/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* 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 char16_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* mStr;
* uint32_t mUint1, mUint2;
* void (*mCallbackFn)();
*
* public:
* uint32_t hash()
* {
* uint32_t hash = HashString(mStr);
* hash = AddToHash(hash, mUint1, mUint2);
* return AddToHash(hash, mCallbackFn);
* }
* };
*
* If you want to hash an nsAString or nsACString, use the HashString functions
* in nsHashKeys.h.
*/
#ifndef mozilla_HashFunctions_h
#define mozilla_HashFunctions_h
#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/Char16.h"
#include "mozilla/MathAlgorithms.h"
#include "mozilla/Types.h"
#include "mozilla/WrappingOperations.h"
#include <stdint.h>
namespace mozilla {
/**
* The golden ratio as a 32-bit fixed-point value.
*/
static const uint32_t kGoldenRatioU32 = 0x9E3779B9U;
namespace detail {
inline uint32_t AddU32ToHash(uint32_t aHash, uint32_t aValue) {
/*
* 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 |aHash| is 0 (as it often is for the beginning of a
* message), the expression
*
* mozilla::WrappingMultiply(kGoldenRatioU32, RotateBitsLeft(aHash, 5))
* |xor|
* aValue
*
* evaluates to |aValue|.
*
* (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 mozilla::WrappingMultiply(kGoldenRatioU32,
RotateLeft(aHash, 5) ^ aValue);
}
/**
* AddUintptrToHash takes sizeof(uintptr_t) as a template parameter.
*/
template <size_t PtrSize>
inline uint32_t AddUintptrToHash(uint32_t aHash, uintptr_t aValue) {
return AddU32ToHash(aHash, static_cast<uint32_t>(aValue));
}
template <>
inline uint32_t AddUintptrToHash<8>(uint32_t aHash, uintptr_t aValue) {
uint32_t v1 = static_cast<uint32_t>(aValue);
uint32_t v2 = static_cast<uint32_t>(static_cast<uint64_t>(aValue) >> 32);
return AddU32ToHash(AddU32ToHash(aHash, 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 T, bool TypeIsNotIntegral = !mozilla::IsIntegral<T>::value,
typename U = typename mozilla::EnableIf<TypeIsNotIntegral>::Type>
MOZ_MUST_USE inline uint32_t AddToHash(uint32_t aHash, T aA) {
/*
* Try to convert |A| to uint32_t implicitly. If this works, great. If not,
* we'll error out.
*/
return detail::AddU32ToHash(aHash, aA);
}
template <typename A>
MOZ_MUST_USE inline uint32_t AddToHash(uint32_t aHash, A* aA) {
/*
* You might think this function should just take a void*. But then we'd only
* catch data pointers and couldn't handle function pointers.
*/
static_assert(sizeof(aA) == sizeof(uintptr_t), "Strange pointer!");
return detail::AddUintptrToHash<sizeof(uintptr_t)>(aHash, uintptr_t(aA));
}
// We use AddUintptrToHash() for hashing all integral types. 8-byte integral
// types are treated the same as 64-bit pointers, and smaller integral types are
// first implicitly converted to 32 bits and then passed to AddUintptrToHash()
// to be hashed.
template <typename T, typename U = typename mozilla::EnableIf<
mozilla::IsIntegral<T>::value>::Type>
MOZ_MUST_USE inline uint32_t AddToHash(uint32_t aHash, T aA) {
return detail::AddUintptrToHash<sizeof(T)>(aHash, aA);
}
template <typename A, typename... Args>
MOZ_MUST_USE uint32_t AddToHash(uint32_t aHash, A aArg, Args... aArgs) {
return AddToHash(AddToHash(aHash, aArg), aArgs...);
}
/**
* 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... Args>
MOZ_MUST_USE inline uint32_t HashGeneric(Args... aArgs) {
return AddToHash(0, aArgs...);
}
namespace detail {
template <typename T>
uint32_t HashUntilZero(const T* aStr) {
uint32_t hash = 0;
for (T c; (c = *aStr); aStr++) {
hash = AddToHash(hash, c);
}
return hash;
}
template <typename T>
uint32_t HashKnownLength(const T* aStr, size_t aLength) {
uint32_t hash = 0;
for (size_t i = 0; i < aLength; i++) {
hash = AddToHash(hash, aStr[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_MUST_USE inline uint32_t HashString(const char* aStr) {
return detail::HashUntilZero(reinterpret_cast<const unsigned char*>(aStr));
}
MOZ_MUST_USE inline uint32_t HashString(const char* aStr, size_t aLength) {
return detail::HashKnownLength(reinterpret_cast<const unsigned char*>(aStr),
aLength);
}
MOZ_MUST_USE
inline uint32_t HashString(const unsigned char* aStr, size_t aLength) {
return detail::HashKnownLength(aStr, aLength);
}
MOZ_MUST_USE inline uint32_t HashString(const char16_t* aStr) {
return detail::HashUntilZero(aStr);
}
MOZ_MUST_USE inline uint32_t HashString(const char16_t* aStr, size_t aLength) {
return detail::HashKnownLength(aStr, aLength);
}
/*
* On Windows, wchar_t is not the same as char16_t, even though it's
* the same width!
*/
#ifdef WIN32
MOZ_MUST_USE inline uint32_t HashString(const wchar_t* aStr) {
return detail::HashUntilZero(aStr);
}
MOZ_MUST_USE inline uint32_t HashString(const wchar_t* aStr, size_t aLength) {
return detail::HashKnownLength(aStr, aLength);
}
#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_MUST_USE extern MFBT_API uint32_t HashBytes(const void* bytes,
size_t aLength);
/**
* A pseudorandom function mapping 32-bit integers to 32-bit integers.
*
* This is for when you're feeding private data (like pointer values or credit
* card numbers) to a non-crypto hash function (like HashBytes) and then using
* the hash code for something that untrusted parties could observe (like a JS
* Map). Plug in a HashCodeScrambler before that last step to avoid leaking the
* private data.
*
* By itself, this does not prevent hash-flooding DoS attacks, because an
* attacker can still generate many values with exactly equal hash codes by
* attacking the non-crypto hash function alone. Equal hash codes will, of
* course, still be equal however much you scramble them.
*
* The algorithm is SipHash-1-3. See <https://131002.net/siphash/>.
*/
class HashCodeScrambler {
struct SipHasher;
uint64_t mK0, mK1;
public:
/** Creates a new scrambler with the given 128-bit key. */
constexpr HashCodeScrambler(uint64_t aK0, uint64_t aK1)
: mK0(aK0), mK1(aK1) {}
/**
* Scramble a hash code. Always produces the same result for the same
* combination of key and hash code.
*/
uint32_t scramble(uint32_t aHashCode) const {
SipHasher hasher(mK0, mK1);
return uint32_t(hasher.sipHash(aHashCode));
}
private:
struct SipHasher {
SipHasher(uint64_t aK0, uint64_t aK1) {
// 1. Initialization.
mV0 = aK0 ^ UINT64_C(0x736f6d6570736575);
mV1 = aK1 ^ UINT64_C(0x646f72616e646f6d);
mV2 = aK0 ^ UINT64_C(0x6c7967656e657261);
mV3 = aK1 ^ UINT64_C(0x7465646279746573);
}
uint64_t sipHash(uint64_t aM) {
// 2. Compression.
mV3 ^= aM;
sipRound();
mV0 ^= aM;
// 3. Finalization.
mV2 ^= 0xff;
for (int i = 0; i < 3; i++) sipRound();
return mV0 ^ mV1 ^ mV2 ^ mV3;
}
MOZ_NO_SANITIZE_UNSIGNED_OVERFLOW
void sipRound() {
mV0 += mV1;
mV1 = RotateLeft(mV1, 13);
mV1 ^= mV0;
mV0 = RotateLeft(mV0, 32);
mV2 += mV3;
mV3 = RotateLeft(mV3, 16);
mV3 ^= mV2;
mV0 += mV3;
mV3 = RotateLeft(mV3, 21);
mV3 ^= mV0;
mV2 += mV1;
mV1 = RotateLeft(mV1, 17);
mV1 ^= mV2;
mV2 = RotateLeft(mV2, 32);
}
uint64_t mV0, mV1, mV2, mV3;
};
};
} /* namespace mozilla */
#endif /* mozilla_HashFunctions_h */