#+TITLE: ScrollBox
#+STARTUP: content
#+FILETAGS: :cl-tty:container:

* Overview

ScrollBox is a container component that handles content larger than the
viewport. It provides scroll offsets, viewport culling (only renders
visible children), and scrollbar rendering.

* Implementation

#+BEGIN_SRC lisp :tangle ../src/components/scrollbox.lisp
(in-package #:cl-tty.container)

(defclass scroll-box (dirty-mixin)
  ((children :initform nil :initarg :children :accessor scroll-box-children :type list)
   (scroll-y :initform 0 :initarg :scroll-y :accessor scroll-box-scroll-y :type fixnum)
   (scroll-x :initform 0 :initarg :scroll-x :accessor scroll-box-scroll-x :type fixnum)
   (sticky-scroll-p :initform t :initarg :sticky-scroll-p :accessor sticky-scroll-p :type boolean)
   (layout-node :initform (make-layout-node) :accessor scroll-box-layout-node)))

(defun make-scroll-box (&key (children nil) (scroll-y 0) (scroll-x 0) sticky-scroll-p)
  (make-instance 'scroll-box
    :children children :scroll-y scroll-y :scroll-x scroll-x
    :sticky-scroll-p (if (null sticky-scroll-p) t sticky-scroll-p)))

(defmethod component-children ((sb scroll-box)) (scroll-box-children sb))
(defmethod component-layout-node ((sb scroll-box)) (scroll-box-layout-node sb))

(defun clamp-scroll (sb)
  (let* ((ln (scroll-box-layout-node sb))
         (viewport-h (if ln (layout-node-height ln) 0))
         (viewport-w  (if ln (layout-node-width ln) 0))
         (content-h (scroll-box-content-height sb))
         (content-w  (scroll-box-content-width sb)))
    (setf (scroll-box-scroll-y sb) (max 0 (min (scroll-box-scroll-y sb) (- content-h viewport-h))))
    (setf (scroll-box-scroll-x sb) (max 0 (min (scroll-box-scroll-x sb) (- content-w viewport-w))))))

(defun scroll-by (sb dy dx)
  (incf (scroll-box-scroll-y sb) dy) (incf (scroll-box-scroll-x sb) dx)
  (clamp-scroll sb) (mark-dirty sb))

(defun scroll-box-content-height (sb)
  (reduce #'+ (scroll-box-children sb)
          :key (lambda (c) (let ((ln (component-layout-node c))) (if ln (max 1 (layout-node-height ln)) 1)))
          :initial-value 0))

(defun scroll-box-content-width (sb)
  (reduce #'max (scroll-box-children sb)
          :key (lambda (c) (let ((ln (component-layout-node c))) (if ln (max 1 (layout-node-width ln)) 1)))
          :initial-value 0))

(defmethod render ((sb scroll-box) backend)
  (let* ((ln (scroll-box-layout-node sb))
         (vx 0) (vy 0)
         (vw (if ln (layout-node-width ln) 80))
         (vh (if ln (layout-node-height ln) 24))
         (sy (scroll-box-scroll-y sb))
         (sx (scroll-box-scroll-x sb)))
    (dolist (child (scroll-box-children sb))
      (let* ((cln (component-layout-node child)) (ch (if cln (layout-node-height cln) 1)) (cy vy))
        (when (and (< (- cy sy) vh) (> (+ (- cy sy) ch) 0))
          (let ((orig-x (if cln (layout-node-x cln) 0)) (orig-y (if cln (layout-node-y cln) 0)))
            (when cln (setf (layout-node-x cln) (- vx sx) (layout-node-y cln) (- vy sy)))
            (unwind-protect (render child backend)
              (when cln (setf (layout-node-x cln) orig-x (layout-node-y cln) orig-y)))))
        (incf vy ch)))
    (draw-scrollbars sb backend vw vh)))

(defun scrollbar-thumb (scroll-pos viewport-size content-size)
  (if (> content-size viewport-size) (/ (float scroll-pos) (- content-size viewport-size)) 0.0))

(defun draw-scrollbars (sb backend viewport-w viewport-h)
  (let* ((content-h (scroll-box-content-height sb)) (content-w (scroll-box-content-width sb))
         (sy (scroll-box-scroll-y sb)) (sx (scroll-box-scroll-x sb))
         (ln (scroll-box-layout-node sb)) (ox (if ln (layout-node-x ln) 0)) (oy (if ln (layout-node-y ln) 0)))
    (when (> content-h viewport-h)
      (let* ((thumb (scrollbar-thumb sy viewport-h content-h)) (thumb-pos (round (* thumb viewport-h))))
        (draw-rect backend (+ ox (1- viewport-w)) oy 1 viewport-h :bg :bright-black)
        (draw-text backend (+ ox (1- viewport-w)) (+ oy thumb-pos) "█" nil nil)))
    (when (> content-w viewport-w)
      (let* ((thumb (scrollbar-thumb sx viewport-w content-w)) (thumb-pos (round (* thumb viewport-w))))
        (draw-rect backend ox (+ oy (1- viewport-h)) viewport-w 1 :bg :bright-black)
        (draw-text backend (+ ox thumb-pos) (+ oy (1- viewport-h)) "█" nil nil)))))

(defun update-sticky-scroll (sb)
  (when (sticky-scroll-p sb)
    (let* ((content-h (scroll-box-content-height sb))
           (ln (scroll-box-layout-node sb)) (viewport-h (if ln (layout-node-height ln) 24)))
      (when (>= (scroll-box-scroll-y sb) (- content-h viewport-h 1))
        (setf (scroll-box-scroll-y sb) (max 0 (- content-h viewport-h)))))))
#+END_SRC