Add A-factor to E-factor conversion to SuperMemo import.

The SuperMemo XML importer assumed that the A-factors used by
SuperMemo map 1-1 with E-factors (which correspond to Ease in
Anki). This resulted in too large E-factors for imported items.

This change adds an A-factor to E-factor conversion to the
importer.

anki/importing/supermemo_xml.py

@@ -142,6 +142,28 @@ class SupermemoXmlImporter(NoteImporter):
 
         return unicode(btflsoup(s, selfClosingTags=['br','hr','img','wbr'], convertEntities=btflsoup.HTML_ENTITIES))
 
+    def _afactor2efactor(self, af):
+        # Adapted from <http://www.supermemo.com/beta/xml/xml-core.htm>
+
+        # Ranges for A-factors and E-factors
+        af_min = 1.2
+        af_max = 6.9
+        ef_min = 1.3
+        ef_max = 3.3
+
+        # Sanity checks for the A-factor
+        if af < af_min:
+            af = af_min
+        elif af > af_max:
+            af = af_max
+
+        # Scale af to the range 0..1
+        af_scaled = (af - af_min) / (af_max - af_min)
+        # Rescale to the interval ef_min..ef_max
+        ef = ef_min + af_scaled * (ef_max - ef_min)
+
+        return ef
+
 ## DEFAULT IMPORTER METHODS
 
     def foreignNotes(self):
@@ -191,7 +213,7 @@ class SupermemoXmlImporter(NoteImporter):
             nextDue = tLastrep + (float(item.Interval) * 86400.0)
             remDays = int((nextDue - time.time())/86400)
             card.due = self.col.sched.today+remDays
-            card.factor = int(float(item.AFactor.replace(',','.'))*1000)
+            card.factor = int(self._afactor2efactor(float(item.AFactor.replace(',','.')))*1000)
             note.cards[0] = card
 
         # categories & tags