Day9a.rb

def neighbors(grid, x, y)
  ns = []
  ns << grid[y][x - 1] if x > 0
  ns << grid[y][x + 1] if x + 1 < grid[y].length
  ns << grid[y - 1][x] if y > 0
  ns << grid[y + 1][x] if y + 1 < grid.length
  ns
end

map = $<.map {|l| l.chomp.chars.map(&:to_i)}

risk = 0
map.each_with_index do |row, y|
  row.each_with_index do |height, x|
    ns = neighbors(map, x, y)
    next unless height < ns.min
    risk += height + 1
  end
end

print risk