{-|
    Tracks terminal state so that new terminal connections can be brought
    up to speed.
-}
module Urbit.Vere.Term.Logic
    ( SpinnerCause(..), St, Ev(..), Ef(..)
    , init
    , step
    , drawState
    , fromTermEv
    , toTermEv
    ) where

import Urbit.Prelude hiding (init)

import Data.Sequence (Seq((:<|)))

import qualified Urbit.Arvo          as Arvo
import qualified Urbit.Vere.Term.API as Term


--------------------------------------------------------------------------------

data SpinnerCause = User | Event Text
  deriving (Show)

type SpinnerState = Maybe SpinnerCause

{-|
    %line -- Output a line above the edit line.
    %spin -- Set the spinner state.
    %bell -- Ring a bell (no change to the state).
    %draw -- Redraw the current line (no change to the state).
    %move -- Move the cursor position.
    %edit -- Set the edit line, moving the cursor to the end.
    %more -- Write the edit line to history, and clear it.
-}
data Ev
    = EvLine Text
    | EvSlog (Atom, Tank)
    | EvSpin SpinnerState
    | EvMove Word
    | EvBell
    | EvDraw
    | EvEdit Text
    | EvMore
  deriving (Show)

data Ef
    = EfClear
    | EfWrite Text
    | EfShift Int
    | EfRing
    | EfSpin SpinnerState
  deriving (Show)

data History
    = HistoryText Text
    | HistorySlog (Atom, Tank)
  deriving (Show)

data St = St
    { sHistory :: Seq History
    , sLine    :: Text
    , sCurPos  :: Word
    , sSpinner :: SpinnerState
    }
  deriving (Show)

--------------------------------------------------------------------------------

init :: St
init = St mempty "" 0 Nothing

{-|
    When we process `EvMore`, we need to append a newline to the end of
    the current line. During normal play, the ENTER key inserts the
    newline for us, so we need to recreate that newline when we rebuild
    the state for a new terminal connection.
-}
step :: St -> Ev -> St
step st@St{..} = \case
    EvLine t -> st & recordText t
    EvSlog s -> st & recordSlog s
    EvSpin s -> st { sSpinner = s }
    EvMove w -> st { sCurPos = min w (word $ length sLine) }
    EvEdit t -> st { sLine = t, sCurPos = word (length t) }
    EvMore   -> st { sLine = "", sCurPos = 0 } & recordText (sLine <> "\n")
    EvBell   -> st
    EvDraw   -> st
  where
    word :: Integral i => i -> Word
    word = fromIntegral

    recordText :: Text -> St -> St
    recordText t st@St{..} = st {
      sHistory = trim (sHistory |> (HistoryText t))
      }

    recordSlog :: (Atom, Tank) -> St -> St
    recordSlog t st@St{..} = st {
      sHistory = trim (sHistory |> (HistorySlog t))
      }

    trim :: Seq a -> Seq a
    trim s | length s < 20 = s
    trim (_ :<| s)         = s
    trim s                 = s

drawState :: St -> [Ev]
drawState St{..} = hist <> out <> cur <> spin
  where
    hist = fmap drawHistory $ toList sHistory
    out  = if null sLine   then [] else [EvEdit sLine]
    cur  = if 0 == sCurPos then [] else [EvMove $ fromIntegral $ sCurPos]
    spin = maybe [] (singleton . EvSpin . Just) sSpinner

    drawHistory (HistoryText t) = EvLine t
    drawHistory (HistorySlog s) = EvSlog s


-- Conversion ------------------------------------------------------------------

fromBlit :: Arvo.Blit -> Maybe Ev
fromBlit = \case
    Arvo.Hop w  -> Just $ EvMove $ fromIntegral w
    Arvo.Bel () -> Just EvBell
    Arvo.Clr () -> Just EvDraw
    Arvo.Lin s  -> Just $ EvEdit (pack s)
    Arvo.Mor () -> Just EvMore
    _           -> Nothing

toCause :: Maybe Cord -> SpinnerCause
toCause Nothing         = User
toCause (Just (Cord c)) = Event c

fromCause :: SpinnerCause -> Maybe Cord
fromCause User      = Nothing
fromCause (Event t) = Just (Cord t)

fromTermEv :: Term.Ev -> [Ev]
fromTermEv = \case
    Term.Blits bs -> catMaybes (fromBlit <$> bs)
    Term.Trace t  -> [EvLine $ unCord t]
    Term.Blank    -> [EvLine ""]
    Term.Spinr s  -> [EvSpin $ toCause <$> s]
    Term.Slog s   -> [EvSlog s]

toTermEv :: Ev -> Term.Ev
toTermEv = \case
    EvLine "" -> Term.Blank
    EvLine t  -> Term.Trace (Cord t)
    EvSlog s  -> Term.Slog s
    EvSpin s  -> Term.Spinr (fromCause <$> s)
    EvMove w  -> Term.Blits [Arvo.Hop $ fromIntegral w]
    EvBell    -> Term.Blits [Arvo.Bel ()]
    EvDraw    -> Term.Blits [Arvo.Clr ()]
    EvEdit t  -> Term.Blits [Arvo.Lin $ unpack t]
    EvMore    -> Term.Blits [Arvo.Mor ()]