;;; center-content.el --- Center buffer content -*- lexical-binding: t; -*-

;;; Commentary:

;; This package provides a minor mode `center-content-mode' that centers the
;; content of the current buffer both horizontally and vertically.

;;; Code:

(require 'face-remap)

(defgroup center-content ()
  "Center buffer content."
  :group 'convenience
  :prefix "center-content-")

(defcustom center-content-horizontal t
  "Enable horizontal centering."
  :type 'boolean
  :group 'center-content)

(defcustom center-content-vertical t
  "Enable vertical centering."
  :type 'boolean
  :group 'center-content)

(defcustom center-content-vertical-by-overlay nil
  "Enable vertical by using overlays.

  Using overlays for vertical centering may give some undesirable
  effects on the first line. By default, vertical centering is achieved
  by using setting `header-line-format'."
  :type 'boolean
  :group 'center-content)

(defcustom center-content-hide-mode-line nil
  "Hide mode-line when centering."
  :type 'boolean
  :group 'center-content)

(defvar-local center-content--horizontal-overlay nil
  "Overlay used to add left margin for horizontal centering.")

(defvar-local center-content--vertical-overlay nil
  "Overlay used to add top margin for vertical centering.")

(defvar-local center-content--original-header-line-format header-line-format
  "Stores the original `header-line-format` to restore upon disabling.")

(defvar-local center-content--original-mode-line-format mode-line-format
  "Stores the original `mode-line-format` to restore upon disabling.")

(defvar-local center-content--header-line-face-remap-cookie nil
  "Cookie for the face remapping of `header-line`.")

(defun center-content--content-pixel-size ()
  "Return the size of the content of the visible text in pixels."
  (window-text-pixel-size (selected-window) (window-start) (window-end nil t)))

(defun center-content--calculate-left-margin (content-width)
  "Calculate the left margin relative to CONTENT-WIDTH."
  (let ((window-width (window-pixel-width))
        (column-width (string-pixel-width " ")))
    (/ (max 0 (- window-width content-width)) (* 2 column-width))))

(defun center-content--calculate-top-margin (content-height)
  "Calculate the top margin relative to CONTENT-HEIGHT."
  (let ((window-height (window-pixel-height))
        (line-height (default-line-height)))
    (/ (max 0 (- window-height content-height)) (* 2 line-height))))

(defun center-content--horizontal (content-width)
  "Add overlay for horizontal centering relative to CONTENT-WIDTH."
  (let* ((left-margin (center-content--calculate-left-margin content-width))
         (overlay (make-overlay (point-min) (point-max)))
         (space (propertize " " 'display `(space :width ,left-margin))))
    (overlay-put overlay 'line-prefix space)
    (setq center-content--horizontal-overlay overlay)))

(defun center-content--vertical (content-height)
  "Add overlay for vertical centering relative to CONTENT-HEIGHT."
  (let* ((top-margin (center-content--calculate-top-margin content-height)))
    ;; Save and remap the header-line face to match the default face
    (setq center-content--header-line-face-remap-cookie
          (face-remap-add-relative 'header-line 'default))
    (setq header-line-format (propertize " " 'display `(height ,top-margin)))))

(defun center-content--vertical-by-overlay (content-height)
  "Add overlay for vertical centering relative to CONTENT-HEIGHT."
  (let* ((top-margin (center-content--calculate-top-margin content-height))
         (overlay (make-overlay  (window-start) (window-start) nil t t))
         (space (propertize " " 'display `(space :width 0 :height ,top-margin))))
    (overlay-put overlay 'intangible t)
    (overlay-put overlay 'before-string space)
    (setq center-content--vertical-overlay overlay)))

(defun center-content--enable ()
  "Enable horizontal and vertical centering of content."
  (setq center-content--original-header-line-format header-line-format)
  (setq center-content--original-mode-line-format mode-line-format)
  (let* ((content-size (center-content--content-pixel-size)))
    (when center-content-horizontal
      (center-content--horizontal (car content-size)))
    (when center-content-vertical
      (if center-content-vertical-by-overlay
          (center-content--vertical-by-overlay (cdr content-size))
        (center-content--vertical (cdr content-size))))
    (when center-content-hide-mode-line
      (setq mode-line-format nil))))

(defun center-content--disable ()
  "Disable centering of content."
  ;; Remove horizontal centering overlay
  (when (overlayp center-content--horizontal-overlay)
    (delete-overlay center-content--horizontal-overlay)
    (setq center-content--horizontal-overlay nil))
  ;; Remove vertical centering overlay
  (when (overlayp center-content--vertical-overlay)
    (delete-overlay center-content--vertical-overlay)
    (setq center-content--vertical-overlay nil))
  ;; Restore the header-line face remapping
  (when center-content--header-line-face-remap-cookie
    (face-remap-remove-relative center-content--header-line-face-remap-cookie)
    (setq center-content--header-line-face-remap-cookie nil))
  ;; Restore the header-line-format
  (setq header-line-format center-content--original-header-line-format)
  ;; Restore the original mode-line-format
  (setq mode-line-format center-content--original-mode-line-format))

(defun center-content-update ()
  "Updates the centering if `center-content-mode' is enabled."
  (interactive)
  (when (bound-and-true-p center-content-mode)
    (center-content--disable)
    (center-content--enable)))

;;;###autoload
(define-minor-mode center-content-mode
  "Toggle horizontal and vertical centering of buffer content.

When enabled, the content of the buffer is centered both horizontally
and vertically within the window."
  :init-value nil
  :global nil
  :lighter " Center"
  (if center-content-mode
      (center-content--enable)
    (center-content--disable)))

(provide 'center-content)

;;; center-content.el ends here