Add List.ends_with?/2

Author: sabiwara

CHANGELOG.md

@@ -13,6 +13,7 @@ This release no longer supports WERL (a graphical user interface for the Erlang
   * [Exception] Add `MissingApplicationsError` exception to denote missing applications
   * [Kernel] Update source code parsing to match [UTS #55](https://www.unicode.org/reports/tr55/) latest recommendations. In particular, mixed script is allowed in identifiers as long as they are separate by underscores (`_`), such as `http_сервер`. Previously allowed highly restrictive identifiers, which mixed Latin and other scripts, such as the japanese word for t-shirt, `Tシャツ`, now require the underscore as well
   * [Kernel] Warn on bidirectional confusability in identifiers
+  * [List] Add `List.ends_with?/2`
   * [Macro] Improve `dbg` handling of `if/2`, `unless/2`, and code blocks
   * [Process] Handle arbitrarily high integer values in `Process.sleep/1`
   * [String] Inspect special whitespace and zero-width characters using their Unicode representation

lib/elixir/lib/list.ex

@@ -924,6 +924,44 @@ defmodule List do
   def starts_with?(list, []) when is_list(list), do: true
   def starts_with?(list, [_ | _]) when is_list(list), do: false
 
+  @doc """
+  Returns `true` if `list` ends with the given `suffix` list, otherwise returns `false`.
+
+  If `suffix` is an empty list, it returns `true`.
+
+  ### Examples
+
+      iex> List.ends_with?([1, 2, 3], [2, 3])
+      true
+
+      iex> List.ends_with?([1, 2], [1, 2, 3])
+      false
+
+      iex> List.ends_with?([:alpha], [])
+      true
+
+      iex> List.ends_with?([], [:alpha])
+      false
+
+  """
+  @doc since: "1.18.0"
+  @spec ends_with?(nonempty_list, nonempty_list) :: boolean
+  @spec ends_with?(list, []) :: true
+  @spec ends_with?([], nonempty_list) :: false
+  def ends_with?(list, suffix) do
+    case ends_with_offset(list, suffix) do
+      nil -> false
+      n -> :lists.nthtail(n, list) === suffix
+    end
+  end
+
+  defp ends_with_offset([], [_ | _]), do: nil
+  defp ends_with_offset(rest, []), do: length(rest)
+
+  defp ends_with_offset([_ | tail], [_ | suffix_tail]) do
+    ends_with_offset(tail, suffix_tail)
+  end
+
   @doc """
   Converts a charlist to an atom.
 

lib/elixir/test/elixir/list_test.exs

@@ -292,6 +292,45 @@ defmodule ListTest do
     end
   end
 
+  describe "ends_with?/2" do
+    test "list and prefix are equal" do
+      assert List.ends_with?([], [])
+      assert List.ends_with?([1], [1])
+      assert List.ends_with?([1, 2, 3], [1, 2, 3])
+    end
+
+    test "proper lists" do
+      refute List.ends_with?([2], [1, 2])
+      assert List.ends_with?([1, 2, 3], [2, 3])
+      refute List.ends_with?([2, 3, 4], [1, 2, 3, 4])
+    end
+
+    test "list is empty" do
+      refute List.ends_with?([], [1])
+      refute List.ends_with?([], [1, 2])
+    end
+
+    test "prefix is empty" do
+      assert List.ends_with?([1], [])
+      assert List.ends_with?([1, 2], [])
+      assert List.ends_with?([1, 2, 3], [])
+    end
+
+    test "only accepts proper lists" do
+      message = "no function clause matching in List.ends_with_offset/2"
+
+      assert_raise FunctionClauseError, message, fn ->
+        List.ends_with?([1 | 2], [1 | 2])
+      end
+
+      message = "no function clause matching in List.ends_with_offset/2"
+
+      assert_raise FunctionClauseError, message, fn ->
+        List.ends_with?([1, 2], 1)
+      end
+    end
+  end
+
   test "to_string/1" do
     assert List.to_string([?æ, ?ß]) == "æß"
     assert List.to_string([?a, ?b, ?c]) == "abc"