module Sound.MIDI.ALSA.EventList where
{-
ToDo:
controller mapping

fix laziness issues in splitting and merging
change pattern according to program change events
-}

import Sound.MIDI.ALSA.Common
          (Bundle, EventDataBundle, Time, TimeAbs,
           Handle, Pattern, PatternMulti, Selector,
           sequ, with, incTime,
           singletonBundle, checkController, checkChannel,
           checkProgram, checkPitch,
           SweepState, sweepSpeed, sweepPhase, sweepDepth, sweepCenter,
           updateDur, updateChord, )
import qualified Sound.MIDI.ALSA.Common as Common

import qualified Sound.ALSA.Sequencer.Event as Event
import qualified Sound.ALSA.Exception as Exc

import qualified Sound.MIDI.ALSA as MALSA
import qualified Sound.MIDI.Message.Channel as ChannelMsg
import qualified Sound.MIDI.Message.Channel.Voice as VoiceMsg

import Sound.MIDI.Message.Channel (Channel, )
import Sound.MIDI.Message.Channel.Voice (Velocity, Pitch, Controller, Program, )

import qualified Data.EventList.Relative.TimeBody as EventList
import qualified Data.EventList.Relative.MixedBody as EventListMB
import qualified Data.EventList.Absolute.TimeBody as EventListAbs

import qualified Data.Accessor.Basic as Acc
import Data.Accessor.Basic ((^.), )

import qualified Data.List.HT as ListHT
import qualified Data.List.Match as Match
import Data.Tuple.HT (mapFst, mapSnd, mapPair, )
import Data.Ord.HT (limit, )
import qualified Data.List as List
import Data.Maybe (mapMaybe, )

import qualified Data.Map as Map

import qualified Control.Monad.Trans.State as State
import qualified Control.Monad.Trans.Reader as Reader
import Control.Monad.Trans.Reader (ReaderT, )
import Control.Monad.IO.Class (liftIO, )
import qualified Control.Applicative as App
import Control.Monad (liftM2, guard, )
import qualified Data.Foldable as Fold
import qualified Data.Traversable as Trav

import qualified Numeric.NonNegative.Class as NonNeg

import Data.Int (Int32, )

import System.IO.Unsafe (unsafeInterleaveIO, )

import Prelude hiding (init, filter, )


ioToLazyList :: IO a -> IO [a]
ioToLazyList m =
   let go = unsafeInterleaveIO $ liftM2 (:) m go
   in  go


inputEventsCore :: ReaderT Handle IO [Event.T]
inputEventsCore =
   Reader.ReaderT $ \h ->
   ioToLazyList (Event.input (sequ h))

inputEvents :: ReaderT Handle IO (EventList.T Time Event.Data)
inputEvents =
   fmap (EventList.fromAbsoluteEventList .
         EventListAbs.fromPairList .
         map (\ev -> (Common.timeFromStamp (Event.timestamp ev),
                      Event.body ev))) $
   inputEventsCore


pairListFromRelativeEvents :: EventList.T Time a -> [(TimeAbs,a)]
pairListFromRelativeEvents =
   EventListAbs.toPairList .
   EventListAbs.mapTime Common.deconsTime .
   EventList.toAbsoluteEventList 0

outputEvent :: TimeAbs -> Event.Data -> ReaderT Handle IO ()
outputEvent t ev = Reader.ReaderT $ \h ->
   Event.output (sequ h) (Common.makeEvent h t ev) >>
   Event.drainOutput (sequ h) >>
   return ()

outputEvents :: EventList.T Time Event.Data -> ReaderT Handle IO ()
outputEvents =
   mapM_ (uncurry outputEvent) .
   pairListFromRelativeEvents


{- |
Sends (drain) each event individually
since the events in the bundle might be created in a lazy manner.
-}
outputEventBundles :: EventList.T Time EventDataBundle -> ReaderT Handle IO ()
outputEventBundles =
   mapM_
      (\(t,evs) ->
         flip mapM_ evs (\(dt,ev) ->
            outputEvent (incTime dt t) ev)) .
   pairListFromRelativeEvents

outputEventBundled :: EventList.T Time EventDataBundle -> ReaderT Handle IO ()
outputEventBundled =
   mapM_
      (\(t,evs) -> Reader.ReaderT $ \h ->
         flip mapM_ evs (\(dt,ev) ->
            Event.output (sequ h) (Common.makeEvent h (incTime dt t) ev)) >>
         Event.drainOutput (sequ h) >>
         return ()) .
   pairListFromRelativeEvents


data Trigger a =
     Regular a
   | Trigger

instance Functor Trigger where
   fmap f (Regular a) = Regular (f a)
   fmap _ Trigger     = Trigger

instance Fold.Foldable Trigger where
   foldMap = Trav.foldMapDefault

instance Trav.Traversable Trigger where
   sequenceA (Regular a) = fmap Regular a
   sequenceA Trigger     = App.pure Trigger


type EventDataTrigger = Bundle (Trigger Event.Data)

makeTriggerEvent :: Handle -> TimeAbs -> Trigger Event.Data -> Event.T
makeTriggerEvent h t x =
   case x of
      Regular ev -> Common.makeEvent h t ev
      Trigger -> Common.makeEcho h t (Event.Custom 0 0 0)

makeTriggerEvents :: Handle -> TimeAbs -> EventDataTrigger -> [Event.T]
makeTriggerEvents h t =
   map (\(dt,ev) -> makeTriggerEvent h (incTime dt t) ev)

{- |
This function distinguishes between events from portIn
and events that are generated by us.
Our generated events must also send an echo to the input port
in order to break 'event_input' and thus trigger their delivery.
-}
outputTriggerEvents ::
   EventList.T Time EventDataTrigger ->
   ReaderT Handle IO ()
outputTriggerEvents =
   mapM_
      (\(t,ee) -> Reader.ReaderT $ \h ->
         mapM_
            (\e ->
                Event.output (sequ h) e >>
                Event.drainOutput (sequ h))
            (makeTriggerEvents h t ee)
          >> return ()) .
   pairListFromRelativeEvents

mergeGenerated ::
   EventList.T Time (Bundle a) ->
   EventList.T Time (Bundle a) ->
   EventList.T Time (Bundle (Trigger a))
mergeGenerated gens ins =
   merge
      (fmap (\t -> [(t, Trigger)]) $ EventList.fromPairList $
       ListHT.mapAdjacent (,) (0 : EventList.getTimes gens))
      (fmap (map (mapSnd Regular)) $
       merge gens ins)


{- ToDo: move to eventlist package -}
equidistantEvents :: Time -> [a] -> EventList.T Time a
equidistantEvents dur as =
   case as of
      [] -> EventList.empty
      x:xs ->
         EventList.cons 0 x $
         EventList.fromPairList (map ((,) dur) xs)

whirl :: EventList.T Time EventDataBundle
whirl =
   let dur = 0.125
       notes =
          cycle $ concat $ concatMap (replicate 4) $
             [57, 59, 60, 64] :
             [57, 59, 60, 65] :
             [57, 62, 64, 65] :
             [57, 59, 60, 64] :
             []
       ctrls =
          map (\t -> round (80 + 47 * sin t)) (iterate (0.1+) (0::Double))
       events =
          zipWith (:)
             (map
                (\k -> (0, Event.CtrlEv Event.Controller (Event.Ctrl
                   {Event.ctrlChannel = 0,
                    Event.ctrlParam = 23,
                    Event.ctrlValue = k})))
                ctrls)
             (map
                (\k ->
                   (0,   Event.NoteEv Event.NoteOn  $ Event.simpleNote 0 k 64) :
                   (dur, Event.NoteEv Event.NoteOff $ Event.simpleNote 0 k 64) :
                   [])
                notes)

   in  EventList.cons 0
          [(0, Event.CtrlEv Event.PgmChange (Event.Ctrl
                  {Event.ctrlChannel = 0,
                   Event.ctrlParam = 0,
                   Event.ctrlValue = 5}))] $
       equidistantEvents dur events


mergeGeneratedAtoms ::
   (Time -> a) ->
   EventList.T Time a ->
   EventList.T Time a ->
   EventList.T Time a
mergeGeneratedAtoms trigger gens ins =
   EventList.mergeBy (\_ _ -> True)
      (fmap trigger $ EventList.fromPairList $
       ListHT.mapAdjacent (,) (0 : EventList.getTimes gens))
      (EventList.mergeBy (\_ _ -> True) gens ins)

pattern ::
   Selector i ->
   [i] ->
   Time ->
   EventList.T Time Event.Data ->
   EventList.T Time EventDataTrigger
pattern select ixs dur ins =
   flip State.evalState Map.empty $ Trav.sequenceA $
   mergeGeneratedAtoms
      (\dt -> return [(dt, Trigger)])
      (fmap
          (\n -> State.gets (map (mapSnd Regular) . select n dur . Map.toAscList))
          (equidistantEvents dur ixs))
      (fmap
          (\e ->
             case e of
                Event.NoteEv notePart note -> do
                   State.modify (updateChord notePart note)
                   return []
                _ -> return $ singletonBundle (Regular e))
          ins)


patternTempo ::
   Selector i ->
   [i] ->
   ((Channel,Controller), (Time,Time,Time)) ->
   EventList.T Time Event.Data ->
   EventList.T Time EventDataTrigger
patternTempo select ixs0 ((chan,ctrl), (minDur, defltDur, maxDur)) =
   let recourse dur chord ixs =
          EventList.switchL EventList.empty $ \(time,me) rest ->
          uncurry (EventList.cons time) $
          case me of
             Nothing ->
                case ixs of
                   [] -> ([], recourse dur chord ixs rest)
                   i:ir ->
                      ((dur, Trigger) :
                       map (mapSnd Regular) (select i dur $ Map.toAscList chord),
                       recourse dur chord ir $
                       EventList.insertBy (\_ _ -> True) dur Nothing rest)
             Just e ->
                case e of
                   Event.NoteEv notePart note ->
                      ([],
                       recourse dur (updateChord notePart note chord) ixs rest)
                   Event.CtrlEv Event.Controller param |
                          Common.controllerMatch chan ctrl param ->
                      ([],
                       recourse
                          (updateDur param (minDur,maxDur))
                          chord ixs rest)
                   _ -> (singletonBundle (Regular e),
                         recourse dur chord ixs rest)
   in  recourse defltDur Map.empty ixs0 .
       EventList.insertBy (\_ _ -> True) defltDur Nothing .
       fmap Just


{- |
This allows more complex patterns including pauses,
notes of different lengths and simultaneous notes.
-}
patternMultiTempo ::
   Selector i ->
   EventList.T Int [Common.IndexNote i] ->
   ((Channel,Controller), (Time,Time,Time)) ->
   EventList.T Time Event.Data ->
   EventList.T Time EventDataTrigger
patternMultiTempo select ixs0 ((chan,ctrl), (minDur, defltDur, maxDur)) =
   let recourse dur chord ixs =
          EventList.switchL EventList.empty $ \(time,me) rest ->
          uncurry (EventList.cons time) $
          case me of
             Nothing ->
                EventList.switchL
                   ([], recourse dur chord ixs rest)
                   (\(t,is) ir0 ->
                      let (notes,ir1) =
                             if t>0
                               then ([], EventList.cons (t-1) is ir0)
                               else
                                 (do Common.IndexNote d i <- is
                                     evs <-
                                        select i (fromIntegral d * dur) $
                                        Map.toAscList chord
                                     return (mapSnd Regular evs),
                                  ir0)
                      in  ((dur, Trigger) : notes,
                           recourse dur chord ir1 $
                           EventList.insertBy (\_ _ -> True) dur Nothing rest))
                   ixs
             Just e ->
                case e of
                   Event.NoteEv notePart note ->
                      ([],
                       recourse dur (updateChord notePart note chord) ixs rest)
                   Event.CtrlEv Event.Controller param |
                          Common.controllerMatch chan ctrl param ->
                      ([],
                       recourse
                          (updateDur param (minDur,maxDur))
                          chord ixs rest)
                   _ -> (singletonBundle (Regular e),
                         recourse dur chord ixs rest)
   in  recourse defltDur Map.empty ixs0 .
       EventList.insertBy (\_ _ -> True) defltDur Nothing .
       fmap Just


{- |
Automatically changes the value of a MIDI controller
every @period@ seconds according to a periodic wave.
The wave function is a mapping
from the phase in @[0,1)@
to a controller value in the range @(-1,1)@.
The generation of the wave is controlled by a speed controller
(@minSpeed@ and @maxSpeed@ are in waves per second),
the modulation depth and the center value.
The center controller is also the one where we emit our wave.
That is, when modulation depth is zero
then this effect is almost the same
as forwarding the controller without modification.
The small difference is, that we emit a controller value at a regular pattern,
whereas direct control would mean
that only controller value changes are transfered.

> sweep channel
>    period (speedCtrl, (minSpeed, maxSpeed)) depthCtrl centerCtrl
>    (ctrlRange (-1,1) (sin . (2*pi*)))

We could use the nice Wave abstraction from the synthesizer package,
but that's a heavy dependency because of multi-parameter type classes.
-}
sweep ::
   Channel ->
   Time ->
   (Controller, (Time,Time)) ->
   Controller ->
   Controller ->
   (Double -> Double) ->
   EventList.T Time Event.Data ->
   EventList.T Time EventDataTrigger
sweep chan dur (speedCtrl, (minSpeed, maxSpeed)) depthCtrl centerCtrl
      wave ins =
   flip State.evalState
       (Common.SweepState {
          sweepSpeed =
             realToFrac $ Common.deconsTime $
             dur*(minSpeed+maxSpeed)/2,
          sweepDepth = 64,
          sweepCenter = 64,
          sweepPhase = 0
        }) $
   Trav.sequenceA $
   mergeGeneratedAtoms
      (\dt -> return [(dt, Trigger)])
      (fmap
          (\() -> do
              ev <-
                 State.gets (\s ->
                    Event.CtrlEv Event.Controller $
                    Event.Ctrl {
                       Event.ctrlChannel = MALSA.fromChannel chan,
                       Event.ctrlParam = MALSA.fromController centerCtrl,
                       Event.ctrlValue =
                          round $ limit (0,127) $
                          sweepCenter s + sweepDepth s * wave (sweepPhase s)
                    })
              State.modify (\s ->
                 s{sweepPhase = Common.fraction (sweepPhase s + sweepSpeed s)})
              return $ singletonBundle (Regular ev))
          (equidistantEvents dur $ repeat ()))
      (fmap
          (\e ->
             maybe (return $ singletonBundle (Regular e))
                   (\f -> State.modify f >> return []) $ do
                Event.CtrlEv Event.Controller param <- Just e
                let c = param ^. MALSA.ctrlChannel
                    ctrl = param ^. MALSA.ctrlController
                    x :: Num a => a
                    x = fromIntegral (Event.ctrlValue param)
                guard (c==chan)
                lookup ctrl $
                   (speedCtrl,
                    \s -> s{sweepSpeed =
                       realToFrac $ Common.deconsTime $ (dur *) $
                       minSpeed + (maxSpeed-minSpeed) * x/127}) :
                   (depthCtrl,  \s -> s{sweepDepth = x}) :
                   (centerCtrl, \s -> s{sweepCenter = x}) :
                   [])
          ins)


-- * combinators

{- |
The function maintains empty bundles
in order to maintain laziness breaks.
These breaks are import for later merging of the streams.
-}
filter ::
   (a -> Bool) ->
   State.State
      (EventList.T Time (Bundle a))
      (EventList.T Time (Bundle a))
filter p = State.state $
   EventList.foldrPair
      (\t evs ->
         let (evsT,evsF) =
                List.partition (p . snd) evs
         in  mapPair
                (EventList.cons t evsT,
                 EventList.cons t evsF))
      (EventList.empty, EventList.empty)

filterSimple ::
   (a -> Bool) ->
   EventList.T Time (Bundle a) ->
   EventList.T Time (Bundle a)
filterSimple p =
   EventList.foldrPair
      (\t evs ->
         EventList.cons t (List.filter (p . snd) evs))
      EventList.empty

{-
merge ::
   EventList.T Time (Bundle a) ->
   EventList.T Time (Bundle a) ->
   EventList.T Time (Bundle a)
merge x y =
{-
   fmap concat $
   EventList.collectCoincident $
-}
   EventList.mergeBy (\_ _ -> True) x y
-}

merge ::
   EventList.T Time (Bundle a) ->
   EventList.T Time (Bundle a) ->
   EventList.T Time (Bundle a)
merge x0 y0 =
   flip (EventList.switchL y0) x0 $ \(tx,bx) rx ->
   flip (EventList.switchL x0) y0 $ \(ty,by) ry ->
      let (tz, ~(bz, rz)) =
             mapSnd
                (\ ~(b,d) ->
                   if b
                     then
                       mapFst (bx++) $
                       if d == NonNeg.zero
                         then (by, merge rx ry)
                         else ([], merge rx (EventList.cons d by ry))
                     else
                       (by, merge (EventList.cons d bx rx) ry)) $
             NonNeg.split tx ty
      in  EventList.cons tz bz rz


-- * run filters

process ::
   (EventList.T Time Event.Data ->
    EventList.T Time EventDataTrigger) ->
   ReaderT Handle IO ()
process f = do
   Common.startQueue
   outputTriggerEvents . f =<< inputEvents

processSimple ::
   (EventList.T Time Event.Data ->
    EventList.T Time EventDataBundle) ->
   ReaderT Handle IO ()
processSimple f = do
   Common.startQueue
   outputEventBundles . f =<< inputEvents


runWhirl :: ReaderT Handle IO ()
runWhirl =
   process
      ({-
       we must prepend the trigger event,
       otherwise 'mergeGenerated' makes us wait for the first user event
       -}
       EventList.cons 0 [(0,Trigger)] .
       mergeGenerated whirl .
       fmap singletonBundle)

runDelay :: ReaderT Handle IO ()
runDelay =
   processSimple (fmap (Common.delayAdd 50 0.3))

runKeyboardSplit :: ReaderT Handle IO ()
runKeyboardSplit =
   processSimple $
   uncurry merge .
   State.runState (do
      low <-
         filter (\e ->
            (checkChannel (ChannelMsg.toChannel 0 ==) e &&
             checkPitch   (VoiceMsg.toPitch 60 >) e) ||
             checkController (VoiceMsg.toController 91 ==) e ||
             checkController (VoiceMsg.toController 93 ==) e)
      return $
         fmap (mapMaybe (\(t,p) -> fmap ((,) t) $ Common.transpose 12 p) .
               map (mapSnd (Common.setChannel (ChannelMsg.toChannel 1)))) low) .
   fmap singletonBundle

runKeyboardSplitLow :: ReaderT Handle IO ()
runKeyboardSplitLow =
   processSimple $
   fmap (mapMaybe (\(t,p) -> fmap ((,) t) $ Common.transpose 12 p) .
         map (mapSnd (Common.setChannel (ChannelMsg.toChannel 1)))) .
   filterSimple (\e ->
      (checkChannel (ChannelMsg.toChannel 0 ==) e &&
       checkPitch   (VoiceMsg.toPitch 60 >) e) ||
      checkController (VoiceMsg.toController 91 ==) e ||
      checkController (VoiceMsg.toController 93 ==) e) .
   fmap singletonBundle

runKeyboardSplitHigh :: ReaderT Handle IO ()
runKeyboardSplitHigh =
   processSimple $
--   fmap (map (mapSnd (setChannel (ChannelMsg.toChannel 0)))) .
   filterSimple (\e ->
      (checkChannel (ChannelMsg.toChannel 0 ==) e &&
       checkPitch   (VoiceMsg.toPitch 60 <=) e) ||
      checkController (const True) e ||
      checkProgram (const True) e) .
   fmap singletonBundle

{- this defers events occasionally
   EventList.collectCoincident .
   fmap ((,) 0)
-}

runNote :: Channel -> Time -> Velocity -> Pitch -> ReaderT Handle IO ()
runNote chan dur vel pit =
   let note =
          Event.simpleNote
             (MALSA.fromChannel chan)
             (MALSA.fromPitch pit)
             (MALSA.fromVelocity vel)
   in  do outputEvent 0
             (Event.NoteEv Event.NoteOn note)
          outputEvent (incTime dur 0)
             (Event.NoteEv Event.NoteOff note)

runKey :: Channel -> Bool -> Velocity -> Pitch -> ReaderT Handle IO ()
runKey chan noteOn vel pit =
   outputEvent 0
      (Event.NoteEv
         (if noteOn then Event.NoteOn else Event.NoteOff)
         (Event.simpleNote
             (MALSA.fromChannel chan)
             (MALSA.fromPitch pit)
             (MALSA.fromVelocity vel)))

runController :: Channel -> Controller -> Int -> ReaderT Handle IO ()
runController chan ctrl val =
   outputEvent 0
      (Event.CtrlEv Event.Controller $
       Event.Ctrl {
          Event.ctrlChannel = MALSA.fromChannel chan,
          Event.ctrlParam = MALSA.fromController ctrl,
          Event.ctrlValue = fromIntegral val
       })

runProgram :: Channel -> Program -> ReaderT Handle IO ()
runProgram chan pgm =
   outputEvent 0
      (Event.CtrlEv Event.PgmChange $
       Event.Ctrl {
          Event.ctrlChannel = MALSA.fromChannel chan,
          Event.ctrlParam = 0,
          Event.ctrlValue = MALSA.fromProgram pgm
       })

{- |
> runCyclePrograms (map VoiceMsg.toProgram [8..12])
-}
runCyclePrograms :: [Program] -> ReaderT Handle IO ()
runCyclePrograms pgms =
   processSimple
      (flip State.evalState (cycle pgms) .
       Trav.traverse (Common.traverseProgramsSeek (length pgms)))

{- |
> runProgramsAsBanks [8,4,4]
-}
runProgramsAsBanks :: [Int32] -> ReaderT Handle IO ()
runProgramsAsBanks ns =
   processSimple
      (fmap singletonBundle .
       flip State.evalState (Match.replicate ns 0) .
       Trav.traverse (Common.programsAsBanks ns))

{- |
> runPattern 0.12 (cycleUp 4)
-}
runPattern ::
   Time ->
   Pattern i ->
   ReaderT Handle IO ()
runPattern dur pat =
   process (uncurry pattern pat dur)

{- |
> runPatternTempo 0.12 (cycleUp 4)

> runPatternTempo 0.2 (selectFromOctaveChord, cycle [0,1,2,0,1,2,0,1])
-}
runPatternTempo ::
   Time ->
   Pattern i ->
   ReaderT Handle IO ()
runPatternTempo dur pat =
   process
      (uncurry patternTempo pat
         (Common.defaultTempoCtrl, (0.5*dur, dur, 1.5*dur)))


{- |
> runPatternMultiTempo 0.1 (selectFromLimittedChord, let pat = [item 0 1] ./ 1 /. [item 1 1] ./ 2 /. [item 1 1] ./ 1 /. [item 0 1] ./ 2 /. pat in 0 /. pat)
-}
runPatternMultiTempo ::
   Time ->
   PatternMulti i ->
   ReaderT Handle IO ()
runPatternMultiTempo dur pat =
   process
      (uncurry patternMultiTempo pat
         (Common.defaultTempoCtrl, (0.5*dur, dur, 1.5*dur)))


runFilterSweep ::
   ReaderT Handle IO ()
runFilterSweep =
   process
      (sweep (ChannelMsg.toChannel 1)
         0.01 (VoiceMsg.toController 72, (0.1, 1))
         (VoiceMsg.toController 73) (VoiceMsg.toController 91)
         (sin . (2*pi*)))




main :: IO ()
main = (with $ do
   liftIO $ putStrLn "Please connect me to a synth"
   liftIO $ getLine
   Common.startQueue
   liftIO . mapM_ print =<< inputEventsCore
   outputEvents =<< inputEvents
   outputEventBundles whirl
   outputEvents . EventList.mapMaybe (Common.transpose 1) =<< inputEvents)

  `Exc.catch` \e ->
      putStrLn $ "alsa_exception: " ++ Exc.show e



{-
stateless map:
   only send an output event if there was an input event
   without maintaining a state

   change channel, program, transposition
   split stream according to channel, program, pitch
   merge two streams

   convert key-press events to controller changes
      this way we can control the filter frequency of a resonant lowpass
      or we can control an external analogue synthesizer
   convert keypress intervals to a gate signal with according velocity
      this way we can generate an envelope for the resonance of a lowpass
      or we can control an external analogue synthesizer

stateful map:
   only send an output event if there was an input event
   while maintaining a state

   add a bass tone to a chord
   hold a key or a chord until the next one is played
   play something on releasing the keys
      this way we could control Guitar strokes up and down
   cycle through a set of instruments for each note played
      This way we can play syllables of a word like To-ma-ten-sa-lat.

pattern:
   use a separate clock at which events can be scheduled
   patterns can be implemented by applying a stateful map to a beat stream

   play tones of current chord upwards, downwards, ping-pong or randomly
   generate pattern consisting of the current tone and the tone one octave above it
   generate repeated keystrokes for the key that is constantly pressed
   play current chord repeatedly in randomly chosen inversions
   patterns like parity of ones in binary numbers (cf. my Flip song)
   record and replay events in a loop
   generate MIDI controller events according to a function of time
   generate MIDI controller events by a random pattern,
      this may be used to control the cutoff frequency of a resonant filter
   delay events
   echo: play a note multiple times with a certain delay and decreasing velocity
   use the last n played notes for a pattern
      with lazy access to a list, we can simulate a queue
-}