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+/* -*- 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 uint32_t (a.k.a.
+ * mozilla::HashNumber), 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:
+ * HashNumber hash()
+ * {
+ * HashNumber 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>
+#include <type_traits>
+
+namespace mozilla {
+
+using HashNumber = uint32_t;
+static const uint32_t kHashNumberBits = 32;
+
+/**
+ * The golden ratio as a 32-bit fixed-point value.
+ */
+static const HashNumber kGoldenRatioU32 = 0x9E3779B9U;
+
+/*
+ * Given a raw hash code, h, return a number that can be used to select a hash
+ * bucket.
+ *
+ * This function aims to produce as uniform an output distribution as possible,
+ * especially in the most significant (leftmost) bits, even though the input
+ * distribution may be highly nonrandom, given the constraints that this must
+ * be deterministic and quick to compute.
+ *
+ * Since the leftmost bits of the result are best, the hash bucket index is
+ * computed by doing ScrambleHashCode(h) / (2^32/N) or the equivalent
+ * right-shift, not ScrambleHashCode(h) % N or the equivalent bit-mask.
+ */
+constexpr HashNumber ScrambleHashCode(HashNumber h) {
+ /*
+ * Simply returning h would not cause any hash tables to produce wrong
+ * answers. But it can produce pathologically bad performance: The caller
+ * right-shifts the result, keeping only the highest bits. The high bits of
+ * hash codes are very often completely entropy-free. (So are the lowest
+ * bits.)
+ *
+ * So we use Fibonacci hashing, as described in Knuth, The Art of Computer
+ * Programming, 6.4. This mixes all the bits of the input hash code h.
+ *
+ * The value of goldenRatio is taken from the hex expansion of the golden
+ * ratio, which starts 1.9E3779B9.... This value is especially good if
+ * values with consecutive hash codes are stored in a hash table; see Knuth
+ * for details.
+ */
+ return mozilla::WrappingMultiply(h, kGoldenRatioU32);
+}
+
+namespace detail {
+
+MOZ_NO_SANITIZE_UNSIGNED_OVERFLOW
+constexpr HashNumber RotateLeft5(HashNumber aValue) {
+ return (aValue << 5) | (aValue >> 27);
+}
+
+constexpr HashNumber AddU32ToHash(HashNumber 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, RotateLeft5(aHash))
+ * |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,
+ RotateLeft5(aHash) ^ aValue);
+}
+
+/**
+ * AddUintptrToHash takes sizeof(uintptr_t) as a template parameter.
+ */
+template <size_t PtrSize>
+constexpr HashNumber AddUintptrToHash(HashNumber aHash, uintptr_t aValue) {
+ return AddU32ToHash(aHash, static_cast<uint32_t>(aValue));
+}
+
+template <>
+inline HashNumber AddUintptrToHash<8>(HashNumber 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 = !std::is_integral_v<T>,
+ bool TypeIsNotEnum = !std::is_enum_v<T>,
+ std::enable_if_t<TypeIsNotIntegral && TypeIsNotEnum, int> = 0>
+[[nodiscard]] inline HashNumber AddToHash(HashNumber 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>
+[[nodiscard]] inline HashNumber AddToHash(HashNumber 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, std::enable_if_t<std::is_integral_v<T>, int> = 0>
+[[nodiscard]] constexpr HashNumber AddToHash(HashNumber aHash, T aA) {
+ return detail::AddUintptrToHash<sizeof(T)>(aHash, aA);
+}
+
+template <typename T, std::enable_if_t<std::is_enum_v<T>, int> = 0>
+[[nodiscard]] constexpr HashNumber AddToHash(HashNumber aHash, T aA) {
+ // Hash using AddUintptrToHash with the underlying type of the enum type
+ using UnderlyingType = typename std::underlying_type<T>::type;
+ return detail::AddUintptrToHash<sizeof(UnderlyingType)>(
+ aHash, static_cast<UnderlyingType>(aA));
+}
+
+template <typename A, typename... Args>
+[[nodiscard]] HashNumber AddToHash(HashNumber 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>
+[[nodiscard]] inline HashNumber HashGeneric(Args... aArgs) {
+ return AddToHash(0, aArgs...);
+}
+
+/**
+ * Hash successive |*aIter| until |!*aIter|, i.e. til null-termination.
+ *
+ * This function is *not* named HashString like the non-template overloads
+ * below. Some users define HashString overloads and pass inexactly-matching
+ * values to them -- but an inexactly-matching value would match this overload
+ * instead! We follow the general rule and don't mix and match template and
+ * regular overloads to avoid this.
+ *
+ * If you have the string's length, call HashStringKnownLength: it may be
+ * marginally faster.
+ */
+template <typename Iterator>
+[[nodiscard]] constexpr HashNumber HashStringUntilZero(Iterator aIter) {
+ HashNumber hash = 0;
+ for (; auto c = *aIter; ++aIter) {
+ hash = AddToHash(hash, c);
+ }
+ return hash;
+}
+
+/**
+ * Hash successive |aIter[i]| up to |i == aLength|.
+ */
+template <typename Iterator>
+[[nodiscard]] constexpr HashNumber HashStringKnownLength(Iterator aIter,
+ size_t aLength) {
+ HashNumber hash = 0;
+ for (size_t i = 0; i < aLength; i++) {
+ hash = AddToHash(hash, aIter[i]);
+ }
+ return hash;
+}
+
+/**
+ * The HashString overloads below do just what you'd expect.
+ *
+ * These functions are non-template functions so that users can 1) overload them
+ * with their own types 2) in a way that allows implicit conversions to happen.
+ */
+[[nodiscard]] inline HashNumber HashString(const char* aStr) {
+ // Use the |const unsigned char*| version of the above so that all ordinary
+ // character data hashes identically.
+ return HashStringUntilZero(reinterpret_cast<const unsigned char*>(aStr));
+}
+
+[[nodiscard]] inline HashNumber HashString(const char* aStr, size_t aLength) {
+ // Delegate to the |const unsigned char*| version of the above to share
+ // template instantiations.
+ return HashStringKnownLength(reinterpret_cast<const unsigned char*>(aStr),
+ aLength);
+}
+
+[[nodiscard]] inline HashNumber HashString(const unsigned char* aStr,
+ size_t aLength) {
+ return HashStringKnownLength(aStr, aLength);
+}
+
+[[nodiscard]] constexpr HashNumber HashString(const char16_t* aStr) {
+ return HashStringUntilZero(aStr);
+}
+
+[[nodiscard]] inline HashNumber HashString(const char16_t* aStr,
+ size_t aLength) {
+ return HashStringKnownLength(aStr, aLength);
+}
+
+/**
+ * HashString overloads for |wchar_t| on platforms where it isn't |char16_t|.
+ */
+template <typename WCharT, typename = typename std::enable_if<
+ std::is_same<WCharT, wchar_t>::value &&
+ !std::is_same<wchar_t, char16_t>::value>::type>
+[[nodiscard]] inline HashNumber HashString(const WCharT* aStr) {
+ return HashStringUntilZero(aStr);
+}
+
+template <typename WCharT, typename = typename std::enable_if<
+ std::is_same<WCharT, wchar_t>::value &&
+ !std::is_same<wchar_t, char16_t>::value>::type>
+[[nodiscard]] inline HashNumber HashString(const WCharT* aStr, size_t aLength) {
+ return HashStringKnownLength(aStr, aLength);
+}
+
+/**
+ * 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.
+ */
+[[nodiscard]] extern MFBT_API HashNumber 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.
+ */
+ HashNumber scramble(HashNumber aHashCode) const {
+ SipHasher hasher(mK0, mK1);
+ return HashNumber(hasher.sipHash(aHashCode));
+ }
+
+ static constexpr size_t offsetOfMK0() {
+ return offsetof(HashCodeScrambler, mK0);
+ }
+
+ static constexpr size_t offsetOfMK1() {
+ return offsetof(HashCodeScrambler, mK1);
+ }
+
+ 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;
+ }
+
+ void sipRound() {
+ mV0 = WrappingAdd(mV0, mV1);
+ mV1 = RotateLeft(mV1, 13);
+ mV1 ^= mV0;
+ mV0 = RotateLeft(mV0, 32);
+ mV2 = WrappingAdd(mV2, mV3);
+ mV3 = RotateLeft(mV3, 16);
+ mV3 ^= mV2;
+ mV0 = WrappingAdd(mV0, mV3);
+ mV3 = RotateLeft(mV3, 21);
+ mV3 ^= mV0;
+ mV2 = WrappingAdd(mV2, mV1);
+ mV1 = RotateLeft(mV1, 17);
+ mV1 ^= mV2;
+ mV2 = RotateLeft(mV2, 32);
+ }
+
+ uint64_t mV0, mV1, mV2, mV3;
+ };
+};
+
+} /* namespace mozilla */
+
+#endif /* mozilla_HashFunctions_h */
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