Xmas tree kata

Tree.scala

package xmas

object Tree {
  def apply(n: Int) = {
    val width = n * 2 - 1
    def center(line: String) = {
      val padding = " " * ((width - line.length) / 2)
      padding ++ line ++ padding
    }.mkString
    def line(row: Int) = row * 2 + 1
    center("*") +: Seq.tabulate(n) { row => center("0" * line(row)) }
  }
}

TreeTest.scala

package xmas

import org.scalatest.matchers.MustMatchers
import org.scalatest.FlatSpec

class TreeTest extends FlatSpec with MustMatchers{

  "An empty tree" must "have just a star" in {
    Tree(0) must equal (Seq("*"))
  }

  "A minimal tree" must "have a star and a zero" in {
    Tree(1) must equal (Seq("*", "0"))
  }

  "A 2-tree" must "have 3 rows and 3 columns" in {
    Tree(2) must equal (Seq(" * ",
                            " 0 ",
                            "000"))
  }

  "A 5-tree" must "have 6 rows and 9 columns" in {
    Tree(5) must equal (Seq(
    "    *    ",
    "    0    ",
    "   000   ",
    "  00000  ",
    " 0000000 ",
    "000000000"
    ))
  }
}