The Tor Browser: mfbt/HashFunctions.h@b8a032363ba2 (annotated)

mfbt/HashFunctions.h@b8a032363ba2 (annotated)

mfbt/HashFunctions.h

Thu, 22 Jan 2015 13:21:57 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Thu, 22 Jan 2015 13:21:57 +0100
branch: TOR_BUG_9701
changeset 15: b8a032363ba2
permissions: -rw-r--r--

Incorporate requested changes from Mozilla in review:
https://bugzilla.mozilla.org/show_bug.cgi?id=1123480#c6

 /* -*- 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 */

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mfbt/HashFunctions.h@b8a032363ba2 (annotated)

mfbt/HashFunctions.h