export const PAGE_GAP = 24;
export const MAX_RENDERED_PAGES = 9;
export const MAX_CANVAS_PIXELS = 1_500_000;

export function buildPageLayout(
  count,
  width,
  ratios = {},
  defaultRatio = Math.SQRT2,
) {
  const offsets = [],
    heights = [];
  let totalHeight = 0;
  for (let index = 0; index < count; index++) {
    offsets.push(totalHeight);
    const height = width * (ratios[index + 1] || defaultRatio);
    heights.push(height);
    totalHeight += height + PAGE_GAP;
  }
  return {
    offsets,
    heights,
    totalHeight: Math.max(0, totalHeight - PAGE_GAP),
    count,
  };
}

export function pageAtOffset(layout, offset) {
  let low = 0,
    high = layout.count - 1;
  while (low < high) {
    const middle = Math.ceil((low + high) / 2);
    // Scroll offsets can round down by a fraction of a CSS pixel at browser zoom.
    if (layout.offsets[middle] <= offset + 0.5) low = middle;
    else high = middle - 1;
  }
  return low + 1;
}

export function readingPosition(layout, offset) {
  const page = pageAtOffset(layout, Math.max(0, offset));
  const height = layout.heights[page - 1] || 1;
  return {
    page,
    pageOffset: Math.max(
      0,
      Math.min(1, (offset - layout.offsets[page - 1]) / height),
    ),
  };
}

export function positionOffset(layout, position) {
  const page = Math.max(1, Math.min(layout.count, position.page));
  return (
    (layout.offsets[page - 1] || 0) +
    (position.pageOffset || 0) * (layout.heights[page - 1] || 0)
  );
}

export function navigationScrollTop(layout, position, padding, maximumTop) {
  return Math.max(
    0,
    Math.min(maximumTop, padding + positionOffset(layout, position)),
  );
}

export function renderedWindow(layout, top, height) {
  if (!layout.count) return { start: 1, end: 0 };
  const start = Math.max(1, pageAtOffset(layout, Math.max(0, top)) - 1);
  const end = Math.min(
    layout.count,
    pageAtOffset(layout, top + height) + 1,
    start + MAX_RENDERED_PAGES - 1,
  );
  return { start, end };
}

export function canvasPixelRatio(width, height, deviceRatio = 1) {
  return Math.min(
    deviceRatio,
    1.5,
    Math.sqrt(MAX_CANVAS_PIXELS / Math.max(1, width * height)),
  );
}