Add seq-intersection and seq-difference to the seq library

* lisp/emacs-lisp/seq.el (seq-intersection, seq-difference): New
functions.

* test/automated/seq-tests.el: Add tests for seq-intersection and
seq-difference.

* doc/lispref/sequences.texi: Add documentation for seq-intersection
and seq-difference.

doc/lispref/sequences.texi

@@ -723,6 +723,35 @@ contain less elements than @var{n}.  @var{n} must be an integer.  If
 @end example
 @end defun
 
+@defun seq-intersection sequence1 sequence2 &optional function
+  This function returns a list of the elements that appear both in
+@var{sequence1} and @var{sequence2}.  If the optional argument
+@var{function} is non-@code{nil}, it is a function of two arguments to
+use to compare elements instead of the default @code{equal}.
+
+@example
+@group
+(seq-intersection [2 3 4 5] [1 3 5 6 7])
+@result {} (3 5)
+@end group
+@end example
+@end defun
+
+
+@defun seq-difference sequence1 sequence2 &optional function
+  This function returns a list of the elements that appear in
+@var{sequence1} but not in @var{sequence2}.  If the optional argument
+@var{function} is non-@code{nil}, it is a function of two arguments to
+use to compare elements instead of the default @code{equal}.
+
+@example
+@group
+(seq-difference '(2 3 4 5) [1 3 5 6 7])
+@result {} (2 4)
+@end group
+@end example
+@end defun
+
 @defun seq-group-by function sequence
   This function separates the elements of @var{sequence} into an alist
 whose keys are the result of applying @var{function} to each element
@@ -761,7 +790,6 @@ of type @var{type}.  @var{type} can be one of the following symbols:
 @end example
 @end defun
 
-
 @defmac seq-doseq (var sequence [result]) body@dots{}
 @cindex sequence iteration
   This macro is like @code{dolist}, except that @var{sequence} can be a list,

lisp/emacs-lisp/seq.el

@@ -4,7 +4,7 @@
 
 ;; Author: Nicolas Petton <nicolas@petton.fr>
 ;; Keywords: sequences
-;; Version: 1.3
+;; Version: 1.4
 ;; Package: seq
 
 ;; Maintainer: emacs-devel@gnu.org
@@ -240,6 +240,26 @@ negative integer or 0, nil is returned."
         (setq seq (seq-drop seq n)))
       (nreverse result))))
 
+(defun seq-intersection (seq1 seq2 &optional testfn)
+  "Return a list of the elements that appear in both SEQ1 and SEQ2.
+Equality is defined by TESTFN if non-nil or by `equal' if nil."
+  (seq-reduce (lambda (acc elt)
+                (if (seq-contains-p seq2 elt testfn)
+                    (cons elt acc)
+                  acc))
+              (seq-reverse seq1)
+              '()))
+
+(defun seq-difference (seq1 seq2 &optional testfn)
+  "Return a list of th elements that appear in SEQ1 but not in SEQ2.
+Equality is defined by TESTFN if non-nil or by `equal' if nil."
+  (seq-reduce (lambda (acc elt)
+                (if (not (seq-contains-p seq2 elt testfn))
+                    (cons elt acc)
+                  acc))
+              (seq-reverse seq1)
+              '()))
+
 (defun seq-group-by (function seq)
   "Apply FUNCTION to each element of SEQ.
 Separate the elements of SEQ into an alist using the results as
@@ -318,6 +338,11 @@ This is an optimization for lists in `seq-take-while'."
       (setq n (+ 1 n)))
     n))
 
+(defun seq--activate-font-lock-keywords ()
+  "Activate font-lock keywords for some symbols defined in seq."
+  (font-lock-add-keywords 'emacs-lisp-mode
+                          '("\\<seq-doseq\\>")))
+
 (defalias 'seq-copy #'copy-sequence)
 (defalias 'seq-elt #'elt)
 (defalias 'seq-length #'length)
@@ -325,5 +350,7 @@ This is an optimization for lists in `seq-take-while'."
 (defalias 'seq-each #'seq-do)
 (defalias 'seq-map #'mapcar)
 
+(add-to-list 'emacs-lisp-mode-hook #'seq--activate-font-lock-keywords)
+
 (provide 'seq)
 ;;; seq.el ends here

test/automated/seq-tests.el

@@ -250,5 +250,31 @@ Evaluate BODY for each created sequence.
     (should (same-contents-p list vector))
     (should (vectorp vector))))
 
+(ert-deftest test-seq-intersection ()
+  (let ((v1 [2 3 4 5])
+        (v2 [1 3 5 6 7]))
+    (should (same-contents-p (seq-intersection v1 v2)
+                             '(3 5))))
+  (let ((l1 '(2 3 4 5))
+        (l2 '(1 3 5 6 7)))
+    (should (same-contents-p (seq-intersection l1 l2)
+                             '(3 5))))
+  (let ((v1 [2 4 6])
+        (v2 [1 3 5]))
+    (should (seq-empty-p (seq-intersection v1 v2)))))
+
+(ert-deftest test-seq-difference ()
+  (let ((v1 [2 3 4 5])
+        (v2 [1 3 5 6 7]))
+    (should (same-contents-p (seq-difference v1 v2)
+                             '(2 4))))
+  (let ((l1 '(2 3 4 5))
+        (l2 '(1 3 5 6 7)))
+    (should (same-contents-p (seq-difference l1 l2)
+                             '(2 4))))
+  (let ((v1 [2 4 6])
+        (v2 [2 4 6]))
+    (should (seq-empty-p (seq-difference v1 v2)))))
+
 (provide 'seq-tests)
 ;;; seq-tests.el ends here