updated docs

docs/source/drag_and_drop_input.rst

@@ -212,6 +212,93 @@ for same number of draggables, anyof is equal to unordered_equal
 
 If we have can_reuse=true, than one must use only long form of correct answer.
 
+
+Grading logic
+-------------
+
+1. User answer and correct answer populated to the same form::
+
+    group_id: group_draggables, group_targets, group_rule
+
+Group_id is ordinal number, for every dict in correct answer incremental
+group_id is assigned: 0, 1, 2, ...
+
+Draggables from user answer are added to same group_id where identical draggables
+from correct answer are::
+
+    If correct_draggables[group_0] = [t1, t2] then
+       user_draggables[group_0] are all draggables t1 and t2 from user answer:
+        [t1] or [t1, t2] or [t1, t2, t2] etc..
+
+2. For every group user_draggables, if 'number' not in rule, set() is applicated,
+if 'number' not in rule, set is not applicated::
+
+    from [t1, t2, t3, t3] -> [t1, t2, ,t3]
+
+Set() and 'number' are needed only for case of reusable draggables,
+for other cases there are no equal draggables in list, so set() does nothing.
+
+Usege of set() operation allows easily create rule for case of
+``any number of same draggable can be dragged to some targets``::
+
+    {
+            'draggables': ['draggable_1'],
+            'targets': ['target3', 'target6', 'target9'],
+            'rule': 'anyof'
+    }
+
+'number' rule is used for the case of reusable draggables, when one want to
+fix number of draggable to drag. In this example only two instances of
+draggables_1 are allowed to be dragged::
+
+    {
+            'draggables': ['draggable_1', 'draggable_1'],
+            'targets': ['target3', 'target6', 'target9'],
+            'rule': 'anyof+number'
+    }
+
+Note, that in using rule 'exact', one does not need 'number', because you can't
+recognize from user interface which reusable draggable on which target.
+Absurd example::
+
+        {
+            'draggables': ['draggable_1', 'draggable_1', 'draggable_2'],
+            'targets': ['target3', 'target6', 'target9'],
+            'rule': 'exact'
+    }
+
+
+Correct handling of upper example is create different rule for draggable_1 and
+draggable_2
+
+For 'unordered_equal' (or 'exact') we not need 'number' if you have only same
+draggable in group, as targets length will provide contraing for the number of
+draggables::
+
+    {
+            'draggables': ['draggable_1'],
+            'targets': ['target3', 'target6', 'target9'],
+            'rule': 'unordered_equal'
+    }
+
+This means that only three draggaggables 'draggable_1' can be dragged.
+
+But if you have more that one different reusable draggable in lis, you may use
+'number' rule::
+
+    {
+            'draggables': ['draggable_1', 'draggable_1', 'draggable_2'],
+            'targets': ['target3', 'target6', 'target9'],
+            'rule': 'unordered_equal+number'
+    }
+
+If not use number, draggables list will be setted to  ['draggable_1', 'draggable_2']
+
+For every group, at this step, draggables lists are equal
+
+3. For every group, lists of targets are compared using rule for that group.
+
+
 Logic flow
 ----------