Test Problem

urinal.rb

class Urinal
  def initialize(occupied, line)
    @occupied = occupied
    @line = line
  end

  def select
    scores = stalls.map { |n| [score(n), n] }
    best = scores.sort.last
    best.first < 0 ? nil : best.last
  end

  private

  def score(n)
    return never_choose_this if occupied?(n)
    result = 1000
    result += apply_rule1(n)
    result += apply_rule2(n) if rule2_upheld?
    result += apply_rule3(n) if rule3_upheld?
    result
  end

  def apply_rule1(n)
    n
  end

  def apply_rule2(n)
    neighbors?(n) ? neighbor_penalty : 0
  end

  def rule2_upheld?
    @rule2_upheld ||= stalls.all? { |n| !occupied?(n) || !neighbors?(n) }
  end

  def apply_rule3(n)
    double_neighbors?(n) ? double_neighbor_penalty : 0
  end

  def rule3_upheld?
    @rule3_upheld ||= stalls.all? { |n| !occupied?(n) || !double_neighbors?(n) }
  end

  def neighbor_penalty
    line? ? -10 : -10000
  end

  def double_neighbor_penalty
    line? ? -20 : -20000
  end

  def neighbors?(n)
    occupied?(n-1) || occupied?(n+1)
  end

  def double_neighbors?(n)
    occupied?(n-1) && occupied?(n+1)
  end

  def occupied?(n)
    if n < 0 || n > last_stall
      nil
    else
      @occupied[n]
    end
  end

  def stalls
    (0..last_stall)
  end

  def last_stall
    @occupied.size-1
  end

  def line?
    @line
  end

  def never_choose_this
    -1_000_000_000_000_000_000
  end
end

urinal_spec.rb

require 'rspec/given'
require 'urinal'

describe Urinal do
  Given(:urinal) { Urinal.new(occupied, has_line) }

  Given(:stalls) { "....." }
  Given(:has_line) { false }

  Given(:occupied) { stalls.split(//).map { |s| s.downcase == 'x' } }
  Given(:outcome) { out = stalls.dup; out[selection] = 'o' if selection; out }

  When(:selection) { urinal.select }

  describe "Rule 1: farthest from the door" do
    context "when empty" do
      Then { outcome.should == '....o' }
    end
    context "when some occupied" do
      Given(:stalls) { "..x.." }
      Then { outcome.should == '..x.o' }
    end
  end

  describe "Rule 2: not next to a dude" do
    context "when given a choice" do
      Given(:stalls) { "....x" }
      Then { outcome.should == '..o.x' }
    end

    context "when there is not other choice" do
      Given(:stalls) { "x.x.x" }
      Then { outcome.should == 'x.x.x' }

      context "and when there is a line" do
        Given(:has_line) { true }
        Then { outcome.should == 'x.xox' }
      end

      context "and when rule 2 is broken" do
        Given(:stalls) { ".x.x.xx" }
        Then { outcome.should == 'ox.x.xx' }
      end
    end
  end

  describe "Rule 3: really don't stand next to two dudes" do
    Given(:has_line) { true }

    context "when given a choice" do
      Given(:stalls) { ".x.x" }
      Then { outcome.should == 'ox.x' }
    end

    context "and when rule 2 is broken" do
      Given(:stalls) { ".x.xxx" }
      Then { outcome.should == '.xoxxx' }
    end
  end
end