defmodule Zoneinfo do
  @moduledoc """
  Elixir time zone support for your OS-supplied time zone database

  Tell Elixir to use this as the default time zone database by running:

  ```elixir
  Calendar.put_time_zone_database(Zoneinfo.TimeZoneDatabase)
  ```

  Time zone data is loaded from the path returned by `tzpath/0`. The default
  is to use `/usr/share/zoneinfo`, but that may be changed by setting the
  `$TZPATH` environment or adding the following to your project's `config.exs`:

  ```elixir
  config :zoneinfo, tzpath: "/custom/location"
  ```

  Call `time_zones/0` to get the list of supported time zones.
  """

  @doc """
  Return all known time zones

  This function scans the path returned by `tzpath/0` for all time zones and
  performs a basic check on each file. It may not be fast. It will not return
  the aliases that zoneinfo uses for backwards compatibility even though they
  may still work.
  """
  @spec time_zones() :: [String.t()]
  def time_zones() do
    path = Path.expand(tzpath())

    Path.join(path, "**")
    |> Path.wildcard()
    # Filter out directories and symlinks to old names of time zones
    |> Enum.filter(fn f -> File.lstat!(f, time: :posix).type == :regular end)
    # Filter out anything that doesn't look like a TZif file
    |> Enum.filter(&contains_tzif?/1)
    # Fix up the remaining paths to look like time zones
    |> Enum.map(&String.replace_leading(&1, path <> "/", ""))
  end

  @doc """
  Return the path to the time zone files
  """
  @spec tzpath() :: binary()
  def tzpath() do
    with nil <- Application.get_env(:zoneinfo, :tzpath),
         nil <- System.get_env("TZPATH") do
      "/usr/share/zoneinfo"
    end
  end

  defp contains_tzif?(path) do
    case File.open(path, [:read], &contains_tzif_helper/1) do
      {:ok, result} -> result
      _error -> false
    end
  end

  defp contains_tzif_helper(io) do
    with buff when is_binary(buff) and byte_size(buff) == 8 <- IO.binread(io, 8),
         {:ok, _version} <- Zoneinfo.TZif.version(buff) do
      true
    else
      _anything -> false
    end
  end
end