class Rack::Deflater

encoding.
Note that despite the name, Deflater does not support the deflate
is empty.
code is one that doesn’t allow an entity body, or when the body
directive of ‘no-transform’ is present, when the response status
and allowed. For example no encoding is made when a cache
This middleware automatically detects when encoding is supported
* identity (no transformation)
* gzip
Currently supported encodings:
usually for purposes of compression.
This middleware enables content encoding of http responses,

def call(env) 

def call(env)
  status, headers, body = response = @app.call(env)
  unless should_deflate?(env, status, headers, body)
    return response
  end
  request = Request.new(env)
  encoding = Utils.select_best_encoding(%w(gzip identity),
                                        request.accept_encoding)
  # Set the Vary HTTP header.
  vary = headers["vary"].to_s.split(",").map(&:strip)
  unless vary.include?("*") || vary.any?{|v| v.downcase == 'accept-encoding'}
    headers["vary"] = vary.push("Accept-Encoding").join(",")
  end
  case encoding
  when "gzip"
    headers['content-encoding'] = "gzip"
    headers.delete(CONTENT_LENGTH)
    mtime = headers["last-modified"]
    mtime = Time.httpdate(mtime).to_i if mtime
    response[2] = GzipStream.new(body, mtime, @sync)
    response
  when "identity"
    response
  else # when nil
    # Only possible encoding values here are 'gzip', 'identity', and nil
    message = "An acceptable encoding for the requested resource #{request.fullpath} could not be found."
    bp = Rack::BodyProxy.new([message]) { body.close if body.respond_to?(:close) }
    [406, { CONTENT_TYPE => "text/plain", CONTENT_LENGTH => message.length.to_s }, bp]
  end
end

def initialize(app, options = {})

Defaults to +true+.
latency for time-sensitive streaming applications, but hurts compression and throughput.
:sync :: determines if the stream is going to be flushed after every chunk. Flushing after every chunk reduces
:include :: a list of content types that should be compressed. By default, all content types are compressed.
such as when it is an +IO+ instance.
However, be aware that calling `body.each` inside the block will break cases where `body.each` is not idempotent,
(e.g use Rack::Deflater, :if => lambda { |*, body| sum=0; body.each { |i| sum += i.length }; sum > 512 }).
:if :: a lambda enabling / disabling deflation based on returned boolean value

Creates Rack::Deflater middleware. Options: 
def initialize(app, options = {})
  @app = app
  @condition = options[:if]
  @compressible_types = options[:include]
  @sync = options.fetch(:sync, true)
end

def should_deflate?(env, status, headers, body)

Whether the body should be compressed. 
def should_deflate?(env, status, headers, body)
  # Skip compressing empty entity body responses and responses with
  # no-transform set.
  if Utils::STATUS_WITH_NO_ENTITY_BODY.key?(status.to_i) ||
      /\bno-transform\b/.match?(headers[CACHE_CONTROL].to_s) ||
      headers['content-encoding']&.!~(/\bidentity\b/)
    return false
  end
  # Skip if @compressible_types are given and does not include request's content type
  return false if @compressible_types && !(headers.has_key?(CONTENT_TYPE) && @compressible_types.include?(headers[CONTENT_TYPE][/[^;]*/]))
  # Skip if @condition lambda is given and evaluates to false
  return false if @condition && !@condition.call(env, status, headers, body)
  # No point in compressing empty body, also handles usage with
  # Rack::Sendfile.
  return false if headers[CONTENT_LENGTH] == '0'
  true
end