(defun snapshot-timemachine--osx-timemachine-parse-date (date)
  "Parse a DATE like \"2016-05-27-140240\" as a time object."
  (let* ((two "\\([0-9][0-9]\\)")
         (time-re
          (format "\\([0-9]\\{4\\}\\)-%s-%s-%s%s%s" two two two two two)))
    (cl-macrolet ((m (n) `(string-to-number (match-string ,n date))))
      (string-match time-re date)
      (encode-time (m 6) (m 5) (m 4) (m 3) (m 2) (m 1)))))


(defun snapshot-timemachine-osx-timemachine-snapshot-finder (file)
  "Find snapshots of FILE made by Time Machine (OS X only).
Uses the tmutil utility to get a list of Time Machine backups.

For example, say FILE is
\"/home/thomas/.emacs.\"

And tmutil listbackups returns:
\"/Volumes/TM/Backups.backupdb/mac/2015-12-23-120613\"
\"/Volumes/TM/Backups.backupdb/mac/2016-01-03-151443\"
...

The snapshots of the file will be
\"/Volumes/TM/Backups.backupdb/mac/2015-12-23-120613/ssd/home/thomas/.emacs\"
\"/Volumes/TM/Backups.backupdb/mac/2016-01-03-151443/ssd/home/thomas/.emacs\"
...

Note the presence of \"ssd\", the name of the boot disk."
  ;; TODO check if Time Machine is enabled, not just if we're on OS X
  (if (not (executable-find "tmutil"))
      (error "Time Machine not found")
    (let ((backup-list (shell-command-to-string "tmutil listbackups"))
          (id 0)
          (file (expand-file-name file)) ;; e.g., replace ~ with /Users/user
          ;; TODO currently hardcoded
          (boot-disk "ssd"))
      (cl-loop for snapshot-dir in (split-string backup-list "\n" t)
               for date-str = (file-name-nondirectory snapshot-dir)
               for date = (snapshot-timemachine--osx-timemachine-parse-date
                           date-str)
               for path = (format "%s/%s%s" snapshot-dir boot-disk file)
               when (file-exists-p path)
               collect (make-snapshot
                        :id (cl-incf id)
                        :name (number-to-string id)
                        :file path
                        :date date)))))

(setq snapshot-timemachine-snapshot-finder
      #'snapshot-timemachine-osx-timemachine-snapshot-finder)