Split hash - seperate key has for bxor with value

src/leveled_codec.erl

@@ -176,7 +176,8 @@
 %% speed can be gained if just the segment ID is known - but more can be 
 %% gained should the extended hash (with the second element) is known
 segment_hash(Key) when is_binary(Key) ->
-    {segment_hash, SegmentID, ExtraHash} = leveled_tictac:keyto_segment48(Key),
+    {segment_hash, SegmentID, ExtraHash, _AltHash}
+        = leveled_tictac:keyto_segment48(Key),
     {SegmentID, ExtraHash};
 segment_hash({?RIAK_TAG, Bucket, Key, null}) 
                                     when is_binary(Bucket), is_binary(Key) ->

src/leveled_tictac.erl

@@ -66,7 +66,6 @@
             fetch_leaves/2,
             merge_trees/2,
             get_segment/2,
-            tictac_hash/2,
             export_tree/1,
             import_tree/1,
             valid_size/1,
@@ -108,10 +107,14 @@
                         level2 :: any() % an array - but OTP compatibility
                         }).
 
--type tictactree() :: #tictactree{}.
--type segment48() :: {segment_hash, integer(), integer()}.
--type tree_extract() :: {binary(), integer(), integer(), integer(), binary()}.
--type tree_size() :: xxsmall|xsmall|small|medium|large|xlarge.
+-type tictactree() ::
+    #tictactree{}.
+-type segment48() ::
+    {segment_hash, non_neg_integer(), non_neg_integer(), non_neg_integer()}.
+-type tree_extract() ::
+    {binary(), integer(), integer(), integer(), binary()}.
+-type tree_size() :: 
+    xxsmall|xsmall|small|medium|large|xlarge.
 
 -export_type([tictactree/0, segment48/0, tree_size/0]).
 
@@ -327,7 +330,7 @@ get_segment(Hash, TreeSize) ->
 %% erlang:phash2.  If an exportable hash of the value is required this should
 %% be managed through the add_kv ExtractFun providing a pre-prepared Hash.
 tictac_hash(BinKey, Val) when is_binary(BinKey) ->
-    HashKey = keyto_segment32(BinKey),
+    {HashKeyToSeg, AltHashKey} = keyto_doublesegment32(BinKey),
     HashVal = 
         case Val of 
             {is_hash, HashedVal} ->
@@ -335,13 +338,23 @@ tictac_hash(BinKey, Val) when is_binary(BinKey) ->
             _ ->
                 erlang:phash2(Val)
         end,
-    {HashKey, HashKey bxor HashVal}.
+    {HashKeyToSeg, AltHashKey bxor HashVal}.
+
+-spec keyto_doublesegment32(binary())
+                                    -> {non_neg_integer(), non_neg_integer()}.
+%% @doc
+%% Used in tictac_hash/2 to provide an alternative hash of the key to bxor with
+%% the value, as well as the segment hash to locate the leaf of the tree to be
+%% updated
+keyto_doublesegment32(BinKey) when is_binary(BinKey) ->
+    Segment48 = keyto_segment48(BinKey),
+    {keyto_segment32(Segment48), element(4, Segment48)}.
 
 -spec keyto_segment32(any()) -> integer().
 %% @doc
 %% The first 16 bits of the segment hash used in the tictac tree should be 
 %% made up of the segment ID part (which is used to accelerate queries)
-keyto_segment32({segment_hash, SegmentID, ExtraHash}) 
+keyto_segment32({segment_hash, SegmentID, ExtraHash, _AltHash}) 
                         when is_integer(SegmentID), is_integer(ExtraHash) ->
     (ExtraHash band 65535) bsl 16 + SegmentID;
 keyto_segment32(BinKey) when is_binary(BinKey) ->
@@ -354,9 +367,11 @@ keyto_segment32(Key) ->
 %% Produce a segment with an Extra Hash part - for tictac use most of the 
 %% ExtraHash will be discarded
 keyto_segment48(BinKey) ->
-    <<SegmentID:16/integer, ExtraHash:32/integer, _Rest/binary>> = 
-        crypto:hash(md5, BinKey),
-    {segment_hash, SegmentID, ExtraHash}.
+    <<SegmentID:16/integer,
+        ExtraHash:32/integer,
+        AltHash:32/integer,
+        _Rest/binary>> =  crypto:hash(md5, BinKey),
+    {segment_hash, SegmentID, ExtraHash, AltHash}.
 
 -spec generate_segmentfilter_list(list(integer()), tree_size())  
                                                     -> false|list(integer()).