import Data.List

type ButtonSeqDB = [(ButtonSeq, Name)]
type ButtonSeq = [String]
type Name = String

-- Default DB of button sequences.
buttonSeqDB :: ButtonSeqDB
buttonSeqDB =
  [ (["down", "forward", "punch"], "hadoken")
  , (["forward", "punch"], "charger")
  , (["up", "punch"], "uppercut")
  ]

register :: ButtonSeq -> Name -> ButtonSeqDB -> ButtonSeqDB
register sequence name db
  | null sequence = db
  | alreadyExists = db
  | otherwise = (sequence, name) : db
  where
  alreadyExists = elem (sequence, name) db

onButtons :: ButtonSeq -> ButtonSeqDB -> [Name]
onButtons buttonHist db = map snd
  . fst
  . foldl step ([], db)
  $ reverse buttonHist
  where
  step acc@(foundSoFar, dbRem) button
    | null remaining = acc
    | otherwise =
      ( foundSoFar ++ entriesComplete
      , dbRem'
      )
    where
    remaining = filter ((==button) . last . fst)
      $ filter (not . null . fst) dbRem
    dbRem' = map (\(a, b) -> (init a, b)) remaining
    entriesComplete = filter (null . fst) dbRem'