import { useCallback, useEffect, useRef, useState, type ReactNode } from "react";

type Props = {
  direction?: "horizontal" | "vertical";
  initialRatio?: number;
  storageKey?: string;
  minPx?: number;
  minSecondPx?: number;
  className?: string;
  children: [ReactNode, ReactNode];
};

function clamp(n: number, lo: number, hi: number) {
  return Math.min(hi, Math.max(lo, n));
}

export function SplitPane({
  direction = "vertical",
  initialRatio = 0.45,
  storageKey,
  minPx = 120,
  minSecondPx = minPx,
  className = "",
  children,
}: Props) {
  const [a, b] = children;
  const rootRef = useRef<HTMLDivElement | null>(null);
  const [size, setSize] = useState(0);
  const [ratio, setRatio] = useState(() => {
    if (storageKey) {
      const raw = localStorage.getItem(storageKey);
      const n = raw ? Number(raw) : NaN;
      if (Number.isFinite(n) && n > 0.1 && n < 0.9) return n;
    }
    return initialRatio;
  });
  const dragging = useRef(false);

  useEffect(() => {
    const root = rootRef.current;
    if (!root) return;
    const measure = () => setSize(direction === "vertical" ? root.clientHeight : root.clientWidth);
    const observer = new ResizeObserver(measure);
    observer.observe(root);
    measure();
    return () => observer.disconnect();
  }, [direction]);

  useEffect(() => {
    if (storageKey) localStorage.setItem(storageKey, String(ratio));
  }, [ratio, storageKey]);

  const onPointerMove = useCallback(
    (e: PointerEvent) => {
      if (!dragging.current || !rootRef.current) return;
      const rect = rootRef.current.getBoundingClientRect();
      if (direction === "vertical") {
        const y = e.clientY - rect.top;
        if (rect.height) setRatio(clamp(y / rect.height, minPx / rect.height, 1 - (minSecondPx + 8) / rect.height));
      } else {
        const x = e.clientX - rect.left;
        if (rect.width) setRatio(clamp(x / rect.width, minPx / rect.width, 1 - (minSecondPx + 8) / rect.width));
      }
    },
    [direction, minPx, minSecondPx],
  );

  const stop = useCallback(() => {
    dragging.current = false;
    document.body.style.cursor = "";
    document.body.style.userSelect = "";
  }, []);

  useEffect(() => {
    window.addEventListener("pointermove", onPointerMove);
    window.addEventListener("pointerup", stop);
    return () => {
      window.removeEventListener("pointermove", onPointerMove);
      window.removeEventListener("pointerup", stop);
    };
  }, [onPointerMove, stop]);

  const vert = direction === "vertical";
  // A stored ratio can become unusable after shrinking the window. Reserve room
  // for both panes and the separator at the current measured size.
  const firstSize = size ? Math.min(Math.max(size * ratio, Math.min(minPx, size / 2)), Math.max(0, size - minSecondPx - 8)) : undefined;
  return (
    <div
      ref={rootRef}
      className={`flex min-h-0 min-w-0 flex-1 ${vert ? "flex-col" : "flex-row"} ${className}`}
    >
      <div className="flex min-h-0 min-w-0 shrink-0 flex-col overflow-hidden" style={vert ? { height: firstSize ?? `${ratio * 100}%` } : { width: firstSize ?? `${ratio * 100}%` }}>
        {a}
      </div>
      <div
        role="separator"
        aria-orientation={vert ? "horizontal" : "vertical"}
        title="拖动调整；双击复位"
        onPointerDown={(e) => {
          e.preventDefault();
          dragging.current = true;
          document.body.style.cursor = vert ? "row-resize" : "col-resize";
          document.body.style.userSelect = "none";
        }}
        onDoubleClick={() => setRatio(initialRatio)}
        className={
          vert
            ? "group relative z-10 h-2 shrink-0 cursor-row-resize"
            : "group relative z-10 w-2 shrink-0 cursor-col-resize"
        }
        style={{ background: "transparent" }}
      >
        <div
          className={
            vert
              ? "absolute inset-x-0 top-1/2 h-px -translate-y-1/2 bg-[var(--border)] transition group-hover:h-1 group-hover:bg-[var(--accent)]"
              : "absolute inset-y-0 left-1/2 w-px -translate-x-1/2 bg-[var(--border)] transition group-hover:w-1 group-hover:bg-[var(--accent)]"
          }
        />
      </div>
      <div className="flex min-h-0 min-w-0 flex-1 flex-col overflow-hidden">{b}</div>
    </div>
  );
}