Find prime triples whose sum approximates the given value

find-prime-triple.rb

# Given a sum, it returns prime numbers that approximately add up to the sum.
# Example: 100 -> [29, 31, 37] (sums to 97)
require 'prime'

def find_prime_triple(target)
  return [2, 3, 5] if target <= 4
  
  middle_prime = nil
  i = 0
  while !middle_prime do
    if Prime.prime?(target + i)
      middle_prime = target + i
    elsif Prime.prime?(target - i)
      middle_prime = target - i
    else
      i += 1
    end
  end

  i = 2
  while !Prime.prime?(middle_prime - i) do
    i += 2
  end
  left_prime = middle_prime - i

  i = 2
  while !Prime.prime?(middle_prime + i) do
    i += 2
  end
  right_prime = middle_prime + i

  return [left_prime, middle_prime, right_prime]
end

size = ARGV[0].to_i
if size > 0
  puts find_prime_triple(size / 3)
else
  puts "Usage: ruby find-prime-triple.rb SUM"
end