From 5c110716cfda6e616a795edd12f2012b132dca9f Mon Sep 17 00:00:00 2001
From: Joris
Date: Sun, 2 Apr 2017 17:51:12 +0200
Subject: Add a chart on payments by month by categories

---
 src/client/Utils/List.elm | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

(limited to 'src/client/Utils/List.elm')

diff --git a/src/client/Utils/List.elm b/src/client/Utils/List.elm
index cc57d9f..8e26e85 100644
--- a/src/client/Utils/List.elm
+++ b/src/client/Utils/List.elm
@@ -1,9 +1,11 @@
 module Utils.List exposing
   ( groupBy
   , mean
+  , links
   )
 
 import Dict
+import Maybe.Extra as Maybe
 
 groupBy : (a -> comparable) -> List a -> List (comparable, List a)
 groupBy f xs =
@@ -15,3 +17,20 @@ groupBy f xs =
 
 mean : List Int -> Int
 mean xs = (List.sum xs) // (List.length xs)
+
+links : List a -> List (a, a)
+links xs =
+  let reversed = List.reverse xs
+  in  List.foldr
+        (\x acc ->
+          case Maybe.map Tuple.first (List.head acc) of
+            Just y ->
+              (x, y) :: acc
+            _ ->
+              acc
+        )
+        (case reversed of
+          x :: y :: _ -> [(y, x)]
+          _ -> []
+        )
+        (List.reverse << List.drop 2 <| reversed)
-- 
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