Support nanoc environments

Introduce basic support for nanoc environment:
- nanoc command is updated to include --env argument
- config are overriden using the active environment if any
- unless define in environment, output_dir is {{output_dir}}/{{env_name}}

Setting environments is done in nanoc.yaml using the `environments` property.
Example usage is:

```
output_dir: output

environments:
  default: &default
    base_url: ...
    ...
  development:
    <<: *default
    base_url: ...
  production:
    <<: *default
    base_url: ...
  yet_another_env:
    <<: *default
    base_url: ...
    output_dir: build
```

Selecting working environment can be done:
- using environment variable `NANOC_ENVIRONMENT`
- using `nanoc --env=[<value>]`

lib/nanoc/base/entities/configuration.rb

@@ -31,10 +31,14 @@ class Configuration
       string_pattern_type: 'glob',
     }.freeze
 
+    attr_reader :env
+
     # Creates a new configuration with the given hash.
     #
     # @param [Hash] hash The actual configuration hash
-    def initialize(hash = {})
+    # @param [Symbol, String] env The active environment for this configuration
+    def initialize(hash = {}, env = nil)
+      @env = env
       @wrapped = hash.__nanoc_symbolize_keys_recursively
     end
 
@@ -47,6 +51,21 @@ def with_defaults
       self.class.new(new_wrapped)
     end
 
+    def with_environment(env = nil)
+      return self unless @wrapped.key? :environments
+
+      # Set active environment
+      env ||= ENV.fetch 'NANOC_ENVIRONMENT', :default
+
+      # Load given environment configuration
+      env_config = @wrapped[:environments][env.to_sym] || {}
+
+      # Handle specific properties
+      env_config[:output_dir] ||= File.join(@wrapped[:output_dir].to_s, env.to_s)
+
+      self.class.new(@wrapped, env).merge(env_config)
+    end
+
     def to_h
       @wrapped
     end
@@ -76,11 +95,11 @@ def []=(key, value)
     end
 
     def merge(hash)
-      self.class.new(@wrapped.merge(hash.to_h))
+      self.class.new(@wrapped.merge(hash.to_h), @env)
     end
 
     def without(key)
-      self.class.new(@wrapped.reject { |k, _v| k == key })
+      self.class.new(@wrapped.reject { |k, _v| k == key }, @env)
     end
 
     def update(hash)

lib/nanoc/base/repos/checksum_store.rb

@@ -6,7 +6,7 @@ module Nanoc::Int
   class ChecksumStore < ::Nanoc::Int::Store
     # @param [Nanoc::Int::Site] site
     def initialize(site: nil)
-      super('tmp/checksums', 1)
+      super(File.join('tmp', (site.config.env if site).to_s, 'checksums'), 1)
 
       @site = site
 

lib/nanoc/base/repos/compiled_content_cache.rb

@@ -4,8 +4,8 @@ module Nanoc::Int
   #
   # @api private
   class CompiledContentCache < ::Nanoc::Int::Store
-    def initialize
-      super('tmp/compiled_content', 2)
+    def initialize(env: nil)
+      super(File.join('tmp', env.to_s, 'compiled_content'), 2)
 
       @cache = {}
     end

lib/nanoc/base/repos/config_loader.rb

@@ -40,7 +40,7 @@ def new_from_cwd
       apply_parent_config(
         Nanoc::Int::Configuration.new(YAML.load_file(filename)),
         [filename],
-      ).with_defaults
+      ).with_defaults.with_environment
     end
 
     # @api private

lib/nanoc/base/repos/dependency_store.rb

@@ -5,8 +5,8 @@ class DependencyStore < ::Nanoc::Int::Store
     attr_reader :objects
 
     # @param [Array<Nanoc::Int::Item, Nanoc::Int::Layout>] objects
-    def initialize(objects)
-      super('tmp/dependencies', 4)
+    def initialize(objects, env: nil)
+      super(File.join('tmp', env.to_s, 'dependencies'), 4)
 
       @objects = objects
       @graph   = Nanoc::Int::DirectedGraph.new([nil] + @objects)

lib/nanoc/base/repos/rule_memory_store.rb

@@ -4,8 +4,8 @@ module Nanoc::Int
   #
   # @api private
   class RuleMemoryStore < ::Nanoc::Int::Store
-    def initialize
-      super('tmp/rule_memory', 1)
+    def initialize(env: nil)
+      super(File.join('tmp', env.to_s, 'rule_memory'), 1)
 
       @rule_memories = {}
     end

lib/nanoc/base/services/compiler_loader.rb

@@ -2,10 +2,10 @@ module Nanoc::Int
   # @api private
   class CompilerLoader
     def load(site)
-      rule_memory_store = Nanoc::Int::RuleMemoryStore.new
+      rule_memory_store = Nanoc::Int::RuleMemoryStore.new env: site.config.env
 
       dependency_store =
-        Nanoc::Int::DependencyStore.new(site.items.to_a + site.layouts.to_a)
+        Nanoc::Int::DependencyStore.new(site.items.to_a + site.layouts.to_a, env: site.config.env)
 
       checksum_store =
         Nanoc::Int::ChecksumStore.new(site: site)
@@ -25,7 +25,7 @@ def load(site)
         )
 
       params = {
-        compiled_content_cache: Nanoc::Int::CompiledContentCache.new,
+        compiled_content_cache: Nanoc::Int::CompiledContentCache.new(env: site.config.env),
         checksum_store: checksum_store,
         rule_memory_store: rule_memory_store,
         dependency_store: dependency_store,

lib/nanoc/cli/commands/nanoc.rb

@@ -10,6 +10,10 @@
   Nanoc::CLI.debug = true
 end
 
+opt :e, :env, 'set environment', argument: :optional do |value|
+  ENV.store 'NANOC_ENVIRONMENT', value || :default
+end
+
 opt :h, :help, 'show the help message and quit' do |_value, cmd|
   puts cmd.help
   exit 0

spec/nanoc/base/entities/configuration_spec.rb

@@ -1,10 +1,9 @@
 describe Nanoc::Int::Configuration do
-  let(:configuration) { described_class.new(hash) }
-
   let(:hash) { { foo: 'bar' } }
+  let(:config) { described_class.new(hash) }
 
   describe '#key?' do
-    subject { configuration.key?(key) }
+    subject { config.key?(key) }
 
     context 'non-existent key' do
       let(:key) { :donkey }
@@ -16,4 +15,35 @@
       it { is_expected.to be }
     end
   end
+
+  context 'with environments defined' do
+    let(:hash) { { foo: 'bar', environments: { test: { foo: 'test-bar' }, default: { foo: 'default-bar' } } } }
+    let(:config) { described_class.new(hash).with_environment(env) }
+
+    subject { config }
+
+    context 'with existing environment' do
+      let(:env) { :test }
+
+      it 'inherits options from given environment' do
+        expect(subject[:foo]).to eq('test-bar')
+      end
+    end
+
+    context 'with unknown environment' do
+      let(:env) { :wtf }
+
+      it 'does not inherits options from any environment' do
+        expect(subject[:foo]).to eq('bar')
+      end
+    end
+
+    context 'without given environment' do
+      let(:env) { nil }
+
+      it 'inherits options from default environment' do
+        expect(subject[:foo]).to eq('default-bar')
+      end
+    end
+  end
 end

spec/nanoc/base/repos/config_loader_spec.rb

@@ -52,6 +52,25 @@
         expect(subject[:parent_config_file]).to be_nil
       end
     end
+
+    context 'config file present and environment defined' do
+      before do
+        File.write('nanoc.yaml', YAML.dump({ foo: 'bar', environments: { test: { foo: 'test-bar' }, default: { foo: 'default-bar' } } }))
+      end
+
+      it 'returns the configuration' do
+        expect(subject).to be_a(Nanoc::Int::Configuration)
+      end
+
+      it 'has the test environment custom option' do
+        allow(ENV).to receive(:fetch).with('NANOC_ENVIRONMENT', :default).and_return('test')
+        expect(subject[:foo]).to eq('test-bar')
+      end
+
+      it 'has the default environment custom option' do
+        expect(subject[:foo]).to eq('default-bar')
+      end
+    end
   end
 
   describe '.cwd_is_nanoc_site? + .config_filename_for_cwd' do