nearest_to.rb

#!/usr/bin/env ruby

class Array
  def nearest_to(given)
    if given < min
      min
    elsif given > max
      max
    else
      low, high = map do |r|
        [r, self[index(r)+1]]
      end.detect { |x,y| y and (x..y).include?(given) }

      up   = high  - given
      down = given - low

      (up <= down) ? high : low
    end
  end
end

list = [1,5,20]

puts "list is #{list.inspect}"

[0,2,3,7,15,30].each do |given|
  puts "nearest to #{given}: #{list.nearest_to given}"
end