gecko/xpcom/glue/nsTArray.h

/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* ***** BEGIN LICENSE BLOCK *****
 * Version: MPL 1.1/GPL 2.0/LGPL 2.1
 *
 * The contents of this file are subject to the Mozilla Public License Version
 * 1.1 (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.mozilla.org/MPL/
 *
 * Software distributed under the License is distributed on an "AS IS" basis,
 * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
 * for the specific language governing rights and limitations under the
 * License.
 *
 * The Original Code is C++ array template.
 *
 * The Initial Developer of the Original Code is Google Inc.
 * Portions created by the Initial Developer are Copyright (C) 2005
 * the Initial Developer. All Rights Reserved.
 *
 * Contributor(s):
 *  Darin Fisher <darin@meer.net>
 *
 * Alternatively, the contents of this file may be used under the terms of
 * either the GNU General Public License Version 2 or later (the "GPL"), or
 * the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
 * in which case the provisions of the GPL or the LGPL are applicable instead
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 * decision by deleting the provisions above and replace them with the notice
 * and other provisions required by the GPL or the LGPL. If you do not delete
 * the provisions above, a recipient may use your version of this file under
 * the terms of any one of the MPL, the GPL or the LGPL.
 *
 * ***** END LICENSE BLOCK ***** */

#ifndef nsTArray_h__
#define nsTArray_h__

#include "prtypes.h"
#include "nsQuickSort.h"
#include "nsDebug.h"
#include "nsTraceRefcnt.h"
#include NEW_H

//
// This class serves as a base class for nsTArray.  It shouldn't be used
// directly.  It holds common implementation code that does not depend on the
// element type of the nsTArray.
//
class NS_COM_GLUE nsTArray_base {
  public:
    typedef PRUint32 size_type;
    typedef PRUint32 index_type;

    // A special value that is used to indicate an invalid or unknown index
    // into the array.
    enum {
      NoIndex = index_type(-1)
    };

    // @return The number of elements in the array.
    size_type Length() const {
      return mHdr->mLength;
    }

    // @return True if the array is empty or false otherwise.
    PRBool IsEmpty() const {
      return Length() == 0;
    }

    // @return The number of elements that can fit in the array without forcing
    // the array to be re-allocated.  The length of an array is always less
    // than or equal to its capacity.
    size_type Capacity() const {
      return mHdr->mCapacity;
    }

#ifdef DEBUG
    void* DebugGetHeader() const {
      return mHdr;
    }
#endif

  protected:
    nsTArray_base();
    ~nsTArray_base();  

    // Resize the storage if necessary to achieve the requested capacity.
    // @param capacity     The requested number of array elements.
    // @param elementSize  The size of an array element.
    // @return False if insufficient memory is available; true otherwise.
    PRBool EnsureCapacity(size_type capacity, size_type elementSize);

    // Resize the storage to the minimum required amount.
    // @param elementSize  The size of an array element.
    void ShrinkCapacity(size_type elementSize);
    
    // This method may be called to resize a "gap" in the array by shifting
    // elements around.  It updates mLength appropriately.  If the resulting
    // array has zero elements, then the array's memory is free'd.
    // @param start        The starting index of the gap.
    // @param oldLen       The current length of the gap.
    // @param newLen       The desired length of the gap.
    // @param elementSize  The size of an array element.
    void ShiftData(index_type start, size_type oldLen, size_type newLen,
                   size_type elementSize);

    // This method increments the length member of the array's header.
    // Note that mHdr may actually be sEmptyHdr in the case where a
    // zero-length array is inserted into our array. But then n should
    // always be 0.
    void IncrementLength(PRUint32 n) {
      NS_ASSERTION(mHdr != &sEmptyHdr || n == 0, "bad data pointer");
      mHdr->mLength += n;
    }

    // This method inserts blank slots into the array.
    // @param index the place to insert the new elements. This must be no
    //              greater than the current length of the array.
    // @param count the number of slots to insert
    // @param elementSize the size of an array element.
    PRBool InsertSlotsAt(index_type index, size_type count,
                         size_type elementSize);

  protected:

    // NOTE: This method isn't heavily optimized if either array is an
    // nsAutoTArray.
    PRBool SwapArrayElements(nsTArray_base& other, size_type elementSize);

    // Helper function for SwapArrayElements. Ensures that if the array
    // is an nsAutoTArray that it doesn't use the built-in buffer.
    PRBool EnsureNotUsingAutoArrayBuffer(size_type elemSize);

    // We prefix mData with a structure of this type.  This is done to minimize
    // the size of the nsTArray object when it is empty.
    struct Header {
      PRUint32 mLength;
      PRUint32 mCapacity : 31;
      PRUint32 mIsAutoArray : 1;
    };

    // Returns true if this nsTArray is an nsAutoTArray with a built-in buffer.
    PRBool IsAutoArray() {
      return mHdr->mIsAutoArray;
    }

    // Dummy struct to get the compiler to simulate the alignment of
    // nsAutoTArray's and nsAutoTPtrArray's mAutoBuf.
    struct AutoArray {
      Header *mHdr;
      PRUint64 aligned;
    };

    // Returns a Header for the built-in buffer of this nsAutoTArray.
    Header* GetAutoArrayBuffer() {
      NS_ASSERTION(IsAutoArray(), "Should be an auto array to call this");

      return reinterpret_cast<Header*>(&(reinterpret_cast<AutoArray*>(&mHdr))->aligned);
    }

    // Returns true if this is an nsAutoTArray and it currently uses the
    // built-in buffer to store its elements.
    PRBool UsesAutoArrayBuffer() {
      return mHdr->mIsAutoArray && mHdr == GetAutoArrayBuffer();
    }

    // This is not const since we may actually write to it. However we will
    // always write to it the same data that it already contains. See
    // IncrementLength
    static Header sEmptyHdr;

    // The array's elements (prefixed with a Header).  This pointer is never
    // null.  If the array is empty, then this will point to sEmptyHdr.
    Header *mHdr;
};

//
// This class defines convenience functions for element specific operations.
// Specialize this template if necessary.
//
template<class E>
class nsTArrayElementTraits {
  public:
    // Invoke the default constructor in place.
    static inline void Construct(E *e) {
      // Do NOT call "E()"! That triggers C++ "default initialization"
      // which zeroes out POD ("plain old data") types such as regular ints.
      // We don't want that because it can be a performance issue and people
      // don't expect it; nsTArray should work like a regular C/C++ array in
      // this respect.
      new (static_cast<void *>(e)) E;
    }
    // Invoke the copy-constructor in place.
    template<class A>
    static inline void Construct(E *e, const A &arg) {
      new (static_cast<void *>(e)) E(arg);
    }
    // Invoke the destructor in place.
    static inline void Destruct(E *e) {
      e->~E();
    }
};

// This class exists because VC6 cannot handle static template functions.
// Otherwise, the Compare method would be defined directly on nsTArray.
template <class E, class Comparator>
class nsQuickSortComparator {
  public:
    typedef E elem_type;
    // This function is meant to be used with the NS_QuickSort function.  It
    // maps the callback API expected by NS_QuickSort to the Comparator API
    // used by nsTArray.  See nsTArray::Sort.
    static int Compare(const void* e1, const void* e2, void *data) {
      const Comparator* c = reinterpret_cast<const Comparator*>(data);
      const elem_type* a = static_cast<const elem_type*>(e1);
      const elem_type* b = static_cast<const elem_type*>(e2);
      return c->LessThan(*a, *b) ? -1 : (c->Equals(*a, *b) ? 0 : 1);
    }
};

// The default comparator used by nsTArray
template<class A, class B>
class nsDefaultComparator {
  public:
    PRBool Equals(const A& a, const B& b) const {
      return a == b;
    }
    PRBool LessThan(const A& a, const B& b) const {
      return a < b;
    }
};

//
// The templatized array class that dynamically resizes its storage as elements
// are added.  This class is designed to behave a bit like std::vector.
//
// The template parameter specifies the type of the elements (elem_type), and
// has the following requirements:
//
//   elem_type MUST define a copy-constructor.
//   elem_type MAY define operator< for sorting.
//   elem_type MAY define operator== for searching.
//
// For methods taking a Comparator instance, the Comparator must be a class
// defining the following methods:
//
//   class Comparator {
//     public:
//       /** @return True if the elements are equals; false otherwise. */
//       PRBool Equals(const elem_type& a, const elem_type& b) const;
//
//       /** @return True if (a < b); false otherwise. */
//       PRBool LessThan(const elem_type& a, const elem_type& b) const;
//   };
//
// The Equals method is used for searching, and the LessThan method is used
// for sorting.
//
template<class E>
class nsTArray : public nsTArray_base {
  public:
    typedef E                        elem_type;
    typedef nsTArray<E>              self_type;
    typedef nsTArrayElementTraits<E> elem_traits;

    //
    // Finalization method
    //

    ~nsTArray() { Clear(); }

    //
    // Initialization methods
    //

    nsTArray() {}

    // Initialize this array and pre-allocate some number of elements.
    explicit nsTArray(size_type capacity) {
      SetCapacity(capacity);
    }
    
    // The array's copy-constructor performs a 'deep' copy of the given array.
    // @param other  The array object to copy.
    nsTArray(const self_type& other) {
      AppendElements(other);
    }

    // The array's assignment operator performs a 'deep' copy of the given
    // array.  It is optimized to reuse existing storage if possible.
    // @param other  The array object to copy.
    nsTArray& operator=(const self_type& other) {
      ReplaceElementsAt(0, Length(), other.Elements(), other.Length());
      return *this;
    }

    //
    // Accessor methods
    //

    // This method provides direct access to the array elements.
    // @return A pointer to the first element of the array.  If the array is
    // empty, then this pointer must not be dereferenced.
    elem_type* Elements() {
      return reinterpret_cast<elem_type *>(mHdr + 1);
    }

    // This method provides direct, readonly access to the array elements.
    // @return A pointer to the first element of the array.  If the array is
    // empty, then this pointer must not be dereferenced.
    const elem_type* Elements() const {
      return reinterpret_cast<const elem_type *>(mHdr + 1);
    }
    
    // This method provides direct access to the i'th element of the array.
    // The given index must be within the array bounds.
    // @param i  The index of an element in the array.
    // @return   A reference to the i'th element of the array.
    elem_type& ElementAt(index_type i) {
      NS_ASSERTION(i < Length(), "invalid array index");
      return Elements()[i];
    }

    // This method provides direct, readonly access to the i'th element of the
    // array.  The given index must be within the array bounds.
    // @param i  The index of an element in the array.
    // @return   A const reference to the i'th element of the array.
    const elem_type& ElementAt(index_type i) const {
      NS_ASSERTION(i < Length(), "invalid array index");
      return Elements()[i];
    }

    // This method provides direct access to the i'th element of the array in
    // a bounds safe manner. If the requested index is out of bounds the
    // provided default value is returned.
    // @param i  The index of an element in the array.
    // @param def The value to return if the index is out of bounds.
    elem_type& SafeElementAt(index_type i, elem_type& def) {
      return i < Length() ? Elements()[i] : def;
    }

    // This method provides direct access to the i'th element of the array in
    // a bounds safe manner. If the requested index is out of bounds the
    // provided default value is returned.
    // @param i  The index of an element in the array.
    // @param def The value to return if the index is out of bounds.
    const elem_type& SafeElementAt(index_type i, const elem_type& def) const {
      return i < Length() ? Elements()[i] : def;
    }

    // Shorthand for ElementAt(i)
    elem_type& operator[](index_type i) {
      return ElementAt(i);
    }

    // Shorthand for ElementAt(i)
    const elem_type& operator[](index_type i) const {
      return ElementAt(i);
    }

    //
    // Search methods
    //

    // This method searches for the first element in this array that is equal
    // to the given element.
    // @param item   The item to search for.
    // @param comp   The Comparator used to determine element equality.
    // @return       PR_TRUE if the element was found.
    template<class Item, class Comparator>
    PRBool Contains(const Item& item, const Comparator& comp) const {
      return IndexOf(item, 0, comp) != NoIndex;
    }

    // This method searches for the first element in this array that is equal
    // to the given element.  This method assumes that 'operator==' is defined
    // for elem_type.
    // @param item   The item to search for.
    // @return       PR_TRUE if the element was found.
    template<class Item>
    PRBool Contains(const Item& item) const {
      return IndexOf(item) != NoIndex;
    }

    // This method searches for the offset of the first element in this
    // array that is equal to the given element.
    // @param item   The item to search for.
    // @param start  The index to start from.
    // @param comp   The Comparator used to determine element equality.
    // @return       The index of the found element or NoIndex if not found.
    template<class Item, class Comparator>
    index_type IndexOf(const Item& item, index_type start,
                       const Comparator& comp) const {
      const elem_type* iter = Elements() + start, *end = Elements() + Length();
      for (; iter != end; ++iter) {
        if (comp.Equals(*iter, item))
          return iter - Elements();
      }
      return NoIndex;
    }

    // This method searches for the offset of the first element in this
    // array that is equal to the given element.  This method assumes
    // that 'operator==' is defined for elem_type.
    // @param item   The item to search for.
    // @param start  The index to start from.
    // @return       The index of the found element or NoIndex if not found.
    template<class Item>
    index_type IndexOf(const Item& item, index_type start = 0) const {
      return IndexOf(item, start, nsDefaultComparator<elem_type, Item>());
    }

    // This method searches for the offset of the last element in this
    // array that is equal to the given element.
    // @param item   The item to search for.
    // @param start  The index to start from.  If greater than or equal to the
    //               length of the array, then the entire array is searched.
    // @param comp   The Comparator used to determine element equality.
    // @return       The index of the found element or NoIndex if not found.
    template<class Item, class Comparator>
    index_type LastIndexOf(const Item& item, index_type start,
                           const Comparator& comp) const {
      if (start >= Length())
        start = Length() - 1;
      const elem_type* end = Elements() - 1, *iter = end + start + 1;
      for (; iter != end; --iter) {
        if (comp.Equals(*iter, item))
          return iter - Elements();
      }
      return NoIndex;
    }

    // This method searches for the offset of the last element in this
    // array that is equal to the given element.  This method assumes
    // that 'operator==' is defined for elem_type.
    // @param item   The item to search for.
    // @param start  The index to start from.  If greater than or equal to the
    //               length of the array, then the entire array is searched.
    // @return       The index of the found element or NoIndex if not found.
    template<class Item>
    index_type LastIndexOf(const Item& item,
                           index_type start = NoIndex) const {
      return LastIndexOf(item, start, nsDefaultComparator<elem_type, Item>());
    }

    // This method searches for the offset for the element in this array
    // that is equal to the given element. The array is assumed to be sorted.
    // @param item   The item to search for.
    // @param comp   The Comparator used.
    // @return       The index of the found element or NoIndex if not found.
    template<class Item, class Comparator>
    index_type BinaryIndexOf(const Item& item, const Comparator& comp) const {
      index_type low = 0, high = Length();
      while (high > low) {
        index_type mid = (high + low) >> 1;
        if (comp.Equals(ElementAt(mid), item))
          return mid;
        if (comp.LessThan(ElementAt(mid), item))
          low = mid + 1;
        else
          high = mid;
      }
      return NoIndex;
    }

    // This method searches for the offset for the element in this array
    // that is equal to the given element. The array is assumed to be sorted.
    // This method assumes that 'operator==' and 'operator<' are defined.
    // @param item   The item to search for.
    // @return       The index of the found element or NoIndex if not found.
    template<class Item>
    index_type BinaryIndexOf(const Item& item) const {
      return BinaryIndexOf(item, nsDefaultComparator<elem_type, Item>());
    }

    //
    // Mutation methods
    //

    // This method replaces a range of elements in this array.
    // @param start     The starting index of the elements to replace.
    // @param count     The number of elements to replace.  This may be zero to
    //                  insert elements without removing any existing elements.
    // @param array     The values to copy into this array.  Must be non-null,
    //                  and these elements must not already exist in the array
    //                  being modified.
    // @param arrayLen  The number of values to copy into this array.
    // @return          A pointer to the new elements in the array, or null if
    //                  the operation failed due to insufficient memory.
    template<class Item>
    elem_type *ReplaceElementsAt(index_type start, size_type count,
                                 const Item* array, size_type arrayLen) {
      // Adjust memory allocation up-front to catch errors.
      if (!EnsureCapacity(Length() + arrayLen - count, sizeof(elem_type)))
        return nsnull;
      DestructRange(start, count);
      ShiftData(start, count, arrayLen, sizeof(elem_type));
      AssignRange(start, arrayLen, array);
      return Elements() + start;
    }

    // A variation on the ReplaceElementsAt method defined above.
    template<class Item>
    elem_type *ReplaceElementsAt(index_type start, size_type count,
                                 const nsTArray<Item>& array) {
      return ReplaceElementsAt(start, count, array.Elements(), array.Length());
    }

    // A variation on the ReplaceElementsAt method defined above.
    template<class Item>
    elem_type *ReplaceElementsAt(index_type start, size_type count,
                                 const Item& item) {
      return ReplaceElementsAt(start, count, &item, 1);
    }
    
    // A variation on the ReplaceElementsAt method defined above.
    template<class Item>
    elem_type *InsertElementsAt(index_type index, const Item* array,
                                size_type arrayLen) {
      return ReplaceElementsAt(index, 0, array, arrayLen);
    }

    // A variation on the ReplaceElementsAt method defined above.
    template<class Item>
    elem_type *InsertElementsAt(index_type index, const nsTArray<Item>& array) {
      return ReplaceElementsAt(index, 0, array.Elements(), array.Length());
    }

    // A variation on the ReplaceElementsAt method defined above.
    template<class Item>
    elem_type *InsertElementAt(index_type index, const Item& item) {
      return ReplaceElementsAt(index, 0, &item, 1);
    }

    // Insert a new element without copy-constructing. This is useful to avoid
    // temporaries.
    // @return A pointer to the newly inserted element, or null on OOM.
    elem_type* InsertElementAt(index_type index) {
      if (!EnsureCapacity(Length() + 1, sizeof(elem_type)))
         return nsnull;
      ShiftData(index, 0, 1, sizeof(elem_type));
      elem_type *elem = Elements() + index;
      elem_traits::Construct(elem);
      return elem;
    }

    // This method searches for the least index of the greatest
    // element less than or equal to |item|.  If |item| is inserted at
    // this index, the array will remain sorted.  True is returned iff
    // this index is also equal to |item|.  In this case, the returned
    // index may point to the start of multiple copies of |item|.
    // @param item   The item to search for.
    // @param comp   The Comparator used.
    // @outparam idx The index of greatest element <= to |item|
    // @return       True iff |item == array[*idx]|.
    // @precondition The array is sorted
    template<class Item, class Comparator>
    PRBool
    GreatestIndexLtEq(const Item& item,
                      const Comparator& comp,
                      index_type* idx NS_OUTPARAM) const {
      // Nb: we could replace all the uses of "BinaryIndexOf" with this
      // function, but BinaryIndexOf will be oh-so-slightly faster so
      // it's not strictly desired to do.

      // invariant: low <= [idx] < high
      index_type low = 0, high = Length();
      while (high > low) {
        index_type mid = (high + low) >> 1;
        if (comp.Equals(ElementAt(mid), item)) {
          // we might have the array [..., 2, 4, 4, 4, 4, 4, 5, ...]
          // and be searching for "4". it's arbitrary where mid ends
          // up here, so we back it up to the first instance to maintain
          // the "least index ..." we promised above.
          do {
            --mid;
          } while (NoIndex != mid && comp.Equals(ElementAt(mid), item));
          *idx = ++mid;
          return PR_TRUE;
        }
        if (comp.LessThan(ElementAt(mid), item))
          // invariant: low <= idx < high
          low = mid + 1;
        else
          // invariant: low <= idx < high
          high = mid;
      }
      // low <= idx < high, so insert at high ("shifting" high up by
      // 1) to maintain invariant.
      // (or insert at low, since low==high; just a matter of taste here.)
      *idx = high;
      return PR_FALSE;
    }

    // A variation on the GreatestIndexLtEq method defined above.
    template<class Item, class Comparator>
    PRBool
    GreatestIndexLtEq(const Item& item,
                      index_type& idx,
                      const Comparator& comp) const {
      return GreatestIndexLtEq(item, comp, &idx);
    }

    // A variation on the GreatestIndexLtEq method defined above.
    template<class Item>
    PRBool
    GreatestIndexLtEq(const Item& item,
                      index_type& idx) const {
      return GreatestIndexLtEq(item, nsDefaultComparator<elem_type, Item>(), &idx);
    }

    // Inserts |item| at such an index to guarantee that if the array
    // was previously sorted, it will remain sorted after this
    // insertion.
    template<class Item, class Comparator>
    elem_type *InsertElementSorted(const Item& item, const Comparator& comp) {
      index_type index;
      GreatestIndexLtEq(item, comp, &index);
      return InsertElementAt(index, item);
    }

    // A variation on the InsertElementSorted metod defined above.
    template<class Item>
    elem_type *InsertElementSorted(const Item& item) {
      return InsertElementSorted(item, nsDefaultComparator<elem_type, Item>());
    }

    // This method appends elements to the end of this array.
    // @param array     The elements to append to this array.
    // @param arrayLen  The number of elements to append to this array.
    // @return          A pointer to the new elements in the array, or null if
    //                  the operation failed due to insufficient memory.
    template<class Item>
    elem_type *AppendElements(const Item* array, size_type arrayLen) {
      if (!EnsureCapacity(Length() + arrayLen, sizeof(elem_type)))
        return nsnull;
      index_type len = Length();
      AssignRange(len, arrayLen, array);
      IncrementLength(arrayLen);
      return Elements() + len;
    }

    // A variation on the AppendElements method defined above.
    template<class Item>
    elem_type *AppendElements(const nsTArray<Item>& array) {
      return AppendElements(array.Elements(), array.Length());
    }

    // A variation on the AppendElements method defined above.
    template<class Item>
    elem_type *AppendElement(const Item& item) {
      return AppendElements(&item, 1);
    }

    // Append new elements without copy-constructing. This is useful to avoid
    // temporaries.
    // @return A pointer to the newly appended elements, or null on OOM.
    elem_type *AppendElements(size_type count) {
      if (!EnsureCapacity(Length() + count, sizeof(elem_type)))
         return nsnull;
      elem_type *elems = Elements() + Length();
      size_type i;
      for (i = 0; i < count; ++i) {
        elem_traits::Construct(elems + i);
      }
      IncrementLength(count);
      return elems;
    }

    // Append a new element without copy-constructing. This is useful to avoid
    // temporaries.
    // @return A pointer to the newly appended element, or null on OOM.
    elem_type *AppendElement() {
      return AppendElements(1);
    }

    // Move all elements from another array to the end of this array without 
    // calling copy constructors or destructors.
    // @return A pointer to the newly appended elements, or null on OOM.
    template<class Item>
    elem_type *MoveElementsFrom(nsTArray<Item>& array) {
      NS_PRECONDITION(&array != this, "argument must be different array");
      index_type len = Length();
      index_type otherLen = array.Length();
      if (!EnsureCapacity(len + otherLen, sizeof(elem_type)))
        return nsnull;
      memcpy(Elements() + len, array.Elements(), otherLen * sizeof(elem_type));
      IncrementLength(otherLen);      
      array.ShiftData(0, otherLen, 0, sizeof(elem_type));
      return Elements() + len;
    }

    // This method removes a range of elements from this array.
    // @param start  The starting index of the elements to remove.
    // @param count  The number of elements to remove.
    void RemoveElementsAt(index_type start, size_type count) {
      NS_ASSERTION(count == 0 || start < Length(), "Invalid start index");
      NS_ASSERTION(start + count <= Length(), "Invalid length");
      DestructRange(start, count);
      ShiftData(start, count, 0, sizeof(elem_type));
    }

    // A variation on the RemoveElementsAt method defined above.
    void RemoveElementAt(index_type index) {
      RemoveElementsAt(index, 1);
    }

    // A variation on the RemoveElementsAt method defined above.
    void Clear() {
      RemoveElementsAt(0, Length());
    }

    // This helper function combines IndexOf with RemoveElementAt to "search
    // and destroy" the first element that is equal to the given element.
    // @param item  The item to search for.
    // @param comp  The Comparator used to determine element equality.
    // @return PR_TRUE if the element was found
    template<class Item, class Comparator>
    PRBool RemoveElement(const Item& item, const Comparator& comp) {
      index_type i = IndexOf(item, 0, comp);
      if (i == NoIndex)
        return PR_FALSE;

      RemoveElementAt(i);
      return PR_TRUE;
    }

    // A variation on the RemoveElement method defined above that assumes
    // that 'operator==' is defined for elem_type.
    template<class Item>
    PRBool RemoveElement(const Item& item) {
      return RemoveElement(item, nsDefaultComparator<elem_type, Item>());
    }

    // This helper function combines GreatestIndexLtEq with
    // RemoveElementAt to "search and destroy" the first element that
    // is equal to the given element.
    // @param item  The item to search for.
    // @param comp  The Comparator used to determine element equality.
    // @return PR_TRUE if the element was found
    template<class Item, class Comparator>
    PRBool RemoveElementSorted(const Item& item, const Comparator& comp) {
      index_type index;
      PRBool found = GreatestIndexLtEq(item, comp, &index);
      if (found)
        RemoveElementAt(index);
      return found;
    }

    // A variation on the RemoveElementSorted method defined above.
    template<class Item>
    PRBool RemoveElementSorted(const Item& item) {
      return RemoveElementSorted(item, nsDefaultComparator<elem_type, Item>());
    }

    // This method causes the elements contained in this array and the given
    // array to be swapped.
    // NOTE: This method isn't heavily optimized if either array is an
    // nsAutoTArray.
    PRBool SwapElements(self_type& other) {
      return SwapArrayElements(other, sizeof(elem_type));
    }

    //
    // Allocation
    //

    // This method may increase the capacity of this array object by the
    // specified amount.  This method may be called in advance of several
    // AppendElement operations to minimize heap re-allocations.  This method
    // will not reduce the number of elements in this array.
    // @param capacity  The desired capacity of this array.
    // @return True if the operation succeeded; false if we ran out of memory
    PRBool SetCapacity(size_type capacity) {
      return EnsureCapacity(capacity, sizeof(elem_type));
    }

    // This method modifies the length of the array.  If the new length is
    // larger than the existing length of the array, then new elements will be
    // constructed using elem_type's default constructor.  Otherwise, this call
    // removes elements from the array (see also RemoveElementsAt).
    // @param newLen  The desired length of this array.
    // @return        True if the operation succeeded; false otherwise.
    // See also TruncateLength if the new length is guaranteed to be
    // smaller than the old.
    PRBool SetLength(size_type newLen) {
      size_type oldLen = Length();
      if (newLen > oldLen) {
        return InsertElementsAt(oldLen, newLen - oldLen) != nsnull;
      }
      
      TruncateLength(newLen);
      return PR_TRUE;
    }

    // This method modifies the length of the array, but may only be
    // called when the new length is shorter than the old.  It can
    // therefore be called when elem_type has no default constructor,
    // unlike SetLength.  It removes elements from the array (see also
    // RemoveElementsAt).
    // @param newLen  The desired length of this array.
    void TruncateLength(size_type newLen) {
      size_type oldLen = Length();
      NS_ABORT_IF_FALSE(newLen <= oldLen,
                        "caller should use SetLength instead");
      RemoveElementsAt(newLen, oldLen - newLen);
    }

    // This method ensures that the array has length at least the given
    // length.  If the current length is shorter than the given length,
    // then new elements will be constructed using elem_type's default
    // constructor.
    // @param minLen  The desired minimum length of this array.
    // @return        True if the operation succeeded; false otherwise.
    PRBool EnsureLengthAtLeast(size_type minLen) {
      size_type oldLen = Length();
      if (minLen > oldLen) {
        return InsertElementsAt(oldLen, minLen - oldLen) != nsnull;
      }
      return PR_TRUE;
    }

    // This method inserts elements into the array, constructing
    // them using elem_type's default constructor.
    // @param index the place to insert the new elements. This must be no
    //              greater than the current length of the array.
    // @param count the number of elements to insert
    elem_type *InsertElementsAt(index_type index, size_type count) {
      if (!nsTArray_base::InsertSlotsAt(index, count, sizeof(elem_type))) {
        return nsnull;
      }

      // Initialize the extra array elements
      elem_type *iter = Elements() + index, *end = iter + count;
      for (; iter != end; ++iter) {
        elem_traits::Construct(iter);
      }

      return Elements() + index;
    }

    // This method inserts elements into the array, constructing them
    // elem_type's copy constructor (or whatever one-arg constructor
    // happens to match the Item type).
    // @param index the place to insert the new elements. This must be no
    //              greater than the current length of the array.
    // @param count the number of elements to insert.
    // @param item the value to use when constructing the new elements.
    template<class Item>
    elem_type *InsertElementsAt(index_type index, size_type count,
                                const Item& item) {
      if (!nsTArray_base::InsertSlotsAt(index, count, sizeof(elem_type))) {
        return nsnull;
      }

      // Initialize the extra array elements
      elem_type *iter = Elements() + index, *end = iter + count;
      for (; iter != end; ++iter) {
        elem_traits::Construct(iter, item);
      }

      return Elements() + index;
    }

    // This method may be called to minimize the memory used by this array.
    void Compact() {
      ShrinkCapacity(sizeof(elem_type));
    }

    //
    // Sorting
    //

    // This method sorts the elements of the array.  It uses the LessThan
    // method defined on the given Comparator object to collate elements.
    // @param c  The Comparator to used to collate elements.
    template<class Comparator>
    void Sort(const Comparator& comp) {
      NS_QuickSort(Elements(), Length(), sizeof(elem_type),
                   nsQuickSortComparator<elem_type, Comparator>::Compare,
                   const_cast<Comparator*>(&comp));
    }

    // A variation on the Sort method defined above that assumes that
    // 'operator<' is defined for elem_type.
    void Sort() {
      Sort(nsDefaultComparator<elem_type, elem_type>());
    }

  protected:

    // This method invokes elem_type's destructor on a range of elements.
    // @param start  The index of the first element to destroy.
    // @param count  The number of elements to destroy.
    void DestructRange(index_type start, size_type count) {
      elem_type *iter = Elements() + start, *end = iter + count;
      for (; iter != end; ++iter) {
        elem_traits::Destruct(iter);
      }
    }

    // This method invokes elem_type's copy-constructor on a range of elements.
    // @param start   The index of the first element to construct.
    // @param count   The number of elements to construct. 
    // @param values  The array of elements to copy. 
    template<class Item>
    void AssignRange(index_type start, size_type count,
                     const Item *values) {
      elem_type *iter = Elements() + start, *end = iter + count;
      for (; iter != end; ++iter, ++values) {
        elem_traits::Construct(iter, *values);
      }
    }
};

template<class E, PRUint32 N>
class nsAutoTArray : public nsTArray<E> {
  public:
    typedef nsTArray<E> base_type;
    typedef typename base_type::Header Header;
    typedef typename base_type::elem_type elem_type;

    nsAutoTArray() {
      base_type::mHdr = reinterpret_cast<Header*>(&mAutoBuf);
      base_type::mHdr->mLength = 0;
      base_type::mHdr->mCapacity = N;
      base_type::mHdr->mIsAutoArray = 1;

      NS_ASSERTION(base_type::GetAutoArrayBuffer() ==
                   reinterpret_cast<Header*>(&mAutoBuf),
                   "GetAutoArrayBuffer needs to be fixed");
    }

  protected:
    union {
      char mAutoBuf[sizeof(Header) + N * sizeof(elem_type)];
      PRUint64 dummy;
    };
};

// specialization for N = 0. this makes the inheritance model easier for
// templated users of nsAutoTArray.
template<class E>
class nsAutoTArray<E, 0> : public nsTArray<E> {
  public:
    nsAutoTArray() {}
};

#endif  // nsTArray_h__