env/opencv-4.8.0/samples/cpp/tutorial_code/gpu/gpu-thrust-interop/Thrust_interop.hpp

#pragma once
#include <opencv2/core/cuda.hpp>

#include <thrust/iterator/permutation_iterator.h>
#include <thrust/iterator/transform_iterator.h>
#include <thrust/iterator/counting_iterator.h>
#include <thrust/device_ptr.h>

/*
    @Brief step_functor is an object to correctly step a thrust iterator according to the stride of a matrix
*/
//! [step_functor]
template<typename T> struct step_functor : public thrust::unary_function<int, int>
{
    int columns;
    int step;
    int channels;
    __host__ __device__ step_functor(int columns_, int step_, int channels_ = 1) : columns(columns_), step(step_), channels(channels_)	{	};
    __host__ step_functor(cv::cuda::GpuMat& mat)
    {
        CV_Assert(mat.depth() == cv::DataType<T>::depth);
        columns = mat.cols;
        step = mat.step / sizeof(T);
        channels = mat.channels();
    }
    __host__ __device__
        int operator()(int x) const
    {
        int row = x / columns;
        int idx = (row * step) + (x % columns)*channels;
        return idx;
    }
};
//! [step_functor]
//! [begin_itr]
/*
    @Brief GpuMatBeginItr returns a thrust compatible iterator to the beginning of a GPU mat's memory.
    @Param mat is the input matrix
    @Param channel is the channel of the matrix that the iterator is accessing.  If set to -1, the iterator will access every element in sequential order
*/
template<typename T>
thrust::permutation_iterator<thrust::device_ptr<T>, thrust::transform_iterator<step_functor<T>, thrust::counting_iterator<int>>>  GpuMatBeginItr(cv::cuda::GpuMat mat, int channel = 0)
{
    if (channel == -1)
    {
        mat = mat.reshape(1);
        channel = 0;
    }
    CV_Assert(mat.depth() == cv::DataType<T>::depth);
    CV_Assert(channel < mat.channels());
    return thrust::make_permutation_iterator(thrust::device_pointer_cast(mat.ptr<T>(0) + channel),
        thrust::make_transform_iterator(thrust::make_counting_iterator(0), step_functor<T>(mat.cols, mat.step / sizeof(T), mat.channels())));
}
//! [begin_itr]
//! [end_itr]
/*
@Brief GpuMatEndItr returns a thrust compatible iterator to the end of a GPU mat's memory.
@Param mat is the input matrix
@Param channel is the channel of the matrix that the iterator is accessing.  If set to -1, the iterator will access every element in sequential order
*/
template<typename T>
thrust::permutation_iterator<thrust::device_ptr<T>, thrust::transform_iterator<step_functor<T>, thrust::counting_iterator<int>>>  GpuMatEndItr(cv::cuda::GpuMat mat, int channel = 0)
{
    if (channel == -1)
    {
        mat = mat.reshape(1);
        channel = 0;
    }
    CV_Assert(mat.depth() == cv::DataType<T>::depth);
    CV_Assert(channel < mat.channels());
    return thrust::make_permutation_iterator(thrust::device_pointer_cast(mat.ptr<T>(0) + channel),
        thrust::make_transform_iterator(thrust::make_counting_iterator(mat.rows*mat.cols), step_functor<T>(mat.cols, mat.step / sizeof(T), mat.channels())));
}
//! [end_itr]