max_element.hpp

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/*
Copyright (c) 2015, Scott Zuyderduyn
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//----------------------------------------------------------------------------
// algo/max_element.hpp
//
// Extension of std::max_element that recognizes device memory and can be
// called from host or device code.
//
// Author: Scott D. Zuyderduyn, Ph.D. (scott.zuyderduyn@utoronto.ca)
//----------------------------------------------------------------------------
#pragma once
#ifndef ECUDA_ALGO_MAX_ELEMENT_HPP
#define ECUDA_ALGO_MAX_ELEMENT_HPP

#include <algorithm>
#include <vector>

#include "../global.hpp"
#include "../iterator.hpp"

namespace ecuda {

// forward declaration
template<class ForwardIterator> __HOST__ __DEVICE__ inline ForwardIterator max_element( ForwardIterator first, ForwardIterator last );

namespace impl {

template<class ForwardIterator>
__HOST__ __DEVICE__ inline ForwardIterator max_element( ForwardIterator first, ForwardIterator last, ecuda::true_type ) {
    // is an iterator to device memory
    #ifdef __CUDA_ARCH__
    if( first == last ) return last;
    ForwardIterator largest = first;
    ++first;
    for( ; first != last; ++first ) {
        if( *largest < *first ) largest = first;
    }
    return largest;
    #else
    const bool isIteratorContiguous = ecuda::is_same<typename ecuda::iterator_traits<ForwardIterator>::is_contiguous,ecuda::true_type>::value;
    ECUDA_STATIC_ASSERT(isIteratorContiguous,CANNOT_FIND_MAX_ELEMENT_IN_RANGE_REPRESENTED_BY_NONCONTIGUOUS_DEVICE_MEMORY);
    typename ecuda::iterator_traits<ForwardIterator>::difference_type n = ecuda::distance( first, last );
    std::vector<typename ecuda::iterator_traits<ForwardIterator>::value_type> hostVector( n );
    ecuda::copy( first, last, hostVector.begin() );
    const std::size_t index = std::max_element( hostVector.begin(), hostVector.end() ) - hostVector.begin();
    first += index;
    return first;
    #endif
}


template<class ForwardIterator>
__HOST__ __DEVICE__ inline ForwardIterator max_element( ForwardIterator first, ForwardIterator last, ecuda::false_type ) {
    // not an iterator to device memory, delegate to STL
    #ifdef __CUDA_ARCH__
    return last; // can never be called from device code, dummy return to satisfy nvcc
    #else
    return std::max_element( first, last );
    #endif
}

} // namespace impl

template<class ForwardIterator>
__HOST__ __DEVICE__ inline ForwardIterator max_element( ForwardIterator first, ForwardIterator last ) {
    impl::max_element(
        first, last,
        typename ecuda::iterator_traits<ForwardIterator>::is_device_iterator()
    );
}

} // namespace ecuda

#endif