sales.reduceLeft(reduceSalesFunction)

def reduceSalesFunction: (Sale, Sale) => Sale= {
     ???
  }

def getUpdatedSale(sales: List[Sale]): Sale = {
    // if just a single sale, get it
    if (sales.size == 1) sales.iterator.next

    else {
      // get the oldest sale
      val sortedSales = sales.sortWith(_.timestamp < _.timestamp)
      val oldestSale: Sale = sortedSales.min
      val salesMetrics: Metrics = oldestSale.metrics

      // update the oldest sale's metrics
      for (sale <- sortedSales) {
        val metrics: Metrics = sale.metrics
        if (metrics.isMetric_1) salesMetrics.setIsMetric_1(metrics.isMetric_1)
        if (metrics.isMetric_2) salesMetrics.setIsMetric_2(metrics.isMetric_2)
        if (metrics.isMetric_3) salesMetrics.setIsMetric_3(metrics.isMetric_3)
      }

      // if there are metrics with `false` values set the oldest sale's metrics to `false` as well (Metrics is an Enum)
      for (metric <- Metrics.values(); if !salesMetrics.isSet(metric)) {
        salesMetrics.setMetricValue(metric, false)
      }
      oldestSale
    }
  }