module Sprockets::Server

def cache_headers(env, etag)
  headers = {}
  # Set caching headers
  headers["Cache-Control"] = String.new("public")
  headers["ETag"]          = %("#{etag}")
  # If the request url contains a fingerprint, set a long
  # expires on the response
  if path_fingerprint(env["PATH_INFO"])
    headers["Cache-Control"] << ", max-age=31536000, immutable"
  # Otherwise set `must-revalidate` since the asset could be modified.
  else
    headers["Cache-Control"] << ", must-revalidate"
    headers["Vary"] = "Accept-Encoding"
  end
  headers
end

def call(env)
  start_time = Time.now.to_f
  time_elapsed = lambda { ((Time.now.to_f - start_time) * 1000).to_i }
  unless ALLOWED_REQUEST_METHODS.include? env['REQUEST_METHOD']
    return method_not_allowed_response
  end
  msg = "Served asset #{env['PATH_INFO']} -"
  # Extract the path from everything after the leading slash
  path = Rack::Utils.unescape(env['PATH_INFO'].to_s.sub(/^\//, ''))
  # Strip fingerprint
  if fingerprint = path_fingerprint(path)
    path = path.sub("-#{fingerprint}", '')
  end
  # URLs containing a `".."` are rejected for security reasons.
  if forbidden_request?(path)
    return forbidden_response(env)
  end
  if fingerprint
    if_match = fingerprint
  elsif env['HTTP_IF_MATCH']
    if_match = env['HTTP_IF_MATCH'][/^"(\w+)"$/, 1]
  end
  if env['HTTP_IF_NONE_MATCH']
    if_none_match = env['HTTP_IF_NONE_MATCH'][/^"(\w+)"$/, 1]
  end
  # Look up the asset.
  asset = find_asset(path)
  if asset.nil?
    status = :not_found
  elsif fingerprint && asset.etag != fingerprint
    status = :not_found
  elsif if_match && asset.etag != if_match
    status = :precondition_failed
  elsif if_none_match && asset.etag == if_none_match
    status = :not_modified
  else
    status = :ok
  end
  case status
  when :ok
    logger.info "#{msg} 200 OK (#{time_elapsed.call}ms)"
    ok_response(asset, env)
  when :not_modified
    logger.info "#{msg} 304 Not Modified (#{time_elapsed.call}ms)"
    not_modified_response(env, if_none_match)
  when :not_found
    logger.info "#{msg} 404 Not Found (#{time_elapsed.call}ms)"
    not_found_response(env)
  when :precondition_failed
    logger.info "#{msg} 412 Precondition Failed (#{time_elapsed.call}ms)"
    precondition_failed_response(env)
  end
rescue Exception => e
  logger.error "Error compiling asset #{path}:"
  logger.error "#{e.class.name}: #{e.message}"
  case File.extname(path)
  when ".js"
    # Re-throw JavaScript asset exceptions to the browser
    logger.info "#{msg} 500 Internal Server Error\n\n"
    return javascript_exception_response(e)
  when ".css"
    # Display CSS asset exceptions in the browser
    logger.info "#{msg} 500 Internal Server Error\n\n"
    return css_exception_response(e)
  else
    raise
  end
end

def css_exception_response(exception)
  message   = "\n#{exception.class.name}: #{exception.message}"
  backtrace = "\n  #{exception.backtrace.first}"
  body = <<-CSS
    html {
      padding: 18px 36px;
    }
    head {
      display: block;
    }
    body {
      margin: 0;
      padding: 0;
    }
    body > * {
      display: none !important;
    }
    head:after, body:before, body:after {
      display: block !important;
    }
    head:after {
      font-family: sans-serif;
      font-size: large;
      font-weight: bold;
      content: "Error compiling CSS asset";
    }
    body:before, body:after {
      font-family: monospace;
      white-space: pre-wrap;
    }
    body:before {
      font-weight: bold;
      content: "#{escape_css_content(message)}";
    }
    body:after {
      content: "#{escape_css_content(backtrace)}";
    }
  CSS
  [ 200, { "Content-Type" => "text/css; charset=utf-8", "Content-Length" => body.bytesize.to_s }, [ body ] ]
end

def escape_css_content(content)
  content.
    gsub('\\', '\\\\005c ').
    gsub("\n", '\\\\000a ').
    gsub('"',  '\\\\0022 ').
    gsub('/',  '\\\\002f ')
end

def forbidden_request?(path)
  # Prevent access to files elsewhere on the file system
  #
  #     http://example.org/assets/../../../etc/passwd
  #
  path.include?("..") || absolute_path?(path)
end

def forbidden_response(env)
  if head_request?(env)
    [ 403, { "Content-Type" => "text/plain", "Content-Length" => "0" }, [] ]
  else
    [ 403, { "Content-Type" => "text/plain", "Content-Length" => "9" }, [ "Forbidden" ] ]
  end
end

def head_request?(env)
  env['REQUEST_METHOD'] == 'HEAD'
end

def headers(env, asset, length)
  headers = {}
  # Set content length header
  headers["Content-Length"] = length.to_s
  # Set content type header
  if type = asset.content_type
    # Set charset param for text/* mime types
    if type.start_with?("text/") && asset.charset
      type += "; charset=#{asset.charset}"
    end
    headers["Content-Type"] = type
  end
  headers.merge(cache_headers(env, asset.etag))
end

def javascript_exception_response(exception)
  err  = "#{exception.class.name}: #{exception.message}\n  (in #{exception.backtrace[0]})"
  body = "throw Error(#{err.inspect})"
  [ 200, { "Content-Type" => "application/javascript", "Content-Length" => body.bytesize.to_s }, [ body ] ]
end

def method_not_allowed_response
  [ 405, { "Content-Type" => "text/plain", "Content-Length" => "18" }, [ "Method Not Allowed" ] ]
end

def not_found_response(env)
  if head_request?(env)
    [ 404, { "Content-Type" => "text/plain", "Content-Length" => "0", "X-Cascade" => "pass" }, [] ]
  else
    [ 404, { "Content-Type" => "text/plain", "Content-Length" => "9", "X-Cascade" => "pass" }, [ "Not found" ] ]
  end
end

def not_modified_response(env, etag)
  [ 304, cache_headers(env, etag), [] ]
end

def ok_response(asset, env)
  if head_request?(env)
    [ 200, headers(env, asset, 0), [] ]
  else
    [ 200, headers(env, asset, asset.length), asset ]
  end
end

def path_fingerprint(path)
  path[/-([0-9a-f]{7,128})\.[^.]+\z/, 1]
end

def precondition_failed_response(env)
  if head_request?(env)
    [ 412, { "Content-Type" => "text/plain", "Content-Length" => "0", "X-Cascade" => "pass" }, [] ]
  else
    [ 412, { "Content-Type" => "text/plain", "Content-Length" => "19", "X-Cascade" => "pass" }, [ "Precondition Failed" ] ]
  end
end