Improve ContainExactly matcher speed when elements obey transitivity

lib/rspec/matchers/built_in/contain_exactly.rb

@@ -36,6 +36,7 @@ def description
         def generate_failure_message
           message = expected_collection_line
           message += actual_collection_line
+          @extra_items, @missing_items = fast_calculate_extra_missing if @transitive
           message += missing_elements_line unless missing_items.empty?
           message += extra_elements_line unless extra_items.empty?
           message
@@ -72,15 +73,23 @@ def message_line(prefix, collection, surface_descriptions=false)
 
         def match(_expected, _actual)
           return false unless convert_actual_to_an_array
-          match_when_sorted? || (extra_items.empty? && missing_items.empty?)
+
+          matched_when_sorted = match_when_sorted?
+
+          return true if matched_when_sorted
+          return matched_when_sorted if @transitive
+
+          (extra_items.empty? && missing_items.empty?)
         end
 
         # This cannot always work (e.g. when dealing with unsortable items,
         # or matchers as expected items), but it's practically free compared to
         # the slowness of the full matching algorithm, and in common cases this
         # works, so it's worth a try.
         def match_when_sorted?
-          values_match?(safe_sort(expected), safe_sort(actual))
+          @transitive = true # Be optimistic. Let `safe_sort` reveal non-transitivity.
+          @sorted_expected, @sorted_actual = safe_sort(expected), safe_sort(actual)
+          values_match?(@sorted_expected, @sorted_actual)
         end
 
         def convert_actual_to_an_array
@@ -96,6 +105,7 @@ def convert_actual_to_an_array
         def safe_sort(array)
           array.sort
         rescue Support::AllExceptionsExceptOnesWeMustNotRescue
+          @transitive = false
           array
         end
 
@@ -124,6 +134,47 @@ def extra_items
           end
         end
 
+        # We use this to determine extra and missing items between expected
+        # and actual arrays.  This runs in O(n) time which is a big improvement
+        # over the O(n!) work incurred by PairingsMaximizer to evaluate all possible
+        # matchings between arrays
+        # rubocop:disable MethodLength
+        # rubocop:disable Metrics/AbcSize
+        def fast_calculate_extra_missing
+          extra, missing = [], []
+          i, j = 0, 0
+
+          # Use 2-pointer approach to find elements in sorted_actual
+          # that aren't in sorted_expected and vice versa
+          while i < @sorted_actual.size && j < @sorted_expected.size
+            current_actual, current_expected = @sorted_actual[i], @sorted_expected[j]
+
+            if current_actual < current_expected
+              extra << current_actual
+              i += 1
+            elsif current_actual > current_expected
+              missing << current_expected
+              j += 1
+            else
+              i += 1
+              j += 1
+            end
+          end
+
+          while i < @sorted_actual.size
+            extra << @sorted_actual[i]
+            i += 1
+          end
+          while j < @sorted_expected.size
+            missing << @sorted_expected[j]
+            j += 1
+          end
+
+          [extra, missing]
+        end
+        # rubocop:enable MethodLength
+        # rubocop:enable Metrics/AbcSize
+
         def best_solution
           @best_solution ||= pairings_maximizer.find_best_solution
         end

spec/rspec/matchers/built_in/contain_exactly_spec.rb

@@ -95,6 +95,77 @@ def array.send; :sent; end
 end
 
 RSpec.describe "using contain_exactly with expect" do
+  # users have reported using contains_exactly with 50 elements
+  # never finishing!
+  context "with transitive enabled" do
+    require 'benchmark'
+
+    context "with actual and expected containing sortable elements" do
+      shared_examples "runs very fast" do
+        it do
+          time = Benchmark.realtime do
+            subject
+          end
+          # this is in seconds
+          expect(time).to be < 0.3
+        end
+      end
+
+      let(:a) { Array.new(10_000) { rand(10) } }
+
+      context "with a positive expectation" do
+        subject { expect(a).to contain_exactly(*b) }
+
+        context "that is valid" do
+          let(:b) { a.shuffle }
+
+          it "matches" do
+            subject
+          end
+
+          include_examples "runs very fast"
+        end
+
+        context "that is not valid" do
+          let(:b) { Array.new(10_000) { rand(10) } }
+
+          it "fails quickly" do
+            time = Benchmark.realtime do
+              expect { subject }.to fail_with(/expected collection contained/)
+            end
+            expect(time).to be < 1
+          end
+
+        end
+      end
+
+      context "with a negative expectation" do
+        subject { expect(a).not_to contain_exactly(*b) }
+
+        context "that is valid" do
+          let(:b) { Array.new(10_000) { rand(10) } }
+
+          it "does not match" do
+            subject
+          end
+
+          include_examples "runs very fast"
+        end
+
+        context "that is not valid" do
+          let(:b) { a.shuffle }
+
+          it "fails quickly" do
+            time = Benchmark.realtime do
+              expect { expect(a).not_to contain_exactly(*b) }.to fail_with(/not to contain exactly/)
+            end
+            expect(time).to be < 1
+          end
+        end
+      end
+    end
+  end
+
   it "passes a valid positive expectation" do
     expect([1, 2]).to contain_exactly(2, 1)
   end