1 files changed, 57 insertions(+), 28 deletions(-)

M myrddin-mode.el

@@ 145,38 145,67 @@
 ;;; Indentation.
 ;;;
 
+(defun myrddin-mode--preceding-nonempty-line ()
+  "Return the text of and the indentation level of the closest
+preceding line that does not consist entirely of whitespace, or
+NIL if there is no preceding line."
+  (unless (bobp)
+    (save-excursion
+      (let (last-line)
+        (while (not (and last-line (plusp (length (string-trim last-line)))))
+          (previous-line)
+          (back-to-indentation)
+          (setf last-line (buffer-substring-no-properties
+                           (line-beginning-position)
+                           (line-end-position))))
+        (values last-line (/ (current-column) myrddin-indent-offset))))))
+
+
+(defun myrddin-mode--nearest-match-level ()
+  (save-excursion
+    (while (not (or (bobp) (looking-at "match")))
+      (previous-line)
+      (back-to-indentation))
+    (/ (current-column) myrddin-indent-offset)))
+
+(defun myrddin-mode--calculate-indent (prev-line prev-level)
+  (save-excursion
+    (back-to-indentation)
+    (cond
+     ;; Handle match arms by finding the indentation of the nearest match
+     ;; statement.
+     ((looking-at (rx ?|))
+      (* myrddin-indent-offset (myrddin-mode--nearest-match-level)))
+     ;; Deindent any lines beginning with a syntactic element for closing a
+     ;; block, unless it occurs immediately following the beginning of a block.
+     ((looking-at (rx (or "}" ";;" "elif" "else")))
+      (if (string-match-p
+           (rx (eval (cons 'or (cons "{" myrddin-mode-block-keywords))))
+           prev-line)
+          (* myrddin-indent-offset prev-level)
+        (max 0 (* myrddin-indent-offset (1- prev-level)))))
+     ;; Indent any lines immediately following a line containing a syntactic
+     ;; element for opening a block.
+     ((string-match-p
+       (rx (eval (cons 'or (append '("|" "{") myrddin-mode-block-keywords))))
+       prev-line)
+      (* myrddin-indent-offset (1+ prev-level)))
+     (t (* myrddin-indent-offset prev-level)))))
+
 (defun myrddin-mode-indent-line ()
   "Indent current line for Myrddin mode.
 Return the amount the indentation changed by."
   (interactive)
-  (let* ((prev-level (save-excursion
-                       (forward-line -1)
-                       (back-to-indentation)
-                       (/ (current-column) myrddin-indent-offset)))
-         (prev-line (save-excursion
-                      (forward-line -1)
-                      (back-to-indentation)
-                      (buffer-substring-no-properties
-                       (line-beginning-position)
-                       (line-end-position)))))
-    (let ((indent
-           (save-excursion
-             (back-to-indentation)
-             (cond
-              ((looking-at
-                (rx (or "}" ";;" "elif" "else")))
-               (max 0 (* myrddin-indent-offset (1- prev-level))))
-              ((string-match-p
-                (rx (or "{" "elif" "else" "for" "if" "match"
-                        "struct" "trait" "while"))
-                prev-line)
-               (* myrddin-indent-offset (1+ prev-level)))
-              (t (* myrddin-indent-offset prev-level))))))
-
-      (if (<= (current-column) (current-indentation))
-          (indent-line-to indent)
-        (save-excursion (indent-line-to indent)))
-      indent)))
+  (let ((prev-line (myrddin-mode--preceding-nonempty-line)))
+    (if prev-line
+        (cl-destructuring-bind (prev-line prev-level) prev-line
+            (let ((indent (myrddin-mode--calculate-indent prev-line prev-level)))
+              (when indent
+                (if (<= (current-column) (current-indentation))
+                    (indent-line-to indent)
+                  (save-excursion (indent-line-to indent))))))
+      ;; The first line in the file should begin on column 0.
+      (indent-line-to 0))))
 
 
 ;;;