split_sequence_lazily.fs

open System

module Seq =
  let split n (xs:_ seq) =
    if Seq.length xs > n then
      Seq.take n xs, Seq.skip n xs
    else
      xs, Seq.empty
  let chunk n = Seq.unfold(fun xs -> if xs = Seq.empty then None else Some (split n xs))

  let split' n (xs:_ seq) =
    let en = xs.GetEnumerator()
    let next , n = ref true, ref n
    seq {
    while !next && !n > 0 do
      yield! try [en.Current] with _ -> []
      next := en.MoveNext()
      decr n
    },
    seq {
    while !next do 
      yield en.Current
      next:= en.MoveNext () 
    }
  let chunk' n = Seq.unfold(fun xs -> if Seq.isEmpty xs then None else Some (split' n xs))

let range a b = seq { for i in a .. b -> printf "!%d " i; i }
let _print xs = printf "seq ["; Seq.iter (printf "%d ") xs; printf "]"
let print (head,tail) = printf "( "; _print head; printf ", "; _print tail; printfn ")"
let print2 xss = Seq.iter _print xss; printfn ""
 
print <| Seq.split 2 (range 1 1)
// !1 ( seq [!1 1 ], seq [])

print <| Seq.split 4 (range 1 7)
// !1 !2 !3 !4 !5 !6 !7 ( seq [!1 1 !2 2 !3 3 !4 4 ], seq [!1 !2 !3 !4 !5 5 !6 6 !7 7 ])

print2 <| Seq.chunk 2 (range 1 1)
// !1 seq [!1 1 ]

print2 <| Seq.chunk 2 (range 1 5)
// !1 !2 !3 !4 !5 seq [!1 1 !2 2 ]!1 !2 !3 !4 !5 seq [!1 !2 !3 3 !4 4 ]!1 !2 !3 !4 !5 seq [!1 !2 !3 !4 !5 5 ]


print <| Seq.split' 2 (range 1 1)
// ( seq [0 !1 1 ], seq [])

print <| Seq.split' 4 (range 1 7)
// ( seq [0 !1 1 !2 2 !3 3 !4 ], seq [4 !5 5 !6 6 !7 7 ])

print2 <| Seq.chunk' 2 (range 1 1)
// !1 seq [0 !1 1 ]

print2 <| Seq.chunk' 2 (range 1 5)
// !1 seq [0 !1 1 !2 ]seq [0 2 !3 ]seq [0 3 !4 ]seq [0 4 !5 ]seq [0 5 ]