Codility Omega 2013 - Falling Disks

falling_disks.rb

def falling_disks(rings, disks)
  min, disk = rings[0], 0

  rings.each_index do |ring|
    min = rings[ring] if rings[ring] < min
    rings[ring] = min if rings[ring] > min
  end

  (rings.size - 1).downto(0) do |ring|
    disk += 1 if disks[disk] <= rings[ring]
    return disk if disk == disks.size
  end
  disk
end

falling_disks_spec.rb

require 'spec_helper'
require_relative '../falling_disks'

describe '#falling_disks' do
  it 'passes the sanity check' do
    rings, disks = [5, 6, 4, 3, 6, 2, 3], [2, 3, 5, 2, 4]

    falling_disks(rings,disks).should == 4
  end

  it 'drops disks to the bottom' do
    rings, disks = [5, 5, 5], [4, 4, 4]

    falling_disks(rings,disks).should == 3
  end

  it 'only drops the number of rings that can fit' do
    rings, disks = [5, 5, 5], [4, 4, 4, 4]

    falling_disks(rings,disks).should == 3
  end

  it 'only drops disks that can fit' do
    rings, disks = [5, 3, 5], [4, 4, 4, 4]

    falling_disks(rings,disks).should == 1
  end

  it 'only drops as many disks as exist' do
    rings, disks = [5, 5, 5, 5], [4, 4, 4]

    falling_disks(rings,disks).should == 3
  end

  it 'only drops the first ring when only the first will fit' do
    rings, disks = [7, 6, 6, 6, 19, 7, 7, 6, 8, 13, 9, 12], [7, 3, 2, 5, 6]

    falling_disks(rings,disks).should == 1
  end

  it 'will not drop any disks when none will fit' do
    rings, disks  = [6, 15, 5, 17, 5, 20, 16, 17, 10, 13, 8, 10], [7, 4, 4, 3, 1]

    falling_disks(rings,disks).should == 0
  end
end