From 3d596d98948b89299ed7a793c5158fb7c537e7f8 Mon Sep 17 00:00:00 2001
From: Arun Isaac
Date: Sun, 14 Aug 2022 20:04:01 +0530
Subject: Rewrite many to many grouping using functional setters.

* ennum.el (ennum-assoc-delete, ennum-assoc-set, ennum-assoc-prepend):
New functions.
* ennum.el (ennum-many-to-many-group-by): Use ennum-assoc-append
instead of mutating the result association list.
---
 ennum.el | 42 +++++++++++++++++++++++++++++++-----------
 1 file changed, 31 insertions(+), 11 deletions(-)

diff --git a/ennum.el b/ennum.el
index d8a54fb..fa0e794 100644
--- a/ennum.el
+++ b/ennum.el
@@ -612,21 +612,41 @@ value of the last form in BODY."
          (progn ,@body)
        (delete-directory ,temporary-directory t))))
 
+(defun ennum-assoc-delete (key alist)
+  "Delete KEY from ALIST.
+
+Delete all occurrences of KEY in ALIST. This function is purely
+functional. ALIST is not mutated."
+  (seq-remove (pcase-lambda (`(,entry-key . ,_))
+                (equal entry-key key))
+              alist))
+
+(defun ennum-assoc-set (key value alist)
+  "Associate KEY with VALUE in ALIST.
+
+This function is purely functional. ALIST is not mutated. Keys
+are compared using `equal'."
+  (cl-acons key value (ennum-assoc-delete key alist)))
+
+(defun ennum-assoc-prepend (key value alist)
+  "Prepend VALUE to that associated with KEY in ALIST.
+
+This function is purely funcion. ALIST is not mutated."
+  (ennum-assoc-set key
+                   (cons value (map-elt alist key nil 'equal))
+                   alist))
+
 (defun ennum-many-to-many-group-by (function sequence)
   "Apply FUNCTION to each element of SEQUENCE.
 Separate the elements of SEQUENCE into an alist using the results
 as keys. Keys are compared using `equal'."
-  (seq-reduce
-   (lambda (result element)
-     (seq-do
-      (lambda (key)
-        (map-put result key
-                 (cons element (map-elt result key nil 'equal))
-                 'equal))
-      (funcall function element))
-     result)
-   (seq-reverse sequence)
-   nil))
+  (seq-reduce (lambda (result element)
+                (seq-reduce (lambda (result key)
+                              (ennum-assoc-prepend key element result))
+                            (funcall function element)
+                            result))
+              (seq-reverse sequence)
+              nil))
 
 (defun ennum-publish ()
   (interactive)
-- 
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