First cut at Perpetuity in-memory support as a gem.

This is based on perpetuity-mongodb and Christopher Garvis's
pull request (jgaskins/perpetuity#20)
for an in-memory data store.

.gitignore

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+*.gem
+*.rbc
+.bundle
+.config
+.yardoc
+Gemfile.lock
+InstalledFiles
+_yardoc
+coverage
+doc/
+lib/bundler/man
+pkg
+rdoc
+spec/reports
+test/tmp
+test/version_tmp
+tmp

Gemfile

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+source 'https://rubygems.org'
+
+# Specify your gem's dependencies in the perpetuity-memory.gemspec file.
+gemspec

LICENSE.txt

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+Copyright (c) 2013 Jamie Gaskins
+
+MIT License
+
+Permission is hereby granted, free of charge, to any person obtaining
+a copy of this software and associated documentation files (the
+"Software"), to deal in the Software without restriction, including
+without limitation the rights to use, copy, modify, merge, publish,
+distribute, sublicense, and/or sell copies of the Software, and to
+permit persons to whom the Software is furnished to do so, subject to
+the following conditions:
+
+The above copyright notice and this permission notice shall be
+included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

README.md

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+# Perpetuity::Memory
+
+This is the in-memory adapter for Perpetuity.
+
+## Installation
+
+Add this line to your application's Gemfile:
+
+    gem 'perpetuity-memory'
+
+And then execute:
+
+    $ bundle
+
+Or install it yourself as:
+
+    $ gem install perpetuity-memory
+
+## Usage
+
+Put this at the top of your application (or in a Rails initializer):
+
+```ruby
+require 'perpetuity/memory' # Unnecessary if using Rails
+Perpetuity.data_source :memory
+```
+
+For information on using Perpetuity to persist your Ruby objects, see the [main Perpetuity repo](https://github.com/jgaskins/perpetuity).
+
+## Contributing
+
+1. Fork it
+2. Create your feature branch (`git checkout -b my-new-feature`)
+3. Commit your changes (`git commit -am 'Add some feature'`)
+4. Push to the branch (`git push origin my-new-feature`)
+5. Create new Pull Request

Rakefile

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+require "bundler/gem_tasks"

lib/perpetuity/memory.rb

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+require 'perpetuity'
+require 'perpetuity/exceptions/duplicate_key_error'
+require 'perpetuity/attribute'
+require 'set'
+require 'securerandom'
+
+module Perpetuity
+  class Memory
+    def initialize options = {}
+      @cache    = Hash.new
+      @indexes  = Hash.new { |hash, key| hash[key] = active_indexes(key) }
+    end
+
+    def insert klass, attributes, _
+      if attributes.is_a? Array
+        return attributes.map{|attr| insert(klass, attr, _)}
+      end
+
+      unless attributes.has_key? :id
+        attributes[:id] = SecureRandom.uuid
+      end
+
+      # make keys indifferent
+      attributes.default_proc = proc do |h, k|
+        case k
+          when String then sym = k.to_sym; h[sym] if h.key?(sym)
+          when Symbol then str = k.to_s; h[str] if h.key?(str)
+        end
+      end
+
+      collection(klass) << attributes
+      attributes[:id]
+    end
+
+    def count klass, criteria=nil, &block
+      if block_given?
+        collection(klass).select(&block).size
+      elsif criteria
+        collection(klass).select(&criteria).size
+      else
+        collection(klass).size
+      end
+    end
+
+    def delete_all klass
+      collection(klass).clear
+    end
+
+    def first klass
+      collection(klass).first
+    end
+
+    def find klass, id
+      collection(klass).find{|o| o[:id] = id}
+    end
+
+    def retrieve klass, criteria, options = {}
+      collection(klass).find_all(&criteria)
+    end
+
+    def all klass
+      collection(klass)
+    end
+
+    def delete object, klass=nil
+      klass ||= object.class
+      id = object.class == String || !object.respond_to?(:id) ? object : object.id
+
+      collection(klass).each_with_index do |record, index|
+        if record[:id] === id
+          collection(klass).delete_at index
+        end
+      end
+    end
+
+    def update klass, id, new_data
+      collection(klass).each_with_index do |record, index|
+        if record[:id] == id
+          collection(klass)[index] = record.merge(new_data)
+        end
+      end
+    end
+
+    def increment klass, id, attribute, count=1
+      find(klass, id)[attribute] += count
+    end
+
+    def can_serialize? value
+      true
+    end
+
+    def serialize object, mapper
+      Marshal.dump(object)
+    end
+
+    def unserialize data, mapper
+      Marshal.load(data)
+    end
+
+
+    def index klass, attribute, options={}
+      @indexes[klass] ||= Set.new
+    end
+
+    def indexes klass
+      @indexes[klass]
+    end
+
+    def active_indexes klass
+      Set.new
+    end
+
+    def activate_index! klass
+      true
+    end
+
+    def remove_index index
+    end
+
+    def query &block
+      block
+    end
+
+    protected
+
+    def collection klass
+      @cache[klass] = Array.new unless @cache.key? klass
+      @cache[klass]
+    end
+  end
+end

lib/perpetuity/memory/version.rb

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+module Perpetuity
+  module Memory
+    VERSION = "0.1.0"
+  end
+end

perpetuity-memory.gemspec

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+# coding: utf-8
+lib = File.expand_path('../lib', __FILE__)
+$LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
+require 'perpetuity/memory/version'
+
+Gem::Specification.new do |spec|
+  spec.name          = "perpetuity-memory"
+  spec.version       = Perpetuity::Memory::VERSION
+  spec.authors       = ["Jamie Gaskins", "Christopher Garvis", "Craig Buchek"]
+  spec.email         = ["craig@boochtek.com"]
+  spec.description   = %q{In-memory adapter for Perpetuity}
+  spec.summary       = %q{In-memory adapter for Perpetuity}
+  spec.homepage      = "https://github.com/boochtek/perpetuity-memory"
+  spec.license       = "MIT"
+
+  spec.files         = `git ls-files`.split($/)
+  spec.executables   = spec.files.grep(%r{^bin/}) { |f| File.basename(f) }
+  spec.test_files    = spec.files.grep(%r{^(test|spec|features)/})
+  spec.require_paths = ["lib"]
+
+  spec.add_development_dependency "bundler", "~> 1.3"
+  spec.add_development_dependency "rspec"
+  spec.add_development_dependency "rake"
+  spec.add_runtime_dependency "perpetuity", "~> 1.0.0.beta3"
+end

spec/perpetuity/memory_spec.rb

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+require 'perpetuity/memory'
+require 'date'
+
+module Perpetuity
+  describe Memory do
+    let(:klass) { String }
+
+    it 'inserts' do
+      expect { subject.insert klass, { name: 'foo' }, [] }.to change { subject.count klass }.by 1
+    end
+
+    it 'inserts multiple objects' do
+      expect { subject.insert klass, [{name: 'foo'}, {name: 'bar'}], [] }
+        .to change { subject.count klass }.by 2
+    end
+
+    it 'removes all objects' do
+      subject.insert klass, {}, []
+      subject.delete_all klass
+      subject.count(klass).should == 0
+    end
+
+    it 'counts the number of objects' do
+      subject.delete_all klass
+      3.times do
+        subject.insert klass, {}, []
+      end
+      subject.count(klass).should == 3
+    end
+
+    it 'counts the objects matching a query' do
+      subject.delete_all klass
+      1.times { subject.insert klass, { name: 'bar' }, [] }
+      3.times { subject.insert klass, { name: 'foo' }, [] }
+      subject.count(klass) { |o| o[:name] == 'foo' }.should == 3
+    end
+
+    it 'gets the first object' do
+      value = {value: 1}
+      subject.insert klass, value, []
+      subject.first(klass)[:value].should == value['value']
+    end
+
+    it 'gets all objects' do
+      values = [{value: 1}, {value: 2}]
+      subject.insert klass, values, []
+      subject.all(klass).should == values
+    end
+
+    it 'retrieves by ID if the ID is a string' do
+      time = Time.now.utc
+      id = subject.insert Object, {inserted: time}, []
+
+      object = subject.retrieve(Object, subject.query{|o| o[:id] == id.to_s }).first
+      retrieved_time = object["inserted"]
+      retrieved_time.to_f.should be_within(0.001).of time.to_f
+    end
+
+    describe 'serialization' do
+    end
+
+    describe 'indexing' do
+      it 'does nothing' do
+        subject.index(klass, 'anything')
+      end
+    end
+
+    describe 'atomic operations' do
+      after(:all) { subject.delete_all klass }
+
+      it 'increments the value of an attribute' do
+        id = subject.insert klass, { count: 1 }, []
+        subject.increment klass, id, :count
+        subject.increment klass, id, :count, 10
+        query = subject.query { |o| o[:id] == id }
+        subject.retrieve(klass, query).first['count'].should be == 12
+        subject.increment klass, id, :count, -1
+        subject.retrieve(klass, query).first['count'].should be == 11
+      end
+    end
+  end
+end