advent_day_2.rb

# http://adventofcode.com/day/2 - part 1
#
# present_dimensions - String a list of present dimensions, each in the format of
# 1x2x3 where 1 is the length, 2 is the width and 3 is the height,
# in feet.
#
# Returns an integer of the area of wrapping paper required, in square feet.
def required_wrapping_paper(present_dimensions)
  total_area = 0

  presents = present_dimensions.split("\n")

  presents.each do |present|
    length, width, height = present.split("x", 3).map(&:to_i)
    total_area += paper_for_one_gift(length, width, height)
  end

  total_area
end

# http://adventofcode.com/day/2 - part 2
#
# present_dimensions - String a list of present dimensions, each in the format of
# 1x2x3 where 1 is the length, 2 is the width and 3 is the height,
# in feet.
#
# Returns an integer of the length of wrapping paper required, in feet.
def required_ribbon(present_dimensions)
  total_length = 0

  presents = present_dimensions.split("\n")

  presents.each do |present|
    length, width, height = present.split("x", 3).map(&:to_i)
    total_length += ribbon_for_one_gift(length, width, height)
  end

  total_length
end

# determines the area of wrapping paper needed for one gift.
#
# length - Integer length, in feet.
# width - Integer width, in feet.
# height - Integer height, in feet.
#
# Returns an Integer in square feet.
def paper_for_one_gift(length, width, height)
  side1 = length * width
  side2 = width * height
  side3 = length * height

  2 * (side1 + side2 + side3) + [side1, side2, side3].min
end

# determines the length of ribbon needed for one gift.
#
# length - Integer length, in feet.
# width - Integer width, in feet.
# height - Integer height, in feet.
#
# Returns an Integer in feet.
def ribbon_for_one_gift(length, width, height)
  sizes = [length, width, height]

  bow_length = sizes.inject(:*)

  # drop the longest value, since the ribbon uses the shorest perimeter
  2 * ((sizes.sort[0, 2]).inject(:+)) + bow_length
end