pagespeed/kernel/base/atom.h

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 * 
 *   http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */

#ifndef PAGESPEED_KERNEL_BASE_ATOM_H_
#define PAGESPEED_KERNEL_BASE_ATOM_H_

#include <set>

#include "pagespeed/kernel/base/string_util.h"

namespace net_instaweb {

struct CaseFold;
struct CasePreserve;
template <class CharTransform>
class SymbolTable;

// Atoms are idempotent representations of strings, created
// via a symbol table.
class Atom {
 public:
  Atom(const Atom& src) : str_(src.str_) {}
  Atom();
  ~Atom() {}  // atoms are memory-managed by SymbolTables.

  Atom& operator=(const Atom& src) {
    if (&src != this) {
      str_ = src.str_;
    }
    return *this;
  }

  // Returns the address of the canonical StringPiece representing this Atom.
  // The underlying StringPiece object (and its data) are owned by the
  // SymbolTable.
  const StringPiece* Rep() const { return str_; }

  // This is comparing the underlying StringPiece pointers.  It is invalid
  // to compare Atoms from different symbol tables.
  bool operator==(const Atom& sym) const { return str_ == sym.str_; }

  // This is comparing the underlying StringPiece pointers.  It is invalid
  // to compare Atoms from different symbol tables.
  bool operator!=(const Atom& sym) const { return str_ != sym.str_; }

  // SymbolTable is a friend of Symbol because SymbolTable is the
  // only class that has the right to construct a new Atom from
  // a StringPiece*.
  friend class SymbolTable<CaseFold>;
  friend class SymbolTable<CasePreserve>;

 private:
  explicit Atom(const StringPiece* str) : str_(str) {}
  const StringPiece* str_;
};

// Once interned, Atoms are very cheap to put in a set, using
// pointer-comparison.
struct AtomCompare {
  bool operator()(const Atom& a1, const Atom& a2) const {
    // Compares pointers. Note that this assumes we don't overlap the
    // StringPiece's, which is the case for the implementation of SymbolTable.
    return a1.Rep()->data() < a2.Rep()->data();
  }
};

// A set of atoms can be constructed very efficiently.  Note that
// iteration over this set will *not* be in alphabetical order.
typedef std::set<Atom, AtomCompare> AtomSet;

}  // namespace net_instaweb

#endif  // PAGESPEED_KERNEL_BASE_ATOM_H_