xpcom/io/nsSegmentedBuffer.h

   1 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
   2 /* ***** BEGIN LICENSE BLOCK *****
   3  * Version: MPL 1.1/GPL 2.0/LGPL 2.1
   4  *
   5  * The contents of this file are subject to the Mozilla Public License Version
   6  * 1.1 (the "License"); you may not use this file except in compliance with
   7  * the License. You may obtain a copy of the License at
   8  * http://www.mozilla.org/MPL/
   9  *
  10  * Software distributed under the License is distributed on an "AS IS" basis,
  11  * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
  12  * for the specific language governing rights and limitations under the
  13  * License.
  14  *
  15  * The Original Code is mozilla.org code.
  16  *
  17  * The Initial Developer of the Original Code is
  18  * Netscape Communications Corporation.
  19  * Portions created by the Initial Developer are Copyright (C) 1998
  20  * the Initial Developer. All Rights Reserved.
  21  *
  22  * Contributor(s):
  23  *
  24  * Alternatively, the contents of this file may be used under the terms of
  25  * either of the GNU General Public License Version 2 or later (the "GPL"),
  26  * or the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
  27  * in which case the provisions of the GPL or the LGPL are applicable instead
  28  * of those above. If you wish to allow use of your version of this file only
  29  * under the terms of either the GPL or the LGPL, and not to allow others to
  30  * use your version of this file under the terms of the MPL, indicate your
  31  * decision by deleting the provisions above and replace them with the notice
  32  * and other provisions required by the GPL or the LGPL. If you do not delete
  33  * the provisions above, a recipient may use your version of this file under
  34  * the terms of any one of the MPL, the GPL or the LGPL.
  35  *
  36  * ***** END LICENSE BLOCK ***** */
  37
  38 #ifndef nsSegmentedBuffer_h__
  39 #define nsSegmentedBuffer_h__
  40
  41 #include "nsMemory.h"
  42 #include "prclist.h"
  43
  44 class nsSegmentedBuffer
  45 {
  46 public:
  47     nsSegmentedBuffer()
  48         : mSegmentSize(0), mMaxSize(0),
  49           mSegAllocator(nsnull), mSegmentArray(nsnull),
  50           mSegmentArrayCount(0),
  51           mFirstSegmentIndex(0), mLastSegmentIndex(0) {}
  52
  53     ~nsSegmentedBuffer() {
  54         Empty();
  55         NS_IF_RELEASE(mSegAllocator);
  56     }
  57
  58
  59     NS_COM nsresult Init(PRUint32 segmentSize, PRUint32 maxSize,
  60                   nsIMemory* allocator = nsnull);
  61
  62     NS_COM char* AppendNewSegment();   // pushes at end
  63
  64     // returns true if no more segments remain:
  65     PRBool DeleteFirstSegment();  // pops from beginning
  66
  67     // returns true if no more segments remain:
  68     PRBool DeleteLastSegment();  // pops from beginning
  69
  70     // Call Realloc() on last segment.  This is used to reduce memory
  71     // consumption when data is not an exact multiple of segment size.
  72     PRBool ReallocLastSegment(size_t newSize);
  73
  74     NS_COM void Empty();               // frees all segments
  75
  76     inline PRUint32 GetSegmentCount() {
  77         if (mFirstSegmentIndex <= mLastSegmentIndex)
  78             return mLastSegmentIndex - mFirstSegmentIndex;
  79         else
  80             return mSegmentArrayCount + mLastSegmentIndex - mFirstSegmentIndex;
  81     }
  82
  83     inline PRUint32 GetSegmentSize() { return mSegmentSize; }
  84     inline PRUint32 GetMaxSize() { return mMaxSize; }
  85     inline PRUint32 GetSize() { return GetSegmentCount() * mSegmentSize; }
  86
  87     inline char* GetSegment(PRUint32 indx) {
  88         NS_ASSERTION(indx < GetSegmentCount(), "index out of bounds");
  89         PRInt32 i = ModSegArraySize(mFirstSegmentIndex + (PRInt32)indx);
  90         return mSegmentArray[i];
  91     }
  92
  93 protected:
  94     inline PRInt32 ModSegArraySize(PRInt32 n) {
  95         PRUint32 result = n & (mSegmentArrayCount - 1);
  96         NS_ASSERTION(result == n % mSegmentArrayCount,
  97                      "non-power-of-2 mSegmentArrayCount");
  98         return result;
  99     }
 100
 101    inline PRBool IsFull() {
 102         return ModSegArraySize(mLastSegmentIndex + 1) == mFirstSegmentIndex;
 103     }
 104
 105 protected:
 106     PRUint32            mSegmentSize;
 107     PRUint32            mMaxSize;
 108     nsIMemory*       mSegAllocator;
 109     char**              mSegmentArray;
 110     PRUint32            mSegmentArrayCount;
 111     PRInt32             mFirstSegmentIndex;
 112     PRInt32             mLastSegmentIndex;
 113 };
 114
 115 // NS_SEGMENTARRAY_INITIAL_SIZE: This number needs to start out as a
 116 // power of 2 given how it gets used. We double the segment array
 117 // when we overflow it, and use that fact that it's a power of 2
 118 // to compute a fast modulus operation in IsFull.
 119 //
 120 // 32 segment array entries can accommodate 128k of data if segments
 121 // are 4k in size. That seems like a reasonable amount that will avoid
 122 // needing to grow the segment array.
 123 #define NS_SEGMENTARRAY_INITIAL_COUNT 32
 124
 125 #endif // nsSegmentedBuffer_h__