/**
 * Layout math for the BookView two-page reader. Pure, so the flip logic is
 * unit-testable and the component only does DOM measurement.
 *
 * The book renders one column per page. A spread shows `columnCount` pages
 * side by side; flipping advances one page (column) at a time by translating
 * the multicol content horizontally.
 */

/** Horizontal gap (px) between book pages: the book's spine. Keep it equal to
 * or larger than the reader's horizontal padding so adjacent pages never peek
 * into the page margins mid-flip. */
export const BOOK_COLUMN_GAP = 32;

/** Maximum width (px) of the whole two-page book window. On larger viewports
 * the book stops growing and centers itself, so the capped text blocks stay
 * close to the spine instead of drifting apart with huge gutters. Choose a
 * value whose page width (content/2) stays close to the text cap. */
export const BOOK_MAX_WIDTH = 1536;

/** Viewport width (px) below which the book falls back to a single page. */
export const BOOK_SINGLE_COLUMN_BREAKPOINT = 768;

/** Pages shown side by side at the given viewport width. */
export function bookColumnCount(viewportWidth: number): number {
  return viewportWidth > 0 && viewportWidth < BOOK_SINGLE_COLUMN_BREAKPOINT ? 1 : 2;
}

/** Distance between the left edges of consecutive pages. */
export function bookPagePitch(
  viewportWidth: number,
  columnCount: number,
  gap = BOOK_COLUMN_GAP,
): number {
  return (viewportWidth + gap) / columnCount;
}

/** Total number of pages the content spans, given the measured scrollWidth. */
export function bookPageCount(
  scrollWidth: number,
  viewportWidth: number,
  columnCount: number,
  gap = BOOK_COLUMN_GAP,
): number {
  if (viewportWidth <= 0 || scrollWidth <= 0) return 1;
  const pitch = bookPagePitch(viewportWidth, columnCount, gap);
  return Math.max(1, Math.ceil((scrollWidth + gap) / pitch));
}

/** Horizontal translation (positive px) that brings pageIndex's page into view. */
export function bookColumnOffset(
  pageIndex: number,
  viewportWidth: number,
  columnCount: number,
  gap = BOOK_COLUMN_GAP,
): number {
  return pageIndex * bookPagePitch(viewportWidth, columnCount, gap);
}

/**
 * Swipe intent for a touch drag on the book: -1 = previous page, 1 = next
 * page, 0 = no flip. Requires horizontal dominance (a vertical scroll gesture
 * never flips) and travel past a threshold proportional to the page width.
 * Swiping left reveals the next page; swiping right goes back.
 */
export function bookSwipeDirection(
  deltaX: number,
  deltaY: number,
  pageAreaWidth: number,
): -1 | 0 | 1 {
  if (Math.abs(deltaX) <= Math.abs(deltaY)) return 0;
  const threshold = Math.max(48, pageAreaWidth * 0.18);
  if (deltaX <= -threshold) return 1;
  if (deltaX >= threshold) return -1;
  return 0;
}