;;; oc-basic.el --- basic backend for citations  -*- lexical-binding: t; -*-

;; Copyright (C) 2021-2024 Free Software Foundation, Inc.

;; Author: Nicolas Goaziou <mail@nicolasgoaziou.fr>

;; 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:

;; The `basic' citation processor provides "activate", "follow", "export" and
;; "insert" capabilities.

;; "activate" capability reuses default fontification, but provides additional
;; features on both correct and wrong keys according to the bibliography
;; defined in the document.

;; When the mouse is over a known key, it displays the corresponding
;; bibliography entry.  Any wrong key, however, is highlighted with `error'
;; face.  Moreover, moving the mouse onto it displays a list of suggested correct
;; keys, and pressing <mouse-1> on the faulty key will try to fix it according to
;; those suggestions.

;; On a citation key, "follow" capability moves point to the corresponding entry
;; in the current bibliography.  Elsewhere on the citation, it asks the user to
;; follow any of the keys cited there, with completion.

;; "export" capability supports the following citation styles:
;;
;;   - author (a), including caps (c) variant,
;;   - noauthor (na) including bare (b) variant,
;;   - text (t), including bare (b), caps (c), and bare-caps (bc) variants,
;;   - note (ft, including bare (b), caps (c), and bare-caps (bc) variants,
;;   - nocite (n)
;;   - numeric (nb),
;;   - default, including bare (b), caps (c), and bare-caps (bc) variants.
;;
;; It also supports the following styles for bibliography:
;;   - plain
;;   - numeric
;;   - author-year (default)

;; "insert" capability inserts or edits (with completion) citation style or
;; citation reference keys.  In an appropriate place, it offers to insert a new
;; citation.  With a prefix argument, it removes the one at point.

;; It supports bibliography files in BibTeX (".bibtex"), biblatex (".bib") and
;; JSON (".json") format.

;; Disclaimer: this citation processor is meant to be a proof of concept, and
;; possibly a fall-back mechanism when nothing else is available.  It is too
;; limited for any serious use case.

;;; Code:

(require 'org-macs)
(org-assert-version)

(require 'bibtex)
(require 'json)
(require 'map)
(require 'oc)
(require 'seq)
(require 'transient)

(declare-function org-open-at-point "org" (&optional arg))
(declare-function org-open-file "org" (path &optional in-emacs line search))

(declare-function org-element-create "org-element-ast" (type &optional props &rest children))
(declare-function org-element-set "org-element-ast" (old new &optional keep-props))

(declare-function org-element-interpret-data "org-element" (data))
(declare-function org-element-parse-secondary-string "org-element" (string restriction &optional parent))
(declare-function org-element-map "org-element"
                  ( data types fun
                    &optional
                    info first-match no-recursion
                    with-affiliated no-undefer))
(declare-function org-element-property "org-element-ast" (property node))
(declare-function org-element-type-p "org-element-ast" (node types))
(declare-function org-element-contents "org-element-ast" (node))

(declare-function org-export-data "org-export" (data info))
(declare-function org-export-derived-backend-p "org-export" (backend &rest backends))
(declare-function org-export-raw-string "org-export" (contents))

\f
;;; Customization
(defcustom org-cite-basic-sorting-field 'author
  "Field used to sort bibliography items as a symbol, or nil."
  :group 'org-cite
  :package-version '(Org . "9.5")
  :type 'symbol
  :safe #'symbolp)

(defcustom org-cite-basic-author-year-separator ", "
  "String used to separate cites in an author-year configuration."
  :group 'org-cite
  :package-version '(Org . "9.5")
  :type 'string
  :safe #'stringp)

(defcustom org-cite-basic-max-key-distance 2
  "Maximum (Levenshtein) distance between a wrong key and its suggestions."
  :group 'org-cite
  :package-version '(Org . "9.5")
  :type 'integer
  :safe #'integerp)

(defcustom org-cite-basic-author-column-end 25
  "Column where author field ends in completion table, as an integer."
  :group 'org-cite
  :package-version '(Org . "9.5")
  :type 'integer
  :safe #'integerp)

(defcustom org-cite-basic-column-separator "  "
  "Column separator in completion table, as a string."
  :group 'org-cite
  :package-version '(Org . "9.5")
  :type 'string
  :safe #'stringp)

(defcustom org-cite-basic-mouse-over-key-face 'highlight
  "Face used when mouse is over a citation key."
  :group 'org-cite
  :package-version '(Org . "9.5")
  :type 'face
  :safe #'facep)

(defcustom org-cite-basic-follow-ask nil
  "Should `org-cite-basic' ask how to follow citations?

When this option is nil, `org-cite-basic-follow' opens the bibliography entry.
Otherwise, `org-cite-basic-follow' will display a transient menu prompting the 
user for an action. The contents of this menu can be customized in 
`org-cite-basic-follow-actions'."
  :group 'org-cite
  :package-version '(Org . "9.8")
  :type 'boolean)

(defcustom org-cite-basic-follow-actions
  '[["Open"
     ("b" "bibliography entry" (org-cite-basic-goto !citation !prefix))]]
  "Actions in the `org-cite-basic-follow' transient menu.

This option uses the same syntax as `transient-define-prefix', see Info node
`(transient)Binding Suffix and Infix Commands'. In addition, it is possible 
to specify a function call for the COMMAND part, where !citation, 
!prefix, and !citation-key can be used to access those values. 

If COMMAND is a lambda form, it can access the citation and prefix like this:

  (lambda (citation prefix)
     (interactive (transient-scope))
     ...)"
  :group 'org-cite
  :package-version '(Org . "9.8")
  :type 'sexp)

\f
;;; Internal variables
(defvar org-cite-basic--bibliography-cache nil
  "Cache for parsed bibliography files.

This is an association list following the pattern:

  (FILE-ID . ENTRIES)

FILE-ID is a cons cell (FILE . HASH), with FILE being the absolute file name of
the bibliography file, and HASH a hash of its contents.

ENTRIES is a hash table with citation references as keys and fields alist as
values.")

(defvar org-cite-basic--completion-cache (make-hash-table :test #'equal)
  "Cache for key completion table.

This is an a hash-table.")

\f
;;; Internal functions
(defun org-cite-basic--parse-json ()
  "Parse JSON entries in the current buffer.
Return a hash table with citation references as keys and fields alist as values."
  (let ((entries (make-hash-table :test #'equal)))
    (let ((json-array-type 'list)
          (json-key-type 'symbol))
      (dolist (item (json-read))
        (puthash (cdr (assq 'id item))
                 (mapcar (pcase-lambda (`(,field . ,value))
                           (pcase field
                             ((or 'author 'editor)
                              ;; Author and editors are arrays of
                              ;; objects, each of them designing a
                              ;; person.  These objects may contain
                              ;; multiple properties, but for this
                              ;; basic processor, we'll focus on
                              ;; `given' and `family'.
                              ;;
                              ;; For compatibility with BibTeX, add
                              ;; "and" between authors and editors.
                              (cons field
                                    (mapconcat
                                     (lambda (alist)
                                       (concat (alist-get 'family alist)
                                               " "
                                               (alist-get 'given alist)))
                                     value
                                     " and ")))
                             ('issued
                              ;; Date are expressed as an array
                              ;; (`date-parts') or a "string (`raw'
                              ;; or `literal'). In both cases,
                              ;; extract the year and associate it
                              ;; to `year' field, for compatibility
                              ;; with BibTeX format.
                              (let ((date (or (alist-get 'date-parts value)
                                              (alist-get 'literal value)
                                              (alist-get 'raw value))))
                                (cons 'year
                                      (cond
                                       ((consp date)
                                         (let ((year (caar date)))
                                           (cond
                                             ((numberp year) (number-to-string year))
                                             ((stringp year) year)
                                             (t
                                               (error
                                                 "First element of CSL-JSON date-parts should be a number or string, got %s: %S"
                                                 (type-of year) year)))))
                                       ((stringp date)
                                        (replace-regexp-in-string
                                          (rx
                                            (minimal-match (zero-or-more anything))
                                            (group-n 1 (repeat 4 digit))
                                            (zero-or-more anything))
                                          (rx (backref 1))
                                          date))
                                       (t
                                        (error "Unknown CSL-JSON date format: %S"
                                               value))))))
                             (_
                              (cons field value))))
                         item)
                 entries))
      entries)))

(defun org-cite-basic--parse-bibtex (dialect)
  "Parse BibTeX entries in the current buffer.
DIALECT is the BibTeX dialect used.  See `bibtex-dialect'.
Return a hash table with citation references as keys and fields alist as values."
  (let ((entries (make-hash-table :test #'equal))
        (bibtex-sort-ignore-string-entries t))
    (bibtex-set-dialect dialect t)
    ;; Throw an error if bibliography is malformed.
    (unless (bibtex-validate)
      (user-error "Malformed bibliography at %S"
                  (or (buffer-file-name) (current-buffer))))
    (bibtex-map-entries
     (lambda (key &rest _)
       ;; Normalize entries: field names are turned into symbols
       ;; including special "=key=" and "=type=", and consecutive
       ;; white spaces are removed from values.
       (puthash key
                (mapcar
                 (pcase-lambda (`(,field . ,value))
                   (pcase field
                     ("=key=" (cons 'id key))
                     ("=type=" (cons 'type value))
                     (_
                      (cons
                       (intern (downcase field))
                       (replace-regexp-in-string "[ \t\n]+" " " value)))))
                 ;; Parse, substituting the @string replacements.
                 ;; See Emacs bug#56475 discussion.
                 (let ((bibtex-string-files `(,(buffer-file-name)))
                       (bibtex-expand-strings t))
                   (bibtex-parse-entry t)))
                entries)))
    entries))

(defvar org-cite-basic--file-id-cache nil
  "Hash table linking files to their hash.")
(defun org-cite-basic--parse-bibliography (&optional info)
  "List all entries available in the buffer.

Each association follows the pattern

  (FILE . ENTRIES)

where FILE is the absolute file name of the BibTeX file, and ENTRIES is a hash
table where keys are references and values are association lists between fields,
as symbols, and values as strings or nil.

Optional argument INFO is the export state, as a property list."
  (unless (hash-table-p org-cite-basic--file-id-cache)
    (setq org-cite-basic--file-id-cache (make-hash-table :test #'equal)))
  (if (plist-member info :cite-basic/bibliography)
      (plist-get info :cite-basic/bibliography)
    (let ((results nil))
      (dolist (file (org-cite-list-bibliography-files))
        ;; Follow symlinks, to look into modification time of the
        ;; actual file, not its symlink.
        (setq file (file-truename file))
        (when (file-readable-p file)
          (with-temp-buffer
            (when (or (org-file-has-changed-p file)
                      (not (gethash file org-cite-basic--file-id-cache)))
              (insert-file-contents file)
              (set-visited-file-name file t)
              (puthash file (org-buffer-hash) org-cite-basic--file-id-cache))
            (condition-case nil
                (unwind-protect
	            (let* ((file-id (cons file (gethash file org-cite-basic--file-id-cache)))
                           (entries
                            (or (cdr (assoc file-id org-cite-basic--bibliography-cache))
                                (let ((table
                                       (pcase (file-name-extension file)
                                         ("json" (org-cite-basic--parse-json))
                                         ("bib" (org-cite-basic--parse-bibtex 'biblatex))
                                         ("bibtex" (org-cite-basic--parse-bibtex 'BibTeX))
                                         (ext
                                          (user-error "Unknown bibliography extension: %S"
                                                      ext)))))
                                  (push (cons file-id table) org-cite-basic--bibliography-cache)
                                  table))))
                      (push (cons file entries) results))
                  (set-visited-file-name nil t))
              (error (setq org-cite-basic--file-id-cache nil))))))
      (when info (plist-put info :cite-basic/bibliography results))
      results)))

(defun org-cite-basic--key-number (key info)
  "Return number associated to cited KEY.
INFO is the export state, as a property list."
  (let ((predicate
         (org-cite-basic--field-less-p org-cite-basic-sorting-field info)))
    (org-cite-key-number key info predicate)))

(defun org-cite-basic--all-keys ()
  "List all keys available in current bibliography."
  (seq-mapcat (pcase-lambda (`(,_ . ,entries))
                (map-keys entries))
              (org-cite-basic--parse-bibliography)))

(defun org-cite-basic--get-entry (key &optional info)
  "Return BibTeX entry for KEY, as an association list.
When non-nil, INFO is the export state, as a property list."
  (catch :found
    (pcase-dolist (`(,_ . ,entries) (org-cite-basic--parse-bibliography info))
      (let ((entry (gethash key entries)))
        (when entry (throw :found entry))))
    nil))

(defun org-cite-basic--get-field (field entry-or-key &optional info raw)
  "Return FIELD value for ENTRY-OR-KEY, or nil.

FIELD is a symbol.  ENTRY-OR-KEY is either an association list, as returned by
`org-cite-basic--get-entry', or a string representing a citation key.

Optional argument INFO is the export state, as a property list.

Return value may be nil or a string.  If current export backend is derived
from `latex', return a raw string object instead, unless optional
argument RAW is non-nil.

Throw an error if the field value is non-string and non-nil."
  (let ((value
         (cdr
          (assq field
                (pcase entry-or-key
                  ((pred stringp)
                   (org-cite-basic--get-entry entry-or-key info))
                  ((pred consp)
                   entry-or-key)
                  (_
                   (error "Wrong value for ENTRY-OR-KEY: %S" entry-or-key)))))))
    (when (and value (not (stringp value)))
      (error "Non-string bibliography field value: %S" value))
    (if (and value
             (not raw)
             (org-export-derived-backend-p (plist-get info :back-end) 'latex))
        (org-export-raw-string value)
      value)))

(defun org-cite-basic--shorten-names (names)
  "Return a list of family names from a list of full NAMES.
NAMES can be a string or raw string object.

To better accomomodate corporate names, this will only shorten
personal names of the form \"family, given\"."
  (let (names-string raw-p)
    (cond
     ((stringp names) (setq names-string names))
     ((org-element-type-p names 'raw)
      (setq names-string (mapconcat #'identity (org-element-contents names) "")
            raw-p t)))
    (when names-string
      (setq names-string
            (mapconcat
             (lambda (name)
               (if (eq 1 (length name))
                   (cdr (split-string name))
                 (car (split-string name ", "))))
             (split-string names-string " and ")
             ", "))
      (if raw-p (org-export-raw-string names-string)
        names-string))))

(defun org-cite-basic--number-to-suffix (n)
  "Compute suffix associated to number N.
This is used for disambiguation."
  (let ((result nil))
    (apply #'string
           (mapcar (lambda (n) (+ 97 n))
                   (catch :complete
                     (while t
                       (push (% n 26) result)
                       (setq n (/ n 26))
                       (cond
                        ((= n 0) (throw :complete result))
                        ((< n 27) (throw :complete (cons (1- n) result)))
                        ((= n 27) (throw :complete (cons 0 (cons 0 result))))
                        (t nil))))))))

(defun org-cite-basic--get-author (entry-or-key &optional info raw)
  "Return author associated to ENTRY-OR-KEY.

ENTRY-OR-KEY, INFO and RAW arguments are the same arguments as
used in `org-cite-basic--get-field', which see.

Author is obtained from the \"author\" field, if available, or
from the \"editor\" field otherwise."
  (or (org-cite-basic--get-field 'author entry-or-key info raw)
      (org-cite-basic--get-field 'editor entry-or-key info raw)))

(defun org-cite-basic--get-year (entry-or-key info &optional no-suffix)
  "Return year associated to ENTRY-OR-KEY.

ENTRY-OR-KEY is either an association list, as returned by
`org-cite-basic--get-entry', or a string representing a citation
key.  INFO is the export state, as a property list.

Year is obtained from the \"year\" field, if available, or from
the \"date\" field if it starts with a year pattern.

Unlike `org-cite-basic--get-field', this function disambiguates
author-year patterns by adding a letter suffix to the year when
necessary, unless optional argument NO-SUFFIX is non-nil."
  ;; The cache is an association list with the following structure:
  ;;
  ;;    (AUTHOR-YEAR . KEY-SUFFIX-ALIST).
  ;;
  ;; AUTHOR-YEAR is the author year pair associated to current entry
  ;; or key.
  ;;
  ;; KEY-SUFFIX-ALIST is an association (KEY . SUFFIX), where KEY is
  ;; the cite key, as a string, and SUFFIX is the generated suffix
  ;; string, or the empty string.
  (let* ((author (org-cite-basic--get-author entry-or-key info 'raw))
         (year
          (or (org-cite-basic--get-field 'year entry-or-key info 'raw)
              (let ((date
                     (org-cite-basic--get-field 'date entry-or-key info 'raw)))
                (and (stringp date)
                     (string-match (rx string-start
                                       (group (= 4 digit))
                                       (or string-end (not digit)))
                                   date)
                     (match-string 1 date)))))
         (cache-key (cons author year))
         (key
          (pcase entry-or-key
            ((pred stringp) entry-or-key)
            ((pred consp) (cdr (assq 'id entry-or-key)))
            (_ (error "Wrong value for ENTRY-OR-KEY: %S" entry-or-key))))
         (cache (plist-get info :cite-basic/author-date-cache)))
    (pcase (assoc cache-key cache)
      ('nil
       (let ((value (cons cache-key (list (cons key "")))))
         (plist-put info :cite-basic/author-date-cache (cons value cache))
         year))
      (`(,_ . ,alist)
       (let ((suffix
              (or (cdr (assoc key alist))
                  (let ((new (org-cite-basic--number-to-suffix
                              (1- (length alist)))))
                    (push (cons key new) alist)
                    new))))
         (if no-suffix year (concat year suffix)))))))

(defun org-cite-basic--print-bibtex-string (element &optional info)
  "Print Bibtex formatted string ELEMENT, according to Bibtex syntax.
Remove all the {...} that are not a part of LaTeX macros and parse the
LaTeX fragments.  Do nothing when current backend is derived from
LaTeX, according to INFO.

Return updated ELEMENT."
  (if (org-export-derived-backend-p (plist-get info :back-end) 'latex)
      ;; Derived from LaTeX, no need to use manual ad-hoc LaTeX
      ;; parser.
      element
    ;; Convert ELEMENT to anonymous when ELEMENT is string.
    ;; Otherwise, we cannot modify ELEMENT by side effect.
    (when (org-element-type-p element 'plain-text)
      (setq element (org-element-create 'anonymous nil element)))
    ;; Approximately parse LaTeX fragments, assuming Org mode syntax
    ;; (it is close to original LaTeX, and we do not want to
    ;; re-implement complete LaTeX parser here))
    (org-element-map element t
      (lambda (str)
        (when (stringp str)
          (org-element-set
           str
           (org-element-parse-secondary-string
            str '(latex-fragment entity))))))
    ;; Strip the remaining { and }.
    (org-element-map element t
      (lambda (str)
        (when (stringp str)
          (org-element-set str (replace-regexp-in-string "[{}]" "" str)))))
    element))

(defun org-cite-basic--print-entry (entry style &optional info)
  "Format ENTRY according to STYLE string.
ENTRY is an alist, as returned by `org-cite-basic--get-entry'.
Optional argument INFO is the export state, as a property list."
  (let ((author (org-cite-basic--get-author entry info))
        (title (org-cite-basic--get-field 'title entry info))
        (from
         (or (org-cite-basic--get-field 'publisher entry info)
             (org-cite-basic--get-field 'journal entry info)
             (org-cite-basic--get-field 'institution entry info)
             (org-cite-basic--get-field 'school entry info))))
    (org-cite-basic--print-bibtex-string
     (pcase style
       ("plain"
        (let ((year (org-cite-basic--get-year entry info 'no-suffix)))
          (org-cite-concat
           (org-cite-basic--shorten-names author) ". "
           title (and from (list ", " from)) ", " year ".")))
       ("numeric"
        (let ((n (org-cite-basic--key-number (cdr (assq 'id entry)) info))
              (year (org-cite-basic--get-year entry info 'no-suffix)))
          (org-cite-concat
           (format "[%d] " n) author ", "
           (org-cite-emphasize 'italic title)
           (and from (list ", " from)) ", "
           year ".")))
       ;; Default to author-year.  Use year disambiguation there.
       (_
        (let ((year (org-cite-basic--get-year entry info)))
          (org-cite-concat
           author " (" year "). "
           (org-cite-emphasize 'italic title)
           (and from (list ", " from)) "."))))
     info)))

\f
;;; "Activate" capability
(defun org-cite-basic--close-keys (key keys)
  "List cite keys close to KEY in terms of string distance."
  (seq-filter (lambda (k)
                (>= org-cite-basic-max-key-distance
                    (org-string-distance k key)))
              keys))

(defun org-cite-basic--set-keymap (beg end suggestions)
  "Set keymap on citation key between BEG and END positions.

When the key is know, SUGGESTIONS is nil.  Otherwise, it may be
a list of replacement keys, as strings, which will be offered as
substitutes for the unknown key.  Finally, it may be the symbol
`all'."
  (let ((km (make-sparse-keymap)))
    (define-key km (kbd "<mouse-1>")
      (pcase suggestions
        ('nil #'org-open-at-point)
        ('all #'org-cite-insert)
        (_
         (lambda ()
           (interactive)
           (save-excursion
             (goto-char beg)
             (delete-region beg end)
             (insert
              "@"
              (if (= 1 (length suggestions))
                  (car suggestions)
                (completing-read "Did you mean: "
                                 suggestions nil t))))))))
    (put-text-property beg end 'keymap km)))

(defun org-cite-basic-activate (citation)
  "Set various text properties on CITATION object.

Fontify whole citation with `org-cite' face.  Fontify key with `error' face
when it does not belong to known keys.  Otherwise, use `org-cite-key' face.

Moreover, when mouse is on a known key, display the corresponding bibliography.
On a wrong key, suggest a list of possible keys, and offer to substitute one of
them with a mouse click."
  (pcase-let ((`(,beg . ,end) (org-cite-boundaries citation))
              (keys (org-cite-basic--all-keys)))
    (put-text-property beg end 'font-lock-multiline t)
    (add-face-text-property beg end 'org-cite)
    (dolist (reference (org-cite-get-references citation))
      (pcase-let* ((`(,beg . ,end) (org-cite-key-boundaries reference))
                   (key (org-element-property :key reference)))
        ;; Highlight key on mouse over.
        (put-text-property beg end
                           'mouse-face
                           org-cite-basic-mouse-over-key-face)
        (if (member key keys)
            ;; Activate a correct key.  Face is `org-cite-key' and
            ;; `help-echo' displays bibliography entry, for reference.
            ;; <mouse-1> calls `org-open-at-point'.
            (let* ((entry (org-cite-basic--get-entry key))
                   (bibliography-entry
                    (org-element-interpret-data
                     (org-cite-basic--print-entry entry "plain"))))
              (add-face-text-property beg end 'org-cite-key)
              (put-text-property beg end 'help-echo bibliography-entry)
              (org-cite-basic--set-keymap beg end nil))
          ;; Activate a wrong key.  Face is `error', `help-echo'
          ;; displays possible suggestions.
          (add-face-text-property beg end 'error)
          (let ((close-keys (org-cite-basic--close-keys key keys)))
            (when close-keys
              (put-text-property beg end 'help-echo
                                 (concat "Suggestions (mouse-1 to substitute): "
                                         (mapconcat #'identity close-keys " "))))
            ;; When the are close know keys, <mouse-1> provides
            ;; completion to fix the current one.  Otherwise, call
            ;; `org-cite-insert'.
            (org-cite-basic--set-keymap beg end (or close-keys 'all))))))))

\f
;;; "Export" capability
(defun org-cite-basic--format-author-year (citation format-cite format-ref info)
  "Format CITATION object according to author-year format.

FORMAT-CITE is a function of three arguments: the global prefix, the contents,
and the global suffix.  All arguments can be strings or secondary strings.

FORMAT-REF is a function of four arguments: the reference prefix, as a string or
secondary string, the author, the year, and the reference suffix, as a string or
secondary string.

INFO is the export state, as a property list."
  (org-export-data
   (funcall format-cite
            (org-element-property :prefix citation)
            (org-cite-mapconcat
             (lambda (ref)
               (let ((k (org-element-property :key ref))
                     (prefix (org-element-property :prefix ref))
                     (suffix (org-element-property :suffix ref)))
                 (funcall format-ref
                          prefix
                          (or (org-cite-basic--get-author k info) "??")
                          (or (org-cite-basic--get-year k info) "????")
                          suffix)))
             (org-cite-get-references citation)
             org-cite-basic-author-year-separator)
            (org-element-property :suffix citation))
   info))

(defun org-cite-basic--citation-numbers (citation info)
  "Return numbers associated to references in CITATION object.
INFO is the export state as a property list."
  (let* ((numbers
          (sort (mapcar (lambda (k) (org-cite-basic--key-number k info))
                        (org-cite-get-references citation t))
                #'<))
         (last (car numbers))
         (result (list (number-to-string (pop numbers)))))
    ;; Use compact number references, i.e., "1, 2, 3" becomes "1-3".
    (while numbers
      (let ((current (pop numbers))
            (next (car numbers)))
        (cond
         ((and next
               (= current (1+ last))
               (= current (1- next)))
          (unless (equal "-" (car result))
            (push "-" result)))
         ((equal "-" (car result))
          (push (number-to-string current) result))
         (t
          (push (format ", %d" current) result)))
        (setq last current)))
    (apply #'concat (nreverse result))))

(defun org-cite-basic--field-less-p (field info)
  "Return a sort predicate comparing FIELD values for two citation keys.
INFO is the export state, as a property list."
  (and field
       (lambda (a b)
         (org-string<
          (org-cite-basic--get-field field a info 'raw)
          (org-cite-basic--get-field field b info 'raw)
          nil t))))

(defun org-cite-basic--sort-keys (keys info)
  "Sort KEYS by author name.
INFO is the export communication channel, as a property list."
  (let ((predicate (org-cite-basic--field-less-p org-cite-basic-sorting-field info)))
    (if predicate
        (sort keys predicate)
      keys)))

(defun org-cite-basic-export-citation (citation style _ info)
  "Export CITATION object.
STYLE is the expected citation style, as a pair of strings or nil.  INFO is the
export communication channel, as a property list."
  (let ((has-variant-p
         (lambda (variant type)
           ;; Non-nil when style VARIANT has TYPE.  TYPE is either
           ;; `bare' or `caps'.
           (member variant
                   (pcase type
                     ('bare '("bare" "bare-caps" "b" "bc"))
                     ('caps '("caps" "bare-caps" "c" "bc"))
                     (_ (error "Invalid variant type: %S" type)))))))
    (pcase style
      ;; "author" style.
      (`(,(or "author" "a") . ,variant)
       (let ((caps (member variant '("caps" "c"))))
         (org-cite-basic--format-author-year
          citation
          (lambda (p c s) (org-cite-concat p c s))
          (lambda (prefix author _ suffix)
            (org-cite-concat
             prefix
             (if caps (org-cite-capitalize author) author)
             suffix))
          info)))
      ;; "noauthor" style.
      (`(,(or "noauthor" "na") . ,variant)
       (let ((bare? (funcall has-variant-p variant 'bare)))
         (org-cite-basic--format-author-year
          citation
          (lambda (prefix contents suffix)
            (org-cite-concat
             (unless bare? "(")
             prefix
             contents
             suffix
             (unless bare? ")")))
          (lambda (prefix _ year suffix)
            (org-cite-concat prefix year suffix))
          info)))
      ;; "nocite" style.
      (`(,(or "nocite" "n") . ,_) nil)
      ;; "text" and "note" styles.
      (`(,(and (or "text" "note" "t" "ft") style) . ,variant)
       (when (and (member style '("note" "ft"))
                  (not (org-cite-inside-footnote-p citation)))
         (org-cite-adjust-note citation info)
         (org-cite-wrap-citation citation info))
       (let ((bare (funcall has-variant-p variant 'bare))
             (caps (funcall has-variant-p variant 'caps)))
         (org-cite-basic--format-author-year
          citation
          (lambda (p c s) (org-cite-concat p c s))
          (lambda (p a y s)
            (org-cite-concat p
                             (if caps (org-cite-capitalize a) a)
                             (if bare " " " (")
                             y
                             (and (not bare) ")")
                             s))
          info)))
      ;; "numeric" style.
      ;;
      ;; When using this style on citations with multiple references,
      ;; use global affixes and ignore local ones.
      (`(,(or "numeric" "nb") . ,_)
       (pcase-let ((`(,prefix . ,suffix) (org-cite-main-affixes citation)))
         (org-export-data
          (org-cite-concat
           "(" prefix (org-cite-basic--citation-numbers citation info) suffix ")")
          info)))
      ;; Default ("nil") style.
      (`(,_ . ,variant)
       (let ((bare (funcall has-variant-p variant 'bare))
             (caps (funcall has-variant-p variant 'caps)))
         (org-cite-basic--format-author-year
          citation
          (lambda (p c s)
            (org-cite-concat (and (not bare) "(") p c s (and (not bare) ")")))
          (lambda (p a y s)
            (org-cite-concat p (if caps (org-cite-capitalize a) a) ", " y s))
          info)))
      ;; This should not happen.
      (_ (error "Invalid style: %S" style)))))

(defun org-cite-basic-export-bibliography (keys _files style _props backend info)
  "Generate bibliography.
KEYS is the list of cited keys, as strings.  STYLE is the expected bibliography
style, as a string.  BACKEND is the export backend, as a symbol.  INFO is the
export state, as a property list."
  (mapconcat
   (lambda (entry)
     (org-export-data
      (org-cite-make-paragraph
       (and (org-export-derived-backend-p backend 'latex)
            (org-export-raw-string "\\noindent\n"))
       (org-cite-basic--print-entry entry style info))
      info))
   (delq nil
         (mapcar
          (lambda (k) (org-cite-basic--get-entry k info))
          (org-cite-basic--sort-keys keys info)))
   "\n"))

\f
;;; "Follow" capability
(defun org-cite-basic-goto (datum _)
  "Follow citation or citation reference DATUM.
When DATUM is a citation reference, open bibliography entry referencing
the citation key.  Otherwise, select which key to follow among all keys
present in the citation."
  (let* ((key
          (if (org-element-type-p datum 'citation-reference)
              (org-element-property :key datum)
            (pcase (org-cite-get-references datum t)
              (`(,key) key)
              (keys
               (or (completing-read "Select citation key: " keys nil t)
                   (user-error "Aborted"))))))
         (file
          (pcase (seq-find (pcase-lambda (`(,_ . ,entries))
                             (gethash key entries))
                           (org-cite-basic--parse-bibliography))
            (`(,f . ,_) f)
            (_  (user-error "Cannot find citation key: %S" key)))))
    (org-open-file file '(4))
    (pcase (file-name-extension file)
      ("json"
       ;; `rx' can not be used with Emacs <27.1 since `literal' form
       ;; is not supported.
       (let ((regexp (rx-to-string `(seq "\"id\":" (0+ (any "[ \t]")) "\"" ,key "\"") t)))
         (goto-char (point-min))
         (re-search-forward regexp)
         (search-backward "{")))
      (_
       (bibtex-set-dialect)
       (bibtex-search-entry key)))))

(transient-define-prefix org-cite-basic-follow (citation &optional prefix)
  "Follow citation.

This transient is invoked through `org-open-at-point'. 
When `org-open-at-point' is invoked with a negative prefix,
or `org-cite-basic-follow-ask' is non-nil, it will present
a transient menu prompting the user for an action. Otherwise,
it will open the bibliography entry for the citation at point.

Suffixes can not be added to this transient menu using the ordinary
`transient-append-suffix' or `transient-insert-suffix', instead, the
contents of the menu are defined in the variable
`org-cite-basic-follow-actions'."
  [:class transient-columns
          :setup-children org-cite-basic-follow--setup
          :pad-keys t]
  (interactive)
  (if (or org-cite-basic-follow-ask
          (equal prefix '(-4)))
      (transient-setup 'org-cite-basic-follow nil nil
                       :scope (list citation prefix))
    (org-cite-basic-goto citation prefix)))

(defun org-cite-basic-follow--parse-suffix-specification (specification)
  (pcase specification
    ((and val `(,key ,desc (,fn . ,fn-args) . ,other))
     (if (eq fn 'lambda) val
       `(,key ,desc
              (lambda ()
		(interactive)
		(let ((!citation (car (transient-scope)))
                      (!prefix (cadr (transient-scope)))
                      (!citation-key
                       (org-element-property :key (car (transient-scope)))))
                  (,fn ,@fn-args)))
              ,@other)))
    (other other)))

(defun org-cite-basic-follow--setup (_)
  (transient-parse-suffixes
   'org-cite-basic-follow
   (cl-map 'vector
           (lambda (group)
             (cl-map 'vector #'org-cite-basic-follow--parse-suffix-specification
                     group))
           org-cite-basic-follow-actions)))

\f
;;; "Insert" capability
(defun org-cite-basic--complete-style (_)
  "Offer completion for style.
Return chosen style as a string."
  (let* ((styles
          (mapcar (pcase-lambda (`((,style . ,_) . ,_))
                    style)
                  (org-cite-supported-styles))))
    (pcase styles
      (`(,style) style)
      (_ (completing-read "Style (\"\" for default): " styles nil t)))))

(defun org-cite-basic--key-completion-table ()
  "Return completion table for cite keys, as a hash table.

In this hash table, keys are a strings with author, date, and
title of the reference.  Values are the cite keys.

Return nil if there are no bibliography files or no entries."
  ;; Populate bibliography cache.
  (let ((entries (org-cite-basic--parse-bibliography)))
    (cond
     ((null entries) nil)               ;no bibliography files
     ((gethash entries org-cite-basic--completion-cache)
      org-cite-basic--completion-cache)
     (t
      (clrhash org-cite-basic--completion-cache)
      (dolist (key (org-cite-basic--all-keys))
        (let* ((entry (org-cite-basic--get-entry
                       key
                       ;; Supply pre-calculated bibliography to avoid
                       ;; performance degradation.
                       (list :cite-basic/bibliography entries)))
               (completion
                (concat
                 (let ((author (org-cite-basic--get-author entry nil 'raw)))
                   (if author
                       (truncate-string-to-width
                        (replace-regexp-in-string " and " "; " author)
                        org-cite-basic-author-column-end nil ?\s)
                     (make-string org-cite-basic-author-column-end ?\s)))
                 org-cite-basic-column-separator
                 (let ((date (org-cite-basic--get-year entry nil 'no-suffix)))
                   (format "%4s" (or date "")))
                 org-cite-basic-column-separator
                 (org-cite-basic--get-field 'title entry nil 'raw))))
          (puthash completion key org-cite-basic--completion-cache)))
      (unless (map-empty-p org-cite-basic--completion-cache) ;no key
        (puthash entries t org-cite-basic--completion-cache)
        org-cite-basic--completion-cache)))))

(defun org-cite-basic--complete-key (&optional multiple)
  "Prompt for a reference key and return a citation reference string.

When optional argument MULTIPLE is non-nil, prompt for multiple
keys, until one of them is nil.  Then return the list of
reference strings selected.

Raise an error when no bibliography is set in the buffer."
  (let* ((table
          (or (org-cite-basic--key-completion-table)
              (user-error "No bibliography set")))
         (prompt
          (lambda (text)
            (completing-read text table nil t))))
    (if (null multiple)
        (let ((key (gethash (funcall prompt "Key: ") table)))
          (org-string-nw-p key))
      (let* ((keys nil)
             (build-prompt
              (lambda ()
                (if keys
                    (format "Key (empty input exits) %s: "
                            (mapconcat #'identity (reverse keys) ";"))
                  "Key (empty input exits): "))))
        (let ((key (funcall prompt (funcall build-prompt))))
          (while (org-string-nw-p key)
            (push (gethash key table) keys)
            (setq key (funcall prompt (funcall build-prompt)))))
        keys))))

\f
;;; Register processor
(org-cite-register-processor 'basic
  :activate #'org-cite-basic-activate
  :export-citation #'org-cite-basic-export-citation
  :export-bibliography #'org-cite-basic-export-bibliography
  :follow #'org-cite-basic-follow
  :insert  (org-cite-make-insert-processor #'org-cite-basic--complete-key
                                           #'org-cite-basic--complete-style)
  :cite-styles
  '((("author" "a") ("caps" "c"))
    (("noauthor" "na") ("bare" "b"))
    (("nocite" "n"))
    (("note" "ft") ("bare-caps" "bc") ("caps" "c"))
    (("numeric" "nb"))
    (("text" "t") ("bare-caps" "bc") ("caps" "c"))
    (("nil") ("bare" "b") ("bare-caps" "bc") ("caps" "c"))))

(provide 'oc-basic)
;;; oc-basic.el ends here