>From e3c696f73b37da24eb3883d365a72f014a3e52ee Mon Sep 17 00:00:00 2001
From: Damien Cassou <damien@cassou.me>
Date: Fri, 2 Sep 2022 14:54:36 +0200
Subject: [PATCH] Add new function `seq-positions'

* doc/lispref/sequences.texi (Sequence Functions): Document it.

* lisp/emacs-lisp/seq.el (seq-positions): New function.

* lisp/emacs-lisp/shortdoc.el (sequence): Mention it.

* test/lisp/emacs-lisp/seq-tests.el (test-seq-positions): Test it.
---
 doc/lispref/sequences.texi        | 17 +++++++++++++++++
 etc/NEWS                          |  5 +++++
 lisp/emacs-lisp/seq.el            | 13 +++++++++++++
 lisp/emacs-lisp/shortdoc.el       |  3 +++
 test/lisp/emacs-lisp/seq-tests.el |  5 +++++
 5 files changed, 43 insertions(+)

diff --git a/doc/lispref/sequences.texi b/doc/lispref/sequences.texi
index 1f6f80521c..6b198ba731 100644
--- a/doc/lispref/sequences.texi
+++ b/doc/lispref/sequences.texi
@@ -880,6 +880,23 @@ Sequence Functions
 @end example
 @end defun
 
+@defun seq-positions sequence elt &optional testfn
+  This function returns a list of the (zero-based) indices of the
+elements in @var{sequence} that are equal to @var{elt}.  If the
+optional argument @var{testfn} is non-@code{nil}, it is a function of
+two arguments to use instead of the default @code{equal}.
+
+@example
+@group
+(seq-positions '(a b c a d) 'a)
+@result{} (0 3)
+@end group
+@group
+(seq-positions '(a b c a d) 'z)
+@result{} nil
+@end group
+@end example
+@end defun
 
 @defun seq-uniq sequence &optional function
   This function returns a list of the elements of @var{sequence} with
diff --git a/etc/NEWS b/etc/NEWS
index 1512d45fdc..a8fc8dcc34 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -2710,6 +2710,11 @@ The default timeout value can be defined by the new variable
 ** New function 'seq-split'.
 This returns a list of sub-sequences of the specified sequence.
 
++++
+** New function 'seq-positions'.
+This returns a list of the (zero-based) indices of a given element in
+the specified sequence.
+
 +++
 ** 'plist-get', 'plist-put' and 'plist-member' are no longer limited to 'eq'.
 These function now take an optional comparison predicate argument.
diff --git a/lisp/emacs-lisp/seq.el b/lisp/emacs-lisp/seq.el
index b6f0f66e5b..7e8576fffb 100644
--- a/lisp/emacs-lisp/seq.el
+++ b/lisp/emacs-lisp/seq.el
@@ -445,6 +445,19 @@ seq-position
         (setq index (1+ index)))
       nil)))
 
+;;;###autoload
+(cl-defgeneric seq-positions (sequence elt &optional testfn)
+  "Return a list of the (zero-based) indices of ELT in SEQ.
+Equality is defined by the function TESTFN, which defaults to
+`equal'."
+  (let ((result '()))
+    (seq-do-indexed
+     (lambda (e index)
+       (when (funcall (or testfn #'equal) e elt)
+         (push index result)))
+     sequence)
+    (nreverse result)))
+
 ;;;###autoload
 (cl-defgeneric seq-uniq (sequence &optional testfn)
   "Return a list of the elements of SEQUENCE with duplicates removed.
diff --git a/lisp/emacs-lisp/shortdoc.el b/lisp/emacs-lisp/shortdoc.el
index 990dabe351..80b9c0a69e 100644
--- a/lisp/emacs-lisp/shortdoc.el
+++ b/lisp/emacs-lisp/shortdoc.el
@@ -846,6 +846,9 @@ sequence
    :eval (seq-find #'numberp '(a b 3 4 f 6)))
   (seq-position
    :eval (seq-position '(a b c) 'c))
+  (seq-positions
+   :eval (seq-positions '(a b c a d) 'a)
+   :eval (seq-positions '(a b c a d) 'z))
   (seq-length
    :eval (seq-length "abcde"))
   (seq-max
diff --git a/test/lisp/emacs-lisp/seq-tests.el b/test/lisp/emacs-lisp/seq-tests.el
index 1a27467d29..7f06a7e618 100644
--- a/test/lisp/emacs-lisp/seq-tests.el
+++ b/test/lisp/emacs-lisp/seq-tests.el
@@ -482,6 +482,11 @@ test-seq-position
     (should (= (seq-position seq 'a #'eq) 0))
     (should (null (seq-position seq (make-symbol "a") #'eq)))))
 
+(ert-deftest test-seq-positions ()
+  (with-test-sequences (seq '(1 2 3 1 4))
+    (should (equal '(0 3) (seq-positions seq 1)))
+    (should (seq-empty-p (seq-positions seq 9)))))
+
 (ert-deftest test-seq-sort-by ()
   (let ((seq ["x" "xx" "xxx"]))
     (should (equal (seq-sort-by #'seq-length #'> seq)
-- 
2.36.2