Solution for Palantir Interview Challenge

magic_box.rb

def read_input
  line = STDIN.gets
  m, n = line.split(" ").map{|s| s.to_i}

  fingerprints = []
  m.times do
      fingerprints << STDIN.gets.chomp
  end
  return fingerprints
end

def count_occurences(fingerprints)
  counts = Hash.new 0
  fingerprints.each do |fingerprint|
    counts[fingerprint] += 1
  end
  return counts
end

def inverse(f)
  fingerprint = f.dup
  replacements = [ ["P", "X"], ["T", "P"], ["X", "T"] ]
  replacements.each {|replacement| fingerprint.gsub!(replacement[0], replacement[1])}
  return fingerprint
end

def sum_inverses(counts)
  new_counts = Hash.new 0
  counts.each do |fingerprint, count|
    inverse_counts = counts[inverse(fingerprint)]
    new_counts[fingerprint] = count
    if inverse_counts
      new_counts[fingerprint] += inverse_counts
    end
  end
  return new_counts
end

def largest_hash_key(hash)
  hash.max_by{|k,v| v}
end

def output_result(new_counts)
  STDOUT.write(largest_hash_key(new_counts)[1])
end

fingerprints = read_input
counts = count_occurences(fingerprints)
new_counts = sum_inverses(counts)
output_result(new_counts)