oc: improve modseqFromMessageChangeNumber to return an approximation.

Only for scenarios where we hadn't store the cn <-> modseq relationship

OpenChange/MAPIStoreMailFolder.m

@@ -802,13 +802,43 @@ _compareFetchResultsByMODSEQ (id entry1, id entry2, void *data)
   return YES;
 }
 
+
 - (NSNumber *) modseqFromMessageChangeNumber: (NSString *) changeNum
 {
   NSDictionary *mapping;
   NSNumber *modseq;
+  NSEnumerator *enumerator;
+  id key;
+  uint64_t found, target, current, replica_id, current_cn;
+  NSString *closestChangeNum;
 
   mapping = [[versionsMessage properties] objectForKey: @"VersionMapping"];
   modseq = [mapping objectForKey: changeNum];
+  if (modseq) return modseq;
+
+  // Not found from stored change numbers for this folder.
+  // Get the closest modseq for the change number given.
+  // O(n) cost but will be unusual behaviour.
+  target = exchange_globcnt([changeNum unsignedLongLongValue] >> 16);
+  replica_id = [changeNum unsignedLongLongValue] & 0xFFFF;
+  found = 0;
+  enumerator  = [mapping keyEnumerator];
+  while ((key = [enumerator nextObject]))
+    {
+      current_cn = [(NSString *)key unsignedLongLongValue];
+      if ((current_cn & 0xFFFF) != replica_id)
+        continue;
+      current = exchange_globcnt(current_cn >> 16);
+      if (current < target && current > found)
+        found = current;
+    }
+
+  if (found)
+    {
+      closestChangeNum = [NSString stringWithUnsignedLongLong:
+                                   (exchange_globcnt(found) << 16 | replica_id)];
+      modseq = [mapping objectForKey: closestChangeNum];
+    }
 
   return modseq;
 }