Add a dedicated rotation command and keybinding

We need a dedicated command for rotation since when `pdf-tools`
redisplays the image, the `:rotation` image-property is lost.
Pdf-tools redisplays the images when changing pages, zooming in/out,
setting the slice, etc. so this is essential to have.

A variable is used simply because `pdf-tools` already uses a variable to
keep track of the current scaling.

* lisp/pdf-view.el (pdf-view--current-rotation): Add new variable to
keep track of angle.
(pdf-view-create-page): Account for above.
(pdf-view-mode-map): Add the new command under "R".
(pdf-view-rotate): Add new command.

Fixes: #152
Closes: #165

3 files changed, 26 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index a4bec6d..5abb815 100644
--- a/NEWS
+++ b/NEWS
@@ -5,6 +5,9 @@
   + This feature enabled us to change the default ~selection-style~ used in ~pdf-tools~ from =GLYPH= to =WORD=.
   + The change makes highlighting / selecting text *much* snappier. If you want to go back to the old behaviour of selecting by glyph instead of word, or if you want to select a whole line instead, customize ~pdf-view-selection-style~
 
+- Track rotation of pages and add a command to make rotation of pages easy @vizs #165
+  + The new command is bound to =R= in `pdf-view-mode-map`.
+
 ** Functionality fixes and improvements
 - Fix: Saving a PDF when ~buffer-file-name~ is missing will now prompt for a filename (eg: in EWW) @akater #178
 
diff --git a/README.org b/README.org
index c4306b9..961a75a 100644
--- a/README.org
+++ b/README.org
@@ -288,6 +288,8 @@ Note that ~pdf-tools~ renders the PDF as images inside Emacs. This means that al
 | Trim Margins (set slice to bounding box) | ~s b~           |
 | Reset Margins                            | ~s r~           |
 | Reset Zoom                               | ~0~             |
+| Rotate Page                              | ~R~             |
+|------------------------------------------+-----------------|
 
 ** Annotations
 :PROPERTIES:
diff --git a/lisp/pdf-view.el b/lisp/pdf-view.el
index 454effa..afca46f 100644
--- a/lisp/pdf-view.el
+++ b/lisp/pdf-view.el
@@ -226,6 +226,9 @@ regarding display of the region in the later function.")
 (defvar-local pdf-view--hotspot-functions nil
   "Alist of hotspot functions.")
 
+(defvar-local pdf-view--current-rotation nil
+  "Current rotation of the page.")
+
 (defvar-local pdf-view-register-alist nil
   "Local, dedicated register for PDF positions.")
 
@@ -290,6 +293,8 @@ regarding display of the region in the later function.")
     (define-key map (kbd "s m")       'pdf-view-set-slice-using-mouse)
     (define-key map (kbd "s b")       'pdf-view-set-slice-from-bounding-box)
     (define-key map (kbd "s r")       'pdf-view-reset-slice)
+    ;; Rotation.
+    (define-key map (kbd "R")              #'pdf-view-rotate)
     ;; Reconvert
     (define-key map (kbd "C-c C-c")   'doc-view-mode)
     (define-key map (kbd "g")         'revert-buffer)
@@ -586,6 +591,21 @@ For example, (pdf-view-shrink 1.25) decreases size by 20%."
   (setq pdf-view-display-size 1.0)
   (pdf-view-redisplay t))
 
+
+;; * ================================================================== *
+;; * Rotation
+;; * ================================================================== *
+(defun pdf-view-rotate (angle)
+  "Rotate the current page by ANGLE degrees clockwise.
+When called interactively, angle defaults to 90.  Moreover, if
+called interactively with a prefix argument, then rotate
+anti-clockwise."
+  (interactive (list (if current-prefix-arg -90 90)))
+  (setq-local pdf-view--current-rotation
+              (mod (+ (or pdf-view--current-rotation 0)
+                      angle)
+                   360))
+  (pdf-view-redisplay t))
 
 
 ;; * ================================================================== *
@@ -974,6 +994,7 @@ See also `pdf-view-use-imagemagick'."
                     window page size)))
     (pdf-view-create-image data
       :width (car size)
+      :rotation (or pdf-view--current-rotation 0)
       :map hotspots
       :pointer 'arrow)))