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nsSegmentedBuffer.h
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00037 
00038 #ifndef nsSegmentedBuffer_h__
00039 #define nsSegmentedBuffer_h__
00040 
00041 #include "nsMemory.h"
00042 #include "prclist.h"
00043 
00044 class nsSegmentedBuffer
00045 {
00046 public:
00047     nsSegmentedBuffer()
00048         : mSegmentSize(0), mMaxSize(0), 
00049           mSegAllocator(nsnull), mSegmentArray(nsnull),
00050           mSegmentArrayCount(0),
00051           mFirstSegmentIndex(0), mLastSegmentIndex(0) {}
00052 
00053     ~nsSegmentedBuffer() {
00054         Empty();
00055         NS_IF_RELEASE(mSegAllocator);
00056     }
00057 
00058 
00059     NS_COM nsresult Init(PRUint32 segmentSize, PRUint32 maxSize,
00060                   nsIMemory* allocator = nsnull);
00061 
00062     NS_COM char* AppendNewSegment();   // pushes at end
00063 
00064     // returns true if no more segments remain:
00065     PRBool DeleteFirstSegment();  // pops from beginning
00066 
00067     // returns true if no more segments remain:
00068     PRBool DeleteLastSegment();  // pops from beginning
00069 
00070     // Call Realloc() on last segment.  This is used to reduce memory
00071     // consumption when data is not an exact multiple of segment size.
00072     PRBool ReallocLastSegment(size_t newSize);
00073 
00074     NS_COM void Empty();               // frees all segments
00075 
00076     inline PRUint32 GetSegmentCount() {
00077         if (mFirstSegmentIndex <= mLastSegmentIndex)
00078             return mLastSegmentIndex - mFirstSegmentIndex;
00079         else 
00080             return mSegmentArrayCount + mLastSegmentIndex - mFirstSegmentIndex;
00081     }
00082 
00083     inline PRUint32 GetSegmentSize() { return mSegmentSize; }
00084     inline PRUint32 GetMaxSize() { return mMaxSize; }
00085     inline PRUint32 GetSize() { return GetSegmentCount() * mSegmentSize; }
00086 
00087     inline char* GetSegment(PRUint32 indx) {
00088         NS_ASSERTION(indx < GetSegmentCount(), "index out of bounds");
00089         PRInt32 i = ModSegArraySize(mFirstSegmentIndex + (PRInt32)indx);
00090         return mSegmentArray[i];
00091     }
00092 
00093 protected:
00094     inline PRInt32 ModSegArraySize(PRInt32 n) {
00095         PRUint32 result = n & (mSegmentArrayCount - 1);
00096         NS_ASSERTION(result == n % mSegmentArrayCount,
00097                      "non-power-of-2 mSegmentArrayCount");
00098         return result;
00099     }
00100 
00101    inline PRBool IsFull() {
00102         return ModSegArraySize(mLastSegmentIndex + 1) == mFirstSegmentIndex;
00103     }
00104 
00105 protected:
00106     PRUint32            mSegmentSize;
00107     PRUint32            mMaxSize;
00108     nsIMemory*       mSegAllocator;
00109     char**              mSegmentArray;
00110     PRUint32            mSegmentArrayCount;
00111     PRInt32             mFirstSegmentIndex;
00112     PRInt32             mLastSegmentIndex;
00113 };
00114 
00115 // NS_SEGMENTARRAY_INITIAL_SIZE: This number needs to start out as a
00116 // power of 2 given how it gets used. We double the segment array
00117 // when we overflow it, and use that fact that it's a power of 2 
00118 // to compute a fast modulus operation in IsFull.
00119 //
00120 // 32 segment array entries can accommodate 128k of data if segments
00121 // are 4k in size. That seems like a reasonable amount that will avoid
00122 // needing to grow the segment array.
00123 #define NS_SEGMENTARRAY_INITIAL_COUNT 32
00124 
00125 #endif // nsSegmentedBuffer_h__