September 2017 Update

Task/Longest-common-subsequence/Erlang/longest-common-subsequence-4.erl

@@ -0,0 +1,39 @@
+-module(lcs).
+
+%% API exports
+-export([
+        lcs/2
+]).
+
+%%====================================================================
+%% API functions
+%%====================================================================
+
+lcs(A, B) ->
+        {LCS, _Cache} = get_lcs(A, B, [], #{}),
+        lists:reverse(LCS).
+
+%%====================================================================
+%% Internal functions
+%%=====================================================
+
+get_lcs(A, B, Acc, Cache) ->
+        case maps:find({A, B, Acc}, Cache) of
+                {ok, LCS} -> {LCS, Cache};
+                error     ->
+                        {NewLCS, NewCache} = compute_lcs(A, B, Acc, Cache),
+                        {NewLCS, NewCache#{ {A, B, Acc} => NewLCS }}
+        end.
+
+compute_lcs(A, B, Acc, Cache) when length(A) == 0 orelse length(B) == 0 ->
+        {Acc, Cache};
+compute_lcs([Token |ATail], [Token |BTail], Acc, Cache) ->
+        get_lcs(ATail, BTail, [Token |Acc], Cache);
+compute_lcs([_AToken |ATail]=A, [_BToken |BTail]=B, Acc, Cache) ->
+        {LCSA, CacheA} = get_lcs(A, BTail, Acc, Cache),
+        {LCSB, CacheB} = get_lcs(ATail, B, Acc, CacheA),
+        LCS = case length(LCSA) > length(LCSB) of
+                true  -> LCSA;
+                false -> LCSB
+        end,
+        {LCS, CacheB}.

Task/Longest-common-subsequence/Erlang/longest-common-subsequence-5.erl

@@ -0,0 +1,2 @@
+48> lcs:lcs("thisisatest", "testing123testing").
+"tsitest"