;;; el-exiftool.el --- Elisp wrapper around ExifTool -*- lexical-binding: t -*-

;; Elisp wrapper around ExifTool
;; Copyright (C) 2017 by Arun I
;;
;; Author: Arun I <arunisaac@systemreboot.net>
;; Version: 0.1
;; Keywords: data
;; Homepage: https://git.systemreboot.net/el-exiftool
;; Package-Requires: ((emacs "25"))

;; This file is part of el-exiftool.

;; el-exiftool 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.

;; el-exiftool 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 el-exiftool.  If not, see <http://www.gnu.org/licenses/>.

;;; Commentary:
;;
;; el-exiftool is an elisp wrapper around ExifTool.  ExifTool supports
;; reading and writing metadata in various formats including EXIF, XMP
;; and IPTC.
;;
;; There is a significant overhead in loading ExifTool for every
;; command to be exected.  So, el-exiftool starts an ExifTool process
;; in the -stay_open mode, and passes all commands to it.  For more
;; about ExifTool's -stay_open mode, see
;; http://www.sno.phy.queensu.ca/~phil/exiftool/#performance

;;; Code:

(require 'subr-x)
(require 'cl)

(defun el-exiftool--tq-sync-query (tq question regexp)
  "Add a transaction to transaction queue TQ, block and read response.

See `tq-enqueue' for details of arguments QUESTION and REGEXP."
  (let ((response))
    (tq-enqueue tq question regexp nil
		(lambda (closure answer) (setq response answer)))
    (while (not response)
      (accept-process-output))
    response))

(defun el-exiftool-run ()
  "Start an exiftool process if one is not already running.

If an exiftool process is already running, delete it, and create
a new one.  Return the process object of the newly created
process."
  (when-let (exiftool (get-process "exiftool"))
    (delete-process exiftool))
  (start-process "exiftool" "exiftool" "exiftool" "-stay_open" "True" "-@" "-"))

(let ((tq (tq-create (el-exiftool-run))))
  (defun el-exiftool-command (&rest args)
    "Execute a command in the currently running exiftool process.

ARGS are arguments of the command to be run, as provided to the
exiftool command line application."
    (string-trim
     (let ((suffix "{ready}\n"))
       (string-remove-suffix
	suffix (el-exiftool--tq-sync-query
		tq (concat (string-join args "\n")
			   "\n-execute\n")
		suffix))))))

(defun el-exiftool-read (file &rest tags)
  "Read TAGS from FILE, return an alist mapping TAGS to values.

If a tag is not found, return an empty string \"\" as the
value. If no TAGS are specified, read all tags from FILE.

\(fn FILE TAG...)"
  (mapcar
   (lambda (line)
     (cl-destructuring-bind
	 (tag value) (split-string line ": ")
       (let ((value (if (equal value "-") "" value)))
	 (cons tag value))))
   (split-string
    (apply 'el-exiftool-command
	   "-s" "-s" "-f"
	   (append
	    (mapcar (apply-partially 'format "-%s") tags)
	    (list file)))
    "\n+")))

(defun el-exiftool-copy (source destination &rest tags)
  "Copy TAGS from SOURCE file to DESTINATION file.

If no TAGS are specified, copy all tags from SOURCE."
  (apply 'el-exiftool-command
	 "-overwrite_original"
	 "-tagsFromFile" source
	 (append
	  (mapcar (apply-partially 'format "-%s") tags)
	  (list "-all:all" destination)))
  (message "Tags from %s copied to %s" source destination)
  destination)

(defun el-exiftool-write (file &rest tag-value-alist)
  "Write tags to FILE.

The metadata to be written is specified as (TAG . VALUE)
pairs.  Specifying the empty string \"\" for VALUE deletes that
TAG.

\(fn FILE (TAG . VALUE)...)"
  (apply 'el-exiftool-command
	 "-m" "-overwrite_original"
	 (append
	  (mapcar
	   (cl-function
	    (lambda ((tag . value))
	      (format "-%s=%s" tag value)))
	   tag-value-alist)
	  (list file))))

(provide 'el-exiftool)

;;; el-exiftool.el ends here