/-
Copyright (c) 2019 Microsoft Corporation. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Daniel Selsam

Simple queue implemented using two lists.
Note: this is only a temporary placeholder.
-/
prelude
import init.data.array
import init.data.int
universes u v w

structure Queue (α : Type u) :=
(eList dList : List α := [])

namespace Queue

variable {α : Type u}

def enqueue (v : α) (q : Queue α) : Queue α :=
{ eList := v::q.eList .. q }

def enqueueAll (vs : List α) (q : Queue α) : Queue α :=
{ eList := vs ++ q.eList .. q }

def isEmpty (q : Queue α) : Bool :=
q.dList.isEmpty && q.eList.isEmpty

def dequeue? (q : Queue α) : Option (α × Queue α) :=
match q.dList with
| d::ds => some (d, { dList := ds, .. q })
| []    =>
  match q.eList.reverse with
  | []    => none
  | d::ds => some (d, { eList := [], dList := ds })

end Queue