;;; layout — Pure CL Flexbox layout engine

(defpackage :cl-tty.layout
  (:use :cl)
  (:export
   #:layout-node #:make-layout-node
   #:layout-node-add-child #:layout-node-remove-child
   #:layout-node-children
   #:layout-node-x #:layout-node-y
   #:layout-node-width #:layout-node-height
   #:layout-node-direction
   #:compute-layout
   #:vbox #:hbox #:spacer
   ;; For tests
   #:layout-node-parent #:layout-node-fixed-width
   #:layout-node-fixed-height #:normalize-box
   #:box-edge))

(in-package :cl-tty.layout)

(defun normalize-box (spec)
  (cond ((null spec) (list :top 0 :right 0 :bottom 0 :left 0))
        ((numberp spec) (list :top spec :right spec :bottom spec :left spec))
        (t (loop with result = (list :top 0 :right 0 :bottom 0 :left 0)
                 for (key val) on spec by #'cddr
                 do (setf (getf result key) val)
                 finally (return result)))))

(defun box-edge (box edge)
  (or (getf box edge) 0))

(defclass layout-node ()
  ((parent   :initform nil :accessor layout-node-parent)
   (children :initform nil :accessor layout-node-children)
   (x :initform 0 :accessor layout-node-x)
   (y :initform 0 :accessor layout-node-y)
   (width :initform 0 :accessor layout-node-width)
   (height :initform 0 :accessor layout-node-height)
   (direction :initform :column :initarg :direction :accessor layout-node-direction)
   (grow :initform 0 :initarg :grow :accessor layout-node-grow)
   (shrink :initform 1 :initarg :shrink :accessor layout-node-shrink)
   (padding :initform (list :top 0 :right 0 :bottom 0 :left 0) :initarg :padding :accessor layout-node-padding)
   (margin :initform (list :top 0 :right 0 :bottom 0 :left 0) :initarg :margin :accessor layout-node-margin)
   (gap :initform 0 :initarg :gap :accessor layout-node-gap)
   (position-type :initform :relative :initarg :position-type :accessor layout-node-position-type)
   (position-offset :initform nil :initarg :position-offset :accessor layout-node-position-offset)
   (fixed-width :initform nil :initarg :width :accessor layout-node-fixed-width)
   (fixed-height :initform nil :initarg :height :accessor layout-node-fixed-height)))

(defun make-layout-node (&key direction grow shrink padding margin gap
                         position-type position-offset width height)
  (make-instance 'layout-node
    :direction (or direction :column)
    :grow (or grow 0) :shrink (or shrink 1)
    :padding (normalize-box padding) :margin (normalize-box margin)
    :gap (or gap 0)
    :position-type (or position-type :relative)
    :position-offset position-offset
    :width width :height height))

(defun layout-node-add-child (parent child)
  (setf (layout-node-parent child) parent)
  (setf (layout-node-children parent)
        (nconc (layout-node-children parent) (list child)))
  child)

(defun layout-node-remove-child (parent child)
  (setf (layout-node-parent child) nil)
  (setf (layout-node-children parent)
        (delete child (layout-node-children parent)))
  child)

;; ── Solver ─────────────────────────────────────────────────────

(defun distribute-sizes (children avail gap horizontal)
  "Compute child sizes given available space and gap.
HORIZONTAL is non-nil when distributing width (row layout).
Each child starts from its fixed size (if any). Remaining space
is distributed by grow ratio; overflow is reduced by shrink ratio.
Rounding errors are amortized across the first N children."
  (let* ((n (length children))
         (gap-total (* gap (max 0 (1- n))))
         (base (mapcar (lambda (c)
                         (or (if horizontal
                                 (layout-node-fixed-width c)
                                 (layout-node-fixed-height c))
                             0))
                       children))
         (base-total (reduce #'+ base))
         (remaining (- avail base-total gap-total))
         (grow-total (reduce #'+ (mapcar #'layout-node-grow children)))
         (shrink-total (reduce #'+ (mapcar #'layout-node-shrink children))))
    (let ((sizes (mapcar (lambda (c b)
                           (let ((sz b))
                             (when (and (plusp remaining) (plusp grow-total))
                               (incf sz (round (* remaining (/ (layout-node-grow c) grow-total)))))
                             (when (and (minusp remaining) (plusp shrink-total))
                               (decf sz (round (* (abs remaining) (/ (layout-node-shrink c) shrink-total)))))
                             (max 1 sz)))
                         children base)))
      ;; Distribute rounding remainder to first N children so that
      ;; the total of sizes exactly fills avail minus gap-total.
      ;; Only correct when grow or shrink was actually applied —
      ;; otherwise children keep their fixed sizes and may not fill space.
      (when (or (and (plusp remaining) (plusp grow-total))
                (and (minusp remaining) (plusp shrink-total)))
        (let ((delta (- avail gap-total (reduce #'+ sizes))))
          (when (/= delta 0)
            (loop :for i :from 0 :below (min (abs delta) n)
                  :do (incf (nth i sizes) (signum delta))))))
      sizes)))

(defun compute-layout (root available-width available-height)
  "Layout all children of ROOT within the given dimensions.
Recursively computes position and size for every node."
  (labels ((place-children (node x y max-w max-h)
             (let* ((children (layout-node-children node))
                    (is-row (eql (layout-node-direction node) :row))
                    (pl (box-edge (layout-node-padding node) :left))
                    (pt (box-edge (layout-node-padding node) :top))
                    (pr (box-edge (layout-node-padding node) :right))
                    (pb (box-edge (layout-node-padding node) :bottom))
                    (cw (max 0 (- max-w pl pr)))
                    (ch (max 0 (- max-h pt pb)))
                    (gap (layout-node-gap node))
                    (sizes (distribute-sizes children (if is-row cw ch) gap is-row)))
               ;; Position the node (content area starts at padding inset)
               (setf (layout-node-x node) (+ x pl)
                     (layout-node-y node) (+ y pt))
               ;; Place each child sequentially
               (loop :with pos = 0
                     :for child :in children
                     :for size :in sizes
                     :do (if is-row
                             (setf (layout-node-width child) size
                                   (layout-node-x child) (+ x pl pos)
                                   (layout-node-height child) ch
                                   (layout-node-y child) (+ y pt))
                             (setf (layout-node-height child) size
                                   (layout-node-y child) (+ y pt pos)
                                   (layout-node-width child) cw
                                   (layout-node-x child) (+ x pl)))
                         (place-children child
                                         (layout-node-x child)
                                         (layout-node-y child)
                                         (if is-row size cw)
                                         (if is-row ch size))
                         (incf pos (+ size gap)))
               ;; Compute own size from children
               (let ((last-child (car (last children))))
                 (if is-row
                     (setf (layout-node-width node)
                           (or (layout-node-fixed-width node)
                               (if last-child
                                   (+ (layout-node-x node)
                                      (layout-node-width last-child)
                                      pr)
                                   max-w))
                           (layout-node-height node)
                           max-h)
                     (setf (layout-node-height node)
                           (or (layout-node-fixed-height node)
                               (if last-child
                                   (let ((last-y (layout-node-y last-child))
                                         (last-h (layout-node-height last-child)))
                                     (+ last-y last-h pb))
                                   max-h))
                           (layout-node-width node)
                           max-w))))))
    (place-children root 0 0 available-width available-height)
    root))

;; ── Macros ─────────────────────────────────────────────────────

(defmacro vbox ((&key grow shrink padding margin gap width height) &body children)
  (let ((n (gensym)))
    `(let ((,n (make-layout-node :direction :column
                ,@(when grow `(:grow ,grow))
                ,@(when shrink `(:shrink ,shrink))
                ,@(when padding `(:padding ,padding))
                ,@(when margin `(:margin ,margin))
                ,@(when gap `(:gap ,gap))
                ,@(when width `(:width ,width))
                ,@(when height `(:height ,height)))))
       ,@(loop for c in children collect `(layout-node-add-child ,n ,c))
       ,n)))

(defmacro hbox ((&key grow shrink padding margin gap width height) &body children)
  (let ((n (gensym)))
    `(let ((,n (make-layout-node :direction :row
                ,@(when grow `(:grow ,grow))
                ,@(when shrink `(:shrink ,shrink))
                ,@(when padding `(:padding ,padding))
                ,@(when margin `(:margin ,margin))
                ,@(when gap `(:gap ,gap))
                ,@(when width `(:width ,width))
                ,@(when height `(:height ,height)))))
       ,@(loop for c in children collect `(layout-node-add-child ,n ,c))
       ,n)))

(defmacro spacer (&key grow)
  `(make-layout-node :grow ,(or grow 1)))