Get an official GBP-PLN currency exchange rates on the pay-dates using NBP API. Runnable using scala-cli command.

GetRates.scala

//> using scala "2.13.8"
//> using lib "com.typesafe.play::play-ahc-ws-standalone:2.1.3"
//> using lib "com.typesafe.play::play-ws-standalone-json:2.1.3"

import java.time.LocalDate
import java.time.format.DateTimeFormatter

import akka.actor.ActorSystem
import play.api.libs.ws._
import play.api.libs.ws.ahc._
import play.api.libs.json._
import play.api.libs.ws.JsonBodyReadables._

import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.duration.Duration
import scala.concurrent.{Await, Future}
import com.fasterxml.jackson.core.JsonParseException

object GetRates extends App {

  val year = args(0).toInt
  
  println(s"NBP GBP-PLN exchange rates in year $year")

  def getRate(currency: String, date: String): Future[BigDecimal] = {
    val url =
      s"http://api.nbp.pl/api/exchangerates/rates/a/$currency/$date/"
    client
      .url(url)
      .get()
      .map { response =>
        (response.body[JsValue] \ "rates" \ 0 \ "mid").as[BigDecimal]
      }
      .recover { case e: JsonParseException =>
        0
      }
  }

  implicit val system = ActorSystem("GetRates")

  val client = StandaloneAhcWSClient()

  val startDate = LocalDate.of(year, 1, 1)

  val firstDayOfWeek = startDate.getDayOfWeek().getValue()

  val firstFriday =
    if (firstDayOfWeek > 5) startDate.plusDays(12 - firstDayOfWeek)
    else startDate.plusDays(5 - firstDayOfWeek)

  println(
    s"First Friday of the year ${firstFriday.format(DateTimeFormatter.ISO_DATE)}"
  )

  val endDate = LocalDate.of(year, 12, 31)

  val lastFriday =
    endDate.minusDays((endDate.getDayOfWeek().getValue() + 2) % 7)
  println(
    s"Last Friday of the year ${lastFriday.format(DateTimeFormatter.ISO_DATE)}"
  )

  val numberOfWeeks =
    (lastFriday.getDayOfYear() - firstFriday.getDayOfYear()) / 7
  println(s"Number of weeks $numberOfWeeks")

  val rates = Await.result(
    Future.sequence(
      (0 until numberOfWeeks)
        .scanLeft(firstFriday)((a, i) => a.plusDays(7))
        .map(date =>
          getRate("gbp", DateTimeFormatter.ISO_DATE.format(date))
            .map(rate => (date, rate))
        )
    ),
    Duration.create("1m")
  )

  println("Found rates:")

  rates.foreach { case (date, rate) =>
    println(s"${date.format(DateTimeFormatter.ISO_DATE)}\t$rate")
  }

  System.exit(0)
}