#+title: consulting spotify
#+date: <2021-01-08 04:02>
#+filetags: emacs
#+PROPERTY: header-args :tangle yes :comments no :results silent

We have two kinds of interaction with Spotify: via its HTTP API to
perform operations such as search, and via our local DBUS to talk to
client players running in our computer, such as the official client,
[[https://github.com/Spotifyd/spotifyd][spotifyd]] or [[https://mopidy.com/ext/spotify/][mopidy-spotify]].  Our goal is to obtain via the former a
track or album identifier that we can send then to the latter to play,
with emacs completion mechanisms (consult and friends in this case)
providing the glue between both worlds.

Let's start with an umbrella customization group:
#+begin_src emacs-lisp
  ;;; espotify.el - spotify search and play -  -*- lexical-binding: t; -*-

  (defgroup espotify nil
    "Access to Spotify API and clients"
    :group 'multimedia)
#+end_src

* Access to Spotify's API: authentication

  I am stealing most of the ideas on how to establish authenticated
  connections to the Spotify API and performing queries from
  [[https://github.com/Lautaro-Garcia/counsel-spotify][counsel-spotify]], with many simplifications.

  We start defining a couple of end-points:

  #+begin_src emacs-lisp
    (defvar espotify-spotify-api-url "https://api.spotify.com/v1")
    (defvar espotify-spotify-api-authentication-url
      "https://accounts.spotify.com/api/token")
  #+end_src

  And we're going to need as well a client id and secret for our
  application, which i am again defining as variables since i expect
  them to be set in some secure manner instead of via customize:

  #+begin_src emacs-lisp
    (defvar espotify-client-id nil "Spotify application client ID.")
    (defvar espotify-client-secret nil "Spotify application client secret.")
  #+end_src

  To get valid values for them, one just needs to [[https://developer.spotify.com/my-applications][register a Spotify
  application]].  From them we can derive a base64-encoded credentials
  value:

  #+begin_src emacs-lisp
    (defun espotify--basic-auth-credentials ()
      (let ((credential (concat espotify-client-id ":" espotify-client-secret)))
        (concat "Basic " (base64-encode-string credential t))))
  #+end_src

  The return value of the function above is to be used as the
  "Authorization" header of our requests to the authorization
  end-point, which is going to answer with an authorization token
  that we can then use to further requests.  Let's define a function to
  wrap that operation:

  #+begin_src emacs-lisp
    (defun espotify--with-auth-token (callback)
      (let ((url-request-method "POST")
            (url-request-data "&grant_type=client_credentials")
            (url-request-extra-headers
             `(("Content-Type" . "application/x-www-form-urlencoded")
               ("Authorization" . ,(espotify--basic-auth-credentials)))))
         (url-retrieve espotify-spotify-api-authentication-url
                       (lambda (_status)
                         (goto-char url-http-end-of-headers)
                         (funcall callback
                                  (alist-get 'access_token (json-read)))))))
  #+end_src

  For instance:
  #+begin_src emacs-lisp :load no :tangle no
    (espotify--with-auth-token
     (lambda (token) (message "Your token is: %s" token)))
  #+end_src

  obtains an auth token and prints it as a message.  Note that ~body~
  is evaluated asynchronously by ~url-retrieve~, so invocations to
  ~espotify-with-auth-token~ evaluate to the request's buffer and are
  usually discarded.

* Search queries using the Spotify API

  We are interested in performing a search for some ~term~, of items
  of a given ~types~ (~:track~, ~:album~, ~:artist~, etc.), possibly with an
  additional ~filter~.  That's specified in a GET request's URL
  as constructed by this function:

  #+begin_src emacs-lisp
    (defun espotify--make-search-url (term types &optional filter)
      (when (null types)
        (error "Must supply a non-empty list of types to search for"))
      (let ((term (url-encode-url term)))
        (format "%s/search?q=%s&type=%s&limit=50"
                espotify-spotify-api-url
                (if filter (format "%s:%s" filter term) term)
                (mapconcat #'symbol-name types ","))))
  #+end_src

  For instance:

  #+begin_src emacs-lisp :load no :tangle no :results replace
    (espotify--make-search-url "dream blue turtles" '(album))
  #+end_src

  #+RESULTS:
  : https://api.spotify.com/v1/search?q=dream%20blue%20turtles&type=album&limit=50

  If we have an [[*Access to Spotify's API: authentication][authorisation token]] and a search URL in our hands,
  we can use them as in the following helper function, which will
  calls the given callback with the results of the query:

  #+begin_src emacs-lisp
    (defun espotify--with-query-results (token url callback)
      (let ((url-request-extra-headers
             `(("Authorization" . ,(concat "Bearer " token)))))
        (url-retrieve url
                      (lambda (_status)
                        (goto-char url-http-end-of-headers)
                        (funcall callback
                                 (let ((json-array-type 'list))
                                   (json-read)))))))
  #+end_src

  So we can combine this macro with ~espotify--with-auth-token~ in a
  single search function that takes a callback that will be applied
  to a given query, specified as a triple of term, types and filter:

  #+begin_src emacs-lisp
    (defun espotify-search (callback term types &optional filter)
      (let ((query-url (espotify--make-search-url term types filter)))
        (espotify--with-auth-token
         (lambda (token)
           (espotify--with-query-results token query-url callback)))))
  #+end_src

  For instance:
  #+begin_src emacs-lisp :load no :tangle no
    (defvar espotify-query-result nil)
    (espotify-search (lambda (res) (setq espotify-query-result res))
                     "dream blue turtles"
                     '(album artist))
    (sit-for 0)
  #+end_src

  #+begin_src emacs-lisp :load no :tangle no :results replace
    (mapcar 'car espotify-query-result)
  #+end_src

  #+RESULTS:
  | albums | artists |

  So Spotify is returning a results entry per type, which in turn,
  contains an ~items~ with the list of actual results.  So let's
  provide an interface for a callback that takes as many lists of
  items as types it asks for:

  #+begin_src emacs-lisp
    (defun espotify--type-items (res type)
      (alist-get 'items (alist-get (intern (format "%ss" type)) res)))

    (defun espotify-search* (callback term types &optional filter)
      (let* ((types (if (listp types) types (list types)))
             (cb (lambda (res)
                   (let ((its (mapcar (lambda (tp)
                                        (espotify--type-items res tp))
                                      types)))
                     (apply callback its)))))
        (espotify-search cb term types filter)))
  #+end_src

  For example:

  #+begin_src emacs-lisp :load no :tangle no
    (defvar espotify-query-result nil)
    (espotify-search* (lambda (al ar)
                        (message "Found %s albums, %s artists"
                                 (length al) (length ar))
                        (setq espotify-query-result (cons al ar)))
                     "blue turtles"
                     '(album artist))
    (sit-for 0)
    (list (mapcar 'car (car (car espotify-query-result)))
          (mapcar 'car (car (cdr espotify-query-result))))
  #+end_src

  #+RESULTS:
  | album_type    | artists   | available_markets | external_urls | href | id     | images | name       | release_date | release_date_precision | total_tracks | type | uri |
  | external_urls | followers | genres            | href          | id   | images | name   | popularity | type         | uri                    |              |      |     |

  Another strategy would be to search for several types and pass to
  our callback the concatenation of all items:

  #+begin_src emacs-lisp
    (defun espotify-search-all (callback term &optional types filter)
      (let ((types (or types '(album track artist playlist))))
        (espotify-search* (lambda (&rest items)
                            (funcall callback (apply append items)))
                          term
                          types
                          filter)))
  #+end_src

* Sending commands to local players

  Once we now the URI we want to play (that ~uri~ entry in our items),
  sending it to a local player via DBUS is fairly easy.  Let's
  define a couple of customizable variables pointing to the service
  name and bus:

  #+begin_src emacs-lisp
    (defcustom espotify-service-name "mopidy"
      "Name of the DBUS service used by the client we talk to.

    The official Spotify client uses `spotify', but one can also use
    alternative clients such as mopidy or spotifyd."
      :type 'string)

    (defcustom espotify-use-system-bus-p t
      "Whether to access the spotify client using the system DBUS.")
  #+end_src

  and then using the Emacs DBUS API to send methods to it is a
  breeze:

  #+begin_src emacs-lisp
    (defun espotify-call-spotify-via-dbus (method &rest args)
      "Tell Spotify to execute METHOD with ARGS through DBUS."
      (apply #'dbus-call-method `(,(if espotify-use-system-bus-p :system :session)
                                  ,(format "org.mpris.MediaPlayer2.%s"
                                           espotify-service-name)
                                  "/org/mpris/MediaPlayer2"
                                  "org.mpris.MediaPlayer2.Player"
                                  ,method
                                  ,@args)))

    (defun espotify-play-uri (uri)
      (espotify-call-spotify-via-dbus "OpenUri" uri))
  #+end_src

* Search front-end using consult

  I am exploring [[https://github.com/minad/consult][consult.el]] (and friends) to replace ivy/counsel,
  inspired in part by [[https://protesilaos.com/codelog/2021-01-06-emacs-default-completion/][Protesilaos Stavrou's musings]], and liking a
  lot what i see.  Up till now, everything i had with counsel is
  supported, often in better ways, with one exception: completing
  search of spotify albums using [[https://github.com/Lautaro-Garcia/counsel-spotify][counsel-spotify]].  So let's fix that
  by defining an asynchronous consult function that does precisely
  that!

  The top-level command will have this form:

  #+begin_src emacs-lisp
    (defun consult-spotify-by (type &optional filter)
      (consult--read (format "Search %ss: " type)
                     (espotify--search-generator type filter)
                     :lookup 'espotify--consult-lookup
                     :category 'spotify-query-result
                     :initial "#"
                     :require-match t))
  #+end_src

  where we can write an asynchronous generator of search results
  with the helper function:

  #+begin_src emacs-lisp
    (defun espotify--search-generator (type filter)
      (thread-first (consult--async-sink)
        (consult--async-refresh-timer)
        (espotify--async-search type filter)
        (consult--async-throttle)
        (consult--async-split)))
  #+end_src

  The above follows a generic consult pattern, where all functions
  are pre-defined for us except ~espotify--async-search~, an
  asynchronous dispatcher closure that must generate and handle a
  list of candidates, responding to a set of action messages (init,
  reset, get, flush, etc.) [fn:1]  Here's its definition in our
  case:

  #+begin_src emacs-lisp
    (defun espotify--async-search (next type filter)
      (let (candidates)
        (lambda (action)
          (pcase action
            ((or 'setup 'refresh) (funcall next action))
            ('destroy (setq candidates nil) (funcall next 'destroy))
            ((pred stringp)
             (espotify-search-all
              (lambda (x)
                (setq candidates (mapcar 'espotify--format-item x))
                (funcall next candidates))
              action ;; when we receive a string as the action,
                     ;; it's the user input
              type
              filter))
            ('get candidates)
            (_ (funcall next action))))))
  #+end_src

  When processing the results, we format them as a displayable
  string, while hiding in a property the URI that will allow us to
  play the item:

  #+begin_src emacs-lisp
    (defun espotify--format-item (x)
      (propertize (format "%s" (alist-get 'name x)) 'espotify-item x))

    (defun espotify--item (cand)
      (get-text-property 0 'espotify-item cand))

    (defun espotify--uri (cand)
      (alist-get 'uri (espotify--item cand)))
   #+end_src

   and then we make sure that we access that original string when
   consult looks up for it using the ~:lookup~ function, which we can
   simply define as:

   #+begin_src emacs-lisp
     (require 'seq)
     (defun espotify--consult-lookup (_input cands cand)
       (seq-find (lambda (x) (string= cand x)) cands))
   #+end_src


   With that, when we receive the final result from ~consult--read~,
   we can play the selected URI right away:

   #+begin_src emacs-lisp
     (defun espotify--maybe-play (x)
       (when-let (uri (espotify--uri x))
         (espotify-play-uri uri)))
   #+end_src

   And here, finally, are our interactive command to search and play
   albums using consult:

   #+begin_src emacs-lisp
     (defun consult-spotify-album (&optional filter)
       (interactive)
       (espotify--maybe-play (consult-spotify-by 'album filter)))
   #+end_src

   And likewise for playlists, artists and combinations thereof:

  #+begin_src emacs-lisp
     (defun consult-spotify-artist (&optional filter)
       (interactive)
       (espotify--maybe-play (consult-spotify-by 'artist filter)))
  #+end_src

  #+begin_src emacs-lisp
     (defun consult-spotify-track (&optional filter)
       (interactive)
       (espotify--maybe-play (consult-spotify-by 'track filter)))
  #+end_src

  #+begin_src emacs-lisp
     (defun consult-spotify-playlist (&optional filter)
       (interactive)
       (espotify--maybe-play (consult-spotify-by 'playlist filter)))
  #+end_src

* Adding metadata to candidates using Marginalia

  Let's add metadata fields to our candidates, so that packages like
  [[https://github.com/minad/marginalia][Marginalia]] can offer it to consult or selectrum.

  #+begin_src emacs-lisp
    (defun espotify-marginalia-annotate (cand)
      (when-let (x (espotify--item cand))
        (marginalia--fields
         ((alist-get 'type x "") :face marginalia-mode :width 10)
         ((or (alist-get 'name (car (alist-get 'artists x))) "")
          :face 'marginalia-file-name :width 50)
         ((if-let (d (alist-get 'total_tracks x)) (format "%s tracks" d) "")
          :face 'marginalia-size :width 12)
         ((if-let (d (alist-get 'release_date x)) (format "%s" d) "")
          :face 'maginalia-modified :width 10))))

    (add-to-list 'marginalia-annotators-heavy
                 (cons 'spotify-query-result 'espotify-marginalia-annotate))
#+end_src

* Post-amble                                                     :noexport:

  #+begin_src emacs-lisp
    (provide 'espotify)
  #+end_src


* Footnotes

[fn:1] This is an elegant strategy i first learnt about in SICP, many,
many years ago, and i must say that it is very charming to find it
around in the wild!