string_trimmer.ex

defmodule StringTrimmer do
  def trim_utf8(string, len), do: trim(string, len, &utf8_counter/1)

  def trim_utf16(string, len), do: trim(string, len, &utf16_counter/1)

  def trim(string, len, counter) do
    string
    |> String.graphemes()
    |> Enum.reduce_while({0, ""}, fn grapheme, {count, acc} ->
      count = count + counter.(grapheme)

      if count <= len,
        do: {:cont, {count, acc <> grapheme}},
        else: {:halt, {count, acc}}
    end)
    |> elem(1)
  end

  defp utf8_counter(string), do: string |> String.codepoints() |> Enum.count()

  defp utf16_counter(string) do
    string
    |> :unicode.characters_to_binary(:utf8, :utf16)
    |> byte_size()
    |> div(2)
  end
end