Auto merge of #1856 - rubygems:seg-resolver-sorting, r=segiddins

[Resolver] Use improved bundler sorting algorithm

# Description:

Fixes #1851

# Tasks:

- [ ] Describe the problem / feature
- [ ] Write tests
- [ ] Write code to solve the problem
- [ ] Get code review from coworkers / friends

I will abide by the [code of conduct](https://github.com/rubygems/rubygems/blob/master/CODE_OF_CONDUCT.md).

(cherry picked from commit fa87e95)

Author: homu

lib/rubygems/resolver.rb

@@ -253,6 +253,44 @@ def allow_missing?(dependency)
     @soft_missing
   end
 
+  def sort_dependencies(dependencies, activated, conflicts)
+    dependencies.sort_by do |dependency|
+      name = name_for(dependency)
+      [
+        activated.vertex_named(name).payload ? 0 : 1,
+        amount_constrained(dependency),
+        conflicts[name] ? 0 : 1,
+        activated.vertex_named(name).payload ? 0 : search_for(dependency).count,
+      ]
+    end
+  end
+
+  SINGLE_POSSIBILITY_CONSTRAINT_PENALTY = 1_000_000
+  private_constant :SINGLE_POSSIBILITY_CONSTRAINT_PENALTY if defined?(private_constant)
+
+  # returns an integer \in (-\infty, 0]
+  # a number closer to 0 means the dependency is less constraining
+  #
+  # dependencies w/ 0 or 1 possibilities (ignoring version requirements)
+  # are given very negative values, so they _always_ sort first,
+  # before dependencies that are unconstrained
+  def amount_constrained(dependency)
+    @amount_constrained ||= {}
+    @amount_constrained[dependency.name] ||= begin
+      name_dependency = Gem::Dependency.new(dependency.name)
+      dependency_request_for_name = Gem::Resolver::DependencyRequest.new(name_dependency, dependency.requester)
+      all = @set.find_all(dependency_request_for_name).size
+
+      if all <= 1
+        all - SINGLE_POSSIBILITY_CONSTRAINT_PENALTY
+      else
+        search = search_for(dependency).size
+        search - all
+      end
+    end
+  end
+  private :amount_constrained
+
 end
 
 ##

test/rubygems/test_gem_resolver.rb

@@ -521,11 +521,11 @@ def test_raises_when_possibles_are_exhausted
     assert_equal req('>= 0'), dependency.requirement
 
     activated = e.conflict.activated
-    assert_equal 'c-2', activated.full_name
+    assert_equal 'c-1', activated.full_name
 
-    assert_equal dep('c', '>= 2'), activated.request.dependency
+    assert_equal dep('c', '= 1'), activated.request.dependency
 
-    assert_equal [dep('c', '= 1'), dep('c', '>= 2')],
+    assert_equal [dep('c', '>= 2'), dep('c', '= 1')],
                  e.conflict.conflicting_dependencies
   end