group.rb

#!/usr/bin/env ruby

pictures = ARGV.map(&:to_i).sort
desired_group_size = ENV["GROUP_SIZE"].to_i
min_distance, max_distance = pictures.each_cons(2).map { |a, b| b - a }.minmax
groups = pictures.map { |picture| [picture] }

# Calculates distance factor using linear function.
#
# 1.0 when distance == min_distance
# ...
# 0.0 when distance == max_distance

def distance_factor(distance, min_distance, max_distance)
  minmax_distance = (max_distance - min_distance).to_f
  -distance / minmax_distance + max_distance / minmax_distance
end

# Calculates group size factor.
#
# 1.0 if the group_size does not exceed desired_group_size
# 0.5 if the group_size == desired_group_size + 1
# 0.25 if the group_size == desired_group_size + 2
# ...

def group_size_factor(group_size, desired_group_size)
  if group_size <= desired_group_size
    1.0
  else
    2.0 ** (desired_group_size - group_size)
  end
end

groups.each_cons(2) do |first, second|
  distance = second.first - first.last
  group_size = first.size + second.size

  cost = 1
  cost *= distance_factor(distance, min_distance, max_distance)
  cost *= group_size_factor(group_size, desired_group_size)

  if cost > rand
    # move content of the first group to the second group
    second.unshift(*first.shift(first.size))
  end
end

groups.delete_if(&:empty?)

puts groups.inspect