JugReco/JugReco/Index.hpp

0001 // This file is part of the Acts project.
0002 //
0003 // Copyright (C) 2019-2020 CERN for the benefit of the Acts project
0004 //
0005 // This Source Code Form is subject to the terms of the Mozilla Public
0006 // License, v. 2.0. If a copy of the MPL was not distributed with this
0007 // file, You can obtain one at http://mozilla.org/MPL/2.0/.
0008
0009 #pragma once
0010
0011 #include <cstdint>
0012
0013 #include <boost/container/flat_map.hpp>
0014
0015 namespace Jug {
0016
0017 /// Index type to reference elements in a container.
0018 ///
0019 /// We do not expect to have more than 2^32 elements in any given container so a
0020 /// fixed sized integer type is sufficient.
0021 using Index = uint32_t;
0022
0023 /// Store elements that are identified by an index, e.g. in another container.
0024 ///
0025 /// Each index can have zero or more associated elements. A typical case could
0026 /// be to store all generating particles for a hit where the hit is identified
0027 /// by its index in the hit container.
0028 template <typename value_t>
0029 using IndexMultimap = boost::container::flat_multimap<Index, value_t>;
0030
0031 /// Invert the multimap, i.e. from a -> {b...} to b -> {a...}.
0032 ///
0033 /// @note This assumes that the value in the initial multimap is itself a
0034 ///   sortable index-like object, as would be the case when mapping e.g.
0035 ///   hit ids to particle ids/ barcodes.
0036 template <typename value_t>
0037 inline boost::container::flat_multimap<value_t, Index> invertIndexMultimap(
0038     const IndexMultimap<value_t>& multimap) {
0039   using InverseMultimap = boost::container::flat_multimap<value_t, Index>;
0040
0041   // switch key-value without enforcing the new ordering (linear copy)
0042   typename InverseMultimap::sequence_type unordered;
0043   unordered.reserve(multimap.size());
0044   for (auto&& [index, value] : multimap) {
0045     // value is now the key and the index is now the value
0046     unordered.emplace_back(value, index);
0047   }
0048
0049   // adopting the unordered sequence will reestablish the correct order
0050   InverseMultimap inverse;
0051   inverse.adopt_sequence(std::move(unordered));
0052   return inverse;
0053 }
0054
0055 }  // namespace ActsExamples