SudokuChecker.scala

object Sudoku {

  def main(args: Array[String]): Unit = {
    val solvedBoard = Seq(
      9,6,1,5,7,8,4,3,2,
      2,7,3,4,1,9,8,6,5,
      8,5,4,6,2,3,1,7,9,
      3,8,2,7,6,5,9,1,4,
      4,1,7,9,3,2,6,5,8,
      5,9,6,1,8,4,7,2,3,
      7,2,5,8,9,6,3,4,1,
      1,4,8,3,5,7,2,9,6,
      6,3,9,2,4,1,5,8,7
    )

    val unsolvedBoard = Seq(
      1,1,1,4,4,4,7,7,7,
      1,1,1,4,4,4,7,7,7,
      1,1,1,4,4,4,7,7,7,
      2,2,2,5,5,5,8,8,8,
      2,2,2,5,5,5,8,8,8,
      2,2,2,5,5,5,8,8,8,
      3,3,3,6,6,6,9,9,9,
      3,3,3,6,6,6,9,9,9,
      3,3,3,6,6,6,9,9,9
    )

    println(isSolved(solvedBoard))
    println(isSolved(unsolvedBoard))
  }

  def isSolved(values: Seq[Int]): Boolean = {
    require(values.size == 81, "Board must contain 81 cells")

    def solvedVec(v: Vector[Int]) = v.distinct.sorted == Vector.range(1, 10)
    def squareIndices(sr:Int, sc:Int) = for {
      i <- 0 until 3
      j <- 0 until 3
    } yield (sr*3+i,sc*3+j)

    val board = values.toVector.grouped(9).toVector
    def square(sr: Int, sc: Int) = squareIndices(sr, sc).toVector.map { case(r, c) => board(r)(c) }
    val pairs = for {
      r <- 0 until 3
      c <- 0 until 3
    } yield (r, c)

    val rowsSolved = board.forall(solvedVec)
    val columnsSolved = board.transpose.forall(solvedVec)
    val squaresSolved = pairs.forall { case(r, c) => solvedVec(square(r, c)) }

    rowsSolved && columnsSolved && squaresSolved
  }
}