Rewrite the hash functions to be more generic

[libs/core.git] / source / core / hash.h

diff --git a/source/core/hash.h b/source/core/hash.h
index 85ee5dd74c046a4cdbc3b793cfd07d22dca2132f..a7e8cb165722cf7aa38aac417d97c21d2abd5004 100644 (file)
--- a/source/core/hash.h
+++ b/source/core/hash.h
@@ -2,40 +2,105 @@
 #define MSP_CORE_HASH_H_
 
 #include <cstdint>
 #define MSP_CORE_HASH_H_
 
 #include <cstdint>

+#include <limits>

 #include <string>
 
 namespace Msp {
 
 #include <string>
 
 namespace Msp {
 

-/**
-Computes a 32-bit Fowler-Noll-Vo (FNV-1a) hash.  The number of bits can be
-limited to less than 32, in which case XOR-folding is used to reduce the hash
-size.
-*/
-std::uint32_t hash32(const void *, unsigned, unsigned = 32);
+template<unsigned N, unsigned L = 3+(N>8)+(N>16)+(N>32)+(N>64)>
+struct HashTraits;
+
+template<>
+struct HashTraits<64, 6>
+{
+       using HashType = uint64_t;
+       using ResultType = uint64_t;
+       static constexpr HashType prime = 1099511628211ULL;
+       static constexpr HashType offset = 14695981039346656037ULL;
+       static constexpr bool folded = false;
+};
+
+template<>
+struct HashTraits<32, 5>
+{
+       using HashType = uint32_t;
+       using ResultType = uint32_t;
+       static constexpr HashType prime = 16777619;
+       static constexpr HashType offset = 2166136261U;
+       static constexpr bool folded = false;
+};
+
+template<unsigned N>
+struct HashTraits<N, 6>
+{
+       using HashType = uint64_t;
+       using ResultType = uint64_t;
+       static constexpr bool folded = true;
+};
+
+template<unsigned N>
+struct HashTraits<N, 5>
+{
+       using HashType = uint32_t;
+       using ResultType = uint32_t;
+       static constexpr bool folded = true;
+};
+
+template<unsigned N>
+struct HashTraits<N, 4>
+{
+       using HashType = uint32_t;
+       using ResultType = uint16_t;
+       static constexpr bool folded = true;
+};
+
+template<unsigned N>
+struct HashTraits<N, 3>
+{
+       using HashType = uint32_t;
+       using ResultType = uint8_t;
+       static constexpr bool folded = true;
+};
+

 
 /**
 
 /**

-Convenience function to compute a 32-bit hash of a string.
+Manually folds a hash to a smaller size.

 */
 */

-inline unsigned hash32(const std::string &s, unsigned b = 32)
-{ return hash32(s.data(), s.size(), b); }
+template<unsigned N, typename T>
+typename std::enable_if<N!=std::numeric_limits<T>::digits, typename HashTraits<N>::ResultType>::type hash_fold(T hash)
+{ return (hash>>N ^ hash) & ((T(1)<<N)-1); }
+
+template<unsigned N, typename T>
+typename std::enable_if<N==std::numeric_limits<T>::digits, typename HashTraits<N>::ResultType>::type hash_fold(T hash)
+{ return hash; }

 
 /**
 
 /**

-Computes a 64-bit Fowler-Noll-Vo (FNV-1a) hash.  Note that even if bits is
-limited to 32 or less, this does not produce the same result as hash32.
+Computes an N-bit Fowler-Noll-Vo (FNV-1a) hash.  If N is not a native hash
+size, the next larger native hash is computed and XOR-folding is applied to
+the result.
+
+http://en.wikipedia.org/wiki/Fowler-Noll-Vo_hash_function
+http://www.isthe.com/chongo/tech/comp/fnv/index.html

 */
 */

-std::uint64_t hash64(const void *, unsigned, unsigned = 64);
+template<unsigned N>
+typename HashTraits<N>::ResultType hash(const void *data, std::size_t size)
+{
+       constexpr unsigned hash_bits = std::numeric_limits<typename HashTraits<N>::HashType>::digits;
+       constexpr typename HashTraits<N>::HashType prime = HashTraits<hash_bits>::prime;
+       typename HashTraits<N>::HashType result = HashTraits<hash_bits>::offset;
+       for(unsigned i=0; i<size; ++i)
+               result = (result^*(reinterpret_cast<const unsigned char *>(data)+i))*prime;
+       if(HashTraits<N>::folded)
+               result = hash_fold<N>(result);
+       return result;
+}

 
 /**
 
 /**

-Convenience function to compute a 64-bit hash of a string.
+Convenience function to compute an N-bit hash of a string.

 */
 */

-inline std::uint64_t hash64(const std::string &s, unsigned b = 64)
-{ return hash64(s.data(), s.size(), b); }
-
-inline std::uint32_t fold32(std::uint64_t hash)
-{ return hash^(hash>>32); }
-
-inline std::uint16_t fold16(std::uint64_t hash)
-{ return hash^(hash>>16)^(hash>>32)^(hash>>48); }
+template<unsigned N>
+typename HashTraits<N>::ResultType hash(const std::string &str)
+{ return hash<N>(str.data(), str.size()); }

 
 } // namespace Msp
 
 
 } // namespace Msp