diff -r 000000000000 -r 6474c204b198 xpcom/io/nsSegmentedBuffer.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/io/nsSegmentedBuffer.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,89 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsSegmentedBuffer_h__
+#define nsSegmentedBuffer_h__
+
+#include "nsIMemory.h"
+
+class nsSegmentedBuffer
+{
+public:
+    nsSegmentedBuffer()
+        : mSegmentSize(0), mMaxSize(0), 
+          mSegmentArray(nullptr),
+          mSegmentArrayCount(0),
+          mFirstSegmentIndex(0), mLastSegmentIndex(0) {}
+
+    ~nsSegmentedBuffer() {
+        Empty();
+    }
+
+
+    nsresult Init(uint32_t segmentSize, uint32_t maxSize);
+
+    char* AppendNewSegment();   // pushes at end
+
+    // returns true if no more segments remain:
+    bool DeleteFirstSegment();  // pops from beginning
+
+    // returns true if no more segments remain:
+    bool DeleteLastSegment();  // pops from beginning
+
+    // Call Realloc() on last segment.  This is used to reduce memory
+    // consumption when data is not an exact multiple of segment size.
+    bool ReallocLastSegment(size_t newSize);
+
+    void Empty();               // frees all segments
+
+    inline uint32_t GetSegmentCount() {
+        if (mFirstSegmentIndex <= mLastSegmentIndex)
+            return mLastSegmentIndex - mFirstSegmentIndex;
+        else 
+            return mSegmentArrayCount + mLastSegmentIndex - mFirstSegmentIndex;
+    }
+
+    inline uint32_t GetSegmentSize() { return mSegmentSize; }
+    inline uint32_t GetMaxSize() { return mMaxSize; }
+    inline uint32_t GetSize() { return GetSegmentCount() * mSegmentSize; }
+
+    inline char* GetSegment(uint32_t indx) {
+        NS_ASSERTION(indx < GetSegmentCount(), "index out of bounds");
+        int32_t i = ModSegArraySize(mFirstSegmentIndex + (int32_t)indx);
+        return mSegmentArray[i];
+    }
+
+protected:
+    inline int32_t ModSegArraySize(int32_t n) {
+        uint32_t result = n & (mSegmentArrayCount - 1);
+        NS_ASSERTION(result == n % mSegmentArrayCount,
+                     "non-power-of-2 mSegmentArrayCount");
+        return result;
+    }
+
+   inline bool IsFull() {
+        return ModSegArraySize(mLastSegmentIndex + 1) == mFirstSegmentIndex;
+    }
+
+protected:
+    uint32_t            mSegmentSize;
+    uint32_t            mMaxSize;
+    char**              mSegmentArray;
+    uint32_t            mSegmentArrayCount;
+    int32_t             mFirstSegmentIndex;
+    int32_t             mLastSegmentIndex;
+};
+
+// NS_SEGMENTARRAY_INITIAL_SIZE: This number needs to start out as a
+// power of 2 given how it gets used. We double the segment array
+// when we overflow it, and use that fact that it's a power of 2 
+// to compute a fast modulus operation in IsFull.
+//
+// 32 segment array entries can accommodate 128k of data if segments
+// are 4k in size. That seems like a reasonable amount that will avoid
+// needing to grow the segment array.
+#define NS_SEGMENTARRAY_INITIAL_COUNT 32
+
+#endif // nsSegmentedBuffer_h__