substring.js

// Finds the longest common starting substring in an array of strings
function common_substring(data) {
  var i, ch, memo, idx = 0
  do {
    memo = null
    for (i=0; i < data.length; i++) {
      ch = data[i].charAt(idx)
      if (!ch) break
      if (!memo) memo = ch
      else if (ch != memo) break
    }
  } while (i == data.length && idx < data.length && ++idx)

  return (data[0] || '').slice(0, idx)
}

common_substring(['interspecies', 'interstelar', 'interstate'])  //=> 'inters'
common_substring(['throne', 'throne'])                           //=> 'throne'
common_substring(['throne', 'dungeon'])                          //=> ''
common_substring(['cheese'])                                     //=> 'cheese'
common_substring([])                                             //=> ''
common_substring(['prefix', 'suffix'])                           //=> ''

substring.rb

# Finds the longest common starting substring in an array of strings
class Array
  def common_substring
    return self.first.to_s if self.length < 2
    idx = 0
    other = self.dup
    base = other.shift

    begin
      ch = base[idx, 1]
      match = self.all? { |str| str[idx, 1] == ch }
    end while match and idx < base.length and (idx += 1)
    
    base[0,idx]
  end
end

if __FILE__ == $0
  # in case this file is ran with `ruby -rubygems substring.rb`
  # (needs `gem install rspec`)
  require 'spec/autorun'
  
  describe '#common_substring' do
    
    def self.it(msg = nil, &block)
      if msg and !block and msg =~ /extract '(.*?)' from \[(.*?)\]/
        result, data = $1, $2.split(/\s*,\s*/)
        super(msg) { data.common_substring.should == result }
      else
        super
      end
    end
    
    it "should extract 'inters' from [interspecies, interstelar, interstate]"
    it "should extract 'throne' from [throne, throne]"
    it "should extract '' from [throne, dungeon]"
    it "should extract 'cheese' from [cheese]"
    it "should extract '' from []"
    it "should extract '' from [prefix, suffix]"

  end
end