elevation

elevation.rb

class Elevation
  attr_reader :elevations

  def initialize(elevations)
    @elevations = elevations
    highest = 0
    @highest_lefts = @elevations.map { |elevation|
      highest = Math.max(highest, elevation)
    }

    highest = 0
    @highest_rights = @elevations.reverse.map { |elevation|
      highest = Math.max(highest, elevation)
    }.reverse
  end

  def highest_left(n)
    @highest_lefts[n]
  end

  def highest_right(n)
    @highest_rights[n]
  end

  def depth(n)
    high_left = highest_left(n)
    high_right = highest_right(n)
    water_height = Math.min(high_left, high_right)
    water_height - elevations[n]
  end
end

elevation_test.rb

require File.expand_path('../../test_helper',  __FILE__)

ELEVATIONS = [1, 3, 5, 3, 2, 4, 3, 6, 1]
#             0  1  2  3  4  5  6  7  8
class ElevationTest < ActiveSupport::TestCase
  should "be constructed" do
    elevation = Elevation.new(ELEVATIONS)
  end
  should "store data" do
    elevation = Elevation.new(ELEVATIONS)
    assert_equal(ELEVATIONS, elevation.elevations)
  end

  should "compute highest_left" do
    elevation = Elevation.new(ELEVATIONS)
    assert_equal 5, elevation.highest_left(3)
  end

  should "compute highest_right" do
    elevation = Elevation.new(ELEVATIONS)
    assert_equal 6, elevation.highest_right(3)
  end

  should "compute highest_right at edge" do
    elevation = Elevation.new(ELEVATIONS)
    assert_equal 1, elevation.highest_right(8)
  end

  should "compute depth" do
    elevation = Elevation.new(ELEVATIONS)
    assert_equal 2, elevation.depth(3)
  end
end