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+;;; notable.el --- a simple note-taking app -*- lexical-binding: t; -*-
+;;
+;; Copyright (C) 2020 The TVL Contributors
+;;
+;; Author: Vincent Ambo <mail@tazj.in>
+;; Version: 1.0
+;; Package-Requires: (cl-lib dash f rx s subr-x)
+;;
+;;; Commentary:
+;;
+;; This package provides a simple note-taking application which can be
+;; invoked from anywhere in Emacs, with several interactive
+;; note-taking functions included.
+;;
+;; As is tradition for my software, the idea here is to reduce
+;; friction which I see even with tools like `org-capture', because
+;; `org-mode' does a ton of things I don't care about.
+;;
+;; Notable stores its notes in simple JSON files in the folder
+;; specified by `notable-note-dir'.
+
+(require 'cl-lib)
+(require 'dottime)
+(require 'f)
+(require 'ht)
+(require 'rx)
+(require 's)
+(require 'subr-x)
+
+;; User-facing customisation options
+
+(defgroup notable nil
+  "Simple note-taking application."
+  :group 'applications)
+
+;; TODO(tazjin): Use whatever the XDG state dir thing is for these by
+;; default.
+(defcustom notable-note-dir (expand-file-name "~/.notable/")
+  "File path to the directory containing notable's notes."
+  :type 'string
+  :group 'notable)
+
+;; Package internal definitions
+
+(cl-defstruct (notable--note (:constructor notable--make-note))
+  "Structure containing the fields of a single notable note."
+  time    ;; UNIX timestamp at which the note was taken
+  content ;; Textual content of the note
+  )
+
+(defvar notable--note-lock (make-mutex "notable-notes")
+  "Exclusive lock for note operations with shared state.")
+
+(defvar notable--note-regexp
+  (rx "note-"
+      (group (one-or-more (any num)))
+      ".json")
+  "Regular expression to match note file names.")
+
+(defvar notable--next-note
+  (let ((next 0))
+    (dolist (file (f-entries notable-note-dir))
+      (when-let* ((match (string-match notable--note-regexp file))
+                  (id (string-to-number
+                       (match-string 1 file)))
+                  (larger (> id next)))
+        (setq next id)))
+    (+ 1 next))
+  "Next ID to use for notes. Initial value is determined based on
+  the existing notes files.")
+
+(defun notable--serialize-note (note)
+  "Serialise NOTE into JSON format."
+  (check-type note notable--note)
+  (json-serialize (ht ("time" (notable--note-time note))
+                      ("content" (notable--note-content note)))))
+
+(defun notable--deserialize-note (json)
+  "Deserialise JSON into a notable note."
+  (check-type json string)
+  (let ((parsed (json-parse-string json)))
+    (unless (and (ht-contains? parsed "time")
+                 (ht-contains-p parsed "content"))
+      (error "Missing required keys in note structure!"))
+    (notable--make-note :time (ht-get parsed "time")
+                        :content (ht-get parsed "content"))))
+
+(defun notable--next-id ()
+  "Return the next note ID and increment the counter."
+  (with-mutex notable--note-lock
+    (let ((id notable--next-note))
+      (setq notable--next-note (+ 1 id))
+      id)))
+
+(defun notable--note-path (id)
+  (check-type id integer)
+  (f-join notable-note-dir (format "note-%d.json" id)))
+
+(defun notable--archive-path (id)
+  (check-type id integer)
+  (f-join notable-note-dir (format "archive-%d.json" id)))
+
+(defun notable--add-note (content)
+  "Add a note with CONTENT to the note store."
+  (let* ((id (notable--next-id))
+         (note (notable--make-note :time (time-convert nil 'integer)
+                                   :content content))
+         (path (notable--note-path id)))
+    (when (f-exists? path) (error "Note file '%s' already exists!" path))
+    (f-write-text (notable--serialize-note note) 'utf-8 path)
+    (message "Saved note %d" id)))
+
+(defun notable--archive-note (id)
+  "Archive the note with ID."
+  (check-type id integer)
+
+  (unless (f-exists? (notable--note-path id))
+    (error "There is no note with ID %d." id))
+
+  (when (f-exists? (notable--archive-path id))
+    (error "Oh no, a note with ID %d has already been archived!" id))
+
+  (f-move (notable--note-path id) (notable--archive-path id))
+  (message "Archived note with ID %d." id))
+
+(defun notable--list-note-ids ()
+  "List all note IDs (not contents) from `notable-note-dir'"
+  (cl-loop for file in (f-entries notable-note-dir)
+           with res = nil
+           if (string-match notable--note-regexp file)
+           do (push (string-to-number (match-string 1 file)) res)
+           finally return res))
+
+(defun notable--get-note (id)
+  (let ((path (notable--note-path id)))
+    (unless (f-exists? path)
+      (error "No note with ID %s in note storage!" id))
+    (notable--deserialize-note (f-read-text path 'utf-8))))
+
+;; Note view buffer implementation
+
+(defvar-local notable--buffer-note nil "The note ID displayed by this buffer.")
+
+(define-derived-mode notable-note-mode fundamental-mode "notable-note"
+  "Major mode displaying a single Notable note."
+  (set (make-local-variable 'scroll-preserve-screen-position) t)
+  (setq truncate-lines t)
+  (setq buffer-read-only t)
+  (setq buffer-undo-list t))
+
+(setq notable-note-mode-map
+      (let ((map (make-sparse-keymap)))
+        (define-key map "q" 'kill-current-buffer)
+        map))
+
+(defun notable--show-note (id)
+  "Display a single note in a separate buffer."
+  (check-type id integer)
+
+  (let ((note (notable--get-note id))
+        (buffer (get-buffer-create (format "*notable: %d*" id)))
+        (inhibit-read-only t))
+    (with-current-buffer buffer
+      (notable-note-mode)
+      (erase-buffer)
+      (setq notable--buffer-note id)
+      (setq header-line-format
+            (format "Note from %s"
+                    (dottime-format
+                     (seconds-to-time (notable--note-time note))))))
+    (switch-to-buffer buffer)
+    (goto-char (point-min))
+    (insert (notable--note-content note))))
+
+(defun notable--show-note-at-point ()
+  (interactive)
+  (notable--show-note (get-text-property (point) 'notable-note-id)))
+
+(defun notable--archive-note-at-point ()
+  (interactive)
+  (notable--archive-note (get-text-property (point) 'notable-note-id)))
+
+;; Note list buffer implementation
+
+(define-derived-mode notable-list-mode fundamental-mode "notable"
+  "Major mode displaying the Notable note list."
+  ;; TODO(tazjin): `imenu' functions?
+
+  (set (make-local-variable 'scroll-preserve-screen-position) t)
+  (setq truncate-lines t)
+  (setq buffer-read-only t)
+  (setq buffer-undo-list t)
+  (hl-line-mode t))
+
+(setq notable-list-mode-map
+      (let ((map (make-sparse-keymap)))
+        (define-key map "a" 'notable--archive-note-at-point)
+        (define-key map "q" 'kill-current-buffer)
+        (define-key map "g" 'notable-list-notes)
+        (define-key map (kbd "RET") 'notable--show-note-at-point)
+        map))
+
+(defun notable--render-note (id note)
+  (check-type id integer)
+  (check-type note notable--note)
+
+  (let* ((start (point))
+         (date (dottime-format (seconds-to-time
+                                (notable--note-time note))))
+         (first-line (truncate-string-to-width
+                      (car (s-lines (notable--note-content note)))
+                      ;; Length of the window, minus the date prefix:
+                      (- (window-width) (+ 2 (length date)))
+                      nil nil 1)))
+    (insert (propertize (s-concat date "  " first-line)
+                        'notable-note-id id))
+    (insert "\n")))
+
+(defun notable--render-notes (notes)
+  "Retrieve each note in NOTES by ID and insert its contents into
+the list buffer.
+
+For larger notes only the first line is displayed."
+  (dolist (id notes)
+    (notable--render-note id (notable--get-note id))))
+
+;; User-facing functions
+
+(defun notable-take-note (content)
+  "Interactively prompt the user for a note that should be stored
+in Notable."
+  (interactive "sEnter note: ")
+  (check-type content string)
+  (notable--add-note content))
+
+(defun notable-list-notes ()
+  "Open a buffer listing all active notes."
+  (interactive)
+
+  (let ((buffer (get-buffer-create "*notable*"))
+        (notes (notable--list-note-ids))
+        (inhibit-read-only t))
+    (with-current-buffer buffer
+      (notable-list-mode)
+      (erase-buffer)
+      (setq header-line-format "Notable notes"))
+    (switch-to-buffer buffer)
+    (goto-char (point-min))
+    (notable--render-notes notes)))
+
+(provide 'notable)