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+;; papis-citar.el --- Citar backend for Papis -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2025 Jean-Alexandre Barszcz
+
+;; Author: Jean-Alexandre Barszcz <jean-alexandre.barszcz@umontreal.ca>
+;; Version: 1.0
+;; Package-Requires ((citar "1.2.0") papis)
+;; Keywords: Citar, Papis, Bibliography
+;; URL: https://github.com/papis/papis.el
+
+;;; Commentary:
+
+;; This package registers a Citar backend locating bibliography files
+;; and notes about them. Each callback function runs the 'papis'
+;; command at most once.
+
+;; Add this to your configuration:
+
+;; (use-package papis-citar
+;;   :after citar
+;;   :config (papis-citar-setup))
+
+;; TODO Replace Citar's global bibfiles (`citar-bibliography').
+;; Currently, org-cite and Citar need file(s) to get the library data,
+;; but Papis has it's own library database. We could 1) automatically
+;; update a bibfile from Papis on modification, or 2) update it from
+;; emacs on demand by calling the Papis program at each Citar entry
+;; point, or 3) skip the file entirely and populate the entries for
+;; Citar directly from Papis (see `citar--entries').
+
+;;; Code:
+
+(require 'citar)
+(require 'papis)
+
+;;;; Customization
+
+(defgroup papis-citar nil
+  "Citar backend for Papis."
+  :prefix "papis-citar-"
+  :group 'papis
+  )
+
+(defcustom papis-citar-open-note-hook nil
+  "A hook to run when opening a note.
+
+For example, it can be used to move the cursor to a specific place in
+note templates."
+  :type 'hook
+  :group 'papis-citar)
+
+;;;; Internal functions
+
+;; Those functions are mostly "improved" versions of similar functions
+;; in 'papis.el'
+
+(defun papis-citar--cmd (args &optional destination)
+  "Run the papis program with arguments ARGS.
+
+An alternative to `papis--cmd' with more flexible output.
+
+ARGS is the list of arguments passed to the Papis program (sub-command
+included).
+
+DESTINATION uses the same format as `call-process'."
+  ;; TODO Merge with `papis--cmd'
+  (let* ((cmd papis-binary-path)
+         (libopt (when papis-library (list "-l" papis-library)))
+         (args (append libopt args))
+         (cmdstr (string-join (cons cmd args) " ")))
+    (message cmdstr)
+    (apply 'call-process cmd nil destination nil args)))
+
+(defun papis-citar--cmd-to-string (args &optional mixstderr)
+  "Run the Papis program with ARGS, and copy the output to a string.
+
+ARGS is the list of arguments passed to the Papis program (sub-command
+included).
+
+When MIXSTDERR is t, the returned string also includes the error output."
+  (let ((outbuf (generate-new-buffer " *papis-citar-output*")))
+    (with-current-buffer outbuf
+      (papis-citar--cmd args (list outbuf mixstderr))
+      (prog1 (string-trim (buffer-string))
+        (kill-buffer)))))
+
+(defun papis-citar--library-documents ()
+  "Run `papis export' and return hashtables of documents."
+  ;; TODO Merge with `papis-query'
+  (let* ((outbuf (generate-new-buffer " *papis-citar-json-export*"))
+         (dest (list outbuf nil)))
+    (with-current-buffer outbuf
+      (save-excursion
+        (papis-citar--cmd '("export" "--all" "--format" "json") dest))
+      (prog1 (json-parse-buffer :array-type 'list
+                                :null-object nil
+                                :false-object nil)
+        (kill-buffer)))))
+
+(defun papis-citar--doc-notes-path (doc)
+  "Return the path to the note file associated with DOC, if it exists."
+  (when-let* ((dir (papis-doc-get-folder doc))
+              (file (gethash "notes" doc))
+              (path (file-name-concat dir file)))
+    (and (file-exists-p path) path)))
+
+(defun papis-citar--lib-file-paths (doc)
+  "Return the paths to existing library files associated with DOC."
+  (when-let ((dir (papis-doc-get-folder doc))
+             (files (gethash "files" doc)))
+    (mapcan (lambda (file)
+              (let ((path (file-name-concat dir file)))
+                (and (file-exists-p path) (list path))))
+            files)))
+
+;;;; Checking the papis program and its version
+
+(defvar papis-citar--papis-program-version nil
+  "Papis program version, if known. See `papis-citar--check-program'.")
+
+(defcustom papis-citar-skip-program-check nil
+  "When non-nil, don't check the Papis program version or runnability."
+  :type 'boolean
+  :group 'papis-citar)
+
+(defun papis-citar-check-program (&optional min-version)
+  "Run the Papis program and return its version-list. Error if absent.
+
+Return nil when variable `papis-citar-skip-program-check' is non-nil.
+
+If the obtained version is lower than MIN-VERSION, throw an error.
+
+Idempotence: Save the version to variable
+`papis-citar--papis-program-version', to avoid re-running the Papis
+program over and over."
+  (unless papis-citar-skip-program-check
+    (setq papis-citar--papis-program-version
+          (or papis-citar--papis-program-version
+              (let ((output (papis-citar--cmd-to-string '("--version"))))
+                (version-to-list (car (last (string-split output)))))))
+    (when-let ((version papis-citar--papis-program-version)
+               (min-version (if (stringp min-version)
+                                (version-to-list min-version)
+                              min-version)))
+      (when (version-list-< version min-version)
+        (error
+         (format "Papis program version %S lower than required %S"
+                 version min-version))))
+    papis-citar--papis-program-version))
+
+;;;; Callback functions for notes
+
+(defun papis-citar-get-notes (&optional citekeys)
+  "Return a table mapping bibliography keys to their note file path.
+
+If the list CITEKEYS is given, only consider the corresponding documents."
+  (let ((docs (papis-citar--library-documents)) ; Runs papis
+        (table (make-hash-table :test 'equal)))
+    (dolist (doc docs table)
+      (let ((citekey (papis--get-ref doc)))
+        (if (or (not citekeys) (member citekey citekeys))
+            (when-let ((path (papis-citar--doc-notes-path doc)))
+              (puthash citekey (list path) table)))))))
+
+(defun papis-citar-has-notes ()
+  "Return a predicate returning t when CITEKEY has a note file."
+  (let ((notes (papis-citar-get-notes))) ; Runs papis
+    (unless (hash-table-empty-p notes)
+      (lambda (citekey) (and (gethash citekey notes) t)))))
+
+(defun papis-citar-open-note (filename)
+  "Open the note file FILENAME, and run `papis-citar-open-note-hook'."
+  (find-file filename)
+  (run-hooks 'papis-citar-open-note-hook))
+
+(defun papis-citar-create-note (citekey entry)
+  "Create a note file from CITEKEY and ENTRY.
+Defers the actual note creation
+to `citar-note-format-function', so make sure that this is set
+appropriately (see `papis-citar-format-note-default')."
+  (citar--check-configuration 'citar-note-format-function)
+  (funcall citar-note-format-function citekey entry))
+
+(defun papis-citar-format-note-default (citekey &optional _entry)
+  "Create and open a note for CITEKEY, using Papis' template."
+  ;; TODO Merge with papis--ensured-notes-path
+  (let ((created-notes-path
+         (papis-citar--cmd-to-string
+          (list "edit" "--notes" "--editor" "echo" citekey))))
+    (papis-citar-open-note created-notes-path)))
+
+(defconst papis-citar-notes-config
+  (list :name "Papis Notes"
+        :category 'file
+        :items #'papis-citar-get-notes
+        :hasitems #'papis-citar-has-notes
+        :open #'papis-citar-open-note
+        :create #'papis-citar-create-note
+        )
+  "Citar notes configuration for papis.")
+
+(defun papis-citar-notes-setup ()
+  "Assign the right variables to use Papis notes in Citar."
+  (papis-citar-check-program)
+  (customize-set-variable 'citar-note-format-function
+                          #'papis-citar-format-note-default)
+  (citar-register-notes-source 'papis-citar
+                               papis-citar-notes-config)
+  (customize-set-variable 'citar-notes-source
+                          'papis-citar))
+
+;;;; Callback functions for bibliography files
+
+(defun papis-citar-get-library-files (&optional citekeys)
+  "Return a table mapping bibliography keys to their file paths.
+
+If the list CITEKEYS is given, only consider the corresponding documents."
+  (let ((docs (papis-citar--library-documents)) ; Runs papis
+        (table (make-hash-table :test 'equal)))
+    (dolist (doc docs table)
+      (let ((citekey (papis--get-ref doc)))
+        (if (or (not citekeys) (member citekey citekeys))
+            (when-let ((paths (papis-citar--lib-file-paths doc)))
+              (puthash citekey paths table)))))))
+
+(defun papis-citar-has-library-files ()
+  "Return predicate testing whether CITEKEY has library files."
+  (let ((files (papis-citar-get-library-files)))
+    (unless (hash-table-empty-p files)
+      (lambda (citekey) (and (gethash citekey files) t)))))
+
+(defconst papis-citar-files-config
+  (list :items #'papis-citar-get-library-files
+        :hasitems #'papis-citar-has-library-files)
+  "Citar library files configuration for papis.")
+
+(defun papis-citar-library-files-setup ()
+  "Assign the right variables to use Papis library files in Citar."
+  (papis-citar-check-program)
+  (customize-set-variable 'citar-file-sources
+                          (list papis-citar-files-config)))
+
+;; TODO customize variable `citar-add-file-function'
+
+;;;; General Setup
+
+(defun papis-citar-setup ()
+  "Integrate Papis and Citar."
+  (papis-citar-notes-setup)
+  (papis-citar-library-files-setup))
+
+(provide 'papis-citar)
+
+;;; papis-citar.el ends here