comparison.rb

class Comparison
  @@window = 64
  @@prelude = 12

  def self.window
    @@window
  end

  def self.window=(val)
    @@window = val
  end

  def self.prelude
    @@prelude
  end

  def self.prelude=(val)
    @@prelude = val
  end

  def initialize(expected, actual)
    @expected = expected
    @actual = actual
  end

  def same?
    @expected == @actual
  end

  def different_at
    if (@expected.nil? || @actual.nil?)
      0
    else
      i = 0
      while (i < @expected.size && i < @actual.size)
        if @expected[i] != @actual[i]
          break
        end
        i += 1
      end
      return i
    end
  end

  def message
    "Strings differ at position #{different_at}:\n" +
      "expected: #{chunk(@expected)}\n" +
      "  actual: #{chunk(@actual)}\n"
  end

  def chunk(s)
    prefix, middle, suffix = "...", "", "..."

    start = different_at - @@prelude
    if start < 0
      prefix = ""
      start = 0
    end

    stop = start + @@window
    if stop > s.size
      suffix = ""
      stop = s.size
    end

    [prefix, s[start...stop].inspect, suffix].join
  end
end

if Object.const_defined?(:Spec)
  module Spec
    module Matchers
      class OperatorMatcher
      end
      class PositiveOperatorMatcher < OperatorMatcher #:nodoc:
        def __delegate_operator(actual, operator, expected)
          if actual.__send__(operator, expected)
            true
          elsif expected.is_a?(String) && actual.is_a?(String) && (operator == '==')
            fail_with_message Comparison.new(expected, actual).message
          elsif ['==', '===', '=~'].include?(operator)
            fail_with_message("expected: #{expected.inspect},\n     got: #{actual.inspect} (using #{operator})")
          else
            fail_with_message("expected: #{operator} #{expected.inspect},\n     got: #{operator.gsub(/./, ' ')} #{actual.inspect}")
          end
        end
      end
    end
  end
end

comparison_spec.rb

here = File.dirname(__FILE__); require File.expand_path("#{here}/../spec_helper")

describe Comparison do
  before do
    @old_window = Comparison.window
    Comparison.window = 16
    @old_prelude = Comparison.prelude
    Comparison.prelude = 8
  end

  after do
    Comparison.window = @old_window
  end

  describe '#same?' do
    it "says identical empty strings are the same" do
      comparison = Comparison.new("", "")
      comparison.same?.should be_true
    end

    it "says identical non-empty strings are the same" do
      comparison = Comparison.new("abc", "abc")
      comparison.same?.should be_true
    end

    it "says two nils are the same" do
      comparison = Comparison.new(nil, nil)
      comparison.same?.should be_true
    end

    it "says a string is different from a different string" do
      comparison = Comparison.new("abc", "xyz")
      comparison.same?.should be_false
    end

    it "says a string is different from nil" do
      comparison = Comparison.new("abc", nil)
      comparison.same?.should be_false
    end

    it "says nil is different from a string" do
      comparison = Comparison.new(nil, "abc")
      comparison.same?.should be_false
    end
  end

  describe '#different_at' do
    describe "returns the location where two strings differ" do
      it "at the beginning of the strings" do
        Comparison.new("abc", "xyz").different_at.should == 0
      end

      it "at the middle of the strings" do
        Comparison.new("abc", "ayz").different_at.should == 1
      end

      it "when the expected string is longer" do
        Comparison.new("abcd", "abc").different_at.should == 3
      end

      it "when the actual string is longer" do
        Comparison.new("abc", "abcd").different_at.should == 3
      end

      it "with nil as the expected string" do
        Comparison.new(nil, "abc").different_at.should == 0
      end

      it "with nil as the actual string" do
        Comparison.new("abc", nil).different_at.should == 0
      end

    end
  end

  describe '#message' do
    it 'shows the whole of both strings when the difference is near the start' do
      Comparison.new("abc", "xyz").message.should ==
        "Strings differ at position 0:\n" +
          "expected: \"abc\"\n" +
          "  actual: \"xyz\"\n"
    end

    it 'shows ellipses when the difference is in the middle of a long string' do
      Comparison.new("abcdefghijklmnopqrstuvwxyz", "abcdefghijkl*nopqrstuvwxyz").message.should ==
        "Strings differ at position 12:\n" +
          "expected: ...\"efghijklmnopqrst\"...\n" +
          "  actual: ...\"efghijkl*nopqrst\"...\n"
    end

    it 'shows ellipses when the difference is near the beginning of a long string' do
      Comparison.new("abcdefghijklmnopqrstuvwxyz", "a*cdefghijklmnopqrstuvwxyz").message.should ==
        "Strings differ at position 1:\n" +
          "expected: \"abcdefghijklmnop\"...\n" +
          "  actual: \"a*cdefghijklmnop\"...\n"
    end

    it 'shows ellipses when the difference is near the end of a long string' do
      Comparison.new("abcdefghijklmnopqrstuvwxyz", "abcdefghijklmnopqrstuvw*yz").message.should ==
        "Strings differ at position 23:\n" +
          "expected: ...\"pqrstuvwxyz\"\n" +
          "  actual: ...\"pqrstuvw*yz\"\n"
    end

    it 'allows user to override the window size' do
      Comparison.window = 10
      Comparison.new("abcdefghijklmnopqrstuvwxyz", "a*cdefghijklmnopqrstuvwxyz").message.should ==
        "Strings differ at position 1:\n" +
          "expected: \"abcdefghij\"...\n" +
          "  actual: \"a*cdefghij\"...\n"
    end

    it 'allows user to override the prelude size' do
      Comparison.prelude = 2
      Comparison.new("abcdefghijklmnopqrstuvwxyz", "abcdefghijkl*nopqrstuvwxyz").message.should ==
        "Strings differ at position 12:\n" +
          "expected: ...\"klmnopqrstuvwxyz\"...\n" +
          "  actual: ...\"kl*nopqrstuvwxyz\"...\n"
    end

  end

  it "overrides RSpec's == matcher for strings" do
    lambda do
      "xyz".should == "abc"
    end.should raise_error(Spec::Expectations::ExpectationNotMetError, Comparison.new("abc", "xyz").message)
  end
end