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+;;; file-complete-root-relative.el --- Completion style for files  -*- lexical-binding:t -*-
+;;
+;; Copyright (C) 2019 Free Software Foundation, Inc.
+;;
+;; Author: Stephen Leake <stephen_leake@stephe-leake.org>
+;; Maintainer: Stephen Leake <stephen_leake@stephe-leake.org>
+;;
+;; 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 <http://www.gnu.org/licenses/>.
+
+
+;;; Commentary
+
+;; A file completion style in which the root directory is left out of
+;; the completion string displayed to the user.
+;;
+;; We accomplish this by preprocessing the list of absolute file names
+;; to be in that style, in an alist with the original absolute file
+;; names, and do completion on that alist.
+
+(require 'cl-lib)
+
+(defun fc-root-rel-to-alist (root files)
+  "Return a file-root-rel alist with file names from FILES.
+Result is a list (REL-NAME . ABS-NAME), where REL-NAME is ABS-NAME with ROOT deleted.
+An error is signaled if any name in FILES does not begin with ROOT."
+  (let ((root-len (length root))
+	result)
+    (mapc
+     (lambda (abs-name)
+       (unless (string-equal root (substring abs-name 0 root-len))
+	 (error "%s does not begin with %s" abs-name root))
+       (push (cons (substring abs-name root-len) abs-name) result))
+     files)
+    result))
+
+(defun fc-root-rel-completion-table (files string pred action)
+  "Implement a completion table for file names in FILES,
+FILES is a list of (REL-NAME . ABS-NAME).
+
+STRING, PRED, ACTION are completion table arguments."
+  (cond
+   ((eq action 'alist)
+    (cdr (assoc string files)))
+
+   ((eq (car-safe action) 'boundaries)
+    ;; We don't use boundaries; return the default definition.
+    (cons 'boundaries
+	  (cons 0 (length (cdr action)))))
+
+   ((eq action 'metadata)
+    (cons 'metadata
+	  (list
+	   '(alist . t)
+	   '(category . project-file))))
+
+   ((null action)
+    (try-completion string files pred))
+
+   ((eq 'lambda action)
+    (test-completion string files pred))
+
+   ((eq t action)
+    (all-completions string files pred))
+
+   ))
+
+(provide 'file-complete-root-relative)
+;;; file-complete-root-relative.el ends here