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diff --git a/src/ceph/src/librados/snap_set_diff.cc b/src/ceph/src/librados/snap_set_diff.cc
new file mode 100644
index 0000000..fcf42d0
--- /dev/null
+++ b/src/ceph/src/librados/snap_set_diff.cc
@@ -0,0 +1,108 @@
+// -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*-
+// vim: ts=8 sw=2 smarttab
+
+#include <vector>
+
+#include "snap_set_diff.h"
+#include "common/ceph_context.h"
+#include "include/rados/librados.hpp"
+#include "include/interval_set.h"
+#include "common/debug.h"
+
+#define dout_subsys ceph_subsys_rados
+
+/**
+ * calculate intervals/extents that vary between two snapshots
+ */
+void calc_snap_set_diff(CephContext *cct, const librados::snap_set_t& snap_set,
+			librados::snap_t start, librados::snap_t end,
+			interval_set<uint64_t> *diff, uint64_t *end_size,
+                        bool *end_exists, librados::snap_t *clone_end_snap_id)
+{
+  ldout(cct, 10) << "calc_snap_set_diff start " << start << " end " << end
+		 << ", snap_set seq " << snap_set.seq << dendl;
+  bool saw_start = false;
+  uint64_t start_size = 0;
+  diff->clear();
+  *end_size = 0;
+  *end_exists = false;
+  *clone_end_snap_id = 0;
+
+  for (vector<librados::clone_info_t>::const_iterator r = snap_set.clones.begin();
+       r != snap_set.clones.end();
+       ) {
+    // make an interval, and hide the fact that the HEAD doesn't
+    // include itself in the snaps list
+    librados::snap_t a, b;
+    if (r->cloneid == librados::SNAP_HEAD) {
+      // head is valid starting from right after the last seen seq
+      a = snap_set.seq + 1;
+      b = librados::SNAP_HEAD;
+    } else {
+      a = r->snaps[0];
+      // note: b might be < r->cloneid if a snap has been trimmed.
+      b = r->snaps[r->snaps.size()-1];
+    }
+    ldout(cct, 20) << " clone " << r->cloneid << " snaps " << r->snaps
+		   << " -> [" << a << "," << b << "]"
+		   << " size " << r->size << " overlap to next " << r->overlap << dendl;
+
+    if (b < start) {
+      // this is before start
+      ++r;
+      continue;
+    }
+
+    if (!saw_start) {
+      if (start < a) {
+	ldout(cct, 20) << "  start, after " << start << dendl;
+	// this means the object didn't exist at start
+	if (r->size)
+	  diff->insert(0, r->size);
+	start_size = 0;
+      } else {
+	ldout(cct, 20) << "  start" << dendl;
+	start_size = r->size;
+      }
+      saw_start = true;
+    }
+
+    *end_size = r->size;
+    if (end < a) {
+      ldout(cct, 20) << " past end " << end << ", end object does not exist" << dendl;
+      *end_exists = false;
+      diff->clear();
+      if (start_size) {
+	diff->insert(0, start_size);
+      }
+      break;
+    }
+    if (end <= b) {
+      ldout(cct, 20) << " end" << dendl;
+      *end_exists = true;
+      *clone_end_snap_id = b;
+      break;
+    }
+
+    // start with the max(this size, next size), and subtract off any
+    // overlap
+    const vector<pair<uint64_t, uint64_t> > *overlap = &r->overlap;
+    interval_set<uint64_t> diff_to_next;
+    uint64_t max_size = r->size;
+    ++r;
+    if (r != snap_set.clones.end()) {
+      if (r->size > max_size)
+	max_size = r->size;
+    }
+    if (max_size)
+      diff_to_next.insert(0, max_size);
+    for (vector<pair<uint64_t, uint64_t> >::const_iterator p = overlap->begin();
+	 p != overlap->end();
+	 ++p) {
+      diff_to_next.erase(p->first, p->second);
+    }
+    ldout(cct, 20) << "  diff_to_next " << diff_to_next << dendl;
+    diff->union_of(diff_to_next);
+    ldout(cct, 20) << "  diff now " << *diff << dendl;
+  }
+}