diff -r 000000000000 -r 6474c204b198 xpcom/string/src/nsTSubstringTuple.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/string/src/nsTSubstringTuple.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,95 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 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/. */
+
+
+  /**
+   * computes the aggregate string length
+   */
+
+nsTSubstringTuple_CharT::size_type
+nsTSubstringTuple_CharT::Length() const
+  {
+    uint32_t len;
+    if (mHead)
+      len = mHead->Length();
+    else
+      len = TO_SUBSTRING(mFragA).Length();
+
+    return len + TO_SUBSTRING(mFragB).Length();
+  }
+
+
+  /**
+   * writes the aggregate string to the given buffer.  bufLen is assumed
+   * to be equal to or greater than the value returned by the Length()
+   * method.  the string written to |buf| is not null-terminated.
+   */
+
+void
+nsTSubstringTuple_CharT::WriteTo( char_type *buf, uint32_t bufLen ) const
+  {
+    const substring_type& b = TO_SUBSTRING(mFragB);
+
+    NS_ASSERTION(bufLen >= b.Length(), "buffer too small");
+    uint32_t headLen = bufLen - b.Length();
+    if (mHead)
+      {
+        mHead->WriteTo(buf, headLen);
+      }
+    else
+      {
+        const substring_type& a = TO_SUBSTRING(mFragA);
+
+        NS_ASSERTION(a.Length() == headLen, "buffer incorrectly sized");
+        char_traits::copy(buf, a.Data(), a.Length());
+      }
+
+    char_traits::copy(buf + headLen, b.Data(), b.Length());
+
+#if 0
+    // we need to write out data into |buf|, ending at |buf+bufLen|.  so our
+    // data needs to precede |buf+bufLen| exactly.  we trust that the buffer
+    // was properly sized!
+
+    const substring_type& b = TO_SUBSTRING(mFragB);
+
+    NS_ASSERTION(bufLen >= b.Length(), "buffer is too small");
+    char_traits::copy(buf + bufLen - b.Length(), b.Data(), b.Length());
+
+    bufLen -= b.Length();
+
+    if (mHead)
+      {
+        mHead->WriteTo(buf, bufLen);
+      }
+    else
+      {
+        const substring_type& a = TO_SUBSTRING(mFragA);
+        NS_ASSERTION(bufLen == a.Length(), "buffer is too small");
+        char_traits::copy(buf, a.Data(), a.Length());
+      }
+#endif
+  }
+
+
+  /**
+   * returns true if this tuple is dependent on (i.e., overlapping with)
+   * the given char sequence.
+   */
+
+bool
+nsTSubstringTuple_CharT::IsDependentOn( const char_type *start, const char_type *end ) const
+  {
+    // we start with the right-most fragment since it is faster to check.
+
+    if (TO_SUBSTRING(mFragB).IsDependentOn(start, end))
+      return true;
+
+    if (mHead)
+      return mHead->IsDependentOn(start, end);
+
+    return TO_SUBSTRING(mFragA).IsDependentOn(start, end);
+  }