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305 lines
8.2 KiB
C++
305 lines
8.2 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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/* Utilities for hashing. */
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/*
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* This file exports functions for hashing data down to a 32-bit value,
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* including:
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*
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* - HashString Hash a char* or char16_t/wchar_t* of known or unknown
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* length.
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*
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* - HashBytes Hash a byte array of known length.
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*
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* - HashGeneric Hash one or more values. Currently, we support uint32_t,
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* types which can be implicitly cast to uint32_t, data
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* pointers, and function pointers.
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*
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* - AddToHash Add one or more values to the given hash. This supports the
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* same list of types as HashGeneric.
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*
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*
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* You can chain these functions together to hash complex objects. For example:
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*
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* class ComplexObject
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* {
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* char* mStr;
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* uint32_t mUint1, mUint2;
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* void (*mCallbackFn)();
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*
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* public:
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* uint32_t hash()
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* {
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* uint32_t hash = HashString(mStr);
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* hash = AddToHash(hash, mUint1, mUint2);
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* return AddToHash(hash, mCallbackFn);
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* }
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* };
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*
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* If you want to hash an nsAString or nsACString, use the HashString functions
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* in nsHashKeys.h.
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*/
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#ifndef mozilla_HashFunctions_h
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#define mozilla_HashFunctions_h
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#include "mozilla/Assertions.h"
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#include "mozilla/Attributes.h"
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#include "mozilla/Char16.h"
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#include "mozilla/Types.h"
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#include <stdint.h>
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#ifdef __cplusplus
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namespace mozilla {
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/**
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* The golden ratio as a 32-bit fixed-point value.
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*/
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static const uint32_t kGoldenRatioU32 = 0x9E3779B9U;
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inline uint32_t
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RotateBitsLeft32(uint32_t aValue, uint8_t aBits)
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{
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MOZ_ASSERT(aBits < 32);
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return (aValue << aBits) | (aValue >> (32 - aBits));
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}
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namespace detail {
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inline uint32_t
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AddU32ToHash(uint32_t aHash, uint32_t aValue)
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{
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/*
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* This is the meat of all our hash routines. This hash function is not
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* particularly sophisticated, but it seems to work well for our mostly
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* plain-text inputs. Implementation notes follow.
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*
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* Our use of the golden ratio here is arbitrary; we could pick almost any
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* number which:
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*
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* * is odd (because otherwise, all our hash values will be even)
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*
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* * has a reasonably-even mix of 1's and 0's (consider the extreme case
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* where we multiply by 0x3 or 0xeffffff -- this will not produce good
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* mixing across all bits of the hash).
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*
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* The rotation length of 5 is also arbitrary, although an odd number is again
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* preferable so our hash explores the whole universe of possible rotations.
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*
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* Finally, we multiply by the golden ratio *after* xor'ing, not before.
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* Otherwise, if |aHash| is 0 (as it often is for the beginning of a
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* message), the expression
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*
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* (kGoldenRatioU32 * RotateBitsLeft(aHash, 5)) |xor| aValue
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*
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* evaluates to |aValue|.
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*
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* (Number-theoretic aside: Because any odd number |m| is relatively prime to
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* our modulus (2^32), the list
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*
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* [x * m (mod 2^32) for 0 <= x < 2^32]
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*
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* has no duplicate elements. This means that multiplying by |m| does not
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* cause us to skip any possible hash values.
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*
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* It's also nice if |m| has large-ish order mod 2^32 -- that is, if the
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* smallest k such that m^k == 1 (mod 2^32) is large -- so we can safely
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* multiply our hash value by |m| a few times without negating the
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* multiplicative effect. Our golden ratio constant has order 2^29, which is
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* more than enough for our purposes.)
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*/
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return kGoldenRatioU32 * (RotateBitsLeft32(aHash, 5) ^ aValue);
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}
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/**
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* AddUintptrToHash takes sizeof(uintptr_t) as a template parameter.
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*/
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template<size_t PtrSize>
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inline uint32_t
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AddUintptrToHash(uint32_t aHash, uintptr_t aValue);
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template<>
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inline uint32_t
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AddUintptrToHash<4>(uint32_t aHash, uintptr_t aValue)
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{
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return AddU32ToHash(aHash, static_cast<uint32_t>(aValue));
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}
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template<>
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inline uint32_t
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AddUintptrToHash<8>(uint32_t aHash, uintptr_t aValue)
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{
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/*
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* The static cast to uint64_t below is necessary because this function
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* sometimes gets compiled on 32-bit platforms (yes, even though it's a
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* template and we never call this particular override in a 32-bit build). If
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* we do aValue >> 32 on a 32-bit machine, we're shifting a 32-bit uintptr_t
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* right 32 bits, and the compiler throws an error.
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*/
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uint32_t v1 = static_cast<uint32_t>(aValue);
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uint32_t v2 = static_cast<uint32_t>(static_cast<uint64_t>(aValue) >> 32);
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return AddU32ToHash(AddU32ToHash(aHash, v1), v2);
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}
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} /* namespace detail */
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/**
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* AddToHash takes a hash and some values and returns a new hash based on the
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* inputs.
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*
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* Currently, we support hashing uint32_t's, values which we can implicitly
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* convert to uint32_t, data pointers, and function pointers.
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*/
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template<typename A>
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MOZ_MUST_USE inline uint32_t
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AddToHash(uint32_t aHash, A aA)
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{
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/*
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* Try to convert |A| to uint32_t implicitly. If this works, great. If not,
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* we'll error out.
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*/
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return detail::AddU32ToHash(aHash, aA);
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}
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template<typename A>
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MOZ_MUST_USE inline uint32_t
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AddToHash(uint32_t aHash, A* aA)
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{
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/*
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* You might think this function should just take a void*. But then we'd only
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* catch data pointers and couldn't handle function pointers.
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*/
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static_assert(sizeof(aA) == sizeof(uintptr_t), "Strange pointer!");
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return detail::AddUintptrToHash<sizeof(uintptr_t)>(aHash, uintptr_t(aA));
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}
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template<>
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MOZ_MUST_USE inline uint32_t
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AddToHash(uint32_t aHash, uintptr_t aA)
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{
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return detail::AddUintptrToHash<sizeof(uintptr_t)>(aHash, aA);
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}
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template<typename A, typename... Args>
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MOZ_MUST_USE uint32_t
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AddToHash(uint32_t aHash, A aArg, Args... aArgs)
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{
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return AddToHash(AddToHash(aHash, aArg), aArgs...);
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}
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/**
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* The HashGeneric class of functions let you hash one or more values.
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*
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* If you want to hash together two values x and y, calling HashGeneric(x, y) is
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* much better than calling AddToHash(x, y), because AddToHash(x, y) assumes
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* that x has already been hashed.
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*/
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template<typename... Args>
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MOZ_MUST_USE inline uint32_t
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HashGeneric(Args... aArgs)
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{
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return AddToHash(0, aArgs...);
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}
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namespace detail {
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template<typename T>
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uint32_t
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HashUntilZero(const T* aStr)
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{
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uint32_t hash = 0;
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for (T c; (c = *aStr); aStr++) {
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hash = AddToHash(hash, c);
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}
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return hash;
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}
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template<typename T>
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uint32_t
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HashKnownLength(const T* aStr, size_t aLength)
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{
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uint32_t hash = 0;
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for (size_t i = 0; i < aLength; i++) {
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hash = AddToHash(hash, aStr[i]);
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}
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return hash;
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}
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} /* namespace detail */
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/**
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* The HashString overloads below do just what you'd expect.
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*
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* If you have the string's length, you might as well call the overload which
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* includes the length. It may be marginally faster.
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*/
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MOZ_MUST_USE inline uint32_t
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HashString(const char* aStr)
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{
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return detail::HashUntilZero(reinterpret_cast<const unsigned char*>(aStr));
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}
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MOZ_MUST_USE inline uint32_t
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HashString(const char* aStr, size_t aLength)
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{
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return detail::HashKnownLength(reinterpret_cast<const unsigned char*>(aStr), aLength);
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}
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MOZ_MUST_USE
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inline uint32_t
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HashString(const unsigned char* aStr, size_t aLength)
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{
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return detail::HashKnownLength(aStr, aLength);
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}
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MOZ_MUST_USE inline uint32_t
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HashString(const char16_t* aStr)
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{
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return detail::HashUntilZero(aStr);
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}
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MOZ_MUST_USE inline uint32_t
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HashString(const char16_t* aStr, size_t aLength)
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{
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return detail::HashKnownLength(aStr, aLength);
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}
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/*
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* On Windows, wchar_t is not the same as char16_t, even though it's
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* the same width!
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*/
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#ifdef WIN32
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MOZ_MUST_USE inline uint32_t
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HashString(const wchar_t* aStr)
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{
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return detail::HashUntilZero(aStr);
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}
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MOZ_MUST_USE inline uint32_t
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HashString(const wchar_t* aStr, size_t aLength)
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{
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return detail::HashKnownLength(aStr, aLength);
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}
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#endif
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/**
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* Hash some number of bytes.
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*
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* This hash walks word-by-word, rather than byte-by-byte, so you won't get the
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* same result out of HashBytes as you would out of HashString.
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*/
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MOZ_MUST_USE extern MFBT_API uint32_t
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HashBytes(const void* bytes, size_t aLength);
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} /* namespace mozilla */
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#endif /* __cplusplus */
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#endif /* mozilla_HashFunctions_h */
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