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// Copyright (c) 2003 Google Inc.
//
// Permission is hereby granted, free of charge, to any person or organization
// obtaining a copy of the software and accompanying documentation covered by
// this license (the "Software") to use, reproduce, display, distribute,
// execute, and transmit the Software, and to prepare derivative works of the
// Software, and to permit third-parties to whom the Software is furnished to
// do so, all subject to the following:
//
// The copyright notices in the Software and this entire statement, including
// the above license grant, this restriction and the following disclaimer,
// must be included in all copies of the Software, in whole or in part, and
// all derivative works of the Software, unless such copies or derivative
// works are solely in the form of machine-executable object code generated by
// a source language processor.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
// SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
// FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT, TORT OR OTHERWISE,
// ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
// DEALINGS IN THE SOFTWARE.
//
// linked_ptr.h
// Author: Dan Egnor
//
// A "smart" pointer type with reference tracking.  Every pointer to a
// particular object is kept on a circular linked list.  When the last pointer
// to an object is destroyed or reassigned, the object is deleted.
//
// Used properly, this deletes the object when the last reference goes away.
// There are several caveats:
// - Like all reference counting schemes, cycles lead to leaks.
// - Each smart pointer is actually two pointers (8 bytes instead of 4).
// - Every time a pointer is assigned, the entire list of pointers to that
//   object is traversed.  This class is therefore NOT SUITABLE when there
//   will often be more than two or three pointers to a particular object.
// - References are only tracked as long as linked_ptr<> objects are copied.
//   If a linked_ptr<> is converted to a raw pointer and back, BAD THINGS
//   will happen (double deletion).
//
// A good use of this class is storing object references in STL containers.
// You can safely put linked_ptr<> in a vector<>.
// Other uses may not be as good.
//
// Note: If you use an incomplete type with linked_ptr<>, the class
// *containing* linked_ptr<> must have a constructor and destructor (even
// if they do nothing!).
//
// Bill Gibbons suggested we use something like this.  Yonat Sharon has
// a different (less useful IMHO) implementation at ootips.org.
//
// Thread Safety:
//   A linked_ptr is NOT thread safe. Copying a linked_ptr object is
//   effectively a read-write operation.

#ifndef UTIL_GTL_LINKED_PTR_H__
#define UTIL_GTL_LINKED_PTR_H__

#include <assert.h>

// This is used internally by all instances of linked_ptr<>.  It needs to be
// a non-template class because different types of linked_ptr<> can refer to
// the same object (linked_ptr<Superclass>(obj) vs linked_ptr<Subclass>(obj)).
// So, it needs to be possible for different types of linked_ptr to participate
// in the same circular linked list, so we need a single class type here.
//
// DO NOT USE THIS CLASS DIRECTLY YOURSELF.  Use linked_ptr<T>.
class linked_ptr_internal {
 public:
  // Create a new circle that includes only this instance.
  void join_new() {
    next_ = this;
  }

  // Join an existing circle.
  void join(linked_ptr_internal const* ptr) {
    linked_ptr_internal const* p = ptr;
    while (p->next_ != ptr) p = p->next_;
    p->next_ = this;
    next_ = ptr;
  }

  // Leave whatever circle we're part of.  Returns true iff we were the
  // last member of the circle.  Once this is done, you can join() another.
  bool depart() {
    if (next_ == this) return true;
    linked_ptr_internal const* p = next_;
    while (p->next_ != this) p = p->next_;
    p->next_ = next_;
    return false;
  }

 private:
  mutable linked_ptr_internal const* next_;
};

template <typename T>
class linked_ptr {
 public:
  typedef T element_type;

  // Take over ownership of a raw pointer.  This should happen as soon as
  // possible after the object is created.
  explicit linked_ptr(T* ptr = NULL) { capture(ptr); }
  ~linked_ptr() { depart(); }

  // Copy an existing linked_ptr<>, adding ourselves to the list of references.
  template <typename U> linked_ptr(linked_ptr<U> const& ptr) { copy(&ptr); }
  linked_ptr(linked_ptr const& ptr) { assert(&ptr != this); copy(&ptr); }

  // Assignment releases the old value and acquires the new.
  template <typename U> linked_ptr& operator=(linked_ptr<U> const& ptr) {
    depart();
    copy(&ptr);
    return *this;
  }

  linked_ptr& operator=(linked_ptr const& ptr) {
    if (&ptr != this) {
      depart();
      copy(&ptr);
    }
    return *this;
  }

  // Smart pointer members.
  void reset(T* ptr = NULL) { depart(); capture(ptr); }
  T* get() const { return value_; }
  T* operator->() const { return value_; }
  T& operator*() const { return *value_; }
  // Release ownership of the pointed object and returns it.
  // Sole ownership by this linked_ptr object is required.
  T* release() {
    // !! GOOGLE Gears specific modification !!
    // GCC emits a warning for not using last in a opt build.
    // Warnings are treated as errors.
#ifdef DEBUG
    bool last = link_.depart();
    assert(last);
#else
    link_.depart();
#endif
    T* v = value_;
    value_ = NULL;
    return v;
  }

  bool operator==(T* p) const { return value_ == p; }
  bool operator!=(T* p) const { return value_ != p; }
  template <typename U>
  bool operator==(linked_ptr<U> const& ptr) const {
    return value_ == ptr.get();
  }
  template <typename U>
  bool operator!=(linked_ptr<U> const& ptr) const {
    return value_ != ptr.get();
  }

 private:
  template <typename U>
  friend class linked_ptr;

  T* value_;
  linked_ptr_internal link_;

  void depart() {
    if (link_.depart()) delete value_;
  }

  void capture(T* ptr) {
    value_ = ptr;
    link_.join_new();
  }

  template <typename U> void copy(linked_ptr<U> const* ptr) {
    value_ = ptr->get();
    if (value_)
      link_.join(&ptr->link_);
    else
      link_.join_new();
  }
};

template<typename T> inline
bool operator==(T* ptr, const linked_ptr<T>& x) {
  return ptr == x.get();
}

template<typename T> inline
bool operator!=(T* ptr, const linked_ptr<T>& x) {
  return ptr != x.get();
}

// A function to convert T* into linked_ptr<T>
// Doing e.g. make_linked_ptr(new FooBarBaz<type>(arg)) is a shorter notation
// for linked_ptr<FooBarBaz<type> >(new FooBarBaz<type>(arg))
template <typename T>
linked_ptr<T> make_linked_ptr(T* ptr) {
  return linked_ptr<T>(ptr);
}

#endif // UTIL_GTL_LINKED_PTR_H__