defmodule TimeZoneInfo.TimeZoneDatabase do
  @moduledoc """
  Implementation of the `Calendar.TimeZoneDatabase` behaviour.
  """

  alias TimeZoneInfo.DataStore
  alias TimeZoneInfo.NaiveDateTimeUtil, as: NaiveDateTime
  alias TimeZoneInfo.Transformer.RuleSet

  @behaviour Calendar.TimeZoneDatabase

  @compile {:inline, gap: 4, convert: 1, to_wall: 1, to_wall: 2}

  @seconds_per_day 24 * 60 * 60
  @microseconds_per_second 1_000_000
  @parts_per_day @seconds_per_day * @microseconds_per_second

  def time_zone_periods_from_wall_datetime(naive_datetime, time_zone) do
    naive_datetime
    |> NaiveDateTime.to_gregorian_seconds()
    |> periods_from_wall_gregorian_seconds(time_zone, naive_datetime)
  end

  def time_zone_period_from_utc_iso_days(iso_days, time_zone) do
    iso_days
    |> iso_days_to_gregorian_seconds()
    |> period_from_utc_gregorian_seconds(time_zone, iso_days)
  end

  defp periods_from_wall_gregorian_seconds(at_wall_seconds, time_zone, at_wall_date_time) do
    case DataStore.get_transitions(time_zone) do
      {:ok, transitions} ->
        transitions
        |> find_transitions(at_wall_seconds)
        |> to_periods(at_wall_seconds, at_wall_date_time)

      {:error, :transitions_not_found} ->
        {:error, :time_zone_not_found}
    end
  end

  defp period_from_utc_gregorian_seconds(gregorian_seconds, time_zone, date_time) do
    case DataStore.get_transitions(time_zone) do
      {:ok, transitions} ->
        transitions
        |> find_period(gregorian_seconds)
        |> to_period(date_time)

      {:error, :transitions_not_found} ->
        {:error, :time_zone_not_found}
    end
  end

  defp find_period(transitions, timestamp) do
    Enum.find_value(transitions, fn {at, period} ->
      with true <- at <= timestamp, do: period
    end)
  end

  defp find_transitions(transitions, at_wall) do
    Enum.reduce_while(transitions, nil, fn
      transition_a, {:none, transition_b, transition_c} ->
        {:halt, {transition_a, transition_b, transition_c}}

      {at_utc, _} = transition, _ when at_utc > at_wall ->
        {:cont, transition}

      transition, nil ->
        {:cont, {:none, transition, :none}}

      transition, last_transition ->
        {:cont, {:none, transition, last_transition}}
    end)
  end

  defp to_period({_, _, {_, _}} = transition_rules, {_, {_, _}} = iso_days) do
    to_period(transition_rules, NaiveDateTime.from_iso_days(iso_days))
  end

  defp to_period(
         {utc_offset, rule_name, {_, _} = format},
         %Elixir.NaiveDateTime{} = naive_datetime
       ) do
    case DataStore.get_rules(rule_name) do
      {:ok, rules} ->
        rules
        |> transitions(utc_offset, format, naive_datetime.year)
        |> find_period(NaiveDateTime.to_gregorian_seconds(naive_datetime))
        |> to_period(nil)

      {:error, :rules_not_found} ->
        {:error, :time_zone_not_found}
    end
  end

  defp to_period({utc_offset, std_offset, zone_abbr}, _),
    do: {:ok, %{utc_offset: utc_offset, std_offset: std_offset, zone_abbr: zone_abbr}}

  defp to_periods({:none, {_at, {utc_offset, std_offset, zone_abbr}}, :none}, _, _) do
    {:ok, %{utc_offset: utc_offset, std_offset: std_offset, zone_abbr: zone_abbr}}
  end

  defp to_periods({:none, a, b}, at_wall, at_wall_datetime) do
    to_periods({a, b}, at_wall, at_wall_datetime)
  end

  defp to_periods({a, b, :none}, at_wall, at_wall_datetime) do
    to_periods({a, b}, at_wall, at_wall_datetime)
  end

  defp to_periods(
         {_transition, {_, {utc_offset, rule_name, format}}},
         at_wall,
         at_wall_datetime
       )
       when is_binary(rule_name) do
    calculate_periods(utc_offset, rule_name, format, at_wall, at_wall_datetime)
  end

  defp to_periods({transition_a, transition_b}, at_wall, _at_wall_datetime) do
    at_wall_b = to_wall(transition_b)
    at_wall_ba = to_wall(transition_b, transition_a)

    cond do
      at_wall_b <= at_wall && at_wall < at_wall_ba ->
        {:ambiguous, convert(transition_a), convert(transition_b)}

      at_wall_ba <= at_wall && at_wall < at_wall_b ->
        gap(transition_a, at_wall_ba, transition_b, at_wall_b)

      at_wall < at_wall_b ->
        {:ok, convert(transition_a)}

      true ->
        {:ok, convert(transition_b)}
    end
  end

  defp to_periods(
         {_transition_a, _transition_b, {_, {utc_offset, rule_name, format}}},
         at_wall,
         at_wall_datetime
       )
       when is_binary(rule_name) and is_integer(utc_offset) do
    calculate_periods(utc_offset, rule_name, format, at_wall, at_wall_datetime)
  end

  defp to_periods({transition_a, transition_b, transition_c}, at_wall, _at_wall_datetime) do
    at_wall_a = to_wall(transition_a)
    at_wall_ba = to_wall(transition_b, transition_a)
    at_wall_b = to_wall(transition_b)
    at_wall_cb = to_wall(transition_c, transition_b)
    at_wall_c = to_wall(transition_c)

    cond do
      at_wall >= at_wall_c && at_wall < at_wall_cb ->
        {:ambiguous, convert(transition_b), convert(transition_c)}

      at_wall >= at_wall_c ->
        {:ok, convert(transition_c)}

      at_wall >= at_wall_cb ->
        gap(transition_b, at_wall_cb, transition_c, at_wall_c)

      at_wall >= at_wall_b && at_wall < at_wall_ba ->
        {:ambiguous, convert(transition_a), convert(transition_b)}

      at_wall >= at_wall_b ->
        {:ok, convert(transition_b)}

      at_wall >= at_wall_ba ->
        gap(transition_a, at_wall_ba, transition_b, at_wall_b)

      at_wall >= at_wall_a ->
        {:ok, convert(transition_a)}
    end
  end

  defp calculate_periods(utc_offset, rule_name, format, at_wall_seconds, at_wall_datetime) do
    case DataStore.get_rules(rule_name) do
      {:ok, rules} ->
        rules
        |> transitions(utc_offset, format, at_wall_datetime.year)
        |> find_transitions(at_wall_seconds)
        |> to_periods(at_wall_seconds, at_wall_datetime)

      {:error, :rules_not_found} ->
        {:error, :time_zone_not_found}
    end
  end

  defp transitions(rules, utc_offset, format, year) do
    rules
    |> to_rule_set(year)
    |> RuleSet.transitions(utc_offset, format)
    |> Enum.map(fn {at, period} -> {NaiveDateTime.to_gregorian_seconds(at), period} end)
  end

  @spec to_rule_set([TimeZoneInfo.rule()], Calendar.year()) :: [TimeZoneInfo.transition()]
  defp to_rule_set(rules, year) do
    Enum.flat_map(rules, fn {{month, day, time}, time_standard, std_offset, letters} ->
      Enum.into((year - 1)..(year + 1), [], fn year ->
        at = NaiveDateTime.from_iana(year, month, day, time)
        {at, {time_standard, std_offset, letters}}
      end)
    end)
    |> NaiveDateTime.sort()
  end

  defp gap(transition_a, at_a, transition_b, at_b) do
    limit_a = NaiveDateTime.from_gregorian_seconds(at_a)
    limit_b = NaiveDateTime.from_gregorian_seconds(at_b)
    {:gap, {convert(transition_a), limit_a}, {convert(transition_b), limit_b}}
  end

  defp to_wall({at, {utc_offset, std_offset, _}}),
    do: at + utc_offset + std_offset

  defp to_wall({at, {_, _, _}}, {_, {utc_offset, std_offset, _}}),
    do: at + utc_offset + std_offset

  defp convert({_, {utc_offset, std_offset, zone_abbr}}),
    do: %{utc_offset: utc_offset, std_offset: std_offset, zone_abbr: zone_abbr}

  defp iso_days_to_gregorian_seconds({days, {parts_in_day, @parts_per_day}}) do
    div(days * @parts_per_day + parts_in_day, @microseconds_per_second)
  end
end