文字列比較

0_diff.scala

@annotation.tailrec
def diff(s1: Seq[Char], s2: Seq[Char], acc: StringBuilder = new StringBuilder()): String = {
  (s1, s2) match {
    case (Nil    , Nil)               => acc.toString
    case (x +: _ , Nil)               => acc.append(s"[$x][$$END$$]").toString
    case (Nil    , y +: _)            => acc.append(s"[$$END$$][$y]").toString
    case (x +: xs, y +: ys) if x == y => diff(xs, ys, acc.append(x))
    case (x +: _ , y +: _)            => acc.append(s"[$x][$y]").toString
  }
}

1_repl.scala

scala> diff("abc", "abcd")
res0: String = abc[$END$][d]

scala> diff("abcd", "abce")
res1: String = abc[d][e]

scala>

再帰なしで頑張ってみました。

def diff(s1: String, s2: String) = {
  val (matched, unmatched) = s1 zip s2 partition {case(c1, c2) => (c1 == c2)}
  if (s1.size == s2.size && s1.size == matched.size) s1
  else s1.substring(0,matched.size) + (
     (s1.size - s2.size) match {
       case x if x > 0 =>  s"[${s1.last}][$$END$$]" 
       case x if x < 0 =>  s"[$$END$$][${s2.last}]" 
       case _ => s"[${unmatched(0)._1}][${unmatched(0)._2}]"
    })
}

バグ修正＆ちょっとだけ整理……SMLかぶれ色出てるかも

def diff(s1: String, s2: String) = {
  val (matched, unmatched) = s1 zip s2 partition { case(c1, c2) => (c1 == c2) }
  matched.unzip._1.mkString + (
    (s1.size - s2.size) match {
      case x if x > 0 =>  s"[${matched.last._1}][$$END$$]"
      case x if x < 0 =>  s"[$$END$$][${matched.last._1}]"
      case _ if (s1.size == matched.size) => ""
      case _          => s"[${unmatched(0)._1}][${unmatched(0)._2}]"
    })
}```