import { useEffect } from "react";

/** How close to the left edge a swipe must start to count as opening one. */
const EDGE_PX = 28;
/** How far it must travel sideways before it is a swipe rather than a tap. */
const TRAVEL_PX = 60;
/** And how straight: past this much vertical movement it is a scroll. */
const SLOPE = 1.2;

interface EdgeSwipe {
  enabled: boolean;
  isOpen: boolean;
  onOpen: () => void;
  onClose: () => void;
}

/**
 * Dragging in from the left edge shows the sidebar; dragging it back hides it.
 *
 * The gesture every app on a phone has, and the reason it has to be a gesture
 * here too: the button that opens the sidebar sits in the top corner, which is
 * the one part of a tall screen a thumb cannot reach.
 *
 * Three rules keep it from firing by accident, and each is there for something
 * that went wrong without it:
 *
 *   • it must **start at the edge**, or every sideways drag inside a note — a
 *     text selection, a table being scrolled — would open the sidebar;
 *   • it must travel far enough to be meant, or a tap that slides by two pixels
 *     counts;
 *   • it must be more sideways than downwards, or scrolling a long note with a
 *     thumb near the edge opens it constantly.
 *
 * Listening on `document` rather than on the sidebar: the gesture starts where
 * the sidebar is *not*, which is the whole point of it.
 */
export function useEdgeSwipe({ enabled, isOpen, onOpen, onClose }: EdgeSwipe): void {
  useEffect(() => {
    if (!enabled) return undefined;

    let startX = 0;
    let startY = 0;
    let tracking = false;

    const onStart = (e: TouchEvent) => {
      if (e.touches.length !== 1) return; // a pinch is not a swipe
      const t = e.touches[0];
      startX = t.clientX;
      startY = t.clientY;
      // Closed: only from the edge. Open: anywhere on the panel, because the
      // whole panel is the thing being pushed back.
      tracking = isOpen ? startX < window.innerWidth * 0.8 : startX <= EDGE_PX;
    };

    const onMove = (e: TouchEvent) => {
      if (!tracking || e.touches.length !== 1) return;
      const t = e.touches[0];
      const dx = t.clientX - startX;
      const dy = t.clientY - startY;
      if (Math.abs(dy) > Math.abs(dx) * SLOPE) {
        tracking = false; // a scroll, not a swipe
        return;
      }
      if (!isOpen && dx > TRAVEL_PX) {
        tracking = false;
        onOpen();
      } else if (isOpen && dx < -TRAVEL_PX) {
        tracking = false;
        onClose();
      }
    };

    const stop = () => {
      tracking = false;
    };

    // Passive: this never calls preventDefault — the page must keep scrolling
    // normally while the gesture is being judged.
    document.addEventListener("touchstart", onStart, { passive: true });
    document.addEventListener("touchmove", onMove, { passive: true });
    document.addEventListener("touchend", stop, { passive: true });
    document.addEventListener("touchcancel", stop, { passive: true });
    return () => {
      document.removeEventListener("touchstart", onStart);
      document.removeEventListener("touchmove", onMove);
      document.removeEventListener("touchend", stop);
      document.removeEventListener("touchcancel", stop);
    };
  }, [enabled, isOpen, onOpen, onClose]);
}