"use client";

import { useEffect, useRef, useState, type KeyboardEvent, type ReactNode } from "react";
import { motion, useReducedMotion, type PanInfo } from "motion/react";

export type DragGridItem = {
  id: string;
  /** Read out by screen readers while the tile moves. */
  label: string;
  /** Not for links or buttons: the whole tile is the drag handle. */
  content: ReactNode;
};

type DragGridProps = {
  items: DragGridItem[];
  /** Called with the new order. Store it and pass it back as items. */
  onChange: (items: DragGridItem[]) => void;
  /** Smallest tile width in pixels. The grid fits as many columns as it can. */
  minSize?: number;
  /** Space between tiles in pixels. */
  gap?: number;
  /** Ring colour on the tile being moved. */
  accent?: string;
  /** Name read out by screen readers. */
  label?: string;
  /** Replaces the tile look. */
  className?: string;
};

const moved = (list: DragGridItem[], from: number, to: number) => {
  const next = [...list];
  next.splice(to, 0, next.splice(from, 1)[0]);
  return next;
};

const clamp = (n: number, max: number) => Math.min(Math.max(n, 0), max);

/* DragGrid: pick up a tile and the others slide out of its way. Space lifts a tile, arrows move it, Space drops it. */
export function DragGrid({
  items,
  onChange,
  minSize = 96,
  gap = 12,
  accent = "#a78bfa",
  label = "Sortable tiles",
  className = "aspect-square rounded-xl border border-white/10 bg-white/5 text-white",
}: DragGridProps) {
  const reduce = useReducedMotion();
  const grid = useRef<HTMLDivElement>(null);
  const [lifted, setLifted] = useState<string | null>(null);
  const [status, setStatus] = useState("");

  // React may move the DOM node on reorder, which drops focus.
  useEffect(() => {
    if (lifted) grid.current?.querySelector<HTMLElement>(`[data-id="${CSS.escape(lifted)}"]`)?.focus();
  }, [items, lifted]);

  const columns = () => (grid.current ? getComputedStyle(grid.current).gridTemplateColumns.split(" ").length : 1);

  // Slot comes from grid maths, not from the tiles, because they are mid-animation.
  const onDrag = (id: string, info: PanInfo) => {
    if (!grid.current) return;
    const box = grid.current.getBoundingClientRect();
    const cols = columns();
    const rows = Math.ceil(items.length / cols);
    const col = clamp(Math.floor((info.point.x - window.scrollX - box.left) / ((box.width + gap) / cols)), cols - 1);
    const row = clamp(Math.floor((info.point.y - window.scrollY - box.top) / ((box.height + gap) / rows)), rows - 1);
    const to = Math.min(row * cols + col, items.length - 1);
    const from = items.findIndex((item) => item.id === id);
    if (to !== from) onChange(moved(items, from, to));
  };

  const onKey = (e: KeyboardEvent, item: DragGridItem, index: number) => {
    if (e.key === " " || e.key === "Enter") {
      e.preventDefault();
      const up = lifted !== item.id;
      setLifted(up ? item.id : null);
      setStatus(`${item.label} ${up ? "lifted. Use arrow keys, then Space to drop." : `dropped at position ${index + 1}.`}`);
      return;
    }
    if (lifted !== item.id) return;
    if (e.key === "Escape") {
      setLifted(null);
      setStatus(`${item.label} dropped at position ${index + 1}.`);
      return;
    }
    const cols = columns();
    const step = { ArrowLeft: -1, ArrowRight: 1, ArrowUp: -cols, ArrowDown: cols }[e.key];
    if (!step) return;
    e.preventDefault();
    const to = index + step;
    if (to < 0 || to >= items.length) return;
    onChange(moved(items, index, to));
    setStatus(`${item.label}, position ${to + 1} of ${items.length}.`);
  };

  return (
    <div
      ref={grid}
      role="group"
      aria-label={label}
      className="grid"
      style={{ gap, gridTemplateColumns: `repeat(auto-fill, minmax(${minSize}px, 1fr))` }}
    >
      {items.map((item, i) => (
        <motion.div
          key={item.id}
          layout
          drag
          dragSnapToOrigin
          dragMomentum={false}
          onDrag={(_, info) => onDrag(item.id, info)}
          whileDrag={{ scale: 1.06, zIndex: 10, boxShadow: `0 0 0 2px ${accent}, 0 16px 32px rgb(0 0 0 / 0.4)` }}
          transition={reduce ? { duration: 0 } : { type: "spring", stiffness: 420, damping: 34 }}
          role="button"
          tabIndex={0}
          aria-pressed={lifted === item.id}
          aria-label={`${item.label}, position ${i + 1} of ${items.length}. Press Space to move.`}
          data-id={item.id}
          onKeyDown={(e) => onKey(e, item, i)}
          onBlur={() => setLifted((id) => (id === item.id ? null : id))}
          style={lifted === item.id ? { boxShadow: `0 0 0 2px ${accent}` } : undefined}
          className={`relative grid cursor-grab touch-none select-none place-items-center active:cursor-grabbing ${className}`}
        >
          {item.content}
        </motion.div>
      ))}
      <p role="status" className="sr-only">
        {status}
      </p>
    </div>
  );
}