export interface ParsedSubject {
  /** Filename from the quoted part of the subject. */
  name: string
  /** Segment number `k` from `(k/N)`. */
  segment: number
  /** Segment count `N` from `(k/N)`. */
  totalSegments: number
  /** File counter `[i/M]`, when present. */
  fileIndex?: number
  fileTotal?: number
  /**
   * The subject with the segment counter removed. Identical for every segment
   * of one uploaded file, so it is part of the file identity (see fileKey).
   */
  base: string
}

const SEGMENT_COUNTER = /\((\d+)\/(\d+)\)/g
const FILE_COUNTER = /\[(\d+)\/(\d+)\]/

/**
 * Parses a binary post subject such as
 * `[07/45] - "Some.Release.part06.rar" yEnc (12/137) 51315352`.
 *
 * Returns null for subjects without a quoted filename and segment counter
 * (obfuscated posts), which need their yEnc header read from the body instead.
 */
export function parseSubject(subject: string): ParsedSubject | null {
  const nameEnd = subject.lastIndexOf('"')
  const nameStart = nameEnd > 0 ? subject.lastIndexOf('"', nameEnd - 1) : -1
  if (nameStart === -1) return null
  const name = subject.slice(nameStart + 1, nameEnd)
  if (!name) return null

  // The segment counter is the last "(k/N)" after the filename; a file size may follow it.
  let counter: RegExpExecArray | undefined
  for (const match of subject.matchAll(SEGMENT_COUNTER)) {
    if (match.index > nameEnd) counter = match
  }
  if (!counter) return null

  const segment = Number.parseInt(counter[1], 10)
  const totalSegments = Number.parseInt(counter[2], 10)
  if (totalSegments < 1 || segment < 1 || segment > totalSegments) return null

  const base = (subject.slice(0, counter.index) + subject.slice(counter.index + counter[0].length))
    .replace(/\s+/g, ' ')
    .trim()

  const fileCounter = FILE_COUNTER.exec(subject.slice(0, nameStart))
  return {
    name,
    segment,
    totalSegments,
    fileIndex: fileCounter ? Number.parseInt(fileCounter[1], 10) : undefined,
    fileTotal: fileCounter ? Number.parseInt(fileCounter[2], 10) : undefined,
    base,
  }
}