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+;;; coq-diffs.el --- highlight text marked with XML-like tags for Coq diffs
+
+;; This file is part of Proof General.
+
+;; Portions © Copyright 2019  Jim Fehrle
+
+;; Author:      Jim Fehrle <jim.fehrle@gmail.com>
+
+;; License:  BSD-3 (3-Clause BSD License)
+
+;;; Commentary:
+;; 
+
+(require 'coq-db)
+
+;;; Code:
+
+(defun coq-insert-with-face (str face)
+  (let ((start (point)))
+    (insert str)
+    (if face
+      (overlay-put (span-make start (point-max)) 'face face))))
+
+(defun coq-insert-tagged-text (str)
+"Insert text into the current buffer applying faces specified by tags.
+
+For example '<diff.added>foo</diff.added>' inserts 'foo' in the buffer
+and applies the appropriate face.
+
+coq-tag-map defines the mapping from tag name to face."
+  (let* ((len (length str))
+         (off 0)
+	     (fstack)
+	     (rhs))
+    (while (< off len)
+      (string-match "^\\([ \t]*\\)\\(.*\n?\\)" str off)
+      (setq off (match-end 0))
+      (coq-insert-with-face (match-string 1 str) nil)  ;; begin-line white space
+      (setq rhs (match-string 2 str))
+      (string-match "[ \t\n]*$" rhs)
+      (let* ((end-white (match-string 0 rhs))  ;; end-line white space
+             (line (substring rhs 0 (- (length rhs) (length end-white))))
+             (llen (length line))
+             (loff 0))
+        (while (< loff llen)
+          (if (> loff 0)
+            (aset line (1- loff) ?\n))   ;; only way to get an anchored search midstring
+          (cond
+            ; make sure that a) the matched string is never the empty string, and
+            ; b) that every non-empty string has a match
+            ((string-match "^<\\(/?\\)\\([a-zA-Z\\.]+\\)>" line loff)    ;; tag
+              (let* ((end-mark (match-string 1 line))
+                     (tag (match-string 2 line))
+                     (face (cdr (assoc tag coq-tag-map))))
+                (if face
+                  (setq fstack (if (equal end-mark "") (cons face fstack) (cdr fstack)))
+                  (coq-insert-with-face (match-string 0 line) (car fstack)))))    ;; unknown tag, show as-is
+            ((string-match "^<?[^<\n]+" line loff)    ;; text
+              (coq-insert-with-face (match-string 0 line) (car fstack))))
+          (setq loff (match-end 0)))
+        (coq-insert-with-face end-white nil)))))    ; end-line white space
+
+(provide 'coq-diffs)
+
+;;; coq-diffs.el ends here