interface TocEntry {
  title: string
  url: string
  items?: TocEntry[]
}

interface FlattenedTocEntry {
  title: string
  url: string
  depth: number
}

export const flattenToc = (
  entries: TocEntry[] = [],
  depth = 0,
): FlattenedTocEntry[] =>
  entries.reduce<FlattenedTocEntry[]>(
    (acc, entry) =>
      acc.concat(
        { title: entry.title, url: entry.url, depth },
        flattenToc(entry.items, depth + 1),
      ),
    [],
  )