From 8f99c3953c6bea7b23ec04c60a658dd11ea8bdb4 Mon Sep 17 00:00:00 2001
From: Matt Sturgeon <matt@sturgeon.me.uk>
Date: Mon, 19 Aug 2024 00:45:47 +0100
Subject: [PATCH] lib/util: add `groupListBySize`

Splits up a list into many sub-lists based on the given max-size.

e.g.
```nix
groupListBySize 2 [ 1 2 3 4 5 ]
=> [ [ 1 2 ] [ 3 4 ] [ 5 ] ]
```
---
 lib/helpers.nix     |  1 +
 lib/utils.nix       | 19 +++++++++++++++++++
 tests/lib-tests.nix | 41 +++++++++++++++++++++++++++++++++++++++++
 3 files changed, 61 insertions(+)

diff --git a/lib/helpers.nix b/lib/helpers.nix
index 86ac4346..030ca873 100644
--- a/lib/helpers.nix
+++ b/lib/helpers.nix
@@ -71,6 +71,7 @@ let
       concatNonEmptyLines
       emptyTable
       enableExceptInTests
+      groupListBySize
       hasContent
       ifNonNull'
       listToUnkeyedAttrs
diff --git a/lib/utils.nix b/lib/utils.nix
index 43b22fce..7c548f68 100644
--- a/lib/utils.nix
+++ b/lib/utils.nix
@@ -147,4 +147,23 @@ rec {
       ${string}
       EOF
     '';
+
+  # Split a list into a several sub-list, each with a max-size of `size`
+  groupListBySize =
+    size: list:
+    reverseList (
+      foldl' (
+        lists: item:
+        let
+          first = head lists;
+          rest = drop 1 lists;
+        in
+        if lists == [ ] then
+          [ [ item ] ]
+        else if length first < size then
+          [ (first ++ [ item ]) ] ++ rest
+        else
+          [ [ item ] ] ++ lists
+      ) [ ] list
+    );
 }
diff --git a/tests/lib-tests.nix b/tests/lib-tests.nix
index 7eb3ab3c..abaecb23 100644
--- a/tests/lib-tests.nix
+++ b/tests/lib-tests.nix
@@ -333,6 +333,47 @@ let
         "MIxEd"
       ];
     };
+
+    testGroupListBySize = {
+      expr = {
+        empty = helpers.groupListBySize 5 [ ];
+        "5/5" = helpers.groupListBySize 5 (lib.genList lib.id 5);
+        "13/5" = helpers.groupListBySize 5 (lib.genList lib.id 13);
+      };
+      expected = {
+        empty = [ ];
+        "5/5" = [
+          [
+            0
+            1
+            2
+            3
+            4
+          ]
+        ];
+        "13/5" = [
+          [
+            0
+            1
+            2
+            3
+            4
+          ]
+          [
+            5
+            6
+            7
+            8
+            9
+          ]
+          [
+            10
+            11
+            12
+          ]
+        ];
+      };
+    };
   };
 in
 if results == [ ] then