/**
 * Shared drag-and-drop helpers for tab chips. The payload (which tab, from
 * which tile) travels as a private MIME type so unrelated drags are ignored.
 */
export const TAB_DND_MIME = 'application/x-textile-tab';

export type TabDragPayload = { tileId: string; tabId: string };

export function setTabDrag(dt: DataTransfer, payload: TabDragPayload): void {
  dt.setData(TAB_DND_MIME, JSON.stringify(payload));
  dt.effectAllowed = 'move';
}

export function hasTabDrag(dt: DataTransfer): boolean {
  return dt.types.includes(TAB_DND_MIME);
}

export function readTabDrag(dt: DataTransfer): TabDragPayload | null {
  const raw = dt.getData(TAB_DND_MIME);
  if (!raw) return null;
  try {
    return JSON.parse(raw) as TabDragPayload;
  } catch {
    return null;
  }
}

/**
 * Drop zone within a tile body. Edges split the tile (a new tile on that side);
 * center moves the tab into the tile.
 */
export type DropZone = 'center' | 'left' | 'right' | 'top' | 'bottom';

/** Pointer must be within this fraction of an edge to pick that edge's zone. */
const EDGE_FRACTION = 0.25;

/**
 * Classify a pointer position (relative to a tile body of `width`×`height`)
 * into a drop zone: the nearest edge when within `EDGE_FRACTION` of it, else
 * center. Pure for testing.
 */
export function computeDropZone(
  x: number,
  y: number,
  width: number,
  height: number,
): DropZone {
  if (width <= 0 || height <= 0) return 'center';
  const left = x / width;
  const right = 1 - left;
  const top = y / height;
  const bottom = 1 - top;
  const nearest = Math.min(left, right, top, bottom);
  if (nearest >= EDGE_FRACTION) return 'center';
  if (nearest === left) return 'left';
  if (nearest === right) return 'right';
  if (nearest === top) return 'top';
  return 'bottom';
}