The Tor Browser / file revision

 /* -*- 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 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 nsHashKeys.h.

  */

 #ifndef mozilla_HashFunctions_h

 #define mozilla_HashFunctions_h

 #include "mozilla/Assertions.h"

 #include "mozilla/Attributes.h"

 #include "mozilla/Char16.h"

 #include "mozilla/Types.h"

 #include <stdint.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.

    */

   static_assert(sizeof(a) == sizeof(uintptr_t),

                 "Strange pointer!");

   return detail::AddUintptrToHash<sizeof(uintptr_t)>(hash, uintptr_t(a));

 }

 template<>

 MOZ_WARN_UNUSED_RESULT

 inline uint32_t

 AddToHash(uint32_t hash, uintptr_t a)

 {

   return detail::AddUintptrToHash<sizeof(uintptr_t)>(hash, 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);

 }

 #ifdef MOZ_CHAR16_IS_NOT_WCHAR

 MOZ_WARN_UNUSED_RESULT

 inline uint32_t

 HashString(const char16_t* str)

 {

   return detail::HashUntilZero(str);

 }

 MOZ_WARN_UNUSED_RESULT

 inline uint32_t

 HashString(const char16_t* str, size_t length)

 {

   return detail::HashKnownLength(str, length);

 }

 #endif

 /*

  * On Windows, wchar_t (char16_t) 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 */