import Wayland.Native
import Wayland.Basic.PosixSignal
import Wayland.Basic.Duration
import Wayland.Basic.Lens

/-!
# The Display Monad

All event handlers and effectful code is run within the `DisplayM` monad, which encapsulates the connection to a
Wayland server. Its main purpose is managing its event loop -- processing timers, signal handlers, and Wayland events.
Furthermore, it manages a global state `σ` and allows focussing on a relevant part of that state using
lenses.
-/

variable {σ α : Type}

namespace Wayland

/-- A token managing a single source of events such as a timer or a signal handler. This can be used to unregister the
event. -/
def EventSource : Type := Native.EventSource

/-- Internal handler interface passed as a message to the event loop. -/
private abbrev Handler (σ : Type) := StateT σ IO Unit

/-- Access to the display and event loop, providing the main functionality. -/
structure Display (σ : Type) where
  private disp : Native.Display
  private addTimer : (delayMs : UInt32) → (msg : Handler σ) → IO EventSource
  private addSignal : (signum : UInt32) → (msg : Handler σ) → IO EventSource
  private remove : (source : EventSource) → IO Unit
  private quit : IO Unit

/-- The display monad: A monad extending the `IO` monad with Wayland display related operations as well as state
management for a state type σ. -/
def DisplayM (σ : Type) : Type → Type :=
  ReaderT (Display σ) (StateT σ IO)

deriving instance Monad for DisplayM σ
deriving instance MonadState σ for DisplayM σ

private instance : MonadLift (ReaderT (Display σ) (StateT σ IO)) (DisplayM σ) where
  monadLift := id

/-- Connect to a Wayland server and start the application's main loop.

Parameters:
· `setUp`: Executed on startup before launching the event loop.
· `initialState`: The initial state value within the monad.
· `name`: Socket name for connecting to the Wayland server (by default, `wayland-0` is used). -/
partial def DisplayM.run' (setUp : DisplayM σ Unit) (initialState : σ) (name : Option String := .none) : IO σ := do
  let disp ← Native.Display.new name
  let loop ← Native.EventLoop.new
  _ ← loop.addDisplay disp
  let global : Display σ :=
    { disp := disp
      addTimer := loop.addTimer
      addSignal := loop.addSignal
      remove := loop.remove
      quit := loop.quit }
  let rec inner : StateT σ IO Unit := do
    match (← loop.poll) with
    | .none => return
    | .some msg => msg; inner
  let main : StateT σ IO Unit := do
    setUp.run global
    inner
  main.run initialState |>.map Prod.snd

/-- Connect to a Wayland server and start the application's main loop.

Parameters:
· `setUp`: Executed on startup before launching the event loop.
· `initialState`: The initial state value within the monad.
· `name`: Socket name for connecting to the Wayland server (by default, `wayland-0` is used). -/
def DisplayM.run (setUp : DisplayM σ Unit) (initialState : σ) (name : Option String := .none) : IO Unit :=
  DisplayM.run' setUp initialState (name := name) |> discard

/-- Lift an `IO` operation into the Display monad.

Note that `DisplayM` does not provide an instance for `MonadLift IO` to make access to `IO` more explicit. This should
not be used often. -/
def DisplayM.liftIO (io : IO α) : DisplayM σ α :=
  io

/-- Request the application to stop processing events and exit the event loop.

This can be sent at any time and stops the event loop before any further events can be processed. -/
def DisplayM.requestQuit : DisplayM σ Unit := do
  (← read).quit

/-- Register a (POSIX) signal handler.

When the given `signal` is caught, the `handler` is executed as a part of the event loop. The returned event source can
be used to unregister the handler. -/
def DisplayM.registerSignal (signal : PosixSignal) (handler : DisplayM σ Unit) : DisplayM σ EventSource := do
  let global ← read
  global.addSignal signal.signum (ReaderT.run handler global)

/-- Register a (one-shot) timer.

After the given `timeout`, the `handler` is executed as a part of the event loop. The returned event source can be used
to cancel the timer. -/
def DisplayM.registerTimer (timeout : Duration) (handler : DisplayM σ Unit) : DisplayM σ EventSource := do
  let global ← read
  global.addTimer timeout.toMilliseconds (ReaderT.run handler global)

/-- Unregister a previously registered event source.

If the handler is currently active, it will run to completion, but the event is no longer processed as a part of the
event loop. -/
def DisplayM.unregister (source : EventSource) : DisplayM σ Unit := do
  (← read).remove source

private def zoomState {σ₁ σ₂ : Type} {m : Type → Type} [Monad m] (lens : Lens σ₁ σ₂) (inner : StateT σ₂ m α) : StateT σ₁ m α := do
  let oldOuter ← get
  let (res, newInner) ← inner.run (lens.get oldOuter)
  set (lens.set newInner oldOuter)
  return res

/-- Zoom into a specific part of the state using a lens.

This is possible because handlers can only ever be submitted in the `DisplayM` monad, but never retrieved. -/
def DisplayM.zoom {σ₁ σ₂ : Type} (lens : Lens σ₁ σ₂) (inner : DisplayM σ₂ α) : DisplayM σ₁ α := do
  let global₁ : Display σ₁ ← read
  let global₂ : Display σ₂ :=
    { disp := global₁.disp
      addTimer := fun x y ↦ global₁.addTimer x (zoomState lens y)
      addSignal := fun x y ↦ global₁.addSignal x (zoomState lens y)
      remove := global₁.remove
      quit := global₁.quit }
  zoomState lens (ReaderT.run inner global₂)

end Wayland