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+;;; imenu-conf.el --- Flatten imenu                  -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2023  Jean Sévère Durand
+
+;; Author: Jean Sévère Durand <durand@jsdurand.xyz>
+;; Keywords: convenience, maint, outlines
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; Flatten the imenu list if needed.
+
+;;; Code:
+
+(require 'imenu)
+
+(defvar imenu-flatten-p t
+  "Whether or not to flatten the indices in imenu.")
+
+(defun imenu-flatten-prefix (parent-prefix child-prefix)
+  "A helper function to concatenate prefixes."
+  (cond
+   ((and
+     (stringp parent-prefix)
+     (not (string-empty-p parent-prefix)))
+    ;; We can experiment with different styles, perhaps.
+    (format "%s/%s" parent-prefix child-prefix))
+   (child-prefix)))
+
+(defun imenu-flatten (alist)
+  "Flatten the ALIST.
+See the documentation of the variable `imenu--index-alist' to
+know what the flattening does."
+  (let* ((prefix "")
+         ;; Perform a depth-first walk on the tree.
+         (stack (mapcar (lambda (item) (cons prefix item)) alist))
+         temp result)
+    (while (consp stack)        ; `consp' excludes empty lists
+      ;; pop the stack from the front
+      (setq temp (car stack))
+      (setq stack (cdr stack))
+      (cond
+       ((not (imenu--subalist-p (cdr temp)))
+        (setq result
+              (cons
+               (cons
+                (imenu-flatten-prefix (car temp) (cadr temp))
+                (cddr temp))
+               result)))
+       ((let ((prefix (imenu-flatten-prefix (car temp) (cadr temp))))
+          (mapc
+           (lambda (sub-item)
+             (setq stack (cons (cons prefix sub-item) stack)))
+           (cddr temp))))))
+    (nreverse result)))
+
+(defun imenu-flatten-advice (alist)
+  "An advice for `imenu--make-index-alist' that flattens its result.
+The variable `imenu-flatten-p' controls whether the flattening
+actually happens."
+  (cond
+   (imenu-flatten-p (imenu-flatten alist))
+   (alist)))
+
+(advice-add #'imenu--make-index-alist
+            :filter-return #'imenu-flatten-advice)
+
+
+
+(provide 'imenu-conf)
+;;; imenu-conf.el ends here