diff -r 000000000000 -r 6474c204b198 xpcom/ds/StringBuilder.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/ds/StringBuilder.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,84 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* vim:set ts=4 sw=4 sts=4 et cindent: */
+/* 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/. */
+
+/* We would use std::max but MS makes it painful
+// windef.h defines min and max macros that we don't want
+// http://support.microsoft.com/kb/143208
+#ifdef _WIN32
+#define NOMINMAX
+#endif
+*/
+
+#include <stdlib.h>
+#include <string.h>
+#include "nsAlgorithm.h"
+
+/* This is a standard string builder like ones in Java
+   or C#. It uses a doubling allocation strategy
+   to grow when out of capacity.
+
+   This does not use nsTArray because nsTArray starts
+   growing by multiples of page size after it is the
+   size of one page. We want to keep doubling in size
+   so that we can continue to append at high speed even
+   for large strings.
+
+   Eventually, this should be templated for wide characters.
+
+ */
+
+namespace mozilla {
+
+class StringBuilder
+{
+public:
+    StringBuilder() {
+        mCapacity = 16;
+        mLength = 0;
+        mBuffer = static_cast<char*>(malloc(sizeof(char)*mCapacity));
+        mBuffer[0] = '\0';
+    }
+
+    void Append(const char *s) {
+        size_t newLength = strlen(s);
+
+        EnsureCapacity(mLength + newLength+1);
+
+        // copy the entire string including the null terminator
+        memcpy(&mBuffer[mLength], s, newLength+1);
+        mLength += newLength;
+    }
+
+    char *Buffer() {
+        return mBuffer;
+    }
+
+    size_t Length() {
+        return mLength;
+    }
+
+    size_t EnsureCapacity(size_t capacity) {
+        if (capacity > mCapacity) {
+            // make sure we at least double in size
+            mCapacity = XPCOM_MAX(capacity, mCapacity*2);
+            mBuffer = static_cast<char*>(realloc(mBuffer, mCapacity));
+            mCapacity = moz_malloc_usable_size(mBuffer);
+        }
+        return mCapacity;
+    }
+
+    ~StringBuilder()
+    {
+        free(mBuffer);
+    }
+
+private:
+    char *mBuffer;
+    size_t mLength; // the length of the contained string not including the null terminator
+    size_t mCapacity; // the total size of mBuffer
+};
+
+}