[Upstream][Retrieval] Unmerged PR#4 - add longest common prefix function

lib/retrieval.ex

@@ -132,6 +132,44 @@ defmodule Trieval do
     end)
   end
 
+  @doc """
+  Collects all binaries that begin with a given prefix. Returns matching binaries, along
+  with matching binaries' longest common prefix. Example use-case would be for auto-completion.
+
+  ## Examples
+
+        Trieval.new(~w/apple apply ape/) |> Trieval.longest_common_prefix("a")
+        {"ap", ["apple", "apply", "ape"]}
+
+        Trieval.new(~w/apple apply ape ample/) |> Trieval.longest_common_prefix("z")
+        {nil, []}
+
+  """
+
+  def longest_common_prefix(%Trie{trie: trie}, binary) when is_binary(binary) do
+    _longest_common_prefix(trie, binary, binary)
+  end
+
+  defp _longest_common_prefix(trie, <<next, rest :: binary>>, acc) do
+    case Map.has_key?(trie, next) do
+      true  -> _longest_common_prefix(trie[next], rest, acc)
+      false -> {nil, []}
+    end
+  end
+
+  defp _longest_common_prefix(trie, <<>>, acc) do
+    case Enum.count(trie) do
+      1 ->
+        case Map.keys(trie) do
+          [:mark] -> {acc, [acc]}
+          [ch] -> _longest_common_prefix(trie[ch], <<>>, acc <> <<ch>>)
+        end
+      _ ->
+        matches = _prefix(trie, <<>>, acc)
+        {acc, matches}
+    end
+  end
+
   @doc """
   Collects all binaries match a given pattern. Returns either a list of matches
   or an error in the form `{:error, reason}`.

test/retrieval_test.exs

@@ -4,7 +4,8 @@ defmodule TrievalTest do
 
   @test_data ~w/apple apply ape bed between betray cat cold hot
                 warm winter maze smash crush under above people
-                negative poison place out divide zebra extended/
+                negative poison place out divide zebra extended
+                friend friendly fried frieze/
 
   @test_trie Trieval.new(@test_data)
 
@@ -25,6 +26,13 @@ defmodule TrievalTest do
     assert Trieval.prefix(@test_trie, "abc") == []
   end
 
+  test "longest_common_prefix" do
+    assert Trieval.longest_common_prefix(@test_trie, "fr") == {"frie", ["fried", "friendly", "friend", "frieze"]}
+    assert Trieval.longest_common_prefix(@test_trie, "frien") == {"friend", ["friendly", "friend"]}
+    assert Trieval.longest_common_prefix(@test_trie, "winter") == {"winter", ["winter"]}
+    assert Trieval.longest_common_prefix(@test_trie, "abc") == {nil, []}
+  end
+
   test "pattern errors" do
     assert match?({:error, _}, Trieval.pattern(@test_trie, "ab*[^zsd"))
     assert match?({:error, _}, Trieval.pattern(@test_trie, "ab*[^zsd]{}"))