From 4751b51d5e1182975aa002af08a625e4859ec276 Mon Sep 17 00:00:00 2001
From: Damien Cassou <damien@cassou.me>
Date: Sun, 4 Sep 2022 13:21:59 +0200
Subject: 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
(bug#57548).
---
 lisp/emacs-lisp/seq.el      | 17 +++++++++++++++++
 lisp/emacs-lisp/shortdoc.el |  4 ++++
 2 files changed, 21 insertions(+)

(limited to 'lisp/emacs-lisp')

diff --git a/lisp/emacs-lisp/seq.el b/lisp/emacs-lisp/seq.el
index 64197b55e5f..31dcfa98b40 100644
--- a/lisp/emacs-lisp/seq.el
+++ b/lisp/emacs-lisp/seq.el
@@ -459,6 +459,23 @@ Equality is defined by the function TESTFN, which defaults to `equal'."
         (setq index (1+ index)))
       nil)))
 
+;;;###autoload
+(cl-defgeneric seq-positions (sequence elt &optional testfn)
+  "Return indices for which (TESTFN (seq-elt SEQUENCE index) ELT) is non-nil.
+
+TESTFN is a two-argument function which is passed each element of
+SEQUENCE as first argument and ELT as second. TESTFN defaults to
+`equal'.
+
+The result is a list of (zero-based) indices."
+  (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 6a366ec0fc0..2472479bad6 100644
--- a/lisp/emacs-lisp/shortdoc.el
+++ b/lisp/emacs-lisp/shortdoc.el
@@ -846,6 +846,10 @@ A FUNC form can have any number of `:no-eval' (or `:no-value'),
    :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)
+   :eval (seq-positions '(11 5 7 12 9 15) 10 #'>=))
   (seq-length
    :eval (seq-length "abcde"))
   (seq-max
-- 
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