My solution to the Hitting Rock Bottom puzzle (http://puzzlenode.com/puzzles/11-hitting-rock-bottom)

cave.rb

class Cave
  WATER = '~'
  ROCK = '#'
  AIR = ' '
  attr_reader :grid, :flow_points
  attr_accessor :water

  # Create a Cave from the given file.
  def initialize(filename)
    # Keep track of points where water can flow either down or to the right
    @flow_points = []
    lines = File.readlines(filename)
    # Amount of water that can flow into the cave
    @water = lines.shift.to_i - 1
    lines.shift
    row_count = 0
    # Build a grid for the cave.  Coordinate 0,0 is the top left
    @grid = lines.map do |line|
      line.chomp!
      row = line.split(//)
      if row.first == WATER
        # Seed the flow points by grabbing the first water point
        @flow_points << [row_count,0]
      end
      row_count += 1
      row
    end
  end

  # Spew the grid to a string
  def to_s
    ret = "\n"
    @grid.each do |row|
      ret << row.join('') << "\n"
    end
    ret
  end

  # Flow until there is no water left
  def run
    while water > 0
      flow
    end
    self
  end

  # Flow 1 unit of water
  def flow
    fp = @flow_points.last
    if blocked_at?(fp.first+1, fp.last)
      if @grid[fp.first][fp.last+1] == ROCK
        @flow_points.pop
        flow
      else
        new_row = fp.first
        new_col = fp.last + 1
        fp[0] = new_row
        fp[1] = new_col
        @grid[new_row][new_col] = WATER
        @water -= 1
      end
    else
      new_row = fp.first + 1
      new_col = fp.last
      @grid[new_row][new_col] = WATER
      @water -= 1
      @flow_points << [new_row, new_col]
    end
    self
  end

  # Return the water levels at each cave column
  def levels
    num_cols = @grid.first.size
    num_rows = @grid.size
    (0...num_cols).to_a.map do |col|
      sum = 0
      (0...num_rows).each do |row|
        if sum > 0 && @grid[row][col] == AIR
          sum = WATER
          break
        end
        sum += 1 if @grid[row][col] == WATER
      end
      sum
    end.join(" ")
  end

  private

  # Is the water blocked at this point
  def blocked_at?(row,col)
    [WATER, ROCK].include?(@grid[row][col])
  end
end

if __FILE__ == $0
  # Command line: ruby cave.rb <cave_input_file>
  cave = Cave.new(ARGV[0])
  cave.run
  puts "Final Cave State:\n#{cave}"
  puts "Solution:"
  puts cave.levels
end