* lib/rubygems: Update to RubyGems HEAD(c202db2).

this version contains many enhancements see http://git.io/vtNwF
* test/rubygems: ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51092 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/rubygems/resolver/activation_request.rb

@@ -67,6 +67,8 @@ class Gem::Resolver::ActivationRequest
     @spec.full_name
   end
 
+  alias_method :to_s, :full_name
+
   ##
   # The Gem::Specification for this activation request.
 
@@ -169,4 +171,3 @@ class Gem::Resolver::ActivationRequest
   end
 
 end
-

lib/rubygems/resolver/conflict.rb

@@ -157,4 +157,3 @@ end
 # TODO: Remove in RubyGems 3
 
 Gem::Resolver::DependencyConflict = Gem::Resolver::Conflict # :nodoc:
-

lib/rubygems/resolver/dependency_request.rb

@@ -67,6 +67,10 @@ class Gem::Resolver::DependencyRequest
     @dependency.name
   end
 
+  def type
+    @dependency.type
+  end
+
   ##
   # Indicate that the request is for a gem explicitly requested by the user
 
@@ -113,4 +117,3 @@ class Gem::Resolver::DependencyRequest
   end
 
 end
-

lib/rubygems/resolver/git_specification.rb

@@ -23,8 +23,7 @@ class Gem::Resolver::GitSpecification < Gem::Resolver::SpecSpecification
   def install options = {}
     require 'rubygems/installer'
 
-    installer = Gem::Installer.new '', options
-    installer.spec = spec
+    installer = Gem::Installer.for_spec spec, options
 
     yield installer if block_given?
 

lib/rubygems/resolver/molinillo.rb

@@ -0,0 +1 @@
+require 'rubygems/resolver/molinillo/lib/molinillo'

lib/rubygems/resolver/molinillo/lib/molinillo.rb

@@ -0,0 +1,5 @@
+require 'rubygems/resolver/molinillo/lib/molinillo/gem_metadata'
+require 'rubygems/resolver/molinillo/lib/molinillo/errors'
+require 'rubygems/resolver/molinillo/lib/molinillo/resolver'
+require 'rubygems/resolver/molinillo/lib/molinillo/modules/ui'
+require 'rubygems/resolver/molinillo/lib/molinillo/modules/specification_provider'

lib/rubygems/resolver/molinillo/lib/molinillo/dependency_graph.rb

@@ -0,0 +1,266 @@
+require 'set'
+require 'tsort'
+
+module Gem::Resolver::Molinillo
+  # A directed acyclic graph that is tuned to hold named dependencies
+  class DependencyGraph
+    include Enumerable
+
+    # Enumerates through the vertices of the graph.
+    # @return [Array<Vertex>] The graph's vertices.
+    def each
+      vertices.values.each { |v| yield v }
+    end
+
+    include TSort
+
+    alias_method :tsort_each_node, :each
+
+    def tsort_each_child(vertex, &block)
+      vertex.successors.each(&block)
+    end
+
+    # Topologically sorts the given vertices.
+    # @param [Enumerable<Vertex>] vertices the vertices to be sorted, which must
+    #   all belong to the same graph.
+    # @return [Array<Vertex>] The sorted vertices.
+    def self.tsort(vertices)
+      TSort.tsort(
+        lambda { |b| vertices.each(&b) },
+        lambda { |v, &b| (v.successors & vertices).each(&b) }
+      )
+    end
+
+    # A directed edge of a {DependencyGraph}
+    # @attr [Vertex] origin The origin of the directed edge
+    # @attr [Vertex] destination The destination of the directed edge
+    # @attr [Array] requirements The requirements the directed edge represents
+    Edge = Struct.new(:origin, :destination, :requirements)
+
+    # @return [{String => Vertex}] vertices that have no {Vertex#predecessors},
+    #   keyed by by {Vertex#name}
+    attr_reader :root_vertices
+    # @return [{String => Vertex}] the vertices of the dependency graph, keyed
+    #   by {Vertex#name}
+    attr_reader :vertices
+    # @return [Set<Edge>] the edges of the dependency graph
+    attr_reader :edges
+
+    def initialize
+      @vertices = {}
+      @edges = Set.new
+      @root_vertices = {}
+    end
+
+    # Initializes a copy of a {DependencyGraph}, ensuring that all {#vertices}
+    # have the correct {Vertex#graph} set
+    def initialize_copy(other)
+      super
+      @vertices = other.vertices.reduce({}) do |vertices, (name, vertex)|
+        vertices.tap do |hash|
+          hash[name] = vertex.dup.tap { |v| v.graph = self }
+        end
+      end
+      @root_vertices = Hash[@vertices.select { |n, _v| other.root_vertices[n] }]
+      @edges = other.edges.map do |edge|
+        Edge.new(
+          vertex_named(edge.origin.name),
+          vertex_named(edge.destination.name),
+          edge.requirements.dup
+        )
+      end
+    end
+
+    # @return [String] a string suitable for debugging
+    def inspect
+      "#{self.class}:#{vertices.values.inspect}"
+    end
+
+    # @return [Boolean] whether the two dependency graphs are equal, determined
+    #   by a recursive traversal of each {#root_vertices} and its
+    #   {Vertex#successors}
+    def ==(other)
+      root_vertices == other.root_vertices
+    end
+
+    # @param [String] name
+    # @param [Object] payload
+    # @param [Array<String>] parent_names
+    # @param [Object] requirement the requirement that is requiring the child
+    # @return [void]
+    def add_child_vertex(name, payload, parent_names, requirement)
+      is_root = parent_names.include?(nil)
+      parent_nodes = parent_names.compact.map { |n| vertex_named(n) }
+      vertex = vertex_named(name) || if is_root
+                                       add_root_vertex(name, payload)
+                                     else
+                                       add_vertex(name, payload)
+                                     end
+      vertex.payload ||= payload
+      parent_nodes.each do |parent_node|
+        add_edge(parent_node, vertex, requirement)
+      end
+      vertex
+    end
+
+    # @param [String] name
+    # @param [Object] payload
+    # @return [Vertex] the vertex that was added to `self`
+    def add_vertex(name, payload)
+      vertex = vertices[name] ||= Vertex.new(self, name, payload)
+      vertex.tap { |v| v.payload = payload }
+    end
+
+    # @param [String] name
+    # @param [Object] payload
+    # @return [Vertex] the vertex that was added to `self`
+    def add_root_vertex(name, payload)
+      add_vertex(name, payload).tap { |v| root_vertices[name] = v }
+    end
+
+    # Detaches the {#vertex_named} `name` {Vertex} from the graph, recursively
+    # removing any non-root vertices that were orphaned in the process
+    # @param [String] name
+    # @return [void]
+    def detach_vertex_named(name)
+      vertex = vertex_named(name)
+      return unless vertex
+      successors = vertex.successors
+      vertices.delete(name)
+      edges.reject! { |e| e.origin == vertex || e.destination == vertex }
+      successors.each { |v| detach_vertex_named(v.name) unless root_vertices[v.name] || v.predecessors.any? }
+    end
+
+    # @param [String] name
+    # @return [Vertex,nil] the vertex with the given name
+    def vertex_named(name)
+      vertices[name]
+    end
+
+    # @param [String] name
+    # @return [Vertex,nil] the root vertex with the given name
+    def root_vertex_named(name)
+      root_vertices[name]
+    end
+
+    # Adds a new {Edge} to the dependency graph
+    # @param [Vertex] origin
+    # @param [Vertex] destination
+    # @param [Object] requirement the requirement that this edge represents
+    # @return [Edge] the added edge
+    def add_edge(origin, destination, requirement)
+      if origin == destination || destination.path_to?(origin)
+        raise CircularDependencyError.new([origin, destination])
+      end
+      Edge.new(origin, destination, [requirement]).tap { |e| edges << e }
+    end
+
+    # A vertex in a {DependencyGraph} that encapsulates a {#name} and a
+    # {#payload}
+    class Vertex
+      # @return [DependencyGraph] the graph this vertex is a node of
+      attr_accessor :graph
+
+      # @return [String] the name of the vertex
+      attr_accessor :name
+
+      # @return [Object] the payload the vertex holds
+      attr_accessor :payload
+
+      # @return [Arrary<Object>] the explicit requirements that required
+      #   this vertex
+      attr_reader :explicit_requirements
+
+      # @param [DependencyGraph] graph see {#graph}
+      # @param [String] name see {#name}
+      # @param [Object] payload see {#payload}
+      def initialize(graph, name, payload)
+        @graph = graph
+        @name = name
+        @payload = payload
+        @explicit_requirements = []
+      end
+
+      # @return [Array<Object>] all of the requirements that required
+      #   this vertex
+      def requirements
+        incoming_edges.map(&:requirements).flatten + explicit_requirements
+      end
+
+      # @return [Array<Edge>] the edges of {#graph} that have `self` as their
+      #   {Edge#origin}
+      def outgoing_edges
+        graph.edges.select { |e| e.origin.shallow_eql?(self) }
+      end
+
+      # @return [Array<Edge>] the edges of {#graph} that have `self` as their
+      #   {Edge#destination}
+      def incoming_edges
+        graph.edges.select { |e| e.destination.shallow_eql?(self) }
+      end
+
+      # @return [Set<Vertex>] the vertices of {#graph} that have an edge with
+      #   `self` as their {Edge#destination}
+      def predecessors
+        incoming_edges.map(&:origin).to_set
+      end
+
+      # @return [Set<Vertex>] the vertices of {#graph} that have an edge with
+      #   `self` as their {Edge#origin}
+      def successors
+        outgoing_edges.map(&:destination).to_set
+      end
+
+      # @return [Set<Vertex>] the vertices of {#graph} where `self` is an
+      #   {#ancestor?}
+      def recursive_successors
+        successors + successors.map(&:recursive_successors).reduce(Set.new, &:+)
+      end
+
+      # @return [String] a string suitable for debugging
+      def inspect
+        "#{self.class}:#{name}(#{payload.inspect})"
+      end
+
+      # @return [Boolean] whether the two vertices are equal, determined
+      #   by a recursive traversal of each {Vertex#successors}
+      def ==(other)
+        shallow_eql?(other) &&
+          successors == other.successors
+      end
+
+      # @return [Boolean] whether the two vertices are equal, determined
+      #   solely by {#name} and {#payload} equality
+      def shallow_eql?(other)
+        other &&
+          name == other.name &&
+          payload == other.payload
+      end
+
+      alias_method :eql?, :==
+
+      # @return [Fixnum] a hash for the vertex based upon its {#name}
+      def hash
+        name.hash
+      end
+
+      # Is there a path from `self` to `other` following edges in the
+      # dependency graph?
+      # @return true iff there is a path following edges within this {#graph}
+      def path_to?(other)
+        successors.include?(other) || successors.any? { |v| v.path_to?(other) }
+      end
+
+      alias_method :descendent?, :path_to?
+
+      # Is there a path from `other` to `self` following edges in the
+      # dependency graph?
+      # @return true iff there is a path following edges within this {#graph}
+      def ancestor?(other)
+        predecessors.include?(other) || predecessors.any? { |v| v.ancestor?(other) }
+      end
+
+      alias_method :is_reachable_from?, :ancestor?
+    end
+  end
+end

lib/rubygems/resolver/molinillo/lib/molinillo/errors.rb

@@ -0,0 +1,69 @@
+module Gem::Resolver::Molinillo
+  # An error that occurred during the resolution process
+  class ResolverError < StandardError; end
+
+  # An error caused by searching for a dependency that is completely unknown,
+  # i.e. has no versions available whatsoever.
+  class NoSuchDependencyError < ResolverError
+    # @return [Object] the dependency that could not be found
+    attr_accessor :dependency
+
+    # @return [Array<Object>] the specifications that depended upon {#dependency}
+    attr_accessor :required_by
+
+    # @param [Object] dependency @see {#dependency}
+    # @param [Array<Object>] required_by @see {#required_by}
+    def initialize(dependency, required_by = [])
+      @dependency = dependency
+      @required_by = required_by
+      super()
+    end
+
+    def message
+      sources = required_by.map { |r| "`#{r}`" }.join(' and ')
+      message = "Unable to find a specification for `#{dependency}`"
+      message << " depended upon by #{sources}" unless sources.empty?
+      message
+    end
+  end
+
+  # An error caused by attempting to fulfil a dependency that was circular
+  #
+  # @note This exception will be thrown iff a {Vertex} is added to a
+  #   {DependencyGraph} that has a {DependencyGraph::Vertex#path_to?} an
+  #   existing {DependencyGraph::Vertex}
+  class CircularDependencyError < ResolverError
+    # [Set<Object>] the dependencies responsible for causing the error
+    attr_reader :dependencies
+
+    # @param [Array<DependencyGraph::Vertex>] nodes the nodes in the dependency
+    #   that caused the error
+    def initialize(nodes)
+      super "There is a circular dependency between #{nodes.map(&:name).join(' and ')}"
+      @dependencies = nodes.map(&:payload).to_set
+    end
+  end
+
+  # An error caused by conflicts in version
+  class VersionConflict < ResolverError
+    # @return [{String => Resolution::Conflict}] the conflicts that caused
+    #   resolution to fail
+    attr_reader :conflicts
+
+    # @param [{String => Resolution::Conflict}] conflicts see {#conflicts}
+    def initialize(conflicts)
+      pairs = []
+      conflicts.values.flatten.map(&:requirements).flatten.each do |conflicting|
+        conflicting.each do |source, conflict_requirements|
+          conflict_requirements.each do |c|
+            pairs << [c, source]
+          end
+        end
+      end
+
+      super "Unable to satisfy the following requirements:\n\n" \
+        "#{pairs.map { |r, d| "- `#{r}` required by `#{d}`" }.join("\n")}"
+      @conflicts = conflicts
+    end
+  end
+end

lib/rubygems/resolver/molinillo/lib/molinillo/gem_metadata.rb

@@ -0,0 +1,3 @@
+module Gem::Resolver::Molinillo
+  VERSION = '0.3.0'
+end

lib/rubygems/resolver/molinillo/lib/molinillo/modules/specification_provider.rb

@@ -0,0 +1,99 @@
+module Gem::Resolver::Molinillo
+  # Provides information about specifcations and dependencies to the resolver,
+  # allowing the {Resolver} class to remain generic while still providing power
+  # and flexibility.
+  #
+  # This module contains the methods that users of Gem::Resolver::Molinillo must to implement,
+  # using knowledge of their own model classes.
+  module SpecificationProvider
+    # Search for the specifications that match the given dependency.
+    # The specifications in the returned array will be considered in reverse
+    # order, so the latest version ought to be last.
+    # @note This method should be 'pure', i.e. the return value should depend
+    #   only on the `dependency` parameter.
+    #
+    # @param [Object] dependency
+    # @return [Array<Object>] the specifications that satisfy the given
+    #   `dependency`.
+    def search_for(dependency)
+      []
+    end
+
+    # Returns the dependencies of `specification`.
+    # @note This method should be 'pure', i.e. the return value should depend
+    #   only on the `specification` parameter.
+    #
+    # @param [Object] specification
+    # @return [Array<Object>] the dependencies that are required by the given
+    #   `specification`.
+    def dependencies_for(specification)
+      []
+    end
+
+    # Determines whether the given `requirement` is satisfied by the given
+    # `spec`, in the context of the current `activated` dependency graph.
+    #
+    # @param [Object] requirement
+    # @param [DependencyGraph] activated the current dependency graph in the
+    #   resolution process.
+    # @param [Object] spec
+    # @return [Boolean] whether `requirement` is satisfied by `spec` in the
+    #   context of the current `activated` dependency graph.
+    def requirement_satisfied_by?(requirement, activated, spec)
+      true
+    end
+
+    # Returns the name for the given `dependency`.
+    # @note This method should be 'pure', i.e. the return value should depend
+    #   only on the `dependency` parameter.
+    #
+    # @param [Object] dependency
+    # @return [String] the name for the given `dependency`.
+    def name_for(dependency)
+      dependency.to_s
+    end
+
+    # @return [String] the name of the source of explicit dependencies, i.e.
+    #   those passed to {Resolver#resolve} directly.
+    def name_for_explicit_dependency_source
+      'user-specified dependency'
+    end
+
+    # @return [String] the name of the source of 'locked' dependencies, i.e.
+    #   those passed to {Resolver#resolve} directly as the `base`
+    def name_for_locking_dependency_source
+      'Lockfile'
+    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?
+    #
+    # @param [Array<Object>] dependencies
+    # @param [DependencyGraph] activated the current dependency graph in the
+    #   resolution process.
+    # @param [{String => Array<Conflict>}] conflicts
+    # @return [Array<Object>] a sorted copy of `dependencies`.
+    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,
+        ]
+      end
+    end
+
+    # Returns whether this dependency, which has no possible matching
+    # specifications, can safely be ignored.
+    #
+    # @param [Object] dependency
+    # @return [Boolean] whether this dependency can safely be skipped.
+    def allow_missing?(dependency)
+      false
+    end
+  end
+end

lib/rubygems/resolver/molinillo/lib/molinillo/modules/ui.rb

@@ -0,0 +1,63 @@
+module Gem::Resolver::Molinillo
+  # Conveys information about the resolution process to a user.
+  module UI
+    # The {IO} object that should be used to print output. `STDOUT`, by default.
+    #
+    # @return [IO]
+    def output
+      STDOUT
+    end
+
+    # Called roughly every {#progress_rate}, this method should convey progress
+    # to the user.
+    #
+    # @return [void]
+    def indicate_progress
+      output.print '.' unless debug?
+    end
+
+    # How often progress should be conveyed to the user via
+    # {#indicate_progress}, in seconds. A third of a second, by default.
+    #
+    # @return [Float]
+    def progress_rate
+      0.33
+    end
+
+    # Called before resolution begins.
+    #
+    # @return [void]
+    def before_resolution
+      output.print 'Resolving dependencies...'
+    end
+
+    # Called after resolution ends (either successfully or with an error).
+    # By default, prints a newline.
+    #
+    # @return [void]
+    def after_resolution
+      output.puts
+    end
+
+    # Conveys debug information to the user.
+    #
+    # @param [Integer] depth the current depth of the resolution process.
+    # @return [void]
+    def debug(depth = 0)
+      if debug?
+        debug_info = yield
+        debug_info = debug_info.inspect unless debug_info.is_a?(String)
+        output.puts debug_info.split("\n").map { |s| '  ' * depth + s }
+      end
+    end
+
+    # Whether or not debug messages should be printed.
+    # By default, whether or not the `MOLINILLO_DEBUG` environment variable is
+    # set.
+    #
+    # @return [Boolean]
+    def debug?
+      @debug_mode ||= ENV['MOLINILLO_DEBUG']
+    end
+  end
+end

lib/rubygems/resolver/molinillo/lib/molinillo/resolution.rb

@@ -0,0 +1,430 @@
+module Gem::Resolver::Molinillo
+  class Resolver
+    # A specific resolution from a given {Resolver}
+    class Resolution
+      # A conflict that the resolution process encountered
+      # @attr [Object] requirement the requirement that immediately led to the conflict
+      # @attr [{String,Nil=>[Object]}] requirements the requirements that caused the conflict
+      # @attr [Object, nil] existing the existing spec that was in conflict with
+      #   the {#possibility}
+      # @attr [Object] possibility the spec that was unable to be activated due
+      #   to a conflict
+      # @attr [Object] locked_requirement the relevant locking requirement.
+      # @attr [Array<Array<Object>>] requirement_trees the different requirement
+      #   trees that led to every requirement for the conflicting name.
+      Conflict = Struct.new(
+        :requirement,
+        :requirements,
+        :existing,
+        :possibility,
+        :locked_requirement,
+        :requirement_trees
+      )
+
+      # @return [SpecificationProvider] the provider that knows about
+      #   dependencies, requirements, specifications, versions, etc.
+      attr_reader :specification_provider
+
+      # @return [UI] the UI that knows how to communicate feedback about the
+      #   resolution process back to the user
+      attr_reader :resolver_ui
+
+      # @return [DependencyGraph] the base dependency graph to which
+      #   dependencies should be 'locked'
+      attr_reader :base
+
+      # @return [Array] the dependencies that were explicitly required
+      attr_reader :original_requested
+
+      # @param [SpecificationProvider] specification_provider
+      #   see {#specification_provider}
+      # @param [UI] resolver_ui see {#resolver_ui}
+      # @param [Array] requested see {#original_requested}
+      # @param [DependencyGraph] base see {#base}
+      def initialize(specification_provider, resolver_ui, requested, base)
+        @specification_provider = specification_provider
+        @resolver_ui = resolver_ui
+        @original_requested = requested
+        @base = base
+        @states = []
+        @iteration_counter = 0
+      end
+
+      # Resolves the {#original_requested} dependencies into a full dependency
+      #   graph
+      # @raise [ResolverError] if successful resolution is impossible
+      # @return [DependencyGraph] the dependency graph of successfully resolved
+      #   dependencies
+      def resolve
+        start_resolution
+
+        while state
+          break unless state.requirements.any? || state.requirement
+          indicate_progress
+          if state.respond_to?(:pop_possibility_state) # DependencyState
+            debug(depth) { "Creating possibility state for #{requirement} (#{possibilities.count} remaining)" }
+            state.pop_possibility_state.tap { |s| states.push(s) if s }
+          end
+          process_topmost_state
+        end
+
+        activated.freeze
+      ensure
+        end_resolution
+      end
+
+      # @return [Integer] the number of resolver iterations in between calls to
+      #   {#resolver_ui}'s {UI#indicate_progress} method
+      attr_accessor :iteration_rate
+      private :iteration_rate
+
+      # @return [Time] the time at which resolution began
+      attr_accessor :started_at
+      private :started_at
+
+      # @return [Array<ResolutionState>] the stack of states for the resolution
+      attr_accessor :states
+      private :states
+
+      private
+
+      # Sets up the resolution process
+      # @return [void]
+      def start_resolution
+        @started_at = Time.now
+
+        handle_missing_or_push_dependency_state(initial_state)
+
+        debug { "Starting resolution (#{@started_at})" }
+        resolver_ui.before_resolution
+      end
+
+      # Ends the resolution process
+      # @return [void]
+      def end_resolution
+        resolver_ui.after_resolution
+        debug do
+          "Finished resolution (#{@iteration_counter} steps) " \
+          "(Took #{(ended_at = Time.now) - @started_at} seconds) (#{ended_at})"
+        end
+        debug { 'Unactivated: ' + Hash[activated.vertices.reject { |_n, v| v.payload }].keys.join(', ') } if state
+        debug { 'Activated: ' + Hash[activated.vertices.select { |_n, v| v.payload }].keys.join(', ') } if state
+      end
+
+      require 'rubygems/resolver/molinillo/lib/molinillo/state'
+      require 'rubygems/resolver/molinillo/lib/molinillo/modules/specification_provider'
+
+      ResolutionState.new.members.each do |member|
+        define_method member do |*args, &block|
+          current_state = state || ResolutionState.empty
+          current_state.send(member, *args, &block)
+        end
+      end
+
+      SpecificationProvider.instance_methods(false).each do |instance_method|
+        define_method instance_method do |*args, &block|
+          begin
+            specification_provider.send(instance_method, *args, &block)
+          rescue NoSuchDependencyError => error
+            if state
+              vertex = activated.vertex_named(name_for error.dependency)
+              error.required_by += vertex.incoming_edges.map { |e| e.origin.name }
+              error.required_by << name_for_explicit_dependency_source unless vertex.explicit_requirements.empty?
+            end
+            raise
+          end
+        end
+      end
+
+      # Processes the topmost available {RequirementState} on the stack
+      # @return [void]
+      def process_topmost_state
+        if possibility
+          attempt_to_activate
+        else
+          create_conflict if state.is_a? PossibilityState
+          unwind_for_conflict until possibility && state.is_a?(DependencyState)
+        end
+      end
+
+      # @return [Object] the current possibility that the resolution is trying
+      #   to activate
+      def possibility
+        possibilities.last
+      end
+
+      # @return [RequirementState] the current state the resolution is
+      #   operating upon
+      def state
+        states.last
+      end
+
+      # Creates the initial state for the resolution, based upon the
+      # {#requested} dependencies
+      # @return [DependencyState] the initial state for the resolution
+      def initial_state
+        graph = DependencyGraph.new.tap do |dg|
+          original_requested.each { |r| dg.add_root_vertex(name_for(r), nil).tap { |v| v.explicit_requirements << r } }
+        end
+
+        requirements = sort_dependencies(original_requested, graph, {})
+        initial_requirement = requirements.shift
+        DependencyState.new(
+          initial_requirement && name_for(initial_requirement),
+          requirements,
+          graph,
+          initial_requirement,
+          initial_requirement && search_for(initial_requirement),
+          0,
+          {}
+        )
+      end
+
+      # Unwinds the states stack because a conflict has been encountered
+      # @return [void]
+      def unwind_for_conflict
+        debug(depth) { "Unwinding for conflict: #{requirement}" }
+        conflicts.tap do |c|
+          states.slice!((state_index_for_unwind + 1)..-1)
+          raise VersionConflict.new(c) unless state
+          state.conflicts = c
+        end
+      end
+
+      # @return [Integer] The index to which the resolution should unwind in the
+      #   case of conflict.
+      def state_index_for_unwind
+        current_requirement = requirement
+        existing_requirement = requirement_for_existing_name(name)
+        until current_requirement.nil?
+          current_state = find_state_for(current_requirement)
+          return states.index(current_state) if state_any?(current_state)
+          current_requirement = parent_of(current_requirement)
+        end
+
+        until existing_requirement.nil?
+          existing_state = find_state_for(existing_requirement)
+          return states.index(existing_state) if state_any?(existing_state)
+          existing_requirement = parent_of(existing_requirement)
+        end
+        -1
+      end
+
+      # @return [Object] the requirement that led to `requirement` being added
+      #   to the list of requirements.
+      def parent_of(requirement)
+        return nil unless requirement
+        seen = false
+        state = states.reverse_each.find do |s|
+          seen ||= s.requirement == requirement
+          seen && s.requirement != requirement && !s.requirements.include?(requirement)
+        end
+        state && state.requirement
+      end
+
+      # @return [Object] the requirement that led to a version of a possibility
+      #   with the given name being activated.
+      def requirement_for_existing_name(name)
+        return nil unless activated.vertex_named(name).payload
+        states.reverse_each.find { |s| !s.activated.vertex_named(name).payload }.requirement
+      end
+
+      # @return [ResolutionState] the state whose `requirement` is the given
+      #   `requirement`.
+      def find_state_for(requirement)
+        return nil unless requirement
+        states.reverse_each.find { |i| requirement == i.requirement && i.is_a?(DependencyState) }
+      end
+
+      # @return [Boolean] whether or not the given state has any possibilities
+      #   left.
+      def state_any?(state)
+        state && state.possibilities.any?
+      end
+
+      # @return [Conflict] a {Conflict} that reflects the failure to activate
+      #   the {#possibility} in conjunction with the current {#state}
+      def create_conflict
+        vertex = activated.vertex_named(name)
+        requirements = {
+          name_for_explicit_dependency_source => vertex.explicit_requirements,
+          name_for_locking_dependency_source => Array(locked_requirement_named(name)),
+        }
+        vertex.incoming_edges.each { |edge| (requirements[edge.origin.payload] ||= []).unshift(*edge.requirements) }
+        conflicts[name] = Conflict.new(
+          requirement,
+          Hash[requirements.select { |_, r| !r.empty? }],
+          vertex.payload,
+          possibility,
+          locked_requirement_named(name),
+          requirement_trees
+        )
+      end
+
+      # @return [Array<Array<Object>>] The different requirement
+      #   trees that led to every requirement for the current spec.
+      def requirement_trees
+        activated.vertex_named(name).requirements.map { |r| requirement_tree_for(r) }
+      end
+
+      # @return [Array<Object>] the list of requirements that led to
+      #   `requirement` being required.
+      def requirement_tree_for(requirement)
+        tree = []
+        while requirement
+          tree.unshift(requirement)
+          requirement = parent_of(requirement)
+        end
+        tree
+      end
+
+      # Indicates progress roughly once every second
+      # @return [void]
+      def indicate_progress
+        @iteration_counter += 1
+        @progress_rate ||= resolver_ui.progress_rate
+        if iteration_rate.nil?
+          if Time.now - started_at >= @progress_rate
+            self.iteration_rate = @iteration_counter
+          end
+        end
+
+        if iteration_rate && (@iteration_counter % iteration_rate) == 0
+          resolver_ui.indicate_progress
+        end
+      end
+
+      # Calls the {#resolver_ui}'s {UI#debug} method
+      # @param [Integer] depth the depth of the {#states} stack
+      # @param [Proc] block a block that yields a {#to_s}
+      # @return [void]
+      def debug(depth = 0, &block)
+        resolver_ui.debug(depth, &block)
+      end
+
+      # Attempts to activate the current {#possibility}
+      # @return [void]
+      def attempt_to_activate
+        debug(depth) { 'Attempting to activate ' + possibility.to_s }
+        existing_node = activated.vertex_named(name)
+        if existing_node.payload
+          debug(depth) { "Found existing spec (#{existing_node.payload})" }
+          attempt_to_activate_existing_spec(existing_node)
+        else
+          attempt_to_activate_new_spec
+        end
+      end
+
+      # Attempts to activate the current {#possibility} (given that it has
+      # already been activated)
+      # @return [void]
+      def attempt_to_activate_existing_spec(existing_node)
+        existing_spec = existing_node.payload
+        if requirement_satisfied_by?(requirement, activated, existing_spec)
+          new_requirements = requirements.dup
+          push_state_for_requirements(new_requirements)
+        else
+          return if attempt_to_swap_possibility
+          create_conflict
+          debug(depth) { "Unsatisfied by existing spec (#{existing_node.payload})" }
+          unwind_for_conflict
+        end
+      end
+
+      # Attempts to swp the current {#possibility} with the already-activated
+      # spec with the given name
+      # @return [Boolean] Whether the possibility was swapped into {#activated}
+      def attempt_to_swap_possibility
+        swapped = activated.dup
+        swapped.vertex_named(name).payload = possibility
+        return unless swapped.vertex_named(name).requirements.
+            all? { |r| requirement_satisfied_by?(r, swapped, possibility) }
+        attempt_to_activate_new_spec
+      end
+
+      # Attempts to activate the current {#possibility} (given that it hasn't
+      # already been activated)
+      # @return [void]
+      def attempt_to_activate_new_spec
+        satisfied = begin
+          locked_requirement = locked_requirement_named(name)
+          requested_spec_satisfied = requirement_satisfied_by?(requirement, activated, possibility)
+          locked_spec_satisfied = !locked_requirement ||
+            requirement_satisfied_by?(locked_requirement, activated, possibility)
+          debug(depth) { 'Unsatisfied by requested spec' } unless requested_spec_satisfied
+          debug(depth) { 'Unsatisfied by locked spec' } unless locked_spec_satisfied
+          requested_spec_satisfied && locked_spec_satisfied
+        end
+        if satisfied
+          activate_spec
+        else
+          create_conflict
+          unwind_for_conflict
+        end
+      end
+
+      # @param [String] requirement_name the spec name to search for
+      # @return [Object] the locked spec named `requirement_name`, if one
+      #   is found on {#base}
+      def locked_requirement_named(requirement_name)
+        vertex = base.vertex_named(requirement_name)
+        vertex && vertex.payload
+      end
+
+      # Add the current {#possibility} to the dependency graph of the current
+      # {#state}
+      # @return [void]
+      def activate_spec
+        conflicts.delete(name)
+        debug(depth) { 'Activated ' + name + ' at ' + possibility.to_s }
+        vertex = activated.vertex_named(name)
+        vertex.payload = possibility
+        require_nested_dependencies_for(possibility)
+      end
+
+      # Requires the dependencies that the recently activated spec has
+      # @param [Object] activated_spec the specification that has just been
+      #   activated
+      # @return [void]
+      def require_nested_dependencies_for(activated_spec)
+        nested_dependencies = dependencies_for(activated_spec)
+        debug(depth) { "Requiring nested dependencies (#{nested_dependencies.map(&:to_s).join(', ')})" }
+        nested_dependencies.each { |d|  activated.add_child_vertex name_for(d), nil, [name_for(activated_spec)], d }
+
+        push_state_for_requirements(requirements + nested_dependencies)
+      end
+
+      # Pushes a new {DependencyState} that encapsulates both existing and new
+      # requirements
+      # @param [Array] new_requirements
+      # @return [void]
+      def push_state_for_requirements(new_requirements, new_activated = activated.dup)
+        new_requirements = sort_dependencies(new_requirements.uniq, new_activated, conflicts)
+        new_requirement = new_requirements.shift
+        new_name = new_requirement ? name_for(new_requirement) : ''
+        possibilities = new_requirement ? search_for(new_requirement) : []
+        handle_missing_or_push_dependency_state DependencyState.new(
+          new_name, new_requirements, new_activated,
+          new_requirement, possibilities, depth, conflicts.dup
+        )
+      end
+
+      # Pushes a new {DependencyState}.
+      # If the {#specification_provider} says to
+      # {SpecificationProvider#allow_missing?} that particular requirement, and
+      # there are no possibilities for that requirement, then `state` is not
+      # pushed, and the node in {#activated} is removed, and we continue
+      # resolving the remaining requirements.
+      # @param [DependencyState] state
+      # @return [void]
+      def handle_missing_or_push_dependency_state(state)
+        if state.requirement && state.possibilities.empty? && allow_missing?(state.requirement)
+          state.activated.detach_vertex_named(state.name)
+          push_state_for_requirements(state.requirements, state.activated)
+        else
+          states.push state
+        end
+      end
+    end
+  end
+end

lib/rubygems/resolver/molinillo/lib/molinillo/resolver.rb

@@ -0,0 +1,43 @@
+require 'rubygems/resolver/molinillo/lib/molinillo/dependency_graph'
+
+module Gem::Resolver::Molinillo
+  # This class encapsulates a dependency resolver.
+  # The resolver is responsible for determining which set of dependencies to
+  # activate, with feedback from the the {#specification_provider}
+  #
+  #
+  class Resolver
+    require 'rubygems/resolver/molinillo/lib/molinillo/resolution'
+
+    # @return [SpecificationProvider] the specification provider used
+    #   in the resolution process
+    attr_reader :specification_provider
+
+    # @return [UI] the UI module used to communicate back to the user
+    #   during the resolution process
+    attr_reader :resolver_ui
+
+    # @param  [SpecificationProvider] specification_provider
+    #   see {#specification_provider}
+    # @param  [UI] resolver_ui
+    #   see {#resolver_ui}
+    def initialize(specification_provider, resolver_ui)
+      @specification_provider = specification_provider
+      @resolver_ui = resolver_ui
+    end
+
+    # Resolves the requested dependencies into a {DependencyGraph},
+    # locking to the base dependency graph (if specified)
+    # @param [Array] requested an array of 'requested' dependencies that the
+    #   {#specification_provider} can understand
+    # @param [DependencyGraph,nil] base the base dependency graph to which
+    #   dependencies should be 'locked'
+    def resolve(requested, base = DependencyGraph.new)
+      Resolution.new(specification_provider,
+                     resolver_ui,
+                     requested,
+                     base).
+        resolve
+    end
+  end
+end

lib/rubygems/resolver/molinillo/lib/molinillo/state.rb

@@ -0,0 +1,51 @@
+module Gem::Resolver::Molinillo
+  # A state that a {Resolution} can be in
+  # @attr [String] name
+  # @attr [Array<Object>] requirements
+  # @attr [DependencyGraph] activated
+  # @attr [Object] requirement
+  # @attr [Object] possibility
+  # @attr [Integer] depth
+  # @attr [Set<Object>] conflicts
+  ResolutionState = Struct.new(
+    :name,
+    :requirements,
+    :activated,
+    :requirement,
+    :possibilities,
+    :depth,
+    :conflicts
+  )
+
+  class ResolutionState
+    # Returns an empty resolution state
+    # @return [ResolutionState] an empty state
+    def self.empty
+      new(nil, [], DependencyGraph.new, nil, nil, 0, Set.new)
+    end
+  end
+
+  # A state that encapsulates a set of {#requirements} with an {Array} of
+  # possibilities
+  class DependencyState < ResolutionState
+    # Removes a possibility from `self`
+    # @return [PossibilityState] a state with a single possibility,
+    #  the possibility that was removed from `self`
+    def pop_possibility_state
+      PossibilityState.new(
+        name,
+        requirements.dup,
+        activated.dup,
+        requirement,
+        [possibilities.pop],
+        depth + 1,
+        conflicts.dup
+      )
+    end
+  end
+
+  # A state that encapsulates a single possibility to fulfill the given
+  # {#requirement}
+  class PossibilityState < ResolutionState
+  end
+end

lib/rubygems/resolver/specification.rb

@@ -89,7 +89,7 @@ class Gem::Resolver::Specification
 
     gem = source.download spec, destination
 
-    installer = Gem::Installer.new gem, options
+    installer = Gem::Installer.at gem, options
 
     yield installer if block_given?