;;; exif.el --- parsing Exif data in JPEG images -*- lexical-binding: t -*-

;; Copyright (C) 2019 Free Software Foundation, Inc.

;; Author: Lars Magne Ingebrigtsen <larsi@gnus.org>
;; Keywords: images

;; 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:

;; Specification at:

;; https://www.media.mit.edu/pia/Research/deepview/exif.html
;; but it's kinda er not very easy to read.

;; The JPEG format is:
;;
;; FFD8 and then any number of chunks on the format: FFxx SSSS ...,
;; where FFxx is the ID, and SSSS is the length of the chunk plus 2.
;; When you get to ID FFDA, the image itself is over and you can stop
;; parsing.
;;
;; The Exif data is in the TIFF format.  It starts off with the six
;; bytes "Exif^0^0".
;;
;; Then either "II" or "MM", where "II" means little-endian and "MM"
;; means big-endian.  All subsequent numbers should be read in
;; according to this.
;;
;; Next follows two bytes that should always represent 0x2a, and then
;; four bytes that's the offset to where the IFD "image file
;; directory" starts.  (It's an offset from the start of this chunk;
;; i.e., where "II"/"MM" is; all offsets in the TIFF format are from
;; this point.)
;;
;; The IFD starts with two bytes that says how many entries there are
;; in the directory, and then that number of entries follows, and then
;; an offset to the next IFD.

;; Usage: (exif-parse "test.jpg") =>
;; ((:tag 274 :tag-name orientation :format 3 :format-type short :value 1)
;;  (:tag 282 :tag-name x-resolution :format 5 :format-type rational :value
;;        (180 . 1))
;;  (:tag 306 :tag-name date-time :format 2 :format-type ascii
;;   :value "2019:09:21 16:22:13")
;;   ...)

;;; Code:

(require 'cl-lib)

(defvar exif-tag-alist
  '((11 processing-software)
    (271 make)
    (272 model)
    (274 orientation)
    (282 x-resolution)
    (283 y-resolution)
    (296 resolution-unit)
    (305 software)
    (306 date-time))
  "Alist of tag values and their names.")

(defun exif-parse (file)
  "Parse FILE (a JPEG file) and return the Exif data, if any.
The return value is a list of Exif items."
  (when-let ((app1 (cdr (assq #xffe1 (exif--parse-jpeg file)))))
    (exif--parse-exif-chunk app1)))

(defun exif--parse-jpeg (file)
  (with-temp-buffer
    (set-buffer-multibyte nil)
    (insert-file-contents-literally file)
    (unless (= (exif--read-number-be 2) #xffd8) ; SOI (start of image)
      (error "Not a valid JPEG file"))
    (cl-loop for segment = (exif--read-number-be 2)
             for size = (exif--read-number-be 2)
             ;; Stop parsing when we get to SOS (start of stream);
             ;; this is when the image itself starts, and there will
             ;; be no more chunks of interest after that.
             while (not (= segment #xffda))
             collect (cons segment (exif--read-chunk (- size 2))))))

(defun exif--parse-exif-chunk (data)
  (with-temp-buffer
    (set-buffer-multibyte nil)
    (insert data)
    (goto-char (point-min))
    ;; The Exif data is in the APP1 JPEG chunk and starts with
    ;; "Exif\0\0".
    (unless (equal (exif--read-chunk 6) (string ?E ?x ?i ?f ?\0 ?\0))
      (error "Not a valid Exif chunk"))
    (delete-region (point-min) (point))
    (let* ((endian-marker (exif--read-chunk 2))
           (le (cond
                ;; "Motorola" is big-endian.
                ((equal endian-marker "MM")
                 nil)
                ;; "Intel" is little-endian.
                ((equal endian-marker "II")
                 t)
                (t
                 (error "Invalid endian-ness %s" endian-marker)))))
      ;; Another magical number.
      (unless (= (exif--read-number 2 le) #x002a)
        (error "Invalid TIFF header length"))
      (let ((offset (exif--read-number 2 le)))
        ;; Jump to where the IFD (directory) starts and parse it.
        (goto-char (1+ offset))
        (exif--parse-directory le)))))

(defun exif--field-format (number)
  (cl-case number
    (1 (cons 'byte 1))
    (2 (cons 'ascii 1))
    (3 (cons 'short 2))
    (4 (cons 'long 4))
    (5 (cons 'rational 8))
    (otherwise (cons 'unknown 1))))

(defun exif--parse-directory (le)
  (let ((dir
         (cl-loop repeat (exif--read-number 2 le)
                  for tag = (exif--read-number 2 le)
                  for format = (exif--read-number 2 le)
                  for field-format = (exif--field-format format)
                  ;; The actual length is the number in this field
                  ;; times the "inherent" length of the field format
                  ;; (i.e., "long integer" (4 bytes) or "ascii" (1
                  ;; byte)).
                  for length = (* (exif--read-number 4 le)
                                  (cdr field-format))
                  for value = (exif--read-number 4 le)
                  collect (list :tag tag
                                :tag-name (cadr (assq tag exif-tag-alist))
                                :format format
                                :format-type (car field-format)
                                :value (exif--process-value
                                        (if (> length 4)
                                            ;; If the length of the data
                                            ;; is more than 4 bytes, then
                                            ;; it's actually stored after
                                            ;; this directory, and the
                                            ;; value here is just the
                                            ;; offset to use to find the
                                            ;; data.
                                            (buffer-substring
                                             (1+ value) (+ (1+ value) length))
                                          ;; The value is stored
                                          ;; directly in the directory.
                                          value)
                                        (car field-format)
                                        le)))))
    (let ((next (exif--read-number 4 le)))
      (if (> next 0)
          ;; There's more than one directory; if so, jump to it and
          ;; keep parsing.
          (progn
            (goto-char (1+ next))
            (nconc dir (exif--parse-directory le)))
        ;; We've reached the end of the directories.
        dir))))

(defun exif--process-value (value type le)
  "Do type-based post-processing of the value."
  (cl-case type
    ;; Chop off trailing zero byte.
    ('ascii (substring value 0 (1- (length value))))
    ('rational (with-temp-buffer
                 (set-buffer-multibyte nil)
                 (insert value)
                 (goto-char (point-min))
                 (cons (exif--read-number 4 le)
                       (exif--read-number 4 le))))
    (otherwise value)))

(defun exif--read-chunk (bytes)
  "Return BYTES octets from the buffer and advance point that much."
  (prog1
      (buffer-substring (point) (+ (point) bytes))
    (forward-char bytes)))

(defun exif--read-number-be (bytes)
  "Read BYTES octets from the buffer as a chunk of big-endian bytes.
Advance point to after the read bytes."
  (let ((sum 0))
    (dotimes (_ bytes)
      (setq sum (+ (* sum 256) (following-char)))
      (forward-char 1))
    sum))

(defun exif--read-number-le (bytes)
  "Read BYTES octets from the buffer as a chunk of low-endian bytes.
Advance point to after the read bytes."
  (let ((sum 0))
    (dotimes (i bytes)
      (setq sum (+ (* (following-char) (expt 256 i)) sum))
      (forward-char 1))
    sum))

(defun exif--read-number (bytes lower-endian)
  "Read BYTES octets from the buffer with endianness determined by LOWER-ENDIAN.
Advance point to after the read bytes."
  (if lower-endian
      (exif--read-number-le bytes)
    (exif--read-number-be bytes)))

(provide 'exif)

;;; exif.el ends here