Two way merge (sentinel)

twowaymerge.rb (w/ sentinel)

leads to less code.

<<-DOC
  Merge two sorted lists in ascending order (non-descending)
  with a sentinel. Sentinel is just a markup on the original
  lists that's a stop-indicator so that we don't rely on checking
  of the length to stow away the remaining items on the output list.
DOC
module Twowaymerge
  def merge (a,b)
    index_a = 0
    index_b = 0
    c=[]

    a.push(Float::MAX)
    b.push(Float::MAX)

    while (index_a < a.length and index_b < b.length) do
      if a[index_a] <= b[index_b] then
         c << a[index_a]
         index_a=index_a+1
      else
         c << b[index_b]
         index_b=index_b+1
      end
    end
    c.pop
    c
  end
end

Testing:

irb -r ./twowaymerge.rb
irb(main):001:0> include Twowaymerge
=> Object
irb(main):002:0> a = [2, 6, 10]
=> [2, 6, 10]
irb(main):003:0> b = [3, 9, 18]
=> [3, 9, 18]
irb(main):004:0> Twowaymerge.merge(a,b)
=> [2, 3, 6, 9, 10, 18]

Testing is the most important part of coding any algorithm. 

Main test scenarios are:

max(a) < min(b)
max(b) < min (a)
max (a) > min (b)
max (b) > min(a)