;;; project.el --- Operations on the current project  -*- lexical-binding: t; -*-

;; Copyright (C) 2015-2020 Free Software Foundation, Inc.
;; Version: 0.3.0
;; Package-Requires: ((emacs "26.3"))

;; This is a GNU ELPA :core package.  Avoid using functionality that
;; not compatible with the version of Emacs recorded above.

;; 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:

;; NOTE: The project API is still experimental and can change in major,
;; backward-incompatible ways.  Everyone is encouraged to try it, and
;; report to us any problems or use cases we hadn't anticipated, by
;; sending an email to emacs-devel, or `M-x report-emacs-bug'.
;;
;; This file contains generic infrastructure for dealing with
;; projects, some utility functions, and commands using that
;; infrastructure.
;;
;; The goal is to make it easier for Lisp programs to operate on the
;; current project, without having to know which package handles
;; detection of that project type, parsing its config files, etc.
;;
;; Infrastructure:
;;
;; Function `project-current', to determine the current project
;; instance, and 4 (at the moment) generic functions that act on it.
;; This list is to be extended in future versions.
;;
;; Utils:
;;
;; `project-combine-directories' and `project-subtract-directories',
;; mainly for use in the abovementioned generics' implementations.
;;
;; Commands:
;;
;; `project-find-file', `project-find-regexp' and
;; `project-or-external-find-regexp' use the current API, and thus
;; will work in any project that has an adapter.

;;; TODO:

;; * Reliably cache the list of files in the project, probably using
;;   filenotify.el (if supported) to invalidate.  And avoiding caching
;;   if it's not available (manual cache invalidation is not nice).
;;
;; * Build tool related functionality.  Start with a `project-build'
;;   command, which should provide completions on tasks to run, and
;;   maybe allow entering some additional arguments.  This might
;;   be handled better with a separate API, though.  Then we won't
;;   force every project backend to be aware of the build tool(s) the
;;   project is using.
;;
;; * Command to (re)build the tag files in all project roots.  To that
;;   end, we might need to add a way to provide file whitelist
;;   wildcards for each root to limit etags to certain files (in
;;   addition to the blacklist provided by ignores), and/or allow
;;   specifying additional tag regexps.
;;
;; * UI for the user to be able to pick the current project for the
;;   whole Emacs session, independent of the current directory.  Or,
;;   in the more advanced case, open a set of projects, and have some
;;   project-related commands to use them all.  E.g., have a command
;;   to search for a regexp across all open projects.
;;
;; * Support for project-local variables: a UI to edit them, and a
;;   utility function to retrieve a value.  Probably useless without
;;   support in various built-in commands.  In the API, we might get
;;   away with only adding a `project-configuration-directory' method,
;;   defaulting to the project root the current file/buffer is in.
;;   And prompting otherwise.  How to best mix that with backends that
;;   want to set/provide certain variables themselves, is up for
;;   discussion.

;;; Code:

(require 'cl-generic)
(eval-when-compile (require 'subr-x))

(defvar project-find-functions (list #'project-try-vc)
  "Special hook to find the project containing a given directory.
Each functions on this hook is called in turn with one
argument (the directory) and should return either nil to mean
that it is not applicable, or a project instance.")

(defvar project-current-inhibit-prompt nil
  "Non-nil to skip prompting the user in `project-current'.")

;;;###autoload
(defun project-current (&optional maybe-prompt dir)
  "Return the project instance in DIR or `default-directory'.
When no project found in DIR, and MAYBE-PROMPT is non-nil, ask
the user for a different project to look in."
  (unless dir (setq dir default-directory))
  (let ((pr (project--find-in-directory dir)))
    (cond
     (pr)
     ((unless project-current-inhibit-prompt
        maybe-prompt)
      (setq dir (project-prompt-project-dir)
            pr (project--find-in-directory dir))))
    (if pr
        (project--add-to-project-list-front pr)
      (project--remove-from-project-list dir)
      (setq pr (cons 'transient dir)))
    pr))

(defun project--find-in-directory (dir)
  (run-hook-with-args-until-success 'project-find-functions dir))

(cl-defgeneric project-root (project)
  "Return root directory of the current project.

It usually contains the main build file, dependencies
configuration file, etc. Though neither is mandatory.

The directory name must be absolute."
  (car (project-roots project)))

(cl-defgeneric project-roots (project)
  "Return the list containing the current project root.

The function is obsolete, all projects have one main root anyway,
and the rest should be possible to express through
`project-external-roots'."
  ;; FIXME: Can we specify project's version here?
  ;; FIXME: Could we make this affect cl-defmethod calls too?
  (declare (obsolete project-root "0.3.0"))
  (list (project-root project)))

;; FIXME: Add MODE argument, like in `ede-source-paths'?
(cl-defgeneric project-external-roots (_project)
  "Return the list of external roots for PROJECT.

It's the list of directories outside of the project that are
still related to it.  If the project deals with source code then,
depending on the languages used, this list should include the
headers search path, load path, class path, and so on."
  nil)

(cl-defgeneric project-ignores (_project _dir)
  "Return the list of glob patterns to ignore inside DIR.
Patterns can match both regular files and directories.
To root an entry, start it with `./'.  To match directories only,
end it with `/'.  DIR must be either `project-root' or one of
`project-external-roots'."
  ;; TODO: Document and support regexp ignores as used by Hg.
  ;; TODO: Support whitelist entries.
  (require 'grep)
  (defvar grep-find-ignored-files)
  (nconc
   (mapcar
    (lambda (dir)
      (concat dir "/"))
    vc-directory-exclusion-list)
   grep-find-ignored-files))

(defun project--file-completion-table (all-files)
  (lambda (string pred action)
    (cond
     ((eq action 'metadata)
      '(metadata . ((category . project-file))))
     (t
      (complete-with-action action all-files string pred)))))

(cl-defmethod project-root ((project (head transient)))
  (cdr project))

(cl-defgeneric project-files (project &optional dirs)
  "Return a list of files in directories DIRS in PROJECT.
DIRS is a list of absolute directories; it should be some
subset of the project root and external roots.

The default implementation uses `find-program'.  PROJECT is used
to find the list of ignores for each directory."
  (cl-mapcan
   (lambda (dir)
     (project--files-in-directory dir
                                  (project--dir-ignores project dir)))
   (or dirs
       (list (project-root project)))))

(defun project--files-in-directory (dir ignores &optional files)
  (require 'find-dired)
  (require 'xref)
  (defvar find-name-arg)
  (let* ((default-directory dir)
         ;; Make sure ~/ etc. in local directory name is
         ;; expanded and not left for the shell command
         ;; to interpret.
         (localdir (file-local-name (expand-file-name dir)))
         (command (format "%s %s %s -type f %s -print0"
                          find-program
                          localdir
                          (xref--find-ignores-arguments ignores localdir)
                          (if files
                              (concat (shell-quote-argument "(")
                                      " " find-name-arg " "
                                      (mapconcat
                                       #'shell-quote-argument
                                       (split-string files)
                                       (concat " -o " find-name-arg " "))
                                      " "
                                      (shell-quote-argument ")"))"")
                          )))
    (project--remote-file-names
     (sort (split-string (shell-command-to-string command) "\0" t)
           #'string<))))

(defun project--remote-file-names (local-files)
  "Return LOCAL-FILES as if they were on the system of `default-directory'."
  (let ((remote-id (file-remote-p default-directory)))
    (if (not remote-id)
        local-files
      (mapcar (lambda (file)
                (concat remote-id file))
              local-files))))

(defgroup project-vc nil
  "Project implementation based on the VC package."
  :version "25.1"
  :group 'tools)

(defcustom project-vc-ignores nil
  "List of patterns to include in `project-ignores'."
  :type '(repeat string)
  :safe 'listp)

(defcustom project-vc-merge-submodules t
  "Non-nil to consider submodules part of the parent project.

After changing this variable (using Customize or .dir-locals.el)
you might have to restart Emacs to see the effect."
  :type 'boolean
  :package-version '(project . "0.2.0")
  :safe 'booleanp)

;; FIXME: Using the current approach, major modes are supposed to set
;; this variable to a buffer-local value.  So we don't have access to
;; the "external roots" of language A from buffers of language B, which
;; seems desirable in multi-language projects, at least for some
;; potential uses, like "jump to a file in project or external dirs".
;;
;; We could add a second argument to this function: a file extension,
;; or a language name.  Some projects will know the set of languages
;; used in them; for others, like VC-based projects, we'll need
;; auto-detection.  I see two options:
;;
;; - That could be implemented as a separate second hook, with a
;;   list of functions that return file extensions.
;;
;; - This variable will be turned into a hook with "append" semantics,
;;   and each function in it will perform auto-detection when passed
;;   nil instead of an actual file extension.  Then this hook will, in
;;   general, be modified globally, and not from major mode functions.
;;
;; The second option seems simpler, but the first one has the
;; advantage that the user could override the list of languages used
;; in a project via a directory-local variable, thus skipping
;; languages they're not working on personally (in a big project), or
;; working around problems in language detection (the detection logic
;; might be imperfect for the project in question, or it might work
;; too slowly for the user's taste).
(defvar project-vc-external-roots-function (lambda () tags-table-list)
  "Function that returns a list of external roots.

It should return a list of directory roots that contain source
files related to the current buffer.

The directory names should be absolute.  Used in the VC project
backend implementation of `project-external-roots'.")

(defun project-try-vc (dir)
  (let* ((backend (ignore-errors (vc-responsible-backend dir)))
         (root
          (pcase backend
            ('Git
             ;; Don't stop at submodule boundary.
             (or (vc-file-getprop dir 'project-git-root)
                 (let ((root (vc-call-backend backend 'root dir)))
                   (vc-file-setprop
                    dir 'project-git-root
                    (if (and
                         ;; FIXME: Invalidate the cache when the value
                         ;; of this variable changes.
                         project-vc-merge-submodules
                         (project--submodule-p root))
                        (let* ((parent (file-name-directory
                                        (directory-file-name root))))
                          (vc-call-backend backend 'root parent))
                      root)))))
            ('nil nil)
            (_ (ignore-errors (vc-call-backend backend 'root dir))))))
    (and root (cons 'vc root))))

(defun project--submodule-p (root)
  ;; XXX: We only support Git submodules for now.
  ;;
  ;; For submodules, at least, we expect the users to prefer them to
  ;; be considered part of the parent project.  For those who don't,
  ;; there is the custom var now.
  ;;
  ;; Some users may also set up things equivalent to Git submodules
  ;; using "git worktree" (for example).  However, we expect that most
  ;; of them would prefer to treat those as separate projects anyway.
  (let* ((gitfile (expand-file-name ".git" root)))
    (cond
     ((file-directory-p gitfile)
      nil)
     ((with-temp-buffer
        (insert-file-contents gitfile)
        (goto-char (point-min))
        ;; Kind of a hack to distinguish a submodule from
        ;; other cases of .git files pointing elsewhere.
        (looking-at "gitdir: [./]+/\\.git/modules/"))
      t)
     (t nil))))

(cl-defmethod project-root ((project (head vc)))
  (cdr project))

(cl-defmethod project-external-roots ((project (head vc)))
  (project-subtract-directories
   (project-combine-directories
    (mapcar
     #'file-name-as-directory
     (funcall project-vc-external-roots-function)))
   (list (project-root project))))

(cl-defmethod project-files ((project (head vc)) &optional dirs)
  (cl-mapcan
   (lambda (dir)
     (let (backend)
       (if (and (file-equal-p dir (cdr project))
                (setq backend (vc-responsible-backend dir))
                (cond
                 ((eq backend 'Hg))
                 ((and (eq backend 'Git)
                       (or
                        (not project-vc-ignores)
                        (version<= "1.9" (vc-git--program-version)))))))
           (project--vc-list-files dir backend project-vc-ignores)
         (project--files-in-directory
          dir
          (project--dir-ignores project dir)))))
   (or dirs
       (list (project-root project)))))

(declare-function vc-git--program-version "vc-git")
(declare-function vc-git--run-command-string "vc-git")
(declare-function vc-hg-command "vc-hg")

(defun project--vc-list-files (dir backend extra-ignores)
  (pcase backend
    (`Git
     (let ((default-directory (expand-file-name (file-name-as-directory dir)))
           (args '("-z"))
           files)
       ;; Include unregistered.
       (setq args (append args '("-c" "-o" "--exclude-standard")))
       (when extra-ignores
         (setq args (append args
                            (cons "--"
                                  (mapcar
                                   (lambda (i)
                                     (if (string-match "\\./" i)
                                         (format ":!/:%s" (substring i 2))
                                       (format ":!:%s" i)))
                                   extra-ignores)))))
       (setq files
             (mapcar
              (lambda (file) (concat default-directory file))
              (split-string
               (apply #'vc-git--run-command-string nil "ls-files" args)
               "\0" t)))
       ;; Unfortunately, 'ls-files --recurse-submodules' conflicts with '-o'.
       (let* ((submodules (project--git-submodules))
              (sub-files
               (mapcar
                (lambda (module)
                  (when (file-directory-p module)
                    (project--vc-list-files
                     (concat default-directory module)
                     backend
                     extra-ignores)))
                submodules)))
         (setq files
               (apply #'nconc files sub-files)))
       ;; 'git ls-files' returns duplicate entries for merge conflicts.
       ;; XXX: Better solutions welcome, but this seems cheap enough.
       (delete-consecutive-dups files)))
    (`Hg
     (let ((default-directory (expand-file-name (file-name-as-directory dir)))
           args)
       ;; Include unregistered.
       (setq args (nconc args '("-mcardu" "--no-status" "-0")))
       (when extra-ignores
         (setq args (nconc args
                           (mapcan
                            (lambda (i)
                              (list "--exclude" i))
                            extra-ignores))))
       (with-temp-buffer
         (apply #'vc-hg-command t 0 "." "status" args)
         (mapcar
          (lambda (s) (concat default-directory s))
          (split-string (buffer-string) "\0" t)))))))

(defun project--git-submodules ()
  ;; 'git submodule foreach' is much slower.
  (condition-case nil
      (with-temp-buffer
        (insert-file-contents ".gitmodules")
        (let (res)
          (goto-char (point-min))
          (while (re-search-forward "path *= *\\(.+\\)" nil t)
            (push (match-string 1) res))
          (nreverse res)))
    (file-missing nil)))

(cl-defmethod project-ignores ((project (head vc)) dir)
  (let* ((root (cdr project))
          backend)
    (append
     (when (file-equal-p dir root)
       (setq backend (vc-responsible-backend root))
       (mapcar
        (lambda (entry)
          (if (string-match "\\`/" entry)
              (replace-match "./" t t entry)
            entry))
        (vc-call-backend backend 'ignore-completion-table root)))
     (project--value-in-dir 'project-vc-ignores root)
     (mapcar
      (lambda (dir)
        (concat dir "/"))
      vc-directory-exclusion-list))))

(defun project-combine-directories (&rest lists-of-dirs)
  "Return a sorted and culled list of directory names.
Appends the elements of LISTS-OF-DIRS together, removes
non-existing directories, as well as directories a parent of
whose is already in the list."
  (let* ((dirs (sort
                (mapcar
                 (lambda (dir)
                   (file-name-as-directory (expand-file-name dir)))
                 (apply #'append lists-of-dirs))
                #'string<))
         (ref dirs))
    ;; Delete subdirectories from the list.
    (while (cdr ref)
      (if (string-prefix-p (car ref) (cadr ref))
          (setcdr ref (cddr ref))
        (setq ref (cdr ref))))
    (cl-delete-if-not #'file-exists-p dirs)))

(defun project-subtract-directories (files dirs)
  "Return a list of elements from FILES that are outside of DIRS.
DIRS must contain directory names."
  ;; Sidestep the issue of expanded/abbreviated file names here.
  (cl-set-difference files dirs :test #'file-in-directory-p))

(defun project--value-in-dir (var dir)
  (with-temp-buffer
    (setq default-directory dir)
    (let ((enable-local-variables :all))
      (hack-dir-local-variables-non-file-buffer))
    (symbol-value var)))

(declare-function grep-read-files "grep")
(declare-function xref--show-xrefs "xref")
(declare-function xref--find-ignores-arguments "xref")

;;;###autoload
(defun project-find-regexp (regexp)
  "Find all matches for REGEXP in the current project's roots.
With \\[universal-argument] prefix, you can specify the directory
to search in, and the file name pattern to search for.  The
pattern may use abbreviations defined in `grep-files-aliases',
e.g. entering `ch' is equivalent to `*.[ch]'.  As whitespace
triggers completion when entering a pattern, including it
requires quoting, e.g. `\\[quoted-insert]<space>'."
  (interactive (list (project--read-regexp)))
  (require 'xref)
  (require 'grep)
  (let* ((pr (project-current t))
         (files
          (if (not current-prefix-arg)
              (project-files pr)
            (let ((dir (read-directory-name "Base directory: "
                                            nil default-directory t)))
              (project--files-in-directory dir
                                           nil
                                           (grep-read-files regexp))))))
    (xref--show-xrefs
     (apply-partially #'project--find-regexp-in-files regexp files)
     nil)))

(defun project--dir-ignores (project dir)
  (let ((root (project-root project)))
    (if (not (file-in-directory-p dir root))
        (project-ignores nil nil)       ;The defaults.
      (let ((ignores (project-ignores project root)))
        (if (file-equal-p root dir)
            ignores
          ;; FIXME: Update the "rooted" ignores to relate to DIR instead.
          (cl-delete-if (lambda (str) (string-prefix-p "./" str))
                        ignores))))))

;;;###autoload
(defun project-or-external-find-regexp (regexp)
  "Find all matches for REGEXP in the project roots or external roots.
With \\[universal-argument] prefix, you can specify the file name
pattern to search for."
  (interactive (list (project--read-regexp)))
  (require 'xref)
  (let* ((pr (project-current t))
         (files
          (project-files pr (cons
                             (project-root pr)
                             (project-external-roots pr)))))
    (xref--show-xrefs
     (apply-partially #'project--find-regexp-in-files regexp files)
     nil)))

(defun project--find-regexp-in-files (regexp files)
  (unless files
    (user-error "Empty file list"))
  (let ((xrefs (xref-matches-in-files regexp files)))
    (unless xrefs
      (user-error "No matches for: %s" regexp))
    xrefs))

(defun project--process-file-region (start end program
                                     &optional buffer display
                                     &rest args)
  ;; FIXME: This branching shouldn't be necessary, but
  ;; call-process-region *is* measurably faster, even for a program
  ;; doing some actual work (for a period of time). Even though
  ;; call-process-region also creates a temp file internally
  ;; (http://lists.gnu.org/archive/html/emacs-devel/2019-01/msg00211.html).
  (if (not (file-remote-p default-directory))
      (apply #'call-process-region
             start end program nil buffer display args)
    (let ((infile (make-temp-file "ppfr")))
      (unwind-protect
          (progn
            (write-region start end infile nil 'silent)
            (apply #'process-file program infile buffer display args))
        (delete-file infile)))))

(defun project--read-regexp ()
  (let ((sym (thing-at-point 'symbol)))
    (read-regexp "Find regexp" (and sym (regexp-quote sym)))))

;;;###autoload
(defun project-find-file ()
  "Visit a file (with completion) in the current project.
The completion default is the filename at point, if one is
recognized."
  (interactive)
  (let* ((pr (project-current t))
         (dirs (list (project-root pr))))
    (project-find-file-in (thing-at-point 'filename) dirs pr)))

;;;###autoload
(defun project-or-external-find-file ()
  "Visit a file (with completion) in the current project or external roots.
The completion default is the filename at point, if one is
recognized."
  (interactive)
  (let* ((pr (project-current t))
         (dirs (cons
                (project-root pr)
                (project-external-roots pr))))
    (project-find-file-in (thing-at-point 'filename) dirs pr)))

(defcustom project-read-file-name-function #'project--read-file-cpd-relative
  "Function to call to read a file name from a list.
For the arguments list, see `project--read-file-cpd-relative'."
  :type '(choice (const :tag "Read with completion from relative names"
                        project--read-file-cpd-relative)
                 (const :tag "Read with completion from absolute names"
                        project--read-file-absolute)
                 (function :tag "Custom function" nil))
  :version "27.1")

(defun project--read-file-cpd-relative (prompt
                                        all-files &optional predicate
                                        hist default)
  "Read a file name, prompting with PROMPT.
ALL-FILES is a list of possible file name completions.
PREDICATE, HIST, and DEFAULT have the same meaning as in
`completing-read'."
  (let* ((common-parent-directory
          (let ((common-prefix (try-completion "" all-files)))
            (if (> (length common-prefix) 0)
                (file-name-directory common-prefix))))
         (cpd-length (length common-parent-directory))
         (prompt (if (zerop cpd-length)
                     prompt
                   (concat prompt (format " in %s" common-parent-directory))))
         (substrings (mapcar (lambda (s) (substring s cpd-length)) all-files))
         (new-collection (project--file-completion-table substrings))
         (res (project--completing-read-strict prompt
                                               new-collection
                                               predicate
                                               hist default)))
    (concat common-parent-directory res)))

(defun project--read-file-absolute (prompt
                                    all-files &optional predicate
                                    hist default)
  (project--completing-read-strict prompt
                                   (project--file-completion-table all-files)
                                   predicate
                                   hist default))

(defun project-find-file-in (filename dirs project)
  "Complete FILENAME in DIRS in PROJECT and visit the result."
  (let* ((all-files (project-files project dirs))
         (file (funcall project-read-file-name-function
                       "Find file" all-files nil nil
                       filename)))
    (if (string= file "")
        (user-error "You didn't specify the file")
      (find-file file))))

(defun project--completing-read-strict (prompt
                                        collection &optional predicate
                                        hist default)
  ;; Tried both expanding the default before showing the prompt, and
  ;; removing it when it has no matches.  Neither seems natural
  ;; enough.  Removal is confusing; early expansion makes the prompt
  ;; too long.
  (let* ((new-prompt (if (and default (not (string-equal default "")))
                         (format "%s (default %s): " prompt default)
                       (format "%s: " prompt)))
         (res (completing-read new-prompt
                               collection predicate t
                               nil ;; initial-input
                               hist default)))
    (when (and (equal res default)
               (not (test-completion res collection predicate)))
      (setq res
            (completing-read (format "%s: " prompt)
                             collection predicate t res hist nil)))
    res))

;;;###autoload
(defun project-dired ()
  "Open Dired in the current project."
  (interactive)
  (dired (project-root (project-current t))))

;;;###autoload
(defun project-eshell ()
  "Open Eshell in the current project."
  (interactive)
  (let ((default-directory (project-root (project-current t))))
    (eshell t)))

(declare-function fileloop-continue "fileloop" ())

;;;###autoload
(defun project-search (regexp)
  "Search for REGEXP in all the files of the project.
Stops when a match is found.
To continue searching for the next match, use the
command \\[fileloop-continue]."
  (interactive "sSearch (regexp): ")
  (fileloop-initialize-search
   regexp (project-files (project-current t)) 'default)
  (fileloop-continue))

;;;###autoload
(defun project-query-replace-regexp (from to)
  "Query-replace REGEXP in all the files of the project.
Stops when a match is found and prompts for whether to replace it.
If you exit the query-replace, you can later continue the query-replace
loop using the command \\[fileloop-continue]."
  (interactive
   (pcase-let ((`(,from ,to)
                (query-replace-read-args "Query replace (regexp)" t t)))
     (list from to)))
  (fileloop-initialize-replace
   from to (project-files (project-current t)) 'default)
  (fileloop-continue))

;;;###autoload
(defun project-compile ()
  "Run `compile' in the project root."
  (interactive)
  (let* ((pr (project-current t))
         (default-directory (project-root pr)))
    (call-interactively 'compile)))

\f
;;; Project list

(defvar project--list 'unset
  "List of known project directories.")

(defun project--ensure-file-exists (filename)
  "Create an empty file FILENAME if it doesn't exist."
  (unless (file-exists-p filename)
    (with-temp-buffer
      (write-file filename))))

(defun project--read-project-list ()
  "Initialize `project--list' from the project list file."
  (let ((filename (locate-user-emacs-file "project-list")))
    (project--ensure-file-exists filename)
    (with-temp-buffer
      (insert-file-contents filename)
      (let ((dirs (split-string (buffer-string) "\n" t))
            (project-list '()))
        (dolist (dir dirs)
          (cl-pushnew (file-name-as-directory dir)
                      project-list
                      :test #'equal))
        (setq project--list (reverse project-list))))))

(defun project--ensure-read-project-list ()
  "Initialize `project--list' if it hasn't already been."
  (when (eq project--list 'unset)
    (project--read-project-list)))

(defun project--write-project-list ()
  "Persist `project--list' to the project list file."
  (let ((filename (locate-user-emacs-file "project-list")))
    (with-temp-buffer
      (insert (string-join project--list "\n"))
      (write-region nil nil filename nil 'silent))))

(defun project--add-to-project-list-front (pr)
  "Add project PR to the front of the project list and save it.
Return PR."
  (project--ensure-read-project-list)
  (let ((dir (project-root pr)))
    (setq project--list (delete dir project--list))
    (push dir project--list))
  (project--write-project-list)
  pr)

(defun project--remove-from-project-list (pr-dir)
  "Remove directory PR-DIR from the project list.
If the directory was in the list before the removal, save the
result to disk."
  (project--ensure-read-project-list)
  ;; XXX: This hardcodes that the number of roots = 1.
  ;; It's fine, though.
  (when (member pr-dir project--list)
    (setq project--list (delete pr-dir project--list))
    (message "Project `%s' not found; removed from list" pr-dir)
    (project--write-project-list)))

(defun project-prompt-project-dir ()
  "Prompt the user for a directory from known project roots.
The project is chosen among projects known from the project list.
It's also possible to enter an arbitrary directory."
  (project--ensure-read-project-list)
  (let* ((dir-choice "... (choose a dir)")
         (choices
          ;; XXX: Just using this for the category (for the substring
          ;; completion style).
          (project--file-completion-table
           (append project--list `(,dir-choice))))
         (pr-dir (completing-read "Select project: " choices nil t)))
    (if (equal pr-dir dir-choice)
        (read-directory-name "Select directory: " default-directory nil t)
      pr-dir)))

\f
;;; Project switching

;;;###autoload
(defvar project-switch-commands
  '(("f" "Find file" project-find-file)
    ("s" "Find regexp" project-find-regexp)
    ("d" "Dired" project-dired)
    ("e" "Eshell" project-eshell))
  "Alist mapping keys to project switching menu entries.
Used by `project-switch-project' to construct a dispatch menu of
commands available upon \"switching\" to another project.

Each element looks like (KEY LABEL COMMAND), where COMMAND is the
command to run when KEY is pressed.  LABEL is used to distinguish
the choice in the dispatch menu.")

(defun project--keymap-prompt ()
  "Return a prompt for the project swithing dispatch menu."
  (mapconcat
   (pcase-lambda (`(,key ,label))
     (format "[%s] %s"
             (propertize (key-description `(,key)) 'face 'bold)
             label))
   project-switch-commands
   "  "))

;;;###autoload
(defun project-switch-project ()
  "\"Switch\" to another project by running a chosen command.
The available commands are picked from `project-switch-commands'
and presented in a dispatch menu."
  (interactive)
  (let ((dir (project-prompt-project-dir))
        (choice nil))
    (while (not (and choice
                     (or (equal choice (kbd "C-g"))
                         (assoc choice project-switch-commands))))
      (setq choice (read-key-sequence (project--keymap-prompt))))
    (if (equal choice (kbd "C-g"))
        (message "Quit")
      (let ((default-directory dir)
            (project-current-inhibit-prompt t))
        (call-interactively
         (nth 2 (assoc choice project-switch-commands)))))))

(provide 'project)
;;; project.el ends here