From 4e85c4ff277c883edf67d87d5d7d158a49dc4850 Mon Sep 17 00:00:00 2001
From: volth <volth@webmaster.ms>
Date: Sun, 10 Jun 2018 17:31:09 +0000
Subject: [PATCH] lib: add groupBy (#38612)

---
 lib/default.nix |  2 +-
 lib/lists.nix   | 36 ++++++++++++++++++++++++++++++++++++
 2 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/lib/default.nix b/lib/default.nix
index 4ca2e2ea6e3..aca484d9a8e 100644
--- a/lib/default.nix
+++ b/lib/default.nix
@@ -76,7 +76,7 @@ let
       optional optionals toList range partition zipListsWith zipLists
       reverseList listDfs toposort sort naturalSort compareLists take
       drop sublist last init crossLists unique intersectLists
-      subtractLists mutuallyExclusive;
+      subtractLists mutuallyExclusive groupBy groupBy';
     inherit (strings) concatStrings concatMapStrings concatImapStrings
       intersperse concatStringsSep concatMapStringsSep
       concatImapStringsSep makeSearchPath makeSearchPathOutput
diff --git a/lib/lists.nix b/lib/lists.nix
index 5ec97f5a07f..194e1c200ec 100644
--- a/lib/lists.nix
+++ b/lib/lists.nix
@@ -250,6 +250,42 @@ rec {
       else { right = t.right; wrong = [h] ++ t.wrong; }
     ) { right = []; wrong = []; });
 
+  /* Splits the elements of a list into many lists, using the return value of a predicate.
+     Predicate should return a string which becomes keys of attrset `groupBy' returns.
+
+     `groupBy'' allows to customise the combining function and initial value
+
+     Example:
+       groupBy (x: boolToString (x > 2)) [ 5 1 2 3 4 ]
+       => { true = [ 5 3 4 ]; false = [ 1 2 ]; }
+       groupBy (x: x.name) [ {name = "icewm"; script = "icewm &";}
+                             {name = "xfce";  script = "xfce4-session &";}
+                             {name = "icewm"; script = "icewmbg &";}
+                             {name = "mate";  script = "gnome-session &";}
+                           ]
+       => { icewm = [ { name = "icewm"; script = "icewm &"; }
+                      { name = "icewm"; script = "icewmbg &"; } ];
+            mate  = [ { name = "mate";  script = "gnome-session &"; } ];
+            xfce  = [ { name = "xfce";  script = "xfce4-session &"; } ];
+          }
+
+
+     groupBy' allows to customise the combining function and initial value
+
+     Example:
+       groupBy' builtins.add 0 (x: boolToString (x > 2)) [ 5 1 2 3 4 ]
+       => { true = 12; false = 3; }
+  */
+  groupBy' = op: nul: pred: lst:
+    foldl' (r: e:
+              let
+                key = pred e;
+              in
+                r // { ${key} = op (r.${key} or nul) e; }
+           ) {} lst;
+
+  groupBy = groupBy' (sum: e: sum ++ [e]) [];
+
   /* Merges two lists of the same size together. If the sizes aren't the same
      the merging stops at the shortest. How both lists are merged is defined
      by the first argument.