Ranges

ranges.scala

def ranges(ints: Seq[Int]): Seq[(Int, Int)] = ranges(ints.sorted.distinct, Seq())
def ranges(ints: Seq[Int], pairs: Seq[(Int, Int)]): Seq[(Int, Int)] = {
  if (ints.isEmpty) pairs.reverse
  else {
    val (drop, take) = ints.zipWithIndex.partition{case (n,i) => (n == ints.head+i)}
    ranges(take.unzip._1, (drop.unzip._1.head, drop.unzip._1.last) +: pairs)
  }
}

assert(ranges(Seq()) == Seq()) // empty
assert(ranges(Seq(1)) == Seq((1,1))) // single value
assert(ranges(Seq(1,2,3)) == Seq((1,3))) // single range
assert(ranges(Seq(1,2,3,9)) == Seq((1,3),(9,9))) // 1 range, 1 value
assert(ranges(Seq(1,2,3,7,8,9)) == Seq((1,3),(7,9))) // two ranges
assert(ranges(Seq(5,7,3)) == Seq((3,3),(5,5),(7,7))) // distinct values unordered
assert(ranges(Seq(6,4,5)) == Seq((4,6))) // range unordered
assert(ranges(Seq(3,3,4,5,4,5)) == Seq((3,5))) // duplicates