Split up and add tests for two page.el functions

* lisp/textmodes/page.el (page--count-lines-page): New function
extracted from `count-lines-page'.
(count-lines-page): Extract main logic into `page--count-lines-page'.
(page--what-page); New function extracted from `what-page'.
(what-page): Extract main logic into `page--what-page'.

* test/lisp/textmodes/page-tests.el (page-tests-count-lines-page)
(page-tests-what-page): New tests for `page--count-lines-page' and
`page--what-page'.  (Bug#36009)

1 files changed, 18 insertions, 1 deletions

diff --git a/test/lisp/textmodes/page-tests.el b/test/lisp/textmodes/page-tests.el
index 0834d654338..517f1d5a9e5 100644
--- a/test/lisp/textmodes/page-tests.el
+++ b/test/lisp/textmodes/page-tests.el
@@ -82,5 +82,22 @@
     (narrow-to-page -1)
     (should (equal (buffer-string) "bar\n"))))
 
-(provide 'page-tests)
+(ert-deftest page-tests-count-lines-page ()
+  (with-temp-buffer
+    (insert "foo\n\nbar\n\nbaz")
+    (goto-char (point-min))
+    (should (equal (page--count-lines-page) '(1 0 1)))
+    (goto-char (point-max))
+    (should (equal (page--count-lines-page) '(2 2 0)))))
+
+(ert-deftest page-tests-what-page ()
+  (with-temp-buffer
+    (insert "foo\n\nbar\n\nbaz")
+    (goto-char (point-min))
+    (should (equal (page--what-page) '(1 1)))
+    (forward-page)
+    (should (equal (page--what-page) '(2 2)))
+    (forward-page)
+    (should (equal (page--what-page) '(3 4)))))
+
 ;;; page-tests.el ends here