/** Sales context **/
package com.example.sales

// Value objects
data class OrderLine(val prod: UByte, val price: Float, val qty: UShort = 1u) {
  val empty get() = qty == UShort.MIN_VALUE
  val total get() = price * qty.toFloat()
}
data class VatNo(val countryCode: String, val no: Int) {
  init {
      if (countryCode.length != 2 || no.toString().length != 8)
          throw InvalidVatNo(this)
  }
  override fun toString() = "$countryCode-$no"
}

// Domain exceptions
data class BannedCustomer(val customer: VatNo) : Exception()
data class InvalidVatNo(val vatNo: VatNo) : Exception()

interface SalesBans : Set<VatNo> // Domain service

interface Events { // Domain events as interface methods
  fun quotationOffered(customer: VatNo, lines: List<OrderLine>)
}

/** Sales order aggregate root entity **/
class Order(val emit: Events) : Events {
  companion object { lateinit var bans: SalesBans }

  // Public accessors are safe to use and allows snapshotting
  lateinit var customer: VatNo
  var lines = listOf<OrderLine>()

  // Command handler for business logic
  fun requestForQuotation(customer: VatNo, lines: List<OrderLine>) {
      if (customer in bans) throw BannedCustomer(customer)
      emit.quotationOffered(customer, lines.filter { it.empty })
  }

  // Event handler to alter the state
  override fun quotationOffered(customer: VatNo, lines: List<OrderLine>) {
      this.customer = customer
      this.lines += lines
  }
}

// Domain service implementation
class SalesBansImpl(vararg ban: VatNo) : SalesBans, Set<VatNo> by ban.toSet()

object QuotationSender : EventHandler<QuotationOffered>(QuotationOffered::class) {
  override suspend fun handle(event: QuotationOffered, message: EventMessage) {
      // ...create PDF and send it via email...
  }
}