How much lava collects in valleys?

gistfile1.txt

#!/usr/bin/env ruby

points = [ 1, 2, 1, 4, 1, 3, 2, 9, 7, 5, 10, 1, 2, 1, 2, 1 ]

l_max = 0
r_max = 0
l_lava = 0
r_lava = 0
lava = 0
l_peak = 0
r_peak = points.count - 1

0.upto((points.count / 2) - 1) do |i|
  l = points[i]
  r = points[points.count - i - 1]

  if l < l_max then
    l_lava = l_lava + l_max - l
  else
    lava = lava + l_lava
    l_lava = 0
    l_max = l
    l_peak = i
  end

  if r < r_max then
    r_lava = r_lava + r_max - r
  else
    lava = lava + r_lava
    r_lava = 0
    r_max = r
    r_peak = points.count - i - 1
  end
end

crest = (l_max - r_max).abs
width = (l_peak - r_peak).abs - 1

lava = lava + (l_lava + r_lava - (crest * width))

puts "lava = #{lava}"