Add function `string-common-prefix'scratch/string-common-prefix

* lisp/subr.el: (string-common-prefix): New function.

* etc/NEWS:
* doc/lispref/strings.texi:
* doc/lispref/minibuf.texi:
* lisp/emacs-lisp/shortdoc.el:
Documentation.

2 files changed, 34 insertions, 1 deletions

diff --git a/lisp/emacs-lisp/shortdoc.el b/lisp/emacs-lisp/shortdoc.el
index 70583e08dbd..af15d15523d 100644
--- a/lisp/emacs-lisp/shortdoc.el
+++ b/lisp/emacs-lisp/shortdoc.el
@@ -385,8 +385,15 @@ A FUNC form can have any number of `:no-eval' (or `:no-value'),
    :eval (reverse "foo"))
   (substring-no-properties
    :eval (substring-no-properties (propertize "foobar" 'face 'bold) 0 3))
+  (string-common-prefix
+   :eval (string-common-prefix '("foobar" "foozot"))
+   :eval (string-common-prefix '("foobar" "foozot" "gazonk"))
+   :eval (string-common-prefix nil))
   (try-completion
-   :eval (try-completion "foo" '("foobar" "foozot" "gazonk")))
+   :eval (try-completion "f" '("foobar" "foozot" "gazonk"))
+   :eval (try-completion "f" '("foo"))
+   :eval (try-completion "foo" '("foo"))
+   :eval (try-completion "foo" nil))
   "Unicode Strings"
   (string-glyph-split
    :eval (string-glyph-split "Hello, 👼🏻🧑🏼‍🤝‍🧑🏻"))
diff --git a/lisp/subr.el b/lisp/subr.el
index d307a07f05b..a5191ba74f3 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -6129,6 +6129,32 @@ attention to letter-case differences."
          (eq t (compare-strings suffix nil nil
                                 string start-pos nil ignore-case)))))
 
+(defun string-common-prefix (strings &optional ignore-case)
+  "Return the longest common prefix from a collection of STRINGS.
+
+Return \"\" if there is no common prefix or if STRINGS is nil.
+If STRINGS contains exactly one string, return that string.
+
+If IGNORE-CASE is non-nil, letter case is ignored when matching the
+substrings, but no guarantee is made about the letter-case of the return
+value, except that it comes from one of the members of STRINGS.
+
+STRINGS may be a list of strings or any other collection type supported
+by `try-completion' and `all-completions' (which see).  To find the
+common prefix of a subset of STRINGS (filtered by string prefix, regular
+expression, or predicate function), use `all-completions' to perform the
+filtering and pass the result to `string-common-prefix' as STRINGS."
+  ;; Note that `try-completion' is not affected by `completion-styles'.
+  (let* ((completion-ignore-case ignore-case)
+         (completion-regexp-list nil)
+         (prefix (try-completion "" strings)))
+    (if (stringp prefix)
+        prefix
+      ;; nil means there was no match.
+      ;; t means that the "" argument was an exact match.
+      ;; We always return a string, so we treat both cases the same.
+      "")))
+
 (defun bidi-string-mark-left-to-right (str)
   "Return a string that can be safely inserted in left-to-right text.