Emoji: split qualification variation into a module

lib/pleroma/emoji/combinations.ex

@@ -0,0 +1,41 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2022 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Emoji.Combinations do
+  # FE0F is the emoji variation sequence. It is used for fully-qualifying
+  # emoji, and that includes emoji combinations.
+  # This code generates combinations per emoji: for each FE0F, all possible
+  # combinations of the character being removed or staying will be generated.
+  # This is made as an attempt to find all partially-qualified and unqualified
+  # versions of a fully-qualified emoji.
+  # I have found *no cases* for which this would be a problem, after browsing
+  # the entire emoji list in emoji-test.txt. This is safe, and, sadly, most
+  # likely sane too.
+
+  defp qualification_combinations([]), do: [[]]
+
+  defp qualification_combinations(["\uFE0F" | tail]) do
+    tail
+    |> qualification_combinations()
+    |> Enum.flat_map(fn x -> [x, ["\uFE0F" | x]] end)
+  end
+
+  defp qualification_combinations([codepoint | tail]) do
+    tail
+    |> qualification_combinations()
+    |> Enum.map(fn x -> [codepoint | x] end)
+  end
+
+  def variate_emoji_qualification(emoji) when is_binary(emoji) do
+    emoji
+    |> String.codepoints()
+    |> qualification_combinations()
+    |> Enum.map(&List.to_string/1)
+  end
+
+  def variate_emoji_qualification(emoji) when is_list(emoji) do
+    emoji
+    |> Enum.map(fn emoji -> {emoji, variate_emoji_qualification(emoji)} end)
+  end
+end