package core_kernel

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package core_kernel
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open! Import

include Bin_prot.Utils.Make_binable1 (struct
    module Binable = struct
      type 'a t = 'a list [@@deriving bin_io]
    end

    type 'a t = 'a Base.Sequence.t

    let of_binable = Base.Sequence.of_list
    let to_binable = Base.Sequence.to_list
  end)

module Step = struct
  type ('a, 's) t = ('a, 's) Base.Sequence.Step.t =
    | Done
    | Skip of 's
    | Yield of 'a * 's
  [@@deriving bin_io]

  include (
    Base.Sequence.Step :
      module type of struct
      include Base.Sequence.Step
    end
    with type ('a, 's) t := ('a, 's) t)
end

module Merge_with_duplicates_element = struct
  type ('a, 'b) t = ('a, 'b) Base.Sequence.Merge_with_duplicates_element.t =
    | Left of 'a
    | Right of 'b
    | Both of 'a * 'b
  [@@deriving bin_io]

  include (
    Base.Sequence.Merge_with_duplicates_element :
      module type of struct
      include Base.Sequence.Merge_with_duplicates_element
    end
    with type ('a, 'b) t := ('a, 'b) t)
end

include (
  Base.Sequence :
    module type of struct
    include Base.Sequence
  end
  with module Step := Base.Sequence.Step
   and module Merge_with_duplicates_element := Base.Sequence
                                               .Merge_with_duplicates_element)

module type Heap = sig
  type 'a t

  val create : compare:('a -> 'a -> int) -> 'a t
  val add : 'a t -> 'a -> 'a t
  val remove_min : 'a t -> ('a * 'a t) option
end

let merge_all (module Heap : Heap) seqs ~compare =
  let module Merge_all_state = struct
    type 'a t =
      { heap : ('a * 'a Base.Sequence.t) Heap.t
      ; not_yet_in_heap : 'a Base.Sequence.t list
      }
    [@@deriving fields]

    let create = Fields.create
  end
  in
  unfold_step
    ~init:
      (Merge_all_state.create
         ~heap:(Heap.create ~compare:(Base.Comparable.lift compare ~f:fst))
         ~not_yet_in_heap:seqs)
    ~f:(fun { heap; not_yet_in_heap } ->
      match not_yet_in_heap with
      | seq :: not_yet_in_heap ->
        (match Expert.next_step seq with
         | Done -> Skip { not_yet_in_heap; heap }
         | Skip seq -> Skip { not_yet_in_heap = seq :: not_yet_in_heap; heap }
         | Yield (elt, seq) -> Skip { not_yet_in_heap; heap = Heap.add heap (elt, seq) })
      | [] ->
        (match Heap.remove_min heap with
         | None -> Done
         | Some ((elt, seq), heap) -> Yield (elt, { heap; not_yet_in_heap = [ seq ] })))
;;