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+;;; ert-font-lock.el --- ERT Font Lock   -*- lexical-binding: t -*-
+
+;; Copyright (C) 2023 Free Software Foundation, Inc.
+
+;; Author: Vladimir Kazanov
+;; Keywords: lisp, tools
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+;;
+;; ERT Font Lock is an extension to the Emacs Lisp Regression Test
+;; library (ERT) providing a convenient way to check syntax
+;; highlighting provided by font-lock.
+;;
+;; ert-font-lock entry points are functions
+;; `ert-font-lock-test-string' and `ert-font-lock-test-file' and
+;; convenience macros: `ert-font-lock-deftest' and
+;; `ert-font-lock-deftest-file'.
+;;
+;; See unit tests in ert-font-lock-tests.el for usage examples.
+
+;;; Code:
+
+(require 'ert)
+(require 'newcomment)
+(require 'pcase)
+
+(defconst ert-font-lock--assertion-re
+  (rx
+   ;; column specifiers
+   (group (or "^" "<-"))
+   (one-or-more " ")
+   ;; optional negation of the face specification
+   (group (optional "!"))
+   ;; face symbol name
+   (group (one-or-more (or alphanumeric "-" "_" "."))))
+  "An ert-font-lock assertion regex.")
+
+(defun ert-font-lock--validate-major-mode (mode)
+  "Validate if MODE is a valid major mode."
+  (unless (functionp mode)
+    (error "Invalid major mode: %S. Please specify a valid major mode for
+ syntax highlighting tests" mode)))
+
+(defun ert-font-lock--test-body-str (mode str test-name)
+  "Run assertions from STR.
+Argument MODE - major mode to test.
+Argument TEST-NAME - name of the currently running ert test."
+  (ert-font-lock--validate-major-mode mode)
+  (with-temp-buffer
+    (insert str)
+    (funcall mode)
+    (font-lock-ensure)
+    (let ((tests (ert-font-lock--parse-comments)))
+      (ert-font-lock--check-faces tests)))
+  test-name)
+
+(defun ert-font-lock--test-body-file (mode file test-name)
+  "Run assertions from FILE.
+Argument MODE - major mode to test.
+Argument TEST-NAME - name of the currently running ert test."
+  (ert-font-lock--validate-major-mode mode)
+  (ert-font-lock-test-file file mode)
+  test-name)
+
+(defun ert-font-lock--parse-macro-args (doc-keys-mode-arg)
+  "Parse DOC-KEYS-MODE-ARG macro argument list."
+  (let (doc doc-p mode arg)
+
+    (when (stringp (car doc-keys-mode-arg))
+      (setq doc (pop doc-keys-mode-arg)
+            doc-p t))
+
+    (pcase-let
+        ((`(,keys ,mode-arg)
+          (ert--parse-keys-and-body doc-keys-mode-arg)))
+
+      (unless (symbolp (car mode-arg))
+        (error "A major mode symbol expected: %S" (car mode-arg)))
+      (setq mode (pop mode-arg))
+
+      (unless (stringp (car mode-arg))
+        (error "A string or file with assertions expected: %S" (car mode-arg)))
+      (setq arg (pop mode-arg))
+
+      (list doc doc-p keys mode arg))))
+
+;;;###autoload
+(defmacro ert-font-lock-deftest (name &rest docstring-keys-mode-and-str)
+  "Define test NAME (a symbol) using assertions from TEST-STR.
+
+Other than MAJOR-MODE and TEST-STR parameters, this macro accepts
+the same parameters and keywords as `ert-deftest' and is intended
+to be used through `ert'.
+
+\(fn NAME () [DOCSTRING] [:expected-result RESULT-TYPE] \
+[:tags \\='(TAG...)] MAJOR-MODE TEST-STR)"
+  (declare (debug (&define [&name "test@" symbolp]
+                           sexp [&optional stringp]
+                           [&rest keywordp sexp]
+                           symbolp
+                           stringp))
+           (doc-string 3)
+           (indent 2))
+  (pcase-let ((`(,documentation
+                 ,documentation-supplied-p
+                 ,keys ,mode ,arg)
+               (ert-font-lock--parse-macro-args docstring-keys-mode-and-str)))
+
+    `(ert-set-test ',name
+                   (make-ert-test
+                    :name ',name
+                    ,@(when documentation-supplied-p
+                        `(:documentation ,documentation))
+                    ,@(when (map-contains-key keys :expected-result)
+                        `(:expected-result-type ,(map-elt keys :expected-result)))
+                    ,@(when (map-contains-key keys :tags)
+                        `(:tags ,(map-elt keys :tags)))
+                    :body (lambda () (ert-font-lock--test-body-str ',mode ,arg ',name))
+
+                    :file-name ,(or (macroexp-file-name) buffer-file-name)))))
+
+;;;###autoload
+(defmacro ert-font-lock-deftest-file (name &rest docstring-keys-mode-and-file)
+  "Define test NAME (a symbol) using assertions from FILE.
+
+FILE - path to a file with assertions in ERT resource director as
+return by `ert-resource-directory'.
+
+Other than MAJOR-MODE and FILE parameters, this macro accepts the
+same parameters and keywords as `ert-deftest' and is intended to
+be used through `ert'.
+
+\(fn NAME () [DOCSTRING] [:expected-result RESULT-TYPE] \
+[:tags \\='(TAG...)] MAJOR-MODE FILE)"
+  (declare (debug (&define [&name "test@" symbolp]
+                           sexp [&optional stringp]
+                           [&rest keywordp sexp]
+                           symbolp
+                           stringp))
+           (doc-string 3)
+           (indent 2))
+
+  (pcase-let ((`(,documentation
+                 ,documentation-supplied-p
+                 ,keys ,mode ,arg)
+               (ert-font-lock--parse-macro-args docstring-keys-mode-and-file)))
+
+    `(ert-set-test ',name
+                   (make-ert-test
+                    :name ',name
+                    ,@(when documentation-supplied-p
+                        `(:documentation ,documentation))
+                    ,@(when (map-contains-key keys :expected-result)
+                        `(:expected-result-type ,(map-elt keys :expected-result)))
+                    ,@(when (map-contains-key keys :tags)
+                        `(:tags ,(map-elt keys :tags)))
+                    :body (lambda () (ert-font-lock--test-body-file
+                                 ',mode (ert-resource-file ,arg) ',name))
+                    :file-name ,(or (macroexp-file-name) buffer-file-name)))))
+
+(defun ert-font-lock--in-comment-p ()
+  "Check if the current point is inside a comment."
+  (nth 4 (syntax-ppss)))
+
+(defun ert-font-lock--comment-start-p ()
+  "Check if the current point starts a comment."
+  (or
+   ;; regexps use syntax tables so let's check that first
+   (looking-at "\\s<")
+
+   ;; check newcomment.el facilities
+   (and comment-start (looking-at (regexp-quote comment-start)))
+   (and comment-start-skip (looking-at comment-start-skip))
+
+   ;; sometimes comment syntax is just hardcoded
+   (and (derived-mode-p '(c-mode c++-mode java-mode))
+        (looking-at-p "//"))))
+
+(defun ert-font-lock--line-comment-p ()
+  "Return t if the current line is a comment-only line."
+  (syntax-ppss)
+  (save-excursion
+    (beginning-of-line)
+    (skip-syntax-forward " ")
+    ;; skip empty lines
+    (unless (eolp)
+      (or
+       ;; multiline comments
+       (ert-font-lock--in-comment-p)
+
+       ;; single line comments
+       (ert-font-lock--comment-start-p)))))
+
+(defun ert-font-lock--line-assertion-p ()
+  "Return t if the current line contains an assertion."
+  (syntax-ppss)
+  (save-excursion
+    (beginning-of-line)
+    (skip-syntax-forward " ")
+    (re-search-forward ert-font-lock--assertion-re
+                       (line-end-position) t 1)))
+
+(defun ert-font-lock--goto-first-char ()
+  "Move the point to the first character."
+  (beginning-of-line)
+  (skip-syntax-forward " "))
+
+(defun ert-font-lock--get-first-char-column ()
+  "Get the position of the first non-empty char in the current line."
+  (save-excursion
+    (ert-font-lock--goto-first-char)
+    (- (point) (line-beginning-position))))
+
+(defun ert-font-lock--parse-comments ()
+  "Read test assertions from comments in the current buffer."
+  (let ((tests '())
+        (curline 1)
+        (linetocheck -1))
+
+    (goto-char (point-min))
+
+    ;; Go through all lines, for comments check if there are
+    ;; assertions. For non-comment and comment/non-assert lines
+    ;; remember the last line seen.
+    (while (not (eobp))
+      (catch 'nextline
+
+        ;; Not a comment? remember the line, move to the next one
+        (unless (ert-font-lock--line-comment-p)
+          (setq linetocheck curline)
+          (throw 'nextline t))
+
+        ;; A comment. Not an assertion? remember the line to be
+        ;; checked, move to the next line
+        (unless (ert-font-lock--line-assertion-p)
+          (setq linetocheck curline)
+          (throw 'nextline t))
+
+
+        ;; Collect the assertion
+        (when (re-search-forward ert-font-lock--assertion-re
+                                 (line-end-position) t 1)
+
+          (unless (> linetocheck -1)
+            (user-error "Invalid test comment syntax at line %d. Expected a line to test before the comment line" curline))
+
+          ;; construct a test
+          (let* (;; either comment start char column (for arrows) or
+                 ;; caret column
+                 (column-checked (if (equal (match-string-no-properties 1) "^")
+                                     (- (match-beginning 1) (line-beginning-position))
+                                   (ert-font-lock--get-first-char-column)))
+                 ;; negate the face?
+                 (negation (string-equal (match-string-no-properties 2) "!"))
+                 ;; the face that is supposed to be in the position specified
+                 (face (match-string-no-properties 3)))
+
+            (push (list :line-checked linetocheck
+                        :line-assert curline
+                        :column-checked column-checked
+                        :face face
+                        :negation negation)
+                  tests))))
+
+      ;; next line
+      (setq curline (1+ curline))
+      (forward-line 1))
+
+    (reverse tests)))
+
+(defun ert-font-lock--point-at-line-and-column (line column)
+  "Get the buffer position for LINE and COLUMN."
+  (save-excursion
+    (goto-char (point-min))
+    (forward-line (1- line))
+    (move-to-column column)
+    (point)))
+
+(defun ert-font-lock--get-line (line-number)
+  "Return the content of the line specified by LINE-NUMBER."
+  (save-excursion
+    (goto-char (point-min))
+    (forward-line (1- line-number))
+    (buffer-substring-no-properties (line-beginning-position) (line-end-position))))
+
+(defun ert-font-lock--check-faces (tests)
+  "Check if the current buffer is fontified correctly.
+TESTS - tests to run.
+
+The function is meant to be run from within an ERT test."
+  (dolist (test tests)
+    (let* ((line-checked (plist-get test :line-checked))
+           (line-assert (plist-get test :line-assert))
+           (column-checked (plist-get test :column-checked))
+           (expected-face (intern (plist-get test :face)))
+           (negation (plist-get test :negation))
+
+           (actual-face (get-text-property (ert-font-lock--point-at-line-and-column line-checked column-checked) 'face))
+           (line-str (ert-font-lock--get-line line-checked))
+           (line-assert-str (ert-font-lock--get-line line-assert)))
+
+      (when (not (eq actual-face expected-face))
+        (ert-fail
+         (list (format "Expected face %S, got %S on line %d column %d"
+                       expected-face actual-face line-checked column-checked)
+               :line line-str
+               :assert line-assert-str)))
+
+      (when (and negation (eq actual-face expected-face))
+        (ert-fail
+         (list (format "Did not expect face %S face on line %d, column %d"
+                       actual-face line-checked column-checked)
+               :line line-str
+               :assert line-assert-str))))))
+
+;;;###autoload
+(defun ert-font-lock-test-string (test-string mode)
+  "Check font faces in TEST-STRING set by MODE.
+
+The function is meant to be run from within an ERT test."
+  (ert-font-lock--validate-major-mode mode)
+  (with-temp-buffer
+    (insert test-string)
+    (funcall mode)
+    (font-lock-ensure)
+
+    (ert-font-lock--check-faces (ert-font-lock--parse-comments)))
+
+  (ert-pass))
+
+;;;###autoload
+(defun ert-font-lock-test-file (filename mode)
+  "Check font faces in FILENAME set by MODE.
+
+The function is meant to be run from within an ERT test."
+  (ert-font-lock--validate-major-mode mode)
+  (with-temp-buffer
+    (insert-file-contents filename)
+    (funcall mode)
+    (font-lock-ensure)
+
+    (ert-font-lock--check-faces (ert-font-lock--parse-comments)))
+
+  (ert-pass))
+
+
+(provide 'ert-font-lock)
+
+;;; ert-font-lock.el ends here