Introduce serializers wrapper class

Provides a wrapper around a set of serializers with constraints to
prevent breakage.

lib/readthis.rb

@@ -1,5 +1,11 @@
 require 'readthis/cache'
 require 'readthis/version'
+require 'readthis/serializers'
 
 module Readthis
+  extend self
+
+  def serializers
+    @serializers ||= Readthis::Serializers.new
+  end
 end

lib/readthis/serializers.rb

@@ -0,0 +1,49 @@
+require 'readthis/passthrough'
+
+module Readthis
+  SerializersFrozenError = Class.new(Exception)
+  SerializersLimitError  = Class.new(Exception)
+
+  class Serializers
+    BASE_SERIALIZERS = {
+      Marshal     => 0x1,
+      Passthrough => 0x2,
+      JSON        => 0x3
+    }.freeze
+
+    SERIALIZER_LIMIT = 7
+
+    attr_reader :serializers
+
+    def initialize
+      @serializers = BASE_SERIALIZERS.dup
+    end
+
+    def inverted
+      serializers.invert
+    end
+
+    def <<(serializer)
+      case
+      when serializers.frozen?
+        raise SerializersFrozenError
+      when serializers.length >= SERIALIZER_LIMIT
+        raise SerializersLimitError
+      else
+        @serializers[serializer] = flags.max.succ
+      end
+    end
+
+    def freeze!
+      serializers.freeze
+    end
+
+    def modules
+      serializers.keys
+    end
+
+    def flags
+      serializers.values
+    end
+  end
+end

spec/readthis/serializers_spec.rb

@@ -0,0 +1,58 @@
+require 'readthis/serializers'
+
+RSpec.describe Readthis::Serializers do
+  CustomSerializer  = Class.new
+  AnotherSerializer = Class.new
+
+  describe '#<<' do
+    it 'appends new serializers' do
+      serializers = Readthis::Serializers.new
+
+      serializers << CustomSerializer
+
+      expect(serializers.modules).to include(CustomSerializer)
+      expect(serializers.flags).to eq((1..4).to_a)
+      expect(serializers.serializers).to include(
+        CustomSerializer => 4
+      )
+    end
+
+    it 'increments flags' do
+      serializers = Readthis::Serializers.new
+      serializers << CustomSerializer
+      serializers << AnotherSerializer
+
+      expect(serializers.flags).to eq((1..5).to_a)
+    end
+
+    it 'prevents more than seven serializers' do
+      serializers = Readthis::Serializers.new
+
+      expect {
+        10.times { serializers << Class.new }
+      }.to raise_error(Readthis::SerializersLimitError)
+    end
+  end
+
+  describe '#inverted' do
+    it 'inverts the current set of serializers' do
+      serializers = Readthis::Serializers.new
+
+      expect(serializers.inverted).to include(
+        1 => Marshal
+      )
+    end
+  end
+
+  describe '#freeze' do
+    it 'does now allow appending after freeze' do
+      serializers = Readthis::Serializers.new
+
+      serializers.freeze!
+
+      expect {
+        serializers << CustomSerializer
+      }.to raise_error(Readthis::SerializersFrozenError)
+    end
+  end
+end

spec/readthis_spec.rb

@@ -0,0 +1,9 @@
+require 'readthis'
+
+RSpec.describe Readthis do
+  describe '#serializers' do
+    it 'lists currently configured serializers' do
+      expect(Readthis.serializers.keys).to include(Marshal, JSON)
+    end
+  end
+end