feat(emacs-pkgs/notable): Begin implementation by adding note-taking r/1796

I'm writing a note taking system because I don't like any of the
existing ones.

This adds a note-taking function which prompts the user for note
content and persists it in a user-configurable directory.

Next steps are roughly:

* listing notes
* editing notes
* archiving notes

At which point I would consider this functional. The idea is to keep
adding incremental improvements that fit my workflows after that, and
if it ends up being useful for anyone else - great.

Change-Id: I7ae0eb4015511ebdbaeff92d8d32502726eb57c4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1981
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

3 files changed, 124 insertions, 0 deletions

diff --git a/tools/emacs-pkgs/notable/OWNERS b/tools/emacs-pkgs/notable/OWNERS
new file mode 100644
index 000000000000..f7da62ecf709
--- /dev/null
+++ b/tools/emacs-pkgs/notable/OWNERS
@@ -0,0 +1,2 @@
+owners:
+  - tazjin
diff --git a/tools/emacs-pkgs/notable/default.nix b/tools/emacs-pkgs/notable/default.nix
new file mode 100644
index 000000000000..74cdddf6ab4f
--- /dev/null
+++ b/tools/emacs-pkgs/notable/default.nix
@@ -0,0 +1,10 @@
+{ pkgs, ... }:
+
+pkgs.emacsPackages.trivialBuild rec {
+  pname = "notable";
+  version = "1.0";
+  src = ./notable.el;
+  packageRequires = with pkgs.emacsPackages; [
+    dash f ht s
+  ];
+}
diff --git a/tools/emacs-pkgs/notable/notable.el b/tools/emacs-pkgs/notable/notable.el
new file mode 100644
index 000000000000..b0b81da21ecc
--- /dev/null
+++ b/tools/emacs-pkgs/notable/notable.el
@@ -0,0 +1,112 @@
+;;; 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 'dash)
+(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--next-note
+  (let ((next 0)
+        (note-regexp (rx "note-"
+                         (group (one-or-more (any num)))
+                         ".json")))
+    (-each (f-entries notable-note-dir)
+      (lambda (file)
+        (when-let* ((match (string-match 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--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 (f-join notable-note-dir (format "note-%d.json" 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)))
+
+;; 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))
+
+(provide 'notable)