select_max_dot.h

#ifndef LINEARALGEBRA_SRC_MOLPRO_LINALG_ARRAY_UTIL_SELECT_MAX_DOT_H
#define LINEARALGEBRA_SRC_MOLPRO_LINALG_ARRAY_UTIL_SELECT_MAX_DOT_H
#include <cmath>
#include <cstdlib>
#include <queue>
 
namespace molpro::linalg::array::util {
 
template <class X, class Y, typename value_type, typename value_type_abs>
auto select_max_dot(size_t n, const X& x, const Y& y) {
  using std::abs;
  using std::begin;
  using std::end;
  using std::greater;
  using select_pair = std::pair<value_type_abs, size_t>; // value and index
  auto selection = std::priority_queue<select_pair, std::vector<select_pair>, greater<select_pair>>();
  auto ix = begin(x);
  auto iy = begin(y);
  for (size_t i = 0; i < n; ++i, ++ix, ++iy) {
    selection.emplace(abs((*ix) * (*iy)), i);
  }
  for (size_t i = n; i < std::min(x.size(), y.size()); ++i, ++ix, ++iy) {
    selection.emplace(abs((*ix) * (*iy)), i);
    selection.pop();
  }
  auto selection_map = std::map<size_t, value_type_abs>();
  auto m = selection.size();
  for (size_t i = 0; i < m; ++i) {
    selection_map.emplace(selection.top().second, selection.top().first);
    selection.pop();
  }
  return selection_map;
}
 
template <class X, class Y, typename value_type, typename value_type_abs>
auto select_max_dot_iter_sparse(size_t n, const X& x, const Y& y) {
  using std::abs;
  using std::begin;
  using std::end;
  using std::greater;
  using select_pair = std::pair<value_type_abs, size_t>; // value and index
  auto selection = std::priority_queue<select_pair, std::vector<select_pair>, greater<select_pair>>();
  for (auto iy = begin(y); iy != end(y); ++iy) {
    if (iy->first >= x.size())
      continue;
    selection.emplace(abs(x[iy->first] * iy->second), iy->first);
    if (selection.size() > n)
      selection.pop();
  }
  auto selection_map = std::map<size_t, value_type_abs>();
  auto m = selection.size();
  for (size_t i = 0; i < m; ++i) {
    selection_map.emplace(selection.top().second, selection.top().first);
    selection.pop();
  }
  return selection_map;
}
 
template <class X, class Y, typename value_type, typename value_type_abs>
auto select_max_dot_sparse(size_t n, const X& x, const Y& y) {
  using std::abs;
  using std::begin;
  using std::end;
  using std::greater;
  using select_pair = std::pair<value_type_abs, size_t>; // value and index
  auto selection = std::priority_queue<select_pair, std::vector<select_pair>, greater<select_pair>>();
  for (auto ix = begin(x); ix != end(x); ++ix) {
    auto iy = y.find(ix->first);
    if (iy == y.end())
      continue;
    selection.emplace(abs(ix->second * iy->second), ix->first);
    if (selection.size() > n)
      selection.pop();
  }
  auto selection_map = std::map<size_t, value_type_abs>();
  auto m = selection.size();
  for (size_t i = 0; i < m; ++i) {
    selection_map.emplace(selection.top().second, selection.top().first);
    selection.pop();
  }
  return selection_map;
}
} // namespace molpro::linalg::array::util
 
#endif // LINEARALGEBRA_SRC_MOLPRO_LINALG_ARRAY_UTIL_SELECT_MAX_DOT_H