makeValueIndex.h

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#ifndef LARCOREALG_COREUTILS_MAKEVALUEINDEX_H
#define LARCOREALG_COREUTILS_MAKEVALUEINDEX_H

// LArSoft libraries
#include "larcorealg/CoreUtils/DebugUtils.h"
#include "larcorealg/CoreUtils/enumerate.h"
#include "larcorealg/CoreUtils/fromFutureImport.h" // util::pre_std::identity<>

// C/C++ standard library
#include <cstddef> // std::size_t
#include <map>
#include <stdexcept>   // std::runtime_error
#include <string>      // std::to_string()
#include <type_traits> // std::invoke_result_t, std::remove_reference_t

namespace util {

  template <typename Coll, typename Extractor>
  decltype(auto) makeValueIndex(Coll const& coll, Extractor getter);

  template <typename Coll>
  auto makeValueIndex(Coll const& coll)
  {
    return makeValueIndex(coll, util::pre_std::identity());
  }

} // namespace util

//------------------------------------------------------------------------------
//--- template implementation
//------------------------------------------------------------------------------
template <typename Coll, typename Extractor>
decltype(auto) util::makeValueIndex(Coll const& coll, Extractor getter)
{

  using Value_t = typename Coll::value_type;
  using Key_t
#if 0  // this is C++17...
    = std::remove_reference_t<std::invoke_result_t<Extractor, Value_t>>;
#else  // ... and this is what Clang 5.0 understands:
    = std::remove_reference_t<decltype(getter(std::declval<Value_t>()))>;
#endif // 0

    using Map_t = std::map<Key_t, std::size_t>;

  Map_t index;
  for (auto&& [iValue, collValue] : util::enumerate(coll)) {

    Key_t const& key = getter(collValue);
    auto const iKey = index.lower_bound(key);
    if ((iKey != index.end()) && (iKey->first == key)) {
      // no guarantee that `key` supports `std::to_string()`: print only indices
      throw std::runtime_error(std::string(__func__) + ": element #" + std::to_string(iValue) +
                               " has the same key as #" + std::to_string(iKey->second));
    }
    index.emplace_hint(iKey, key, iValue);
  } // for

  return index;

} // util::makeValueIndex()

//------------------------------------------------------------------------------

#endif // LARCOREALG_COREUTILS_MAKEVALUEINDEX_H