import { createCipheriv, createDecipheriv, randomBytes, scryptSync } from 'crypto'

const ALGORITHM = 'aes-256-gcm'
const IV_LENGTH = 12
const TAG_LENGTH = 16

/**
 * Get the encryption key from environment.
 * Falls back to a dev-only key for local development.
 */
function getEncryptionKey(): Buffer {
  const envKey = process.env.PERSONNUMMER_ENCRYPTION_KEY
  if (!envKey) {
    if (process.env.NODE_ENV === 'production') {
      throw new Error('PERSONNUMMER_ENCRYPTION_KEY is required in production')
    }
    // Dev-only deterministic key (NOT safe for production)
    return scryptSync('dev-only-key', 'gnubok-dev-salt', 32)
  }
  // Use scrypt to derive a 32-byte key from the env var
  return scryptSync(envKey, 'gnubok-pnr-salt', 32)
}

/**
 * Encrypt a personnummer for storage.
 * Returns a hex string: iv + ciphertext + authTag
 */
export function encryptPersonnummer(personnummer: string): string {
  const key = getEncryptionKey()
  const iv = randomBytes(IV_LENGTH)
  const cipher = createCipheriv(ALGORITHM, key, iv)

  let encrypted = cipher.update(personnummer, 'utf8', 'hex')
  encrypted += cipher.final('hex')
  const authTag = cipher.getAuthTag()

  return iv.toString('hex') + encrypted + authTag.toString('hex')
}

/**
 * Decrypt a personnummer from storage.
 */
export function decryptPersonnummer(encrypted: string): string {
  const key = getEncryptionKey()
  const ivHex = encrypted.slice(0, IV_LENGTH * 2)
  const authTagHex = encrypted.slice(-TAG_LENGTH * 2)
  const ciphertext = encrypted.slice(IV_LENGTH * 2, -TAG_LENGTH * 2)

  const iv = Buffer.from(ivHex, 'hex')
  const authTag = Buffer.from(authTagHex, 'hex')

  const decipher = createDecipheriv(ALGORITHM, key, iv)
  decipher.setAuthTag(authTag)

  let decrypted = decipher.update(ciphertext, 'hex', 'utf8')
  decrypted += decipher.final('utf8')
  return decrypted
}

/**
 * Extract the last 4 digits of a personnummer for display.
 */
export function extractLast4(personnummer: string): string {
  const digits = personnummer.replace(/\D/g, '')
  return digits.slice(-4)
}

/**
 * Validate a Swedish personnummer (12-digit format: YYYYMMDDNNNN).
 * Checks format + Luhn checksum on last 10 digits.
 */
export function validatePersonnummer(personnummer: string): { valid: boolean; error?: string } {
  const digits = personnummer.replace(/\D/g, '')

  if (digits.length !== 12) {
    return { valid: false, error: 'Personnummer måste vara 12 siffror (ÅÅÅÅMMDDNNNN)' }
  }

  const year = parseInt(digits.slice(0, 4))
  const month = parseInt(digits.slice(4, 6))
  const day = parseInt(digits.slice(6, 8))

  if (year < 1900 || year > 2100) {
    return { valid: false, error: 'Ogiltigt år' }
  }
  if (month < 1 || month > 12) {
    return { valid: false, error: 'Ogiltig månad' }
  }
  if (day < 1 || day > 31) {
    return { valid: false, error: 'Ogiltig dag' }
  }

  // Luhn check on digits 3-12 (YYMMDDNNNN, 10 digits)
  const luhnDigits = digits.slice(2)
  if (!luhnCheck(luhnDigits)) {
    return { valid: false, error: 'Ogiltigt kontrollnummer (Luhn)' }
  }

  return { valid: true }
}

/**
 * Luhn checksum validation for 10-digit string.
 */
function luhnCheck(digits: string): boolean {
  let sum = 0
  for (let i = 0; i < digits.length; i++) {
    let d = parseInt(digits[i])
    // Multiply every other digit by 2, starting from the first
    if (i % 2 === 0) {
      d *= 2
      if (d > 9) d -= 9
    }
    sum += d
  }
  return sum % 10 === 0
}

/**
 * Extract birth date from a 12-digit personnummer.
 */
export function extractBirthDate(personnummer: string): { year: number; month: number; day: number } {
  const digits = personnummer.replace(/\D/g, '')
  return {
    year: parseInt(digits.slice(0, 4)),
    month: parseInt(digits.slice(4, 6)),
    day: parseInt(digits.slice(6, 8)),
  }
}

/**
 * Calculate age at a given date from a personnummer.
 */
export function calculateAge(personnummer: string, atDate: string): number {
  const birth = extractBirthDate(personnummer)
  const [refYear, refMonth, refDay] = atDate.split('-').map(Number)

  let age = refYear - birth.year
  if (refMonth < birth.month || (refMonth === birth.month && refDay < birth.day)) {
    age--
  }
  return age
}

/**
 * Calculate age at the start of a given year.
 * Used for avgifter age tier determination.
 */
export function calculateAgeAtYearStart(personnummer: string, year: number): number {
  return calculateAge(personnummer, `${year}-01-01`)
}

/**
 * Mask personnummer for display: YYYYMMDD-XXXX (birthdate visible, suffix hidden).
 */
export function maskPersonnummer(personnummer: string): string {
  const digits = personnummer.replace(/\D/g, '')
  return `${digits.slice(0, 8)}-XXXX`
}

/**
 * Format personnummer with dash: YYYYMMDD-NNNN
 */
export function formatPersonnummer(personnummer: string): string {
  const digits = personnummer.replace(/\D/g, '')
  return `${digits.slice(0, 8)}-${digits.slice(8)}`
}