Advent of Code Day 2 Part 2

#!/usr/bin/env ruby

# This program is a filter.
# Input file should be fed
# into standard input.

# Remove character at index from string
def puncture( string, index )
  substring1 = string[0,index]
  substring2 = string[index+1,string.length-index-1]
  return substring1 + substring2
end

# Read stdin file into array
codes = Array.new
i = 0
while codes[i] = gets
  i = i + 1
end
# Next line removes ending
# blank line if there is one
# because it tends to cause
# problems later on.
codes.pop if codes[-1].nil?
codes.each{ |code| code.strip! }

# Create hash table for each position
hashes = Array.new(26)
0.upto(25) do |i|
  hashes[i] = Hash.new
  codes.each do |code|
    p = puncture( code, i )
    hashes[i][p] = Array.new unless hashes[i].key?(p)
    hashes[i][p].push( code )
  end
end

# Find keys with multiple values
0.upto(25) do |i|
  hashes[i].each do |p|
    if p[1][1]
      puts puncture( p[1][0], i );
    end
  end
end