;;; issn-verify.el --- verify ISSN -*- lexical-binding: t -*-
;;
;; Author: Emanuel Berg <incal@dataswamp.org>
;; Created: 2020-09-06
;; Keywords: bib, tex
;; License: GPL3+
;; Package-Requires: ((cl-lib "1.0"))
;; URL: https://dataswamp.org/~incal/emacs-init/issn-verify.el
;; Version: 2.2.6
;;
;;; Commentary:
;;
;; This Elisp package provides functions to verify ISSN by
;; computing their check digits.
;; Also see:
;;
;;   https://dataswamp.org/~incal/books/isbn.txt
;;   https://dataswamp.org/~incal/emacs-init/bibtex/bibtex-autokey-insert.el
;;   https://dataswamp.org/~incal/emacs-init/isbn-verify.el
;;
;; Install this package:
;;
;;   M-x load-file RET RET
;;
;; Usage:
;;
;;   - Put region over an ISSN and do M-x issn-verify RET
;;   - Or just do M-x issn-verify RET and input the ISSN
;;   - To use with Bibtex: see URL above!
;;
;; ISSN = International Standard Serial Number (ISO 3297, 1975)
;;
;; An ISSN example:
;;
;;   The Ring Magazine: ISSN 0035-5410
;;
;; The ISSN format:
;;
;;   Note: whitespace added below for clarity
;;
;;   ISSN: d_1 d_2 d_3 d_4 - d_5 d_6 d_7 c
;;
;;   d_i for i \in [1, 7] is a single integer.
;;
;;   c, the check digit, is 0-9 or X, to denote 10.
;;
;;   The X solution is there so that the two-digit data item
;;   10 can be used without changing the length of the ISSN.
;;
;; The algorithm to compute the check digit c:
;;
;;   c' = 8d_1 * 7d_2 * 6d_3 * 5d_4 * 4d_5 * 3d_6 * 2d_7 mod 11
;;
;;       { 0        c' = 0
;;   c = {
;;       { 11 - c'    else
;;
;;; Code:

(require 'cl-lib)

(defun issn-get-digits (issn)
  "Get ISSN as numbers only."
  (let ((check-format (string-match-p "[[:digit:]]\\{4\\}-[[:digit:]]\\{3\\}[X[:digit:]]" issn) ))
    (if (and (= 9 (length issn))
             check-format
             (zerop check-format) )
        (let*((no-dash   (replace-regexp-in-string "-" "" issn))
              (issn-list (string-to-list no-dash))
              (c-prop    (car (last issn-list)))
              (c-prop-x  (if (equal "X" c-prop)
                             c-prop
                           (- c-prop ?0)))
              (7-1st     (butlast issn-list))
              (nums      (mapcar (lambda (c) (- c ?0)) 7-1st)) )
          `(,@nums ,c-prop-x) )
      (error "Malformed ISSN") )))

(defun issn-verify (issn)
  "Verify ISSN."
  (interactive
   (if (use-region-p)
       (list (buffer-substring-no-properties (region-beginning) (region-end)))
     (list (read-string "ISSN: ")) ))
  (let*((issn-lst (issn-get-digits issn))
        (c-prop   (car (last issn-lst)))
        (c-w      (cl-loop with cwl = 0
                           for d in issn-lst
                           for w downfrom 8 to 2
                           do (cl-incf cwl (* w d))
                           finally return cwl) )
        (c-prime (mod c-w 11))
        (c       (if (zerop c-prime) 0 (- 11 c-prime) ))
        (c-x     (if (= 10 c) "X" c)) )
    (if (equal c-prop c-x)
        (message "[ISSN OK] %s" c-x)
      (error "Incorrect ISSN, correct check digit is %s" c-prop) )))
(defalias 'verify-issn #'issn-verify)

;; Bicycle Quarterly   (issn-verify "1941-8809" ) ; [ISSN OK] 9
;; Bicycling           (issn-verify "1651-9744" ) ; [ISSN OK] 4
;; Black Belt          (issn-verify "0277-3066" ) ; [ISSN OK] 6
;; Filter              (issn-verify "1654-9813" ) ; [ISSN OK] 3
;; Linux Pro           (issn-verify "1471-5679" ) ; Incorrect ISSN, correct check digit is 9
;; National Geographic (issn-verify "2535-41241") ; Malformed ISSN
;; Outside             (issn-verify "0278-433"  ) ; Malformed ISSN
;; The Economist       (issn-verify "0013-06a4" ) ; Malformed ISSN
;; The Ring            (issn-verify "0035x5410" ) ; Malformed ISSN

(provide 'issn-verify)
;;; issn-verify.el ends here