wtf/text/StringHash.h

   1 /*
   2  * Copyright (C) 2006, 2007, 2008 Apple Inc. All rights reserved
   3  * Copyright (C) Research In Motion Limited 2009. All rights reserved.
   4  *
   5  * This library is free software; you can redistribute it and/or
   6  * modify it under the terms of the GNU Library General Public
   7  * License as published by the Free Software Foundation; either
   8  * version 2 of the License, or (at your option) any later version.
   9  *
  10  * This library is distributed in the hope that it will be useful,
  11  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  12  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13  * Library General Public License for more details.
  14  *
  15  * You should have received a copy of the GNU Library General Public License
  16  * along with this library; see the file COPYING.LIB.  If not, write to
  17  * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  18  * Boston, MA 02110-1301, USA.
  19  *
  20  */
  21
  22 #ifndef StringHash_h
  23 #define StringHash_h
  24
  25 #include "AtomicString.h"
  26 #include "WTFString.h"
  27 #include <wtf/HashTraits.h>
  28 #include <wtf/StringHashFunctions.h>
  29 #include <wtf/unicode/Unicode.h>
  30
  31 // FIXME: This is a temporary layering violation while we move string code to WTF.
  32 // Landing the file moves in one patch, will follow on with patches to change the namespaces.
  33 namespace WebCore {
  34
  35     // The hash() functions on StringHash and CaseFoldingHash do not support
  36     // null strings. get(), contains(), and add() on HashMap<String,..., StringHash>
  37     // cause a null-pointer dereference when passed null strings.
  38
  39     // FIXME: We should really figure out a way to put the computeHash function that's
  40     // currently a member function of StringImpl into this file so we can be a little
  41     // closer to having all the nearly-identical hash functions in one place.
  42
  43     struct StringHash {
  44         static unsigned hash(StringImpl* key) { return key->hash(); }
  45         static bool equal(const StringImpl* a, const StringImpl* b)
  46         {
  47             if (a == b)
  48                 return true;
  49             if (!a || !b)
  50                 return false;
  51
  52             unsigned aLength = a->length();
  53             unsigned bLength = b->length();
  54             if (aLength != bLength)
  55                 return false;
  56
  57             // FIXME: perhaps we should have a more abstract macro that indicates when
  58             // going 4 bytes at a time is unsafe
  59 #if CPU(ARM) || CPU(SH4)
  60             const UChar* aChars = a->characters();
  61             const UChar* bChars = b->characters();
  62             for (unsigned i = 0; i != aLength; ++i) {
  63                 if (*aChars++ != *bChars++)
  64                     return false;
  65             }
  66             return true;
  67 #else
  68             /* Do it 4-bytes-at-a-time on architectures where it's safe */
  69             const uint32_t* aChars = reinterpret_cast<const uint32_t*>(a->characters());
  70             const uint32_t* bChars = reinterpret_cast<const uint32_t*>(b->characters());
  71
  72             unsigned halfLength = aLength >> 1;
  73             for (unsigned i = 0; i != halfLength; ++i)
  74                 if (*aChars++ != *bChars++)
  75                     return false;
  76
  77             if (aLength & 1 && *reinterpret_cast<const uint16_t*>(aChars) != *reinterpret_cast<const uint16_t*>(bChars))
  78                 return false;
  79
  80             return true;
  81 #endif
  82         }
  83
  84         static unsigned hash(const RefPtr<StringImpl>& key) { return key->hash(); }
  85         static bool equal(const RefPtr<StringImpl>& a, const RefPtr<StringImpl>& b)
  86         {
  87             return equal(a.get(), b.get());
  88         }
  89
  90         static unsigned hash(const String& key) { return key.impl()->hash(); }
  91         static bool equal(const String& a, const String& b)
  92         {
  93             return equal(a.impl(), b.impl());
  94         }
  95
  96         static const bool safeToCompareToEmptyOrDeleted = false;
  97     };
  98
  99     class CaseFoldingHash {
 100     public:
 101         // Paul Hsieh's SuperFastHash
 102         // http://www.azillionmonkeys.com/qed/hash.html
 103         static unsigned hash(const UChar* data, unsigned length)
 104         {
 105             unsigned l = length;
 106             const UChar* s = data;
 107             uint32_t hash = WTF::stringHashingStartValue;
 108             uint32_t tmp;
 109
 110             int rem = l & 1;
 111             l >>= 1;
 112
 113             // Main loop.
 114             for (; l > 0; l--) {
 115                 hash += WTF::Unicode::foldCase(s[0]);
 116                 tmp = (WTF::Unicode::foldCase(s[1]) << 11) ^ hash;
 117                 hash = (hash << 16) ^ tmp;
 118                 s += 2;
 119                 hash += hash >> 11;
 120             }
 121
 122             // Handle end case.
 123             if (rem) {
 124                 hash += WTF::Unicode::foldCase(s[0]);
 125                 hash ^= hash << 11;
 126                 hash += hash >> 17;
 127             }
 128
 129             // Force "avalanching" of final 127 bits.
 130             hash ^= hash << 3;
 131             hash += hash >> 5;
 132             hash ^= hash << 2;
 133             hash += hash >> 15;
 134             hash ^= hash << 10;
 135
 136             // This avoids ever returning a hash code of 0, since that is used to
 137             // signal "hash not computed yet", using a value that is likely to be
 138             // effectively the same as 0 when the low bits are masked.
 139             hash |= !hash << 31;
 140
 141             return hash;
 142         }
 143
 144         static unsigned hash(StringImpl* str)
 145         {
 146             return hash(str->characters(), str->length());
 147         }
 148
 149         static unsigned hash(const char* str, unsigned length)
 150         {
 151             // This hash is designed to work on 16-bit chunks at a time. But since the normal case
 152             // (above) is to hash UTF-16 characters, we just treat the 8-bit chars as if they
 153             // were 16-bit chunks, which will give matching results.
 154
 155             unsigned l = length;
 156             const char* s = str;
 157             uint32_t hash = WTF::stringHashingStartValue;
 158             uint32_t tmp;
 159
 160             int rem = l & 1;
 161             l >>= 1;
 162
 163             // Main loop
 164             for (; l > 0; l--) {
 165                 hash += WTF::Unicode::foldCase(s[0]);
 166                 tmp = (WTF::Unicode::foldCase(s[1]) << 11) ^ hash;
 167                 hash = (hash << 16) ^ tmp;
 168                 s += 2;
 169                 hash += hash >> 11;
 170             }
 171
 172             // Handle end case
 173             if (rem) {
 174                 hash += WTF::Unicode::foldCase(s[0]);
 175                 hash ^= hash << 11;
 176                 hash += hash >> 17;
 177             }
 178
 179             // Force "avalanching" of final 127 bits
 180             hash ^= hash << 3;
 181             hash += hash >> 5;
 182             hash ^= hash << 2;
 183             hash += hash >> 15;
 184             hash ^= hash << 10;
 185
 186             // this avoids ever returning a hash code of 0, since that is used to
 187             // signal "hash not computed yet", using a value that is likely to be
 188             // effectively the same as 0 when the low bits are masked
 189             hash |= !hash << 31;
 190
 191             return hash;
 192         }
 193
 194         static bool equal(const StringImpl* a, const StringImpl* b)
 195         {
 196             if (a == b)
 197                 return true;
 198             if (!a || !b)
 199                 return false;
 200             unsigned length = a->length();
 201             if (length != b->length())
 202                 return false;
 203             return WTF::Unicode::umemcasecmp(a->characters(), b->characters(), length) == 0;
 204         }
 205
 206         static unsigned hash(const RefPtr<StringImpl>& key)
 207         {
 208             return hash(key.get());
 209         }
 210
 211         static bool equal(const RefPtr<StringImpl>& a, const RefPtr<StringImpl>& b)
 212         {
 213             return equal(a.get(), b.get());
 214         }
 215
 216         static unsigned hash(const String& key)
 217         {
 218             return hash(key.impl());
 219         }
 220         static unsigned hash(const AtomicString& key)
 221         {
 222             return hash(key.impl());
 223         }
 224         static bool equal(const String& a, const String& b)
 225         {
 226             return equal(a.impl(), b.impl());
 227         }
 228         static bool equal(const AtomicString& a, const AtomicString& b)
 229         {
 230             return (a == b) || equal(a.impl(), b.impl());
 231         }
 232
 233         static const bool safeToCompareToEmptyOrDeleted = false;
 234     };
 235
 236     // This hash can be used in cases where the key is a hash of a string, but we don't
 237     // want to store the string. It's not really specific to string hashing, but all our
 238     // current uses of it are for strings.
 239     struct AlreadyHashed : IntHash<unsigned> {
 240         static unsigned hash(unsigned key) { return key; }
 241
 242         // To use a hash value as a key for a hash table, we need to eliminate the
 243         // "deleted" value, which is negative one. That could be done by changing
 244         // the string hash function to never generate negative one, but this works
 245         // and is still relatively efficient.
 246         static unsigned avoidDeletedValue(unsigned hash)
 247         {
 248             ASSERT(hash);
 249             unsigned newHash = hash | (!(hash + 1) << 31);
 250             ASSERT(newHash);
 251             ASSERT(newHash != 0xFFFFFFFF);
 252             return newHash;
 253         }
 254     };
 255
 256 }
 257
 258 namespace WTF {
 259
 260     template<> struct HashTraits<WebCore::String> : GenericHashTraits<WebCore::String> {
 261         static const bool emptyValueIsZero = true;
 262         static void constructDeletedValue(WebCore::String& slot) { new (&slot) WebCore::String(HashTableDeletedValue); }
 263         static bool isDeletedValue(const WebCore::String& slot) { return slot.isHashTableDeletedValue(); }
 264     };
 265
 266 }
 267
 268 #endif