third_party/WebKit/Source/wtf/text/StringHash.h

/*
 * Copyright (C) 2006, 2007, 2008, 2012, 2013 Apple Inc. All rights reserved
 * Copyright (C) Research In Motion Limited 2009. All rights reserved.
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Library General Public
 * License as published by the Free Software Foundation; either
 * version 2 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Library General Public License for more details.
 *
 * You should have received a copy of the GNU Library General Public License
 * along with this library; see the file COPYING.LIB.  If not, write to
 * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
 * Boston, MA 02110-1301, USA.
 *
 */

#ifndef StringHash_h
#define StringHash_h

#include "wtf/Allocator.h"
#include "wtf/HashTraits.h"
#include "wtf/StringHasher.h"
#include "wtf/text/AtomicString.h"

namespace WTF {

inline bool HashTraits<String>::isEmptyValue(const String& value) {
  return value.isNull();
}

// The hash() functions on StringHash and CaseFoldingHash do not support null
// strings. get(), contains(), and add() on HashMap<String,..., StringHash>
// cause a null-pointer dereference when passed null strings.

// FIXME: We should really figure out a way to put the computeHash function
// that's currently a member function of StringImpl into this file so we can be
// a little closer to having all the nearly-identical hash functions in one
// place.

struct StringHash {
  STATIC_ONLY(StringHash);
  static unsigned hash(StringImpl* key) { return key->hash(); }
  static inline bool equal(const StringImpl* a, const StringImpl* b) {
    return equalNonNull(a, b);
  }

  static unsigned hash(const RefPtr<StringImpl>& key) { return key->hash(); }
  static bool equal(const RefPtr<StringImpl>& a, const RefPtr<StringImpl>& b) {
    return equal(a.get(), b.get());
  }

  static unsigned hash(const String& key) { return key.impl()->hash(); }
  static bool equal(const String& a, const String& b) {
    return equal(a.impl(), b.impl());
  }

  static const bool safeToCompareToEmptyOrDeleted = false;
};

class CaseFoldingHash {
  STATIC_ONLY(CaseFoldingHash);

 public:
  static unsigned hash(const UChar* data, unsigned length) {
    return StringHasher::computeHashAndMaskTop8Bits<UChar, foldCase<UChar>>(
        data, length);
  }

  static unsigned hash(StringImpl* str) {
    if (str->is8Bit())
      return hash(str->characters8(), str->length());
    return hash(str->characters16(), str->length());
  }

  static unsigned hash(const LChar* data, unsigned length) {
    return StringHasher::computeHashAndMaskTop8Bits<LChar, foldCase<LChar>>(
        data, length);
  }

  static inline unsigned hash(const char* data, unsigned length) {
    return CaseFoldingHash::hash(reinterpret_cast<const LChar*>(data), length);
  }

  static inline unsigned hash(const char* data) {
    return CaseFoldingHash::hash(reinterpret_cast<const LChar*>(data),
                                 strlen(data));
  }

  static inline bool equal(const StringImpl* a, const StringImpl* b) {
    DCHECK(a);
    DCHECK(b);
    // Save one branch inside each StringView by derefing the StringImpl,
    // and another branch inside the compare function by skipping the null
    // checks.
    return equalIgnoringCaseAndNullity(*a, *b);
  }

  static unsigned hash(const RefPtr<StringImpl>& key) {
    return hash(key.get());
  }

  static bool equal(const RefPtr<StringImpl>& a, const RefPtr<StringImpl>& b) {
    return equal(a.get(), b.get());
  }

  static unsigned hash(const String& key) { return hash(key.impl()); }
  static unsigned hash(const AtomicString& key) { return hash(key.impl()); }
  static bool equal(const String& a, const String& b) {
    return equal(a.impl(), b.impl());
  }
  static bool equal(const AtomicString& a, const AtomicString& b) {
    return (a == b) || equal(a.impl(), b.impl());
  }

  static const bool safeToCompareToEmptyOrDeleted = false;

 private:
  // Private so no one uses this in the belief that it will return the
  // correctly-folded code point in all cases (see comment below).
  template <typename T>
  static inline UChar foldCase(T ch) {
    if (std::is_same<T, LChar>::value)
      return StringImpl::latin1CaseFoldTable[ch];
    // It's possible for WTF::Unicode::foldCase() to return a 32-bit value
    // that's not representable as a UChar.  However, since this is rare and
    // deterministic, and the result of this is merely used for hashing, go
    // ahead and clamp the value.
    return static_cast<UChar>(WTF::Unicode::foldCase(ch));
  }
};

// This hash can be used in cases where the key is a hash of a string, but we
// don't want to store the string. It's not really specific to string hashing,
// but all our current uses of it are for strings.
struct AlreadyHashed : IntHash<unsigned> {
  STATIC_ONLY(AlreadyHashed);
  static unsigned hash(unsigned key) { return key; }

  // To use a hash value as a key for a hash table, we need to eliminate the
  // "deleted" value, which is negative one. That could be done by changing
  // the string hash function to never generate negative one, but this works
  // and is still relatively efficient.
  static unsigned avoidDeletedValue(unsigned hash) {
    ASSERT(hash);
    unsigned newHash = hash | (!(hash + 1) << 31);
    ASSERT(newHash);
    ASSERT(newHash != 0xFFFFFFFF);
    return newHash;
  }
};

}  // namespace WTF

using WTF::AlreadyHashed;
using WTF::CaseFoldingHash;
using WTF::StringHash;

#endif