hash.hh

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// -*- tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*-
// vi: set et ts=4 sw=2 sts=2:
// SPDX-FileCopyrightInfo: Copyright (C) DUNE Project contributors, see file LICENSE.md in module root
// SPDX-License-Identifier: LicenseRef-GPL-2.0-only-with-DUNE-exception
#ifndef DUNE_COMMON_HASH_HH
#define DUNE_COMMON_HASH_HH
 
#include <functional>
 
#include <dune/common/typetraits.hh>
 
// ********************************************************************************
// Doxygen documentation
// ********************************************************************************
 
#ifdef DOXYGEN
 
namespace Dune {
 
 
  template<typename T>
  struct hash
  {
 
    std::size_t operator()(const T& t) const
    {
      return hash(t);
    }
 
  };
 
}
 
 
#define DUNE_DEFINE_HASH(template_args,type)
 
 
 
#define DUNE_HASH_TEMPLATE_ARGS(...)
 
 
#define DUNE_HASH_TYPE(...)
 
#else // DOXYGEN - hide all the ugly implementation
 
 
 
// ********************************************************************************
// C++11 support
// ********************************************************************************
 
// import std::hash into Dune namespace
namespace Dune {
 
  using std::hash;
 
}
 
// Macro for defining a std::hash specialization for type.
// This should not be called directly. Call DUNE_DEFINE_HASH
// instead.
#define DUNE_DEFINE_STD_HASH(template_args,type)     \
  namespace std {                                    \
                                                     \
    template<template_args>                          \
    struct hash<type>                                \
    {                                                \
                                                     \
      typedef type argument_type;                    \
      typedef std::size_t result_type;               \
                                                     \
      std::size_t operator()(const type& arg) const  \
      {                                              \
        return hash_value(arg);                      \
      }                                              \
    };                                               \
                                                     \
    template<template_args>                          \
    struct hash<const type>                          \
    {                                                \
                                                     \
      typedef type argument_type;                    \
      typedef std::size_t result_type;               \
                                                     \
      std::size_t operator()(const type& arg) const  \
      {                                              \
        return hash_value(arg);                      \
      }                                              \
    };                                               \
                                                     \
  }                                                  \
 
// Wrapper macro for template arguments.
// This is required because the template arguments can contain commas,
// which will create a macro argument list of unknown length. That in itself
// would not be a problem, but DUNE_DEFINE_HASH has to be called with two argument
// lists of unknown length. So this macro wraps its arguments with parentheses,
// turning it into a single argument. The result is used as the parameter list of
// an expansion macro in the calls to the implementation-specific macros
// for C++11 and TR1. Noto that technically, this trick is only legal for C++11,
// but pretty much every compiler supports variadic macros in C++03 mode, as they
// are part of C99.
#define DUNE_HASH_TEMPLATE_ARGS(...) (__VA_ARGS__)
 
// Wrapper macro for type to be hashed.
// See above for rationale.
#define DUNE_HASH_TYPE(...) (__VA_ARGS__)
 
// Expansion macro for the parenthesized argument lists created by
// DUNE_HASH_TEMPLATE_ARGS and DUNE_HASH_TYPE.
#define DUNE_HASH_EXPAND_VA_ARGS(...) __VA_ARGS__
 
// Define specializations for all discovered hash implementations.
#define DUNE_DEFINE_HASH(template_args,type)                                                  \
  DUNE_DEFINE_STD_HASH(DUNE_HASH_EXPAND_VA_ARGS template_args, DUNE_HASH_EXPAND_VA_ARGS type) \
 
 
#endif // DOXYGEN
 
 
 
// ********************************************************************************
// Some utility functions for combining hashes of member variables.
// ********************************************************************************
 
namespace Dune {
 
  // The following functions are an implementation of the proposed hash extensions for
  // the C++ standard by Peter Dimov
  // (cf. http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2005/n1756.pdf, issue 6.18).
  // They are also contained in the boost::functional::hash library by Daniel James, but
  // that implementation uses boost::hash internally, while we want to use Dune::hash. They
  // are also considered for inclusion in TR2 (then based on std::hash, of course).
 
#ifndef DOXYGEN
 
  // helper struct for providing different hash combining algorithms dependent on
  // the size of size_t.
  // hash_combiner has to be specialized for the size (in bytes) of std::size_t.
  // Specialized versions should provide a method
  //
  // template <typename typeof_size_t, typename T>
  // void operator()(typeof_size_t& seed, const T& arg) const;
  //
  // that will be called by the interface function hash_combine() described further below.
  // The redundant template parameter typeof_size_t is needed to avoid warnings for the
  // unused 64-bit specialization on 32-bit systems.
  //
  // There is no default implementation!
  template<int sizeof_size_t>
  struct hash_combiner;
 
 
  // hash combining for 64-bit platforms.
  template<>
  struct hash_combiner<8>
  {
 
    template<typename typeof_size_t, typename T>
    void operator()(typeof_size_t& seed, const T& arg) const
    {
      static_assert(sizeof(typeof_size_t)==8, "hash_combiner::operator() instantiated with nonmatching type and size");
 
      // The following algorithm for combining two 64-bit hash values is inspired by a similar
      // function in CityHash (http://cityhash.googlecode.com/svn-history/r2/trunk/src/city.h),
      // which is in turn based on ideas from the MurmurHash library. The basic idea is easy to
      // grasp, though: New information is XORed into the existing hash multiple times at different
      // places (using shift operations), and the resulting pattern is spread over the complete
      // range of available bits via multiplication with a "magic" constant. The constants used
      // below (47 and  0x9ddfea08eb382d69ULL) are taken from the CityHash implementation.
      //
      // We opted not to use the mixing algorithm proposed in the C++ working group defect list at
      // http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2005/n1756.pdf, p. 57f. because it
      // has very bad hash distribution properties if you apply it to lists of very small numbers,
      // an application that is frequent in PDELab's ordering framework.
 
      Dune::hash<T> hasher;
      const typeof_size_t kMul = 0x9ddfea08eb382d69ULL;
      typeof_size_t h = hasher(arg);
      typeof_size_t a = (seed ^ h) * kMul;
      a ^= (a >> 47);
      typeof_size_t b = (h ^ a) * kMul;
      b ^= (b >> 47);
      b *= kMul;
      seed = b;
    }
 
  };
 
 
  // hash combining for 32-bit platforms.
  template<>
  struct hash_combiner<4>
  {
 
    template<typename typeof_size_t, typename T>
    void operator()(typeof_size_t& seed, const T& arg) const
    {
      static_assert(sizeof(typeof_size_t)==4, "hash_combiner::operator() instantiated with nonmatching type and size");
 
      // The default algorithm above requires a 64-bit std::size_t. The following algorithm is a
      // 32-bit compatible fallback, again inspired by CityHash and MurmurHash
      // (http://cityhash.googlecode.com/svn-history/r2/trunk/src/city.cc).
      // It uses 32-bit constants and relies on rotation instead of multiplication to spread the
      // mixed bits as that is apparently more efficient on IA-32. The constants used below are again
      // taken from CityHash, in particular from the file referenced above.
 
      Dune::hash<T> hasher;
      const typeof_size_t c1 = 0xcc9e2d51;
      const typeof_size_t c2 = 0x1b873593;
      const typeof_size_t c3 = 0xe6546b64;
      typeof_size_t h = hasher(arg);
      typeof_size_t a = seed * c1;
      a = (a >> 17) | (a << (32 - 17));
      a *= c2;
      h ^= a;
      h = (h >> 19) | (h << (32 - 19));
      seed = h * 5 + c3;
    }
 
  };
 
#endif // DOXYGEN
 
 
  template<typename T>
  inline void hash_combine(std::size_t& seed, const T& arg)
  {
    hash_combiner<sizeof(std::size_t)>()(seed,arg);
  }
 
 
  template<typename It>
  inline std::size_t hash_range(It first, It last)
  {
    std::size_t seed = 0;
    for (; first != last; ++first)
    {
      hash_combine(seed,*first);
    }
    return seed;
  }
 
 
  template<typename It>
  inline void hash_range(std::size_t& seed, It first, It last)
  {
    for (; first != last; ++first)
    {
      hash_combine(seed,*first);
    }
  }
 
} // end namespace Dune
 
#endif // DUNE_COMMON_HASH_HH