Add ties to the set of selections to use (#127)

Will randomly select from the top MaxResults set of ID clusters, and any tied with the sparsest set within the MaxResults.

This means when there is a contiguous set of IDs (e.g. when doing root_confirm with a fully broken root), any set of IDs will be chosen, not just sets within the first MaxResults.  This ensures a better spread of selections across the tree, so that the mismatched_segments will more accurately reflect the overall state of the tree

Author: martinsumner

src/aae_exchange.erl

@@ -993,7 +993,7 @@ intersect_ids(IDs0, IDs1) ->
 %% will wastefully correct the same data - so randomly chose one of the better
 %% lists
 select_ids(IDList, MaxOutput, StateName, ExchangeID, LogLevels)
-                                            when length(IDList) > MaxOutput ->
+        when length(IDList) > MaxOutput ->
     IDList0 = lists:sort(IDList),
     aae_util:log(
         ex005,
@@ -1010,8 +1010,21 @@ select_ids(IDList, MaxOutput, StateName, ExchangeID, LogLevels)
         end,
     {_EndIdx, SpaceIdxL} =
         lists:foldl(FoldFun, {1, []}, lists:zip(IDList1, IDList2)),
-    Selections = 
-        lists:sublist(lists:sort(SpaceIdxL), MaxOutput),
+    Selections =
+        case length(SpaceIdxL) of
+            L when L > MaxOutput ->
+                {MaxResults, TailResults} =
+                    lists:split(MaxOutput, lists:sort(SpaceIdxL)),
+                {LastChosenScore, _LastChosenIdx} = lists:last(MaxResults),
+                TiedSelections =
+                    lists:takewhile(
+                        fun({S, _I}) -> S == LastChosenScore end,
+                        TailResults
+                    ),
+                MaxResults ++ TiedSelections;
+            _ ->
+                lists:sort(SpaceIdxL)
+        end,
     {_ChosenSpace, ChosenIdx} =
         lists:nth(rand:uniform(length(Selections)), Selections),
     lists:sublist(IDList0, ChosenIdx, MaxOutput);
@@ -1026,8 +1039,8 @@ jitter_pause(Timeout) ->
     rand:uniform(Timeout) + Timeout div 2.
 
 
--spec reset({pos_integer(), pos_integer()}) 
-                                        -> {non_neg_integer(), pos_integer()}.
+-spec reset(
+    {pos_integer(), pos_integer()}) -> {non_neg_integer(), pos_integer()}.
 %% @doc
 %% Rest the count back to 0
 reset({Target, Target}) -> {0, Target}. 
@@ -1092,6 +1105,25 @@ select_best_id_rand_test() ->
     ?assertMatch({true, true, true},
                     lists:foldl(F, {false, false, false}, lists:seq(1, 1000))).
 
+
+select_id_withties_test() ->
+    L0 = lists:seq(1, 1000),
+    RL = select_ids(L0, 32, root_confirm, "t0.0", undefined),
+    ?assertMatch(32, length(RL)),
+    Select100 =
+        lists:map(
+            fun(_I) -> 
+                hd(select_ids(L0, 32, root_confirm, "t0.1", undefined))
+            end,
+            lists:seq(1, 100)
+        ),
+    % Expect to see heads evenly distributed
+    % - check we see both above and below 500
+    Above500 = lists:filter(fun(H) -> H > 500 end, Select100),
+    Below500 = lists:filter(fun(H) -> H < 500 end, Select100),
+    ?assert(length(Above500) > 0),
+    ?assert(length(Below500) > 0).
+
 compare_clocks_test() ->
     KV1 = {<<"B1">>, <<"K1">>, [{a, 1}]},
     KV2 = {<<"B1">>, <<"K2">>, [{b, 1}]},