class Sprockets::Index

‘Environment#index`.
`Index` should not be initialized directly. Instead use
is immutable.
behavior is ideal in production environments where the file system
for the instances lifetime. This makes `Index` much faster. This
The expection is that all of its file system methods are cached
`Index` is a special cached version of `Environment`.

def build_asset(path, pathname, options)

Cache asset building in memory and in persisted cache. 
def build_asset(path, pathname, options)
  # Memory cache
  key = cache_key_for(pathname, options)
  if @assets.key?(key)
    @assets[key]
  else
    @assets[key] = begin
      # Persisted cache
      cache_asset(key) do
        super
      end
    end
  end
end

def expire_index!

should bomb.
Index is immutable, any methods that try to clear the cache 
def expire_index!
  raise TypeError, "can't modify immutable index"
end

def file_digest(pathname)

Cache calls to `file_digest` 
def file_digest(pathname)
  key = pathname.to_s
  if @digests.key?(key)
    @digests[key]
  else
    @digests[key] = super
  end
end

def find_asset(path, options = {})

Cache `find_asset` calls 
def find_asset(path, options = {})
  options[:bundle] = true unless options.key?(:bundle)
  if asset = @assets[cache_key_for(path, options)]
    asset
  elsif asset = super
    logical_path_cache_key = cache_key_for(path, options)
    full_path_cache_key    = cache_key_for(asset.pathname, options)
    # Cache on Index
    @assets[logical_path_cache_key] = @assets[full_path_cache_key] = asset
    # Push cache upstream to Environment
    @environment.instance_eval do
      @assets[logical_path_cache_key] = @assets[full_path_cache_key] = asset
    end
    asset
  end
end

def index

No-op return self as index 
def index
  self
end

def initialize(environment) 

def initialize(environment)
  @environment = environment
  if environment.respond_to?(:default_external_encoding)
    @default_external_encoding = environment.default_external_encoding
  end
  # Copy environment attributes
  @logger            = environment.logger
  @context_class     = environment.context_class
  @cache             = environment.cache
  @trail             = environment.trail.index
  @digest            = environment.digest
  @digest_class      = environment.digest_class
  @version           = environment.version
  @mime_types        = environment.mime_types
  @engines           = environment.engines
  @preprocessors     = environment.preprocessors
  @postprocessors    = environment.postprocessors
  @bundle_processors = environment.bundle_processors
  # Initialize caches
  @assets  = {}
  @digests = {}
end