to read from disc; when this function returns
   * false for a given timestep/baseline, none of the samples are required
   * in this pass.
   * This method can only be called after calling @ref StartInversionPass()
   * or @ref StartPredictionPass().
   * @param wStart W-value of range start in units of number of wavelengths.
   * @param wEnd W-value of range end in units of number of wavelengths.
   * @returns true if any of the w-values in the given range are processed in
   * this pass.
   */
  bool IsInLayerRange(double wStart, double wEnd) const {
    size_t rangeStart = layerRangeStart(_curLayerRangeIndex),
           rangeEnd = layerRangeStart(_curLayerRangeIndex + 1),
           l1 = WToLayer(wStart);
    if (l1 >= rangeStart && l1 < rangeEnd) return true;
    size_t l2 = WToLayer(wEnd);
    return (
        (l2 >= rangeStart && l2 < rangeEnd)  // l2 is within the range
        ||
        (l1 < rangeStart && l2 >= rangeEnd)  // l1 is before, l2 is after range
        ||
        (l2 < rangeStart && l1 >= rangeEnd)  // l2 is before, l1 is after range
    );
  }

  /**
   * Number of passes that are required when not all the w-layers fit in memory
   * at once. Valid once @ref PrepareWLayers() has been called.
   * @return Number of passes.
   */
  size_t NPasses() const { return _nPasses; }

#ifndef AVOID_CASACORE
  /**
   * Grid an array of data values for inversion. The data values should have the
   * same uvw-values in meters, i.e., they can only differ in frequency. The
   * dataDescId specifies the frequencies of the array of data. This method
   * requires that the channel frequencies have been specified beforehand, by
   * calling @ref PrepareBand().
   *
   * This function will internally call @ref AddDataSample() for each
   * visibility, hence this function is just for convenience, but is not faster
   * than individual calls to
   * @ref AddDataSample().
   *
   * @param data Array of samples for different channels. The size of this array
   * is given by the band referred to by dataDescId.
   */
  void AddData(const std::complex<float> *data, size_t data_desc_id,
               double u_in_m, double v_in_m, double w_in_m);
#endif

  /**
   * Grid a single visibility value for inversion. This method does not require
   * that the channel frequencies have been specified beforehand.
   * @param sample Visibility value.
   * @param uInLambda U value of UVW coordinate, in number of wavelengths.
   * @param vInLambda V value of UVW coordinate, in number of wavelengths.
   * @param wInLambda W value of UVW coordinate, in number of wavelengths.
   */
  void AddDataSample(std::complex<float> sample, double uInLambda,
                     double vInLambda, double wInLambda);

  /**
   * Initialize a new inversion gridding pass. @ref PrepareWLayers() should have
   * been called beforehand. Each call to @ref StartInversionPass() should be
   * followed by a call to
   * @ref FinishInversionPass().
   * @param passIndex The zero-indexed index of this pass, 0 <= @p passIndex <
   * @ref NPasses().
   */
  void StartInversionPass(size_t passIndex);

  /**
   * Finish an inversion gridding pass. This will perform the Fourier transforms
   * of the currently gridded w-layers, and add each gridded layer to the final
   * image including w-term corrections. Therefore, it can take time.
   * @sa @ref StartInversionPass().
   */
  void FinishInversionPass();

  /**
   * Finalize inversion once all passes are performed.
   * @param multiplication_factor Apply this factor to all pixels. This can be
   * used to normalize the image for the weighting scheme.
   */
  void FinalizeImage(double multiplication_factor);

  /**
   * Initialize gridder for prediction and specify image to predict for.
   * @ref PrepareWLayers() should be called before this method.
   * Note that this method needs to be called with the same image before
   * each pass, i.e., before each call to @ref StartPredictionPass().
   *
   * This method is used for prediction of non-complex (IsComplex()==false)
   * images. Use
   * @ref InitializePrediction(Image, Image) for complex prediction -- see @ref
   * SetIsComplex() for more info.
   *
   * @param image The model image that is to be predicted for.
   */
  void InitializePrediction(aocommon::Image image) {
    initializePrediction(std::move(image), _imageData);
  }

  /**
   * Complex alternative for @ref InitializePrediction(Image).
   *
   * This method should be used for complex images -- see @ref SetIsComplex()
   * for more info.
   * @param real Array of width * height giving the real (model) image values.
   * @param imaginary Array of width * height giving the imaginary (model) image
   * values.
   */
  void InitializePrediction(aocommon::Image real, aocommon::Image imaginary) {
    initializePrediction(std::move(real), _imageData);
    initializePrediction(std::move(imaginary), _imageDataImaginary);
  }

  /**
   * Start a new prediction pass. One of the @ref InitializePrediction() methods
   * should be called before each call to this method. This method will perform
   * the fast Fourier transforms necessary for this pass, so it can take time.
   * @param passIndex Zero-indexed index of prediction pass, 0 <= @p passIndex <
   * @ref NPasses().
   */
  void StartPredictionPass(size_t passIndex);

#ifndef AVOID_CASACORE
  /**
   * Predict the values for all channels in a band. This is a convenience
   * function that will call @ref SampleDataSample() for all data values in the
   * band.
   *
   * If a particular value can not be predicted during this pass (because its
   * w-value belongs to a w-layer of a different pass), its value will be set to
   * NaN. A call to @ref StartPredictionPass() should have been made before
   * calling this method.
   *
   * @param data Array of samples for different channels. The size of this array
   * is given by the band.
   */
  void SampleData(std::complex<float> *data, size_t data_desc_id, double u_in_m,
                  double v_in_m, double w_in_m);
#endif

  /**
   * Predict the value of a single visibility with double precision.
   * If the visibility can not be predicted,
   * because its w-value belongs to a w-layer that is not processed during this
   * pass, it will be given a value of NaN. A call to @ref StartPredictionPass()
   * should have been made before calling this method.
   * @param value Will be set to the predicted visibility value.
   * @param uInLambda U value of UVW coordinate, in number of wavelengths.
   * @param vInLambda V value of UVW coordinate, in number of wavelengths.
   * @param wInLambda W value of UVW coordinate, in number of wavelengths.
   */
  void SampleDataSample(std::complex<float> &value, double uInLambda,
                        double vInLambda, double wInLambda);

  /**
   * Get the image result of inversion. This is an array of size width x height,
   * and can be indexed with [x + y*width]. It is allowed to change this image,
   * e.g. set the horizon to zero before saving to fits. This call is only valid
   * once @ref FinalizeImage() has been called.
   *
   * If a complex image is produced, this image returns the real part. The
   * imaginary part can be acquired with @ref ImaginaryImage().
   */
  aocommon::ImageBase<num_t> RealImage() { return std::move(_imageData[0]); }

  /**
   * Returns the real image that is always converted to a 32-bit float. It is
   * independent of the templated type of the class. It is otherwise equal to
   * @ref RealImage().
   */
  aocommon::Image RealImageFloat();

  /**
   * Get the imaginary part of a complex image after inversion. Otherwise
   * similar to
   * @ref RealImage().
   */
  aocommon::ImageBase<num_t> ImaginaryImage() {
    return std::move(_imageDataImaginary[0]);
  }

  /**
   * Returns the imaginary image that is always converted to a 32-bit float. It
   * is independent of the templated type of the class. It is otherwise equal to
   * @ref ImaginaryImage().
   */
  aocommon::Image ImaginaryImageFloat();

  /**
   * Get the number of threads used when performing the FFTs. The w-layers are
   * divided over the threads such that each thread calculates approximately the
   * same number of w-layers. Therefore, optimally this value equals the number
   * of (free) CPU cores available. If the number of w-layers is smaller than
   * the number of threads, some threads will do nothing. Therefore, when not
   * performing any w-term correction with @ref NWLayers() == 1, this value has
   * no effect.
   *
   * When @ref NWLayers() == 1 , it is still possible to perform multi-threading
   * by setting up fftw to do so.
   *
   * @returns The number of threads used for the FFTs.
   */
  size_t NFFTThreads() const { return _nFFTThreads; }

  /**
   * Set the number of threads used to perform the FFTs.
   * @see @ref NFFTThreads() for an explanation.
   * @param nfftThreads Number of threads used.
   */
  void SetNFFTThreads(size_t nfftThreads) { _nFFTThreads = nfftThreads; }

  /**
   * Get the kernel function and its interpolation method used for gridding.
   * @returns The currently selected gridding mode.
   */
  GriddingKernelMode GetGridMode() const { return _gridMode; }

  /**
   * Set the kernel function and its interpolation method used for gridding.
   * @param mode The new gridding mode.
   */
  void SetGridMode(GriddingKernelMode mode) {
    if (mode != _gridMode) {
      _gridMode = mode;
      if (_gridMode != GriddingKernelMode::NearestNeighbour) makeKernels();
    }
  }

  /**
   * Whether the image produced by inversion or used by prediction is complex.
   * In particular, cross-polarized images like XY and YX have complex values,
   * because the UV grid is no longer Hermitian symmetric.
   * @returns Whether the gridder is setup for complex images.
   */
  bool IsComplex() const { return _isComplex; }

  /**
   * Setup the gridder for real or complex images. See @ref IsComplex().
   * @param isComplex Whether the gridder should produce / predict from
   * complex images.
   */
  void SetIsComplex(bool isComplex) { _isComplex = isComplex; }

  /**
   * Setup the gridder for images with a shifted phase centre. When dl or dm is
   * non-zero, the centre of the involved images is shifted by the given amount.
   * This allows inversion/prediction of images that are centred away from the
   * phase centre, while the projection of the image is kept the same. The
   * benefit of this can be that the phase centre can be set to zenith, thereby
   * minimizing w-values, while the actually imaged field is somewhere else,
   * thereby making it possible to handle non-zenith images with small image
   * sizes and small w-values. A more extended explanation is given in the
   * WSClean paper by [Offringa et al. (2014)](http://arxiv.org/abs/1407.1943).
   * @param dl shift of image centre in 'l' (East) direction
   * @param dm shift of image centre in 'm' (North) direction
   */
  void SetDenormalPhaseCentre(double dl, double dm) {
    _l_shift = dl;
    _m_shift = dm;
  }

  /**
   * Retrieve a gridded uv layer. This function can be called after
   * @ref StartInversionPass() was called, and before @ref FinishInversionPass()
   * is called.
   * @param layerIndex Layer index of the grid, with zero being the first
   * layer of the current pass.
   * @returns The layer, with the currently gridded samples on it.
   */
  const std::complex<num_t> *GetGriddedUVLayer(size_t layerIndex) const {
    return _layeredUVData[layerIndex].Data();
  }

  /**
   * Acquire the antialising gridding kernel. This is mostly a debugging/example
   * function.
   * @param gridMode Type of kernel
   * @param kernel Array of size @p oversampling * size
   * @param oversampling Oversampling factor of kernel
   * @param size UV-cell size of the kernel
   */
  static void GetKernel(GriddingKernelMode gridMode, double *kernel,
                        size_t oversampling, size_t size);

  /**
   * Get width of image as specified during construction. This is the full
   * width, before any trimming has been applied.
   * @returns Width in pixels of image being gridded.
   */
  size_t Width() const { return _width; }

  /**
   * Get height of image as specified during construction. This is the full
   * height, before any trimming has been applied.
   * @returns Height in pixels of image being gridded.
   */
  size_t Height() const { return _height; }

  /**
   * Get angular width of single pixel in radians. The full image has a width of
   * @ref Width() * PixelSizeX(), assuming small angle approximation.
   * @returns Width of a single pixel in radians.
   */
  double PixelSizeX() const { return _pixelSizeX; }

  /**
   * Get angular height of single pixel in radians. The full image has a height
   * of
   * @ref Height() * PixelSizeX(), assuming small angle approximation.
   * @returns Height of a single pixel in radians.
   */
  double PixelSizeY() const { return _pixelSizeY; }

 private:
  WStackingGridder(const WStackingGridder &) = delete;
  WStackingGridder<T> &operator=(const WStackingGridder &) = delete;

  size_t layerRangeStart(size_t layerRangeIndex) const {
    return (_nWLayers * layerRangeIndex) / _nPasses;
  }
  void makeFFTWThreadSafe();
  template <bool IsComplexImpl>
  void projectOnImageAndCorrect(const std::complex<num_t> *source, double w,
                                size_t threadIndex);
  template <bool IsComplexImpl>
  void copyImageToLayerAndInverseCorrect(std::complex<num_t> *dest, double w);
  void initializeSqrtLMLookupTable();
  void initializeSqrtLMLookupTableForSampling();
  void initializeLayeredUVData(size_t n);
  void fftToImageThreadFunction(std::mutex *mutex, std::stack<size_t> *tasks,
                                size_t threadIndex);
  void fftToUVThreadFunction(std::mutex *mutex, std::stack<size_t> *tasks);
  void finalizeImage(double multiplication_factor,
                     std::vector<aocommon::ImageBase<num_t>> &data_array);
  void initializePrediction(
      aocommon::Image image,
      std::vector<aocommon::ImageBase<num_t>> &data_array);

  void makeKernels();
  /**
   * Make the Kaiser Bessel windowed sinc functions.
   * Alpha is a parameter of the Kaiser Bessel window function. Values of alpha
   * correspond with the following functions:
   * -   0 ; the rectangular window
   * -   5 ; similar to the Hamming window
   * -   6 ; similar to the Hann window
   * - 8.6 ; similar to the Blackmann window
   * -  14 ; similar to prolate spheroidal
   */
  static void makeKaiserBesselKernel(std::vector<double> &kernel, double alpha,
                                     size_t overSamplingFactor,
                                     bool withSinc = true);

  static void makeRectangularKernel(std::vector<double> &kernel,
                                    size_t overSamplingFactor);

  static void makeGaussianKernel(std::vector<double> &kernel,
                                 size_t overSamplingFactor,
                                 bool withSinc = true);

  static void makeBlackmanNutallKernel(std::vector<double> &kernel,
                                       size_t overSamplingFactor,
                                       bool withSinc = true);

  static double bessel0(double x, double precision);

  template <bool Inverse>
  void correctImageForKernel(num_t *image) const;

  const size_t _width, _height;
  const double _pixelSizeX, _pixelSizeY;
  size_t _nWLayers, _nPasses, _curLayerRangeIndex;
  double _minW, _maxW, _l_shift, _m_shift;
  bool _isComplex, _imageConjugatePart;
#ifndef AVOID_CASACORE
  aocommon::MultiBandData _bands;
#endif

  GriddingKernelMode _gridMode;
  size_t _overSamplingFactor, _kernelSize;
  std::vector<double> _1dKernel;
  std::vector<std::vector<num_t>> _griddingKernels;

  std::vector<aocommon::ComplexImageBase<num_t>> _layeredUVData;
  std::vector<aocommon::ImageBase<num_t>> _imageData, _imageDataImaginary;
  std::vector<num_t> _sqrtLMLookupTable;
  size_t _nFFTThreads;
};

}  // namespace wsclean

#endif
