[Resolver] Sort dependencies by number of possibilities, improving performance…

[Resolver] Sort dependencies by number of possibilities, improving performance exponentially on large Podfiles

lib/cocoapods/resolver.rb

@@ -122,6 +122,8 @@ module Pod
     end
 
     def search_for(dependency)
+      @search ||= {}
+      @search[dependency] ||= begin
         prerelease_requirement = dependency.
           requirement.
           requirements.
@@ -136,6 +138,8 @@ module Pod
           compact.
           each { |s| s.version.head = dependency.head? }
       end
+      @search[dependency].dup
+    end
 
     def dependencies_for(dependency)
       dependency.all_dependencies
@@ -155,6 +159,24 @@ module Pod
       end
     end
 
+    # Sort dependencies so that the ones that are easiest to resolve are first.
+    # Easiest to resolve is (usually) defined by:
+    #   1) Is this dependency already activated?
+    #   2) How relaxed are the requirements?
+    #   3) Are there any conflicts for this dependency?
+    #   4) How many possibilities are there to satisfy this dependency?
+    #
+    def sort_dependencies(dependencies, activated, conflicts)
+      dependencies.sort_by do |dependency|
+        name = name_for(dependency)
+        [
+          activated.vertex_named(name).payload ? 0 : 1,
+          conflicts[name] ? 0 : 1,
+          search_for(dependency).count,
+        ]
+      end
+    end
+
     # @return [Hash{Podfile::TargetDefinition => Array<Specification>}]
     #         returns the resolved specifications grouped by target.
     #