避免在 If Else 中重复调用 Scala 中的同一函数

Question

我有以下我想优化的代码片段：

import org.joda.time.{DateTime, Days, Months, Weeks, Years}

  @scala.annotation.tailrec
  def optimize(start: DateTime, end: DateTime, acc: List[(DateTime, DateTime)]): List[(DateTime, DateTime)] = {

    def doDaysExist(startDate: DateTime, endDate: DateTime): Boolean = {
      Days.daysBetween(startDate, endDate).getDays != 0
    }

    if (Years.yearsBetween(start, end.plusDays(1)).getYears != 0) {
      val years = Years.yearsBetween(start, end.plusDays(1)).getYears
      val newStart = start.plusYears(years).minusDays(1)
      if (doDaysExist(newStart, end)) {
        optimize(newStart, end, (start, newStart) +: acc)
      } else (start, end) +: acc
    } else if (Months.monthsBetween(start, end.plusDays(1)).getMonths != 0) {
      val months = Months.monthsBetween(start, end.plusDays(1)).getMonths
      val newStart = start.plusMonths(months).minusDays(1)
      if (doDaysExist(newStart, end)) {
        optimize(newStart, end, (start, newStart) +: acc)
      } else (start, end) +: acc
    } else if (Weeks.weeksBetween(start, end.plusDays(1)).getWeeks != 0) {
      val weeks = Weeks.weeksBetween(start, end.plusDays(1)).getWeeks
      val newStart = start.plusWeeks(weeks).minusDays(1)
      if (doDaysExist(newStart, end)) {
        optimize(newStart, end, (start, newStart) +: acc)
      } else (start, end) +: acc
    } else {
      // For sure days
      (start, end) +: acc
    }
  }

可以看出，if 条件和后面的下一个语句是重复的代码。有没有更好的方法让它变得更好、更神秘？我必须逐步遍历这个层次结构，因为我必须首先检查年份，然后是月份，然后是星期，然后是日期。

我可以考虑将重复的代码块提取到 lambda 中，但这足够好吗？我无法考虑将其重写为 Map，因为该代码块在尾递归函数中运行。

Answer 1

像这样的事情怎么样：

def optimize(start: DateTime, end: DateTime, acc: List[(DateTime, DateTime)]): List[(DateTime, DateTime)] = {
  val endPlusDay = end.plusDays(1)
  
  val checks = List(
    {
      val years = Years.yearsBetween(start, endPlusDay).getYears
      val newStart = start.plusYears(years).minusDays(1)
 
      (years -> newStart)
    },
    {
      val months = Months.monthsBetween(start, endPlusDay).getMonths
      val newStart = start.plusMonths(months).minusDays(1)
 
      (months -> newStart)
    },
    {
      val weeks = Weeks.weeksBetween(start, endPlusDay).getWeeks
      val newStart = start.plusWeeks(weeks).minusDays(1)
 
      (weeks -> newStart)
    }
  )
  
  checks.collectFirst {
    case (difference, newStart) if (
      (difference != 0) && (Days.daysBetween(newStart, end) != 0)
    ) =>
      optimize(newStart, end, acc :+ (start, newStart))
  }.getOrElse(acc :+ (start, end))
}

PS：两个建议

1. 使用java.time而不是Joda。
2. 前置并最后反向到累加器而不是追加。

Answer 2

只需预先计算值并将第二个测试移到 if/else 之外：

val years = Years.yearsBetween(start, end.plusDays(1)).getYears
lazy val months = Months.monthsBetween(start, end.plusDays(1)).getMonths
lazy val weeks = Weeks.weeksBetween(start, end.plusDays(1)).getWeeks

val newStart =
  if (years != 0) {
    start.plusYears(years).minusDays(1)
  } else if (months != 0) {
    start.plusMonths(months).minusDays(1)
  } else if (weeks != 0) {
    start.plusWeeks(weeks).minusDays(1)
  } else {
    start
  }

if (doDaysExist(newStart, end)) {
  optimize(newStart, end, (start, newStart) +: acc)
} else {
  (start, end) +: acc
}

使用lazy 可能会也可能不会提高性能，因此请衡量以查看哪个是最好的。