History.hs

{-# LANGUAGE UnicodeSyntax #-}
{-# LANGUAGE ViewPatterns #-}
module NClient.Message.History
  ( History
  , empty, prepend
  , line
  , next, previous
  ) where

import Data.Sequence (Seq, (<|))
import Prelude hiding (lines)
import qualified Data.Sequence as Seq

data History = History { lines ∷ Seq String, current ∷ Int, capacity ∷ Int }

prepend ∷ String → History → History
prepend l h = h { lines = Seq.take (capacity h + 1) $ " " <| l <| Seq.drop 1 (lines h), current = 0 }

empty ∷ Int → History
empty c = History { lines = Seq.empty, current = 0, capacity = c }

line ∷ History → String
line h = lines h `Seq.index` current h

next ∷ String → History → History
next t = tryNext . trySetCurrent t

tryNext ∷ History → History
tryNext h@(History ls i c)
  | i == Seq.length ls - 1 = h
  | otherwise = h { current = succ i }

previous ∷ History → History
previous h@(History _ 0 _) = h
previous h = h { current = pred $ current h }

trySetCurrent ∷ String → History → History
trySetCurrent α h@(current → 0) = h { lines = α <| Seq.drop 1 (lines h) }
trySetCurrent _ h = h