const FRONTMATTER = /^---\n([\s\S]*?)\n---/

export interface Frontmatter {
  readonly raw: string
  readonly fields: Readonly<Record<string, unknown>>
}

/**
 * Splits a leading `---` block off a markdown source.
 *
 * `raw` keeps the delimiters so an index can be rewritten with its own
 * frontmatter untouched, including comments and key order that a parse and
 * re-emit would lose.
 */
export function parseFrontmatter(source: string): Frontmatter | undefined {
  const match = source.match(FRONTMATTER)
  if (!match) return undefined

  const parsed = Bun.YAML.parse(match[1])
  if (typeof parsed !== 'object' || parsed === null || Array.isArray(parsed)) {
    return undefined
  }

  return {
    raw: match[0],
    fields: parsed as Record<string, unknown>,
  }
}

export function readField(
  frontmatter: Frontmatter | undefined,
  key: string,
): string | undefined {
  const value = frontmatter?.fields[key]
  if (typeof value === 'string') return value
  if (typeof value === 'number' || typeof value === 'boolean') {
    return String(value)
  }
  return undefined
}

export async function readFrontmatter(
  path: string,
): Promise<Frontmatter | undefined> {
  return parseFrontmatter(await Bun.file(path).text())
}