Given a pyramid of positive integers, calculate the min sum that can be created by traversing the pyramid and selecting either the left or right element on the next row.

min_pyramid_path.rb

#!/usr/bin/env ruby

# Construct a Pyramid structure
# from text, given the following format
#
# 1
# 1 2
# 1 2 3
# 1 2 3 4
class Pyramid

  attr_reader :size

  def initialize(text)
    @data = text.split("\n").map do |line|
      line.split(" ").map(&:to_i)
    end
    @size = @data.length
  end

  def row(i)
    @data[i]
  end

  def min_path
    sums = @data.clone # Cloning in case we choose to reuse the data

    # Start from the bottom!
    (@size - 2).downto(0).each do |j|
      (0...row(j).length).each do |i|
        sums[j][i] += [sums[j + 1][i], sums[j + 1][i + 1]].min
      end
    end
    # The first element in the sums will be the correct solution since
    # we've been progressively adding elements as we head up the pyramid
    sums[0][0]
  end

  def to_s
    "Pyramid:\n" << @data.map { |line| line.join " " }.join("\n")
  end
end

def calculate_paths_for_files(paths)
  paths.each do |path|
    if File.exists? path
      text = File.read path
      pyramid = Pyramid.new text
      puts pyramid
      puts "Min Path: #{pyramid.min_path}"
    end
    puts ""
  end
end

if __FILE__ == $0
  calculate_paths_for_files ARGV
end

pyramid1.txt

1
1 2
1 2 3
1 2 3 4
1 2 3 4 5

pyramid2.txt

29
51 14
68 17 57
50 27 29 48
16 69 87 40 31
93 38 53 60 4 23