next.exs

defmodule StrNext do
  @character_ranges [
    {'A', 'Z'},
    {'a', 'z'},
    {'0', '9'}
  ]
  @characters Enum.map(@character_ranges, fn {a, z} ->
                hd(a)..hd(z)
                |> Enum.to_list()
              end)
              |> List.flatten()

  def next(str) do
    String.reverse(str)
    |> next_string()
    |> String.reverse()
  end

  def next_string(""), do: <<hd(@characters)>>

  def next_string(<<byte, rest::bitstring>>) do
    case next_character(byte) do
      {c, true} -> <<c>> <> next_string(rest)
      {c, false} -> <<c>> <> rest
    end
  end

  def next_character(c) when c in @characters do
    if (c + 1) in @characters do
      {c + 1, false}
    else
      index = Enum.find_index(@characters, &(&1 == c))

      if c = Enum.at(@characters, index + 1) do
        {c, false}
      else
        {hd(@characters), true}
      end
    end
  end

  def next_character(c) do
    {c, true}
  end
end

# Passes these Ruby `String#succ` cases:
"abce" = StrNext.next("abcd")
"THX1139" = StrNext.next("THX1138")
"<<koalb>>" = StrNext.next("<<koala>>")

# Fails these Ruby `String#succ` cases:
# We don't restrict letters to letters and numbers to numbers.
#   "2000aaa" = StrNext.next("1999zzz")
"1999zz0" = StrNext.next("1999zzz")
# Ditto.
#   "AAAA0000" = StrNext.next("ZZZ9999")
"ZZaAAAA" = StrNext.next("ZZZ9999")
# We don't do punctuation.
#   "**+" = StrNext.next("***")
"A***" = StrNext.next("***")