From fb97af680b92d83f675a28ef5a263c828b61525b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Glen=20M=C3=A9vel?= <glen.mevel@crans.org>
Date: Mon, 25 Apr 2016 00:56:05 +0200
Subject: [PATCH 1/2] add CCList.range_by

---
 src/core/CCList.ml  | 20 ++++++++++++++++++++
 src/core/CCList.mli |  7 +++++++
 2 files changed, 27 insertions(+)

diff --git a/src/core/CCList.ml b/src/core/CCList.ml
index 857512ce..975a0861 100644
--- a/src/core/CCList.ml
+++ b/src/core/CCList.ml
@@ -792,6 +792,26 @@ module Idx = struct
   *)
 end
 
+let range_by ~step i j =
+  if step = 0 then raise (Invalid_argument "CCList.range_by");
+  let rec up i j acc =
+    if i>j then acc else up i (j-step) (j::acc)
+  and down i j acc =
+    if i<j then acc else down i (j-step) (j::acc)
+  in
+  let j = (j - i) / step * step + i in
+  if step > 0 then up i j [] else down i j []
+
+(*$T
+  range_by ~step:1   0 0 = [0]
+  range_by ~step:1   5 0 = []
+  range_by ~step:2   0 4 = [0;2;4]
+  range_by ~step:2   0 5 = [0;2;4]
+  range_by ~step:~-1 0 5 = []
+  range_by ~step:~-2 5 1 = [5;3;1]
+  range_by ~step:~-2 5 0 = [5;3;1]
+*)
+
 let range i j =
   let rec up i j acc =
     if i=j then i::acc else up i (j-1) (j::acc)
diff --git a/src/core/CCList.mli b/src/core/CCList.mli
index 41f4e5d0..8bb1ba7e 100644
--- a/src/core/CCList.mli
+++ b/src/core/CCList.mli
@@ -275,6 +275,13 @@ end
 
 (** {2 Other Constructors} *)
 
+val range_by : step:int -> int -> int -> int t
+(** [range_by ~step i j] iterates on integers from [i] to [j] included,
+    where the difference between successive elements is [step].
+    use a negative [step] for a decreasing list.
+    @raise Invalid_argument if [step=0]
+    @since NEXT_RELEASE *)
+
 val range : int -> int -> int t
 (** [range i j] iterates on integers from [i] to [j] included . It works
     both for decreasing and increasing ranges *)

From f57d7267ac54ac6f942a74c6da78d034b9f013ad Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Glen=20M=C3=A9vel?= <glen.mevel@crans.org>
Date: Mon, 25 Apr 2016 11:39:03 +0200
Subject: [PATCH 2/2] bug fix for CCList.range_by

---
 src/core/CCList.ml | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/src/core/CCList.ml b/src/core/CCList.ml
index 975a0861..80b8578f 100644
--- a/src/core/CCList.ml
+++ b/src/core/CCList.ml
@@ -793,23 +793,29 @@ module Idx = struct
 end
 
 let range_by ~step i j =
-  if step = 0 then raise (Invalid_argument "CCList.range_by");
-  let rec up i j acc =
-    if i>j then acc else up i (j-step) (j::acc)
-  and down i j acc =
-    if i<j then acc else down i (j-step) (j::acc)
+  let rec range i j acc =
+    if i=j then i::acc else range i (j-step) (j::acc)
   in
-  let j = (j - i) / step * step + i in
-  if step > 0 then up i j [] else down i j []
+  if step = 0 then
+    raise (Invalid_argument "CCList.range_by")
+  else if (if step > 0 then i>j else i<j) then
+    []
+  else
+    range i ((j-i)/step*step + i)  []
 
+(* note: the last test checks that no error occurs due to overflows. *)
 (*$T
   range_by ~step:1   0 0 = [0]
   range_by ~step:1   5 0 = []
+  range_by ~step:2   1 0 = []
   range_by ~step:2   0 4 = [0;2;4]
   range_by ~step:2   0 5 = [0;2;4]
+  range_by ~step:~-1 0 0 = [0]
   range_by ~step:~-1 0 5 = []
+  range_by ~step:~-2 0 1 = []
   range_by ~step:~-2 5 1 = [5;3;1]
   range_by ~step:~-2 5 0 = [5;3;1]
+  range_by ~step:max_int 0 2 = [0]
 *)
 
 let range i j =