src/google/protobuf/stubs/hash.h

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// Author: kenton@google.com (Kenton Varda)

#ifndef GOOGLE_PROTOBUF_STUBS_HASH_H__
#define GOOGLE_PROTOBUF_STUBS_HASH_H__

#include <cstring>
#include <string>
#include <unordered_map>
#include <unordered_set>

# define GOOGLE_PROTOBUF_HASH_NAMESPACE_DECLARATION_START \
  namespace google {                                      \
  namespace protobuf {
# define GOOGLE_PROTOBUF_HASH_NAMESPACE_DECLARATION_END }}

namespace google {
namespace protobuf {

template <typename Key>
struct hash : public std::hash<Key> {};

template <typename Key>
struct hash<const Key*> {
  inline size_t operator()(const Key* key) const {
    return reinterpret_cast<size_t>(key);
  }
};

// Unlike the old SGI version, the TR1 "hash" does not special-case char*.  So,
// we go ahead and provide our own implementation.
template <>
struct hash<const char*> {
  inline size_t operator()(const char* str) const {
    size_t result = 0;
    for (; *str != '\0'; str++) {
      result = 5 * result + static_cast<size_t>(*str);
    }
    return result;
  }
};

template<>
struct hash<bool> {
  size_t operator()(bool x) const {
    return static_cast<size_t>(x);
  }
};

template <>
struct hash<std::string> {
  inline size_t operator()(const std::string& key) const {
    return hash<const char*>()(key.c_str());
  }

  static const size_t bucket_size = 4;
  static const size_t min_buckets = 8;
  inline bool operator()(const std::string& a, const std::string& b) const {
    return a < b;
  }
};

template <typename First, typename Second>
struct hash<std::pair<First, Second> > {
  inline size_t operator()(const std::pair<First, Second>& key) const {
    size_t first_hash = hash<First>()(key.first);
    size_t second_hash = hash<Second>()(key.second);

    // FIXME(kenton):  What is the best way to compute this hash?  I have
    // no idea!  This seems a bit better than an XOR.
    return first_hash * ((1 << 16) - 1) + second_hash;
  }

  static const size_t bucket_size = 4;
  static const size_t min_buckets = 8;
  inline bool operator()(const std::pair<First, Second>& a,
                           const std::pair<First, Second>& b) const {
    return a < b;
  }
};

}  // namespace protobuf
}  // namespace google

#endif  // GOOGLE_PROTOBUF_STUBS_HASH_H__