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 // This file is part of the Acts project.
 //
 // Copyright (C) 2020 CERN for the benefit of the Acts project
 //
 // This Source Code Form is subject to the terms of the Mozilla Public
 // License, v. 2.0. If a copy of the MPL was not distributed with this
 // file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 #pragma once
 
 #include "Acts/Geometry/GeometryIdentifier.hpp"
 
 #include <algorithm>
 #include <cassert>
 #include <initializer_list>
 #include <iterator>
 #include <stdexcept>
 #include <utility>
 #include <vector>
 
 namespace Acts {
 
 /// Store values mapped into the geometry hierarchy.
 ///
 /// @tparam value_t stored value type
 ///
 /// The core functionality is to find an equivalent element, i.e. an
 /// identifier-value pair, for a given geometry identifier via
 ///
 ///     auto it = container.find(GeometryIdentifier(...));
 ///     if (it != container.end()) {
 ///         ...
 ///     }
 ///
 /// Trailing zero levels of stored geometry identifiers are used as
 /// broadcast values to refer to higher-level objects within the geometry, e.g.
 /// a geometry identifier with vanishing approach and sensitive index identifies
 /// a layer. An entry will all geometry identifier levels set to zero acts as
 /// the global default value.
 ///
 /// The container also supports range-based iteration over all stored elements
 ///
 ///     for (const auto& element : container) {
 ///         ...
 ///     }
 ///
 /// and index-based access to stored elements and associated geometry
 /// identifiers
 ///
 ///     GeometryIdentifier id3 = container.idAt(3);
 ///     const auto& element4 = container.valueAt(4);
 ///
 /// @note No guarantees are given for the element order when using range-based
 ///   or index-based access. Any apparent ordering must be considered an
 ///   implementation detail and might change.
 ///
 /// Adding elements is potentially expensive as the internal lookup structure
 /// must be updated. In addition, modifying an element in-place could change its
 /// identifier which would also break the lookup. Thus, the container can not be
 /// modified after construction to prevent misuse.
 template <typename value_t>
 class GeometryHierarchyMap {
  public:
   /// Combined geometry identifier and value element. Only used for input.
   using InputElement = typename std::pair<GeometryIdentifier, value_t>;
   using Iterator = typename std::vector<value_t>::const_iterator;
   using Size = typename std::vector<value_t>::size_type;
   using Value = value_t;
 
   /// Construct the container from the given elements.
   ///
   /// @param elements input elements (must be unique with respect to identifier)
   GeometryHierarchyMap(std::vector<InputElement> elements);
   /// Construct the container from an initializer list.
   ///
   /// @param elements input initializer list
   GeometryHierarchyMap(std::initializer_list<InputElement> elements);
 
   // defaulted constructors and assignment operators
   GeometryHierarchyMap() = default;
   GeometryHierarchyMap(const GeometryHierarchyMap&) = default;
   GeometryHierarchyMap(GeometryHierarchyMap&&) = default;
   ~GeometryHierarchyMap() = default;
   GeometryHierarchyMap& operator=(const GeometryHierarchyMap&) = default;
   GeometryHierarchyMap& operator=(GeometryHierarchyMap&&) = default;
 
   /// Return an iterator pointing to the beginning of the stored values.
   Iterator begin() const { return m_values.begin(); }
   /// Return an iterator pointing to the end of the stored values.
   Iterator end() const { return m_values.end(); }
   /// Check if any elements are stored.
   bool empty() const { return m_values.empty(); }
   /// Return the number of stored elements.
   Size size() const { return m_values.size(); }
 
   /// Access the geometry identifier for the i-th element with bounds check.
   ///
   /// @throws std::out_of_range for invalid indices
   GeometryIdentifier idAt(Size index) const { return m_ids.at(index); }
   /// Access the value of the i-th element in the container with bounds check.
   ///
   /// @throws std::out_of_range for invalid indices
   const Value& valueAt(Size index) const { return m_values.at(index); }
 
   /// Find the most specific value for a given geometry identifier.
   ///
   /// This can be either from the element matching exactly to the given geometry
   /// id, if it exists, or from the element for the next available higher level
   /// within the geometry hierarchy.
   ///
   /// @param id geometry identifier for which information is requested
   /// @retval iterator to an existing value
   /// @retval `.end()` iterator if no matching element exists
   Iterator find(GeometryIdentifier id) const;
 
  private:
   // NOTE this class assumes that it knows the ordering of the levels within
   //      the geometry id. if the geometry id changes, this code has to be
   //      adapted too. the asserts ensure that such a change is caught.
   static_assert(GeometryIdentifier().setVolume(1).value() <
                     GeometryIdentifier().setVolume(1).setBoundary(1).value(),
                 "Incompatible GeometryIdentifier hierarchy");
   static_assert(GeometryIdentifier().setBoundary(1).value() <
                     GeometryIdentifier().setBoundary(1).setLayer(1).value(),
                 "Incompatible GeometryIdentifier hierarchy");
   static_assert(GeometryIdentifier().setLayer(1).value() <
                     GeometryIdentifier().setLayer(1).setApproach(1).value(),
                 "Incompatible GeometryIdentifier hierarchy");
   static_assert(GeometryIdentifier().setApproach(1).value() <
                     GeometryIdentifier().setApproach(1).setSensitive(1).value(),
                 "Incompatible GeometryIdentifier hierarchy");
 
   using Identifier = GeometryIdentifier::Value;
 
   // encoded ids for all elements for faster lookup.
   std::vector<Identifier> m_ids;
   // validity bit masks for the ids: which parts to use for comparison
   std::vector<Identifier> m_masks;
   std::vector<Value> m_values;
 
   /// Construct a mask where all leading non-zero levels are set.
   static constexpr Identifier makeLeadingLevelsMask(GeometryIdentifier id) {
     // construct id from encoded value with all bits set
     auto allSet = GeometryIdentifier(~GeometryIdentifier::Value{0u});
     // manually iterate over identifier levels starting from the lowest
     if (id.sensitive() != 0u) {
       // all levels are valid; keep all bits set.
       return allSet.setExtra(0u).value();
     }
     if (id.approach() != 0u) {
       return allSet.setExtra(0u).setSensitive(0u).value();
     }
     if (id.layer() != 0u) {
       return allSet.setExtra(0u).setSensitive(0u).setApproach(0u).value();
     }
     if (id.boundary() != 0u) {
       return allSet.setExtra(0u)
           .setSensitive(0u)
           .setApproach(0u)
           .setLayer(0u)
           .value();
     }
     if (id.volume() != 0u) {
       return allSet.setExtra(0u)
           .setSensitive(0u)
           .setApproach(0u)
           .setLayer(0u)
           .setBoundary(0u)
           .value();
     }
     // no valid levels; all bits are zero.
     return Identifier{0u};
   }
   /// Construct a mask where only the highest level is set.
   static constexpr Identifier makeHighestLevelMask() {
     return makeLeadingLevelsMask(GeometryIdentifier(0u).setVolume(1u));
   }
   /// Compare the two identifiers only within the masked bits.
   static constexpr bool equalWithinMask(Identifier lhs, Identifier rhs,
                                         Identifier mask) {
     return (lhs & mask) == (rhs & mask);
   }
   /// Ensure identifier ordering and uniqueness.
   template <typename iterator_t>
   static void sortAndCheckDuplicates(iterator_t beg, iterator_t end);
 
   /// Fill the container from the input elements.
   ///
   /// This assumes that the elements are ordered and unique with respect to
   /// their identifiers.
   template <typename iterator_t>
   void fill(iterator_t beg, iterator_t end);
 };
 
 // implementations
 
 template <typename value_t>
 inline GeometryHierarchyMap<value_t>::GeometryHierarchyMap(
     std::vector<InputElement> elements) {
   sortAndCheckDuplicates(elements.begin(), elements.end());
   fill(elements.begin(), elements.end());
 }
 
 template <typename value_t>
 inline GeometryHierarchyMap<value_t>::GeometryHierarchyMap(
     std::initializer_list<InputElement> elements)
     : GeometryHierarchyMap(
           std::vector<InputElement>(elements.begin(), elements.end())) {}
 
 template <typename value_t>
 template <typename iterator_t>
 inline void GeometryHierarchyMap<value_t>::sortAndCheckDuplicates(
     iterator_t beg, iterator_t end) {
   // ensure elements are sorted by identifier
   std::sort(beg, end, [=](const auto& lhs, const auto& rhs) {
     return lhs.first < rhs.first;
   });
   // check that all elements have unique identifier
   auto dup = std::adjacent_find(beg, end, [](const auto& lhs, const auto& rhs) {
     return lhs.first == rhs.first;
   });
   if (dup != end) {
     throw std::invalid_argument("Input elements contain duplicates");
   }
 }
 
 template <typename value_t>
 template <typename iterator_t>
 inline void GeometryHierarchyMap<value_t>::fill(iterator_t beg,
                                                 iterator_t end) {
   const auto n = std::distance(beg, end);
   m_ids.clear();
   m_ids.reserve(n);
   m_masks.clear();
   m_masks.reserve(n);
   m_values.clear();
   m_values.reserve(n);
   for (; beg != end; ++beg) {
     m_ids.push_back(beg->first.value());
     m_masks.push_back(makeLeadingLevelsMask(beg->first.value()));
     m_values.push_back(std::move(beg->second));
   }
 }
 
 template <typename value_t>
 inline auto GeometryHierarchyMap<value_t>::find(GeometryIdentifier id) const
     -> Iterator {
   assert((m_ids.size() == m_values.size()) &&
          "Inconsistent container state: #ids != # values");
   assert((m_masks.size() == m_values.size()) &&
          "Inconsistent container state: #masks != #values");
 
   // we can not search for the element directly since the relevant one
   // might be stored at a higher level. ids for higher levels would always
   // be sorted before the requested id. searching for the first element
   // after the requested ensures that we include the full hierarchy.
   const auto it = std::upper_bound(m_ids.begin(), m_ids.end(), id.value());
   auto i = std::distance(m_ids.begin(), it);
 
   // now go up the hierarchy to find the first matching element.
   // example: the container stores four identifiers
   //
   //     2|x|x (volume-only)
   //     2|2|1 (volume, layer, and sensitive)
   //     2|3|x (volume and layer)
   //     2|3|4 (volume, layer, and sensitive)
   //
   // where | marks level boundaries. searching for either 2|3|4, 2|3|7, or
   // 2|4|x would first point to 2|3|4 and thus needs to go up the hierarchy.
   while (0 < i) {
     // index always starts after item of interest due to upper bound search
     --i;
 
     // if the input id does not even match at the highest hierarchy level
     // with the current comparison id, then have reached the end of this
     // hierarchy. having a special check for the highest level avoids an
     // unbounded search window all the way to the beginning of the container for
     // the global default entry.
     if (!equalWithinMask(id.value(), m_ids[i], makeHighestLevelMask())) {
       // check if a global default entry exists
       if (m_ids.front() == Identifier{0u}) {
         return begin();
       } else {
         return end();
       }
     }
 
     // since the search is going backwards in the sorted container, it
     // progresses from more specific to less specific elements. the first
     // match is automatically the appropriate one.
     if (equalWithinMask(id.value(), m_ids[i], m_masks[i])) {
       return std::next(begin(), i);
     }
   }
 
   // all options are exhausted and no matching element was found.
   return end();
 }
 
 }  // namespace Acts

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