defmodule Rsh26pool.JST do
  @moduledoc """
  Helpers for converting between UTC (how timestamps are stored) and
  Japan Standard Time (JST, UTC+9) which is what the UI shows and accepts.

  A full time zone database is not configured for this demo, and JST has no
  daylight saving time, so a fixed +9h offset is sufficient and dependency free.
  """

  @offset_seconds 9 * 3600

  @doc """
  Parses a `datetime-local` form value (e.g. `"2026-07-04T15:30"`), interpreting
  it as wall-clock JST, and returns the corresponding UTC `DateTime`.
  """
  @spec parse_local(String.t()) :: {:ok, DateTime.t()} | :error
  def parse_local(value) when is_binary(value) do
    with {:ok, naive} <- NaiveDateTime.from_iso8601(ensure_seconds(value)) do
      utc =
        naive
        |> DateTime.from_naive!("Etc/UTC")
        |> DateTime.add(-@offset_seconds, :second)
        |> DateTime.truncate(:second)

      {:ok, utc}
    else
      _ -> :error
    end
  end

  def parse_local(_), do: :error

  @doc "Converts a stored UTC `DateTime` to a JST `DateTime` for display."
  @spec to_local(DateTime.t()) :: DateTime.t()
  def to_local(%DateTime{} = dt), do: DateTime.add(dt, @offset_seconds, :second)

  @doc "Formats a stored UTC `DateTime` as a human friendly JST string."
  @spec format(DateTime.t() | nil) :: String.t()
  def format(nil), do: ""

  def format(%DateTime{} = dt) do
    l = to_local(dt)

    "~4..0B/~2..0B/~2..0B ~2..0B:~2..0B"
    |> :io_lib.format([l.year, l.month, l.day, l.hour, l.minute])
    |> to_string()
  end

  defp ensure_seconds(value) do
    if Regex.match?(~r/T\d{2}:\d{2}$/, value), do: value <> ":00", else: value
  end
end