Change hash algorithm for penciller

Switch from magic hash to md5 - to hopefully remove the need for some
of the artificial jumps required to get expected fall positive ratios.

Also split the hash into two 16-bit integers.  We assume that SegmentID
(from the perspective of AAE merkle/tictac trees) will always be at
least 16 bits.  the idea is that hashes should be used in blooms and
indexes such that some advantage can be gained from just knowing the
segmentID - in particular when folding over all the keys in a bucket.

Performance testing has been difficult so far - I think due to “cloud”
mysteries.

src/leveled_pmem.erl

@@ -50,7 +50,8 @@
 %%% API
 %%%============================================================================
 
--spec prepare_for_index(index_array(), integer()|no_lookup) -> index_array().
+-spec prepare_for_index(index_array(), {integer(), integer()}|no_lookup) 
+                                                            -> index_array().
 %% @doc
 %% Add the hash of a key to the index.  This is 'prepared' in the sense that
 %% this index is not use until it is loaded into the main index.
@@ -95,7 +96,7 @@ new_index() ->
 clear_index(_L0Index) ->
     new_index().
 
--spec check_index(integer(), index_array()) -> list(integer()).
+-spec check_index({integer(), integer()}, index_array()) -> list(integer()).
 %% @doc
 %% return a list of positions in the list of cache arrays that may contain the
 %% key associated with the hash being checked
@@ -158,9 +159,9 @@ to_list(Slots, FetchFun) ->
 %% checked (with the most recently received cache being checked first) until a
 %% match is found.
 check_levelzero(Key, PosList, TreeList) ->
-    check_levelzero(Key, leveled_codec:magic_hash(Key), PosList, TreeList).
+    check_levelzero(Key, leveled_codec:segment_hash(Key), PosList, TreeList).
 
--spec check_levelzero(tuple(), integer(), list(integer()), list())
+-spec check_levelzero(tuple(), {integer(), integer()}, list(integer()), list())
                                             -> {boolean(), tuple|not_found}.
 %% @doc
 %% Check for the presence of a given Key in the Level Zero cache, with the
@@ -204,10 +205,10 @@ find_pos(<<0:1/integer, NxtSlot:7/integer, T/binary>>, Hash, PosList, _SlotID) -
     find_pos(T, Hash, PosList, NxtSlot).
 
 
-split_hash(Hash) ->
-    Slot = Hash band 255,
-    H0 = (Hash bsr 8) band 8388607,
-    {Slot, H0}.
+split_hash({SegmentID, ExtraHash}) ->
+    Slot = SegmentID band 255,
+    H0 = (SegmentID bsr 8) bor (ExtraHash bsl 8),
+    {Slot, H0 band 8388607}.
 
 check_slotlist(Key, _Hash, CheckList, TreeList) ->
     SlotCheckFun =
@@ -358,7 +359,7 @@ with_index_test_() ->
 with_index_test2() ->
     IndexPrepareFun =
         fun({K, _V}, Acc) ->
-            H = leveled_codec:magic_hash(K),
+            H = leveled_codec:segment_hash(K),
             prepare_for_index(Acc, H)
         end,
     LoadFun =
@@ -382,7 +383,7 @@ with_index_test2() ->
 
     CheckFun =
         fun({K, V}, {L0Idx, L0Cache}) ->
-            H = leveled_codec:magic_hash(K),
+            H = leveled_codec:segment_hash(K),
             PosList = check_index(H, L0Idx),
             ?assertMatch({true, {K, V}},
                             check_slotlist(K, H, PosList, L0Cache)),