Commit af856405 by Jean-Christophe Filliâtre

mergesort_list: generic code, a bit of cleaning up

parent 981d11a6
 (* Sorting a list of integers using mergesort *) module M module Elt use import int.Int use import list.Length use import list.SortedInt use import list.Append use import list.Permut use export int.Int use export list.Length use export list.Append use export list.Permut let split (l0 : list 'a) type elt predicate le elt elt clone relations.TotalPreOrder with type t = elt, predicate rel = le clone export list.Sorted with type t = elt, predicate le = le end module Merge use export Elt let rec merge (l1 l2: list elt) : list elt requires { sorted l1 /\ sorted l2 } ensures { sorted result /\ permut result (l1 ++ l2) } variant { length l1 + length l2 } = match l1, l2 with | Nil, _ -> l2 | _, Nil -> l1 | Cons x1 r1, Cons x2 r2 -> if le x1 x2 then Cons x1 (merge r1 l2) else Cons x2 (merge l1 r2) end end (* TODO: proof to be completed module EfficientMerge use export Elt use import list.Mem use import list.Reverse use import list.RevAppend let rec merge_aux (acc l1 l2: list elt) : list elt requires { sorted (reverse acc) /\ sorted l1 /\ sorted l2 } requires { forall x y: elt. mem x acc -> mem y l1 -> le x y } requires { forall x y: elt. mem x acc -> mem y l2 -> le x y } ensures { sorted result /\ permut result (acc ++ l1 ++ l2) } variant { length l1 + length l2 } = match l1, l2 with | Nil, _ -> rev_append acc l2 | _, Nil -> rev_append acc l1 | Cons x1 r1, Cons x2 r2 -> if le x1 x2 then merge_aux (Cons x1 acc) r1 l2 else merge_aux (Cons x2 acc) l1 r2 end let merge (l1 l2: list elt) : list elt requires { sorted l1 /\ sorted l2 } ensures { sorted result /\ permut result (l1 ++ l2) } = merge_aux Nil l1 l2 end *) module Mergesort use import Merge let split (l0: list 'a) : (list 'a, list 'a) requires { length l0 >= 2 } ensures { let (l1, l2) = result in ensures { let (l1, l2) = result in 1 <= length l1 /\ 1 <= length l2 /\ permut l0 (l1 ++ l2) } = let rec split_aux (l1 : list 'a) l2 l variant { length l } = let rec split_aux (l1 l2 l: list 'a) : (list 'a, list 'a) requires { length l2 = length l1 \/ length l2 = length l1 + 1 } ensures { let r1, r2 = result in ensures { let r1, r2 = result in (length r2 = length r1 \/ length r2 = length r1 + 1) /\ permut (r1 ++ r2) (l1 ++ (l2 ++ l)) } variant { length l } = match l with | Nil -> (l1, l2) | Cons x r -> split_aux l2 (Cons x l1) r ... ... @@ -25,18 +84,9 @@ module M in split_aux Nil Nil l0 let rec merge l1 l2 variant { length l1 + length l2 } requires { sorted l1 /\ sorted l2 } ensures { sorted result /\ permut result (l1 ++ l2) } = match l1, l2 with | Nil, _ -> l2 | _, Nil -> l1 | Cons x1 r1, Cons x2 r2 -> if x1 <= x2 then Cons x1 (merge r1 l2) else Cons x2 (merge l1 r2) end let rec mergesort l variant { length l } let rec mergesort (l: list elt) : list elt ensures { sorted result /\ permut result l } variant { length l } = match l with | Nil | Cons _ Nil -> l | _ -> let l1, l2 = split l in merge (mergesort l1) (mergesort l2) ... ...
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