gecko/mfbt/HashFunctions.h

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/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* 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/. */
/* Utilities for hashing. */
/*
* This file exports functions for hashing data down to a 32-bit value,
* including:
*
* - HashString Hash a char* or uint16_t/wchar_t* of known or unknown
* length.
*
* - HashBytes Hash a byte array of known length.
*
* - HashGeneric Hash one or more values. Currently, we support uint32_t,
* types which can be implicitly cast to uint32_t, data
* pointers, and function pointers.
*
* - AddToHash Add one or more values to the given hash. This supports the
* same list of types as HashGeneric.
*
*
* You can chain these functions together to hash complex objects. For example:
*
* class ComplexObject
* {
* char* str;
* uint32_t uint1, uint2;
* void (*callbackFn)();
*
* public:
* uint32_t hash() {
* uint32_t hash = HashString(str);
* hash = AddToHash(hash, uint1, uint2);
* return AddToHash(hash, callbackFn);
* }
* };
*
* If you want to hash an nsAString or nsACString, use the HashString functions
* in nsHashKey.h.
*/
#ifndef mozilla_HashFunctions_h_
#define mozilla_HashFunctions_h_
#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/StandardInteger.h"
#ifdef __cplusplus
namespace mozilla {
/**
* The golden ratio as a 32-bit fixed-point value.
*/
static const uint32_t GoldenRatioU32 = 0x9E3779B9U;
inline uint32_t
RotateBitsLeft32(uint32_t value, uint8_t bits)
{
MOZ_ASSERT(bits < 32);
return (value << bits) | (value >> (32 - bits));
}
namespace detail {
inline uint32_t
AddU32ToHash(uint32_t hash, uint32_t value)
{
/*
* This is the meat of all our hash routines. This hash function is not
* particularly sophisticated, but it seems to work well for our mostly
* plain-text inputs. Implementation notes follow.
*
* Our use of the golden ratio here is arbitrary; we could pick almost any
* number which:
*
* * is odd (because otherwise, all our hash values will be even)
*
* * has a reasonably-even mix of 1's and 0's (consider the extreme case
* where we multiply by 0x3 or 0xeffffff -- this will not produce good
* mixing across all bits of the hash).
*
* The rotation length of 5 is also arbitrary, although an odd number is again
* preferable so our hash explores the whole universe of possible rotations.
*
* Finally, we multiply by the golden ratio *after* xor'ing, not before.
* Otherwise, if |hash| is 0 (as it often is for the beginning of a message),
* the expression
*
* (GoldenRatioU32 * RotateBitsLeft(hash, 5)) |xor| value
*
* evaluates to |value|.
*
* (Number-theoretic aside: Because any odd number |m| is relatively prime to
* our modulus (2^32), the list
*
* [x * m (mod 2^32) for 0 <= x < 2^32]
*
* has no duplicate elements. This means that multiplying by |m| does not
* cause us to skip any possible hash values.
*
* It's also nice if |m| has large-ish order mod 2^32 -- that is, if the
* smallest k such that m^k == 1 (mod 2^32) is large -- so we can safely
* multiply our hash value by |m| a few times without negating the
* multiplicative effect. Our golden ratio constant has order 2^29, which is
* more than enough for our purposes.)
*/
return GoldenRatioU32 * (RotateBitsLeft32(hash, 5) ^ value);
}
/**
* AddUintptrToHash takes sizeof(uintptr_t) as a template parameter.
*/
template<size_t PtrSize>
inline uint32_t
AddUintptrToHash(uint32_t hash, uintptr_t value);
template<>
inline uint32_t
AddUintptrToHash<4>(uint32_t hash, uintptr_t value)
{
return AddU32ToHash(hash, static_cast<uint32_t>(value));
}
template<>
inline uint32_t
AddUintptrToHash<8>(uint32_t hash, uintptr_t value)
{
/*
* The static cast to uint64_t below is necessary because this function
* sometimes gets compiled on 32-bit platforms (yes, even though it's a
* template and we never call this particular override in a 32-bit build). If
* we do value >> 32 on a 32-bit machine, we're shifting a 32-bit uintptr_t
* right 32 bits, and the compiler throws an error.
*/
uint32_t v1 = static_cast<uint32_t>(value);
uint32_t v2 = static_cast<uint32_t>(static_cast<uint64_t>(value) >> 32);
return AddU32ToHash(AddU32ToHash(hash, v1), v2);
}
} /* namespace detail */
/**
* AddToHash takes a hash and some values and returns a new hash based on the
* inputs.
*
* Currently, we support hashing uint32_t's, values which we can implicitly
* convert to uint32_t, data pointers, and function pointers.
*/
template<typename A>
MOZ_WARN_UNUSED_RESULT
inline uint32_t
AddToHash(uint32_t hash, A a)
{
/*
* Try to convert |A| to uint32_t implicitly. If this works, great. If not,
* we'll error out.
*/
return detail::AddU32ToHash(hash, a);
}
template<typename A>
MOZ_WARN_UNUSED_RESULT
inline uint32_t
AddToHash(uint32_t hash, A* a)
{
/*
* You might think this function should just take a void*. But then we'd only
* catch data pointers and couldn't handle function pointers.
*/
MOZ_STATIC_ASSERT(sizeof(a) == sizeof(uintptr_t),
"Strange pointer!");
return detail::AddUintptrToHash<sizeof(uintptr_t)>(hash, uintptr_t(a));
}
template<typename A, typename B>
MOZ_WARN_UNUSED_RESULT
uint32_t
AddToHash(uint32_t hash, A a, B b)
{
return AddToHash(AddToHash(hash, a), b);
}
template<typename A, typename B, typename C>
MOZ_WARN_UNUSED_RESULT
uint32_t
AddToHash(uint32_t hash, A a, B b, C c)
{
return AddToHash(AddToHash(hash, a, b), c);
}
template<typename A, typename B, typename C, typename D>
MOZ_WARN_UNUSED_RESULT
uint32_t
AddToHash(uint32_t hash, A a, B b, C c, D d)
{
return AddToHash(AddToHash(hash, a, b, c), d);
}
template<typename A, typename B, typename C, typename D, typename E>
MOZ_WARN_UNUSED_RESULT
uint32_t
AddToHash(uint32_t hash, A a, B b, C c, D d, E e)
{
return AddToHash(AddToHash(hash, a, b, c, d), e);
}
/**
* The HashGeneric class of functions let you hash one or more values.
*
* If you want to hash together two values x and y, calling HashGeneric(x, y) is
* much better than calling AddToHash(x, y), because AddToHash(x, y) assumes
* that x has already been hashed.
*/
template<typename A>
MOZ_WARN_UNUSED_RESULT
inline uint32_t
HashGeneric(A a)
{
return AddToHash(0, a);
}
template<typename A, typename B>
MOZ_WARN_UNUSED_RESULT
inline uint32_t
HashGeneric(A a, B b)
{
return AddToHash(0, a, b);
}
template<typename A, typename B, typename C>
MOZ_WARN_UNUSED_RESULT
inline uint32_t
HashGeneric(A a, B b, C c)
{
return AddToHash(0, a, b, c);
}
template<typename A, typename B, typename C, typename D>
MOZ_WARN_UNUSED_RESULT
inline uint32_t
HashGeneric(A a, B b, C c, D d)
{
return AddToHash(0, a, b, c, d);
}
template<typename A, typename B, typename C, typename D, typename E>
MOZ_WARN_UNUSED_RESULT
inline uint32_t
HashGeneric(A a, B b, C c, D d, E e)
{
return AddToHash(0, a, b, c, d, e);
}
namespace detail {
template<typename T>
uint32_t
HashUntilZero(const T* str)
{
uint32_t hash = 0;
for (T c; (c = *str); str++)
hash = AddToHash(hash, c);
return hash;
}
template<typename T>
uint32_t
HashKnownLength(const T* str, size_t length)
{
uint32_t hash = 0;
for (size_t i = 0; i < length; i++)
hash = AddToHash(hash, str[i]);
return hash;
}
} /* namespace detail */
/**
* The HashString overloads below do just what you'd expect.
*
* If you have the string's length, you might as well call the overload which
* includes the length. It may be marginally faster.
*/
MOZ_WARN_UNUSED_RESULT
inline uint32_t
HashString(const char* str)
{
return detail::HashUntilZero(str);
}
MOZ_WARN_UNUSED_RESULT
inline uint32_t
HashString(const char* str, size_t length)
{
return detail::HashKnownLength(str, length);
}
MOZ_WARN_UNUSED_RESULT
inline uint32_t
HashString(const uint16_t* str)
{
return detail::HashUntilZero(str);
}
MOZ_WARN_UNUSED_RESULT
inline uint32_t
HashString(const uint16_t* str, size_t length)
{
return detail::HashKnownLength(str, length);
}
/*
* On Windows, wchar_t (PRUnichar) is not the same as uint16_t, even though it's
* the same width!
*/
#ifdef WIN32
MOZ_WARN_UNUSED_RESULT
inline uint32_t
HashString(const wchar_t* str)
{
return detail::HashUntilZero(str);
}
MOZ_WARN_UNUSED_RESULT
inline uint32_t
HashString(const wchar_t* str, size_t length)
{
return detail::HashKnownLength(str, length);
}
#endif
/**
* Hash some number of bytes.
*
* This hash walks word-by-word, rather than byte-by-byte, so you won't get the
* same result out of HashBytes as you would out of HashString.
*/
MOZ_WARN_UNUSED_RESULT
extern MFBT_API(uint32_t)
HashBytes(const void* bytes, size_t length);
} /* namespace mozilla */
#endif /* __cplusplus */
#endif /* mozilla_HashFunctions_h_ */