diff --git a/middleman-core/lib/middleman-core/contracts.rb b/middleman-core/lib/middleman-core/contracts.rb
index 349728e2..16fff201 100644
--- a/middleman-core/lib/middleman-core/contracts.rb
+++ b/middleman-core/lib/middleman-core/contracts.rb
@@ -1,5 +1,6 @@
 if ENV['TEST'] || ENV['CONTRACTS'] == 'true'
   require 'contracts'
+  require 'hamster'
 
   module Contracts
     class IsA
@@ -27,21 +28,7 @@ if ENV['TEST'] || ENV['CONTRACTS'] == 'true'
       end
     end
 
-    # class MethodDefined
-    #   def self.[](val)
-    #     @lookup ||= {}
-    #     @lookup[val] ||= new(val)
-    #   end
-
-    #   def initialize(val)
-    #     @val = val
-    #   end
-
-    #   def valid?(val)
-    #     val.method_defined? @val
-    #   end
-    # end
-
+    VectorOf = Contracts::CollectionOf::Factory.new(::Hamster::Vector)
     ResourceList = Contracts::ArrayOf[IsA['Middleman::Sitemap::Resource']]
   end
 else
@@ -125,8 +112,8 @@ else
     class Frozen < Callable
     end
 
-    # class MethodDefined < Callable
-    # end
+    class VectorOf < Callable
+    end
   end
 end
 
diff --git a/middleman-core/lib/middleman-core/core_extensions/collections/step_context.rb b/middleman-core/lib/middleman-core/core_extensions/collections/step_context.rb
index 740b8188..fcf1d11f 100644
--- a/middleman-core/lib/middleman-core/core_extensions/collections/step_context.rb
+++ b/middleman-core/lib/middleman-core/core_extensions/collections/step_context.rb
@@ -1,3 +1,5 @@
+require 'hamster'
+
 module Middleman
   module CoreExtensions
     module Collections
@@ -9,13 +11,13 @@ module Middleman
         attr_reader :descriptors
 
         def initialize
-          @descriptors = []
+          @descriptors = ::Hamster.set
         end
 
         def method_missing(name, *args, &block)
           internal = :"_internal_#{name}"
           if respond_to?(internal)
-            @descriptors << send(internal, *args, &block)
+            @descriptors = @descriptors.add(send(internal, *args, &block))
           else
             super
           end
diff --git a/middleman-core/lib/middleman-core/core_extensions/i18n.rb b/middleman-core/lib/middleman-core/core_extensions/i18n.rb
index 2f5f3bb9..6561621b 100644
--- a/middleman-core/lib/middleman-core/core_extensions/i18n.rb
+++ b/middleman-core/lib/middleman-core/core_extensions/i18n.rb
@@ -12,7 +12,7 @@ class Middleman::CoreExtensions::Internationalization < ::Middleman::Extension
 
   def initialize(*)
     super
-        
+
     require 'i18n'
 
     # Don't fail on invalid locale, that's not what our current
diff --git a/middleman-core/lib/middleman-core/preview_server.rb b/middleman-core/lib/middleman-core/preview_server.rb
index 5b596ff8..2d71f99e 100644
--- a/middleman-core/lib/middleman-core/preview_server.rb
+++ b/middleman-core/lib/middleman-core/preview_server.rb
@@ -179,15 +179,15 @@ module Middleman
           http_opts[:SSLEnable] = true
 
           if ssl_certificate || ssl_private_key
-            raise "You must provide both :ssl_certificate and :ssl_private_key" unless ssl_private_key && ssl_certificate
+            raise 'You must provide both :ssl_certificate and :ssl_private_key' unless ssl_private_key && ssl_certificate
             http_opts[:SSLCertificate] = OpenSSL::X509::Certificate.new File.read ssl_certificate
             http_opts[:SSLPrivateKey] = OpenSSL::PKey::RSA.new File.read ssl_private_key
           else
             # use a generated self-signed cert
             http_opts[:SSLCertName] = [
-                                       %w(CN localhost),
-                                       %w(CN #{host})
-                                      ].uniq
+              %w(CN localhost),
+              %w(CN #{host})
+            ].uniq
           end
         end
 
diff --git a/middleman-core/lib/middleman-core/sitemap/store.rb b/middleman-core/lib/middleman-core/sitemap/store.rb
index b454ecf9..129fa9b5 100644
--- a/middleman-core/lib/middleman-core/sitemap/store.rb
+++ b/middleman-core/lib/middleman-core/sitemap/store.rb
@@ -1,6 +1,7 @@
 # Used for merging results of metadata callbacks
 require 'active_support/core_ext/hash/deep_merge'
 require 'monitor'
+require 'hamster'
 
 # Ignores
 Middleman::Extensions.register :sitemap_ignore, auto_activate: :before_configuration do
@@ -37,6 +38,8 @@ require 'middleman-core/contracts'
 module Middleman
   # Sitemap namespace
   module Sitemap
+    ManipulatorDescriptor = Struct.new :name, :manipulator, :priority, :custom_name
+
     # The Store class
     #
     # The Store manages a collection of Resource objects, which represent
@@ -46,20 +49,21 @@ module Middleman
     class Store
       include Contracts
 
-      # @return [Middleman::Application]
+      Contract IsA['Middleman::Application']
       attr_reader :app
 
+      Contract Num
       attr_reader :update_count
 
       # Initialize with parent app
       # @param [Middleman::Application] app
+      Contract IsA['Middleman::Application'] => Any
       def initialize(app)
         @app = app
         @resources = []
         @update_count = 0
 
-        # TODO: Should this be a set or hash?
-        @resource_list_manipulators = []
+        @resource_list_manipulators = ::Hamster.vector
         @needs_sitemap_rebuild = true
 
         @lock = Monitor.new
@@ -76,24 +80,29 @@ module Middleman
       # @param [Numeric] priority Sets the order of this resource list manipulator relative to the rest. By default this is 50, and manipulators run in the order they are registered, but if a priority is provided then this will run ahead of or behind other manipulators.
       # @param [Symbol] custom_name The method name to execute.
       # @return [void]
-      Contract Symbol, RespondTo['manipulate_resource_list'], Maybe[Num], Maybe[Symbol] => Any
+      Contract Symbol, RespondTo[:manipulate_resource_list], Maybe[Num], Maybe[Symbol] => Any
       def register_resource_list_manipulator(name, manipulator, priority=50, custom_name=nil)
         # The third argument used to be a boolean - handle those who still pass one
         priority = 50 unless priority.is_a? Numeric
-        @resource_list_manipulators << [name, manipulator, priority, custom_name]
+        @resource_list_manipulators = @resource_list_manipulators.push(
+          ManipulatorDescriptor.new(name, manipulator, priority, custom_name)
+        )
+
         # The index trick is used so that the sort is stable - manipulators with the same priority
         # will always be ordered in the same order as they were registered.
         n = 0
         @resource_list_manipulators = @resource_list_manipulators.sort_by do |m|
           n += 1
-          [m[2], n]
+          [m[:priority], n]
         end
+
         rebuild_resource_list!(:registered_new)
       end
 
       # Rebuild the list of resources from scratch, using registed manipulators
       # @return [void]
-      def rebuild_resource_list!(_=nil)
+      Contract Maybe[Symbol] => Any
+      def rebuild_resource_list!(_name=nil)
         @lock.synchronize do
           @needs_sitemap_rebuild = true
         end
@@ -178,11 +187,13 @@ module Middleman
 
           @app.logger.debug '== Rebuilding resource list'
 
-          @resources = @resource_list_manipulators.reduce([]) do |result, (_, manipulator, _, custom_name)|
-            newres = manipulator.send(custom_name || :manipulate_resource_list, result)
+          @resources = @resource_list_manipulators.reduce([]) do |result, m|
+            newres = m[:manipulator].send(m[:custom_name] || :manipulate_resource_list, result)
 
             # Reset lookup cache
             reset_lookup_cache!
+
+            # Rebuild cache
             newres.each do |resource|
               @_lookup_by_path[resource.path] = resource
               @_lookup_by_destination_path[resource.destination_path] = resource
diff --git a/middleman-core/lib/middleman-core/sources.rb b/middleman-core/lib/middleman-core/sources.rb
index 34d2ab8a..10e14988 100644
--- a/middleman-core/lib/middleman-core/sources.rb
+++ b/middleman-core/lib/middleman-core/sources.rb
@@ -1,3 +1,4 @@
+require 'hamster'
 require 'middleman-core/contracts'
 require 'backports/2.0.0/enumerable/lazy'
 
@@ -47,10 +48,10 @@ module Middleman
       @options = options
 
       # Set of procs wanting to be notified of changes
-      @on_change_callbacks = []
+      @on_change_callbacks = ::Hamster.vector
 
       # Global ignores
-      @ignores = {}
+      @ignores = ::Hamster.hash
 
       # Whether we're "running", which means we're in a stable
       # watch state after all initialization and config.
@@ -72,7 +73,8 @@ module Middleman
     # @return [void]
     Contract Symbol, Symbol, Or[Regexp, Proc] => Any
     def ignore(name, type, regex=nil, &block)
-      @ignores[name] = { type: type, validator: (block_given? ? block : regex) }
+      @ignores = @ignores.put(name, type: type,
+                                    validator: (block_given? ? block : regex))
 
       bump_count
       find_new_files! if @running
@@ -238,11 +240,10 @@ module Middleman
     # Add callback to be run on file change or deletion
     #
     # @param [Symbol] type The change type.
-    # @return [Set<CallbackDescriptor>]
-    Contract Symbol, Proc => ArrayOf[CallbackDescriptor]
+    # @return [void]
+    Contract Symbol, Proc => Any
     def on_change(type, &block)
-      @on_change_callbacks << CallbackDescriptor.new(type, block)
-      @on_change_callbacks
+      @on_change_callbacks = @on_change_callbacks.push(CallbackDescriptor.new(type, block))
     end
 
     # Backwards compatible change handler.
@@ -328,7 +329,7 @@ module Middleman
     # @param [Set] callback_descriptors The registered callbacks.
     # @param [Array<Middleman::SourceFile>] files The files that were changed.
     # @return [void]
-    Contract ArrayOf[CallbackDescriptor], ArrayOf[SourceFile], ArrayOf[SourceFile] => Any
+    Contract VectorOf[CallbackDescriptor], ArrayOf[SourceFile], ArrayOf[SourceFile] => Any
     def run_callbacks(callback_descriptors, updated_files, removed_files)
       callback_descriptors.each do |callback|
         if callback[:type] == :all