[ConfigImpact] Handle hash collision

Summary:
Two methods with identical method names but different number/type of args will have the same hash: e.g. `foo(int x)` and `foo(int x, int y)`. For Config Impact analysis, we assumed this type of hash collusion would never happen when we are comparing config-impact reports, but that assumption is wrong as demonstrated by the modified tests.

To deal with these, in cost analysis, we pick the highest degree among the potential collisions. We follow a similar idea here, picking the highest number of unchecked callees.

That has its own disadvantages:
E.g.  giving an example from cost, if we had `foo(int x)` with O(1) before, and after the change, we have also added a linear `foo(int x, int y)`, I think we would introduce a complexity increase.

 Still, it is better than picking only the first/last.

Reviewed By: skcho

Differential Revision: D27156722

fbshipit-source-id: c37388f1c

infer/src/integration/Differential.ml

@@ -405,6 +405,10 @@ module ConfigImpactItem = struct
 
   type change_type = Added | Removed
 
+  let compare_by_unchecked_callees_length {unchecked_callees= u1} {unchecked_callees= u2} =
+    Int.compare (UncheckedCallees.cardinal u1) (UncheckedCallees.cardinal u2)
+
+
   let pp_change_type f x =
     Format.pp_print_string f (match x with Added -> "added" | Removed -> "removed")
 
@@ -449,9 +453,17 @@ module ConfigImpactItem = struct
 
   let issues_of_reports ~(current_config_impact : Jsonbug_t.config_impact_report)
       ~(previous_config_impact : Jsonbug_t.config_impact_report) =
+    let get_biggest report =
+      (* two methods with identical method names but different
+          number/type of args will have the same hash. We pick the one with the biggest unchecked callees. *)
+      Option.value_exn (List.max_elt report ~compare:compare_by_unchecked_callees_length)
+    in
     let fold_aux ~key:_ ~data ((acc_introduced, acc_fixed) as acc) =
       match data with
-      | `Both (current, previous) ->
+      | `Both (current_reports, previous_reports) ->
+          (* current/previous reports cannot be empty. *)
+          let current = get_biggest current_reports in
+          let previous = get_biggest previous_reports in
           let introduced =
             UncheckedCallees.diff current.unchecked_callees previous.unchecked_callees
             |> issue_of ~change_type:Added current.config_impact_item
@@ -470,7 +482,7 @@ module ConfigImpactItem = struct
       List.fold ~init:String.Map.empty config_impact
         ~f:(fun acc ({Jsonbug_t.hash= key; unchecked_callees} as config_impact_item) ->
           let unchecked_callees = UncheckedCallees.decode unchecked_callees in
-          String.Map.add_exn acc ~key ~data:{config_impact_item; unchecked_callees} )
+          String.Map.add_multi acc ~key ~data:{config_impact_item; unchecked_callees} )
     in
     let current_map = map_of_config_impact current_config_impact in
     let previous_map = map_of_config_impact previous_config_impact in