import gleam/erlang/atom
import gleam/erlang/process.{type Pid}
import gleam/function
import gleam/int
import gleam/otp/actor
import gleam/result
import gleam/string
import logging

pub type ClockMessage {
  SetTime
}

type ClockTable {
  MistClock
}

type TableKey {
  DateHeader
}

pub type EtsOpts {
  Set
  Protected
  NamedTable
  ReadConcurrency(Bool)
}

pub fn start(_type, _args) -> Result(Pid, actor.StartError) {
  actor.start_spec(
    actor.Spec(
      init: fn() {
        let subj = process.new_subject()
        let selector =
          process.new_selector()
          |> process.selecting(subj, function.identity)
        ets_new(MistClock, [Set, Protected, NamedTable, ReadConcurrency(True)])
        process.send(subj, SetTime)
        actor.Ready(subj, selector)
      },
      init_timeout: 500,
      loop: fn(msg, state) {
        case msg {
          SetTime -> {
            ets_insert(MistClock, #(DateHeader, date()))
            process.send_after(state, 1000, SetTime)
            actor.continue(state)
          }
        }
      },
    ),
  )
  |> result.map(process.subject_owner)
}

pub fn stop(_state) {
  atom.create_from_string("ok")
}

pub fn get_date() -> String {
  case ets_lookup_element(MistClock, DateHeader, 2) {
    Ok(value) -> value
    _ -> {
      logging.log(logging.Warning, "Failed to lookup date, re-calculating")
      date()
    }
  }
}

/// Returns today's date in a format suitable to be used as an http date header
/// (see here: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Date).
///
fn date() -> String {
  let #(weekday, #(year, month, day), #(hour, minute, second)) = now()

  let weekday = weekday_to_short_string(weekday)
  let year = int.to_string(year) |> string.pad_start(to: 4, with: "0")
  let month = month_to_short_string(month)
  let day = int.to_string(day) |> string.pad_start(to: 2, with: "0")
  let hour = int.to_string(hour) |> string.pad_start(to: 2, with: "0")
  let minute = int.to_string(minute) |> string.pad_start(to: 2, with: "0")
  let second = int.to_string(second) |> string.pad_start(to: 2, with: "0")

  { weekday <> ", " }
  <> { day <> " " <> month <> " " <> year <> " " }
  <> { hour <> ":" <> minute <> ":" <> second <> " GMT" }
}

fn weekday_to_short_string(weekday: Int) -> String {
  case weekday {
    1 -> "Mon"
    2 -> "Tue"
    3 -> "Wed"
    4 -> "Thu"
    5 -> "Fri"
    6 -> "Sat"
    7 -> "Sun"
    _ -> panic as "erlang weekday outside of 1-7 range"
  }
}

fn month_to_short_string(month: Int) -> String {
  case month {
    1 -> "Jan"
    2 -> "Feb"
    3 -> "Mar"
    4 -> "Apr"
    5 -> "May"
    6 -> "Jun"
    7 -> "Jul"
    8 -> "Aug"
    9 -> "Sep"
    10 -> "Oct"
    11 -> "Nov"
    12 -> "Dec"
    _ -> panic as "erlang month outside of 1-12 range"
  }
}

@external(erlang, "mist_ffi", "now")
fn now() -> #(Int, #(Int, Int, Int), #(Int, Int, Int))

@external(erlang, "ets", "new")
fn ets_new(table: ClockTable, opts: List(EtsOpts)) -> ClockTable

@external(erlang, "ets", "insert")
fn ets_insert(table: ClockTable, value: #(TableKey, String)) -> Nil

@external(erlang, "mist_ffi", "ets_lookup_element")
fn ets_lookup_element(
  table: ClockTable,
  key: TableKey,
  position: Int,
) -> Result(String, Nil)