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 // This file is part of the ACTS project.
 //
 // Copyright (C) 2016 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 https://mozilla.org/MPL/2.0/.
 
 #pragma once
 
 #include <array>
 #include <cassert>
 #include <climits>
 #include <ostream>
 #include <type_traits>
 #include <utility>
 
 namespace Acts {
 
 /// A set of (hierarchical) indices bitpacked into a single value.
 ///
 /// The underlying value is split into blocks of bits with variable size.
 /// Each block is a level within the index hierarchy and can be set and
 /// retrieved separately. The encoded MultiIndex can be ordered and compared
 /// for equality. The ordering follows the hierarchy, i.e. indices are
 /// first ordered by the highest level, then within the highest level by the
 /// second level and so on.
 template <typename T, std::size_t... BitsPerLevel>
 class MultiIndex {
  public:
   static_assert(std::is_integral_v<T> && std::is_unsigned_v<T>,
                 "The underlying storage type must be an unsigned integer");
   static_assert(0 < sizeof...(BitsPerLevel),
                 "At least one level must be defined");
   static_assert((sizeof(T) * CHAR_BIT) == (... + BitsPerLevel),
                 "The sum of bits per level must match the underlying storage");
 
   /// The type of their underlying storage value.
   using Value = T;
   enum : std::size_t {
     NumLevels = sizeof...(BitsPerLevel),
   };
 
   /// Construct a MultiIndex with all levels set to zero.
   static constexpr MultiIndex Zeros() { return MultiIndex(0u); }
   /// Construct a MultiIndex from values for multiple level.
   ///
   /// This functionality must be implemented as a static, named constructor
   /// to avoid confusion with other constructors. If it would be implemented
   /// as a regular constructor, constructing a MultiIndex from a single
   /// encoded value and encoding only the first level would have the same
   /// signature and could not be distinguished.
   template <typename... Us>
   static constexpr MultiIndex Encode(Us&&... us) {
     static_assert(sizeof...(Us) <= NumLevels,
                   "Can only encode as many levels as in the MultiIndex");
 
     MultiIndex index(0u);
     std::size_t lvl = 0;
     for (Value val : std::array<Value, sizeof...(Us)>{us...}) {
       index.set(lvl++, val);
     }
     return index;
   }
 
   /// Construct a MultiIndex from an already encoded value.
   constexpr MultiIndex(Value encoded) : m_value(encoded) {}
   /// Construct a default MultiIndex with undefined values for each level.
   MultiIndex() = default;
   MultiIndex(const MultiIndex&) = default;
   MultiIndex(MultiIndex&) = default;
   MultiIndex& operator=(const MultiIndex&) = default;
   MultiIndex& operator=(MultiIndex&&) noexcept = default;
   /// Allow setting the MultiIndex from an already encoded value.
   constexpr MultiIndex& operator=(Value encoded) {
     m_value = encoded;
     return *this;
   }
 
   /// Get the encoded value of all index levels.
   constexpr Value value() const { return m_value; }
   /// Get the value for the index level.
   constexpr Value level(std::size_t lvl) const {
     assert((lvl < NumLevels) && "Index level outside allowed range");
     return (m_value >> shift(lvl)) & mask(lvl);
   }
   /// Set the value of the index level.
   constexpr MultiIndex& set(std::size_t lvl, Value val) {
     assert((lvl < NumLevels) && "Index level outside allowed range");
     // mask of valid bits at the encoded positions for the index level
     Value shiftedMask = (mask(lvl) << shift(lvl));
     // value of the index level shifted to its encoded position
     Value shiftedValue = (val << shift(lvl));
     // combine existing values with the value for the index level
     m_value = (m_value & ~shiftedMask) | (shiftedValue & shiftedMask);
     return *this;
   }
 
   /// Create index with the selected level increased and levels below zeroed.
   constexpr MultiIndex makeNextSibling(std::size_t lvl) const {
     assert((lvl < NumLevels) && "Index level outside allowed range");
     // remove lower levels by shifting the upper levels to the left edge
     Value upper = (m_value >> shift(lvl));
     // increase to create sibling and shift back to zero lower levels again
     return ((upper + 1u) << shift(lvl));
   }
   /// Create index with every level below the selected level maximized.
   constexpr MultiIndex makeLastDescendant(std::size_t lvl) const {
     assert((lvl < NumLevels) && "Index level outside allowed range");
     // mask everything below the selected level
     Value maskLower = (Value{1u} << shift(lvl)) - 1u;
     // replace the masked lower levels w/ ones
     return (m_value & ~maskLower) | maskLower;
   }
 
   /// Get the number of bits for the associated level
   static constexpr std::size_t bits(std::size_t lvl) {
     assert((lvl < NumLevels) && "Index level outside allowed range");
     return s_bits[lvl];
   }
 
  private:
   // per-level mask and right-most bit position for shifting
   static constexpr std::array<std::size_t, NumLevels> s_bits{BitsPerLevel...};
   static constexpr std::size_t shift(std::size_t lvl) {
     std::size_t s = 0u;
     // sum up all bits below the requested level
     for (std::size_t i = (lvl + 1); i < s_bits.size(); ++i) {
       s += s_bits[i];
     }
     return s;
   }
   static constexpr Value mask(std::size_t lvl) {
     return (Value{1u} << s_bits[lvl]) - 1u;
   }
 
   Value m_value;
 
   friend constexpr bool operator<(MultiIndex lhs, MultiIndex rhs) {
     return lhs.m_value < rhs.m_value;
   }
 
   friend constexpr bool operator==(MultiIndex lhs, MultiIndex rhs) {
     return lhs.m_value == rhs.m_value;
   }
 
   friend inline std::ostream& operator<<(std::ostream& os, MultiIndex idx) {
     // one level is always defined
     os << idx.level(0u);
     for (std::size_t lvl = 1; lvl < NumLevels; ++lvl) {
       os << '|' << idx.level(lvl);
     }
     return os;
   }
 };
 
 }  // namespace Acts
 
 // specialize std::hash so MultiIndex can be used e.g. in an unordered_map
 namespace std {
 template <typename Storage, std::size_t... BitsPerLevel>
 struct hash<Acts::MultiIndex<Storage, BitsPerLevel...>> {
   auto operator()(
       Acts::MultiIndex<Storage, BitsPerLevel...> idx) const noexcept {
     return std::hash<Storage>()(idx.value());
   }
 };
 }  // namespace std

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