From 5986955fb6ace1261619b0ad2a5201fe5d4cf420 Mon Sep 17 00:00:00 2001
From: Simon Cruanes <simon.cruanes.2007@m4x.org>
Date: Wed, 28 Mar 2018 20:10:14 -0500
Subject: [PATCH] feat(list): add `{interleave,intersperse}` (closes #191)

---
 src/core/CCList.ml  | 50 ++++++++++++++++++++++++++++++++++++++++++++-
 src/core/CCList.mli | 11 +++++++++-
 2 files changed, 59 insertions(+), 2 deletions(-)

diff --git a/src/core/CCList.ml b/src/core/CCList.ml
index 8a4dbfb0..30ef12a1 100644
--- a/src/core/CCList.ml
+++ b/src/core/CCList.ml
@@ -580,7 +580,7 @@ let sorted_merge ~cmp l1 l2 =
     List.length (sorted_merge ~cmp:CCInt.compare l1 l2) = List.length l1 + List.length l2)
 *)
 
-let sort_uniq (type elt) ~cmp l = List.sort_uniq cmp l
+let sort_uniq ~cmp l = List.sort_uniq cmp l
 
 (*$T
   sort_uniq ~cmp:CCInt.compare [1;2;5;3;6;1;4;2;3] = [1;2;3;4;5;6]
@@ -786,6 +786,54 @@ let sublists_of_len ?(last=fun _ -> None) ?offset n l =
   [[1;2]; [3;4]] (subs 2 [1;2;3;4;5])
 *)
 
+let intersperse x l =
+  let rec aux_direct i x l = match l with
+    | [] -> []
+    | [_] -> l
+    | _ when i=0 -> aux_tailrec [] x l
+    | y :: tail -> y :: x :: aux_direct (i-1) x tail
+  and aux_tailrec acc x l = match l with
+    | [] -> List.rev acc
+    | [y] -> List.rev (y::acc)
+    | y :: tail -> aux_tailrec (x :: y :: acc) x tail
+  in
+  aux_direct 1_000 x l
+
+(*$=
+  [] (intersperse 0 [])
+  [1] (intersperse 0 [1])
+  [1;0;2;0;3;0;4] (intersperse 0 [1;2;3;4])
+*)
+
+(*$Q
+  Q.(pair int (list int)) (fun (x,l) -> \
+    length (intersperse x l) = (if length l <= 1 then length l else 2 * length l-1))
+  Q.(pair int (list int)) (fun (x,l) -> \
+    rev (intersperse x l) = intersperse x (rev l))
+*)
+
+let interleave l1 l2 : _ list =
+  let rec aux acc l1 l2 = match l1, l2 with
+    | [], [] -> List.rev acc
+    | [], _ -> List.rev (List.rev_append l2 acc)
+    | _, [] -> List.rev (List.rev_append l1 acc)
+    | x1 :: tl1, x2 :: tl2 ->
+      aux (x2 :: x1 :: acc) tl1 tl2
+  in
+  aux [] l1 l2
+
+(*$=
+  [1;2;3;4;5]  (interleave [1;3] [2;4;5])
+  [1;2;3] (interleave [1] [2;3])
+*)
+
+(*$Q
+  Q.(pair (small_list int)(small_list int)) (fun (l1,l2) -> \
+    length (interleave l1 l2) = length l1 + length l2)
+  Q.(small_list int) (fun l -> l = interleave [] l)
+  Q.(small_list int) (fun l -> l = interleave l [])
+*)
+
 let take_while p l =
   let rec direct i p l = match l with
     | [] -> []
diff --git a/src/core/CCList.mli b/src/core/CCList.mli
index a9ed88d2..2b8261c5 100644
--- a/src/core/CCList.mli
+++ b/src/core/CCList.mli
@@ -45,7 +45,7 @@ val (@) : 'a t -> 'a t -> 'a t
     Concatenate two lists. *)
 
 val filter : ('a -> bool) -> 'a t -> 'a t
-(** Safe version of {!List.filter}. 
+(** Safe version of {!List.filter}.
     [filter p l] returns all the elements of the list [l]
     that satisfy the predicate [p].  The order of the elements
     in the input list is preserved. *)
@@ -202,6 +202,15 @@ val sublists_of_len :
     @raise Invalid_argument if [offset <= 0] or [n <= 0].
     @since 1.0 *)
 
+val intersperse : 'a -> 'a list -> 'a list
+(** Insert the first argument between every element of the list
+    @since NEXT_RELEASE *)
+
+val interleave : 'a list -> 'a list -> 'a list
+(** [interleave [x1…xn] [y1…ym]] is [x1,y1,x2,y2,…] and finishes with
+    the suffix of the longest list
+    @since NEXT_RELEASE *)
+
 val pure : 'a -> 'a t
 (** [pure] is [return]. *)