Add priority header according to RFC9218.

lib/protocol/http/header/priority.rb

@@ -0,0 +1,55 @@
+# frozen_string_literal: true
+
+# Released under the MIT License.
+# Copyright, 2024, by Samuel Williams.
+
+require_relative "split"
+
+module Protocol
+	module HTTP
+		module Header
+			# Represents the `priority` header, used to indicate the relative importance of an HTTP request.
+			#
+			# The `priority` header allows clients to express their preference for how resources should be prioritized by the server. It supports directives like `u=` to specify the urgency level of a request, and `i` to indicate whether a response can be delivered incrementally. The urgency levels range from 0 (highest priority) to 7 (lowest priority), while the `i` directive is a boolean flag.
+			class Priority < Split
+				# Initialize the priority header with the given value.
+				#
+				# @parameter value [String | Nil] the value of the priority header, if any. The value should be a comma-separated string of directives.
+				def initialize(value = nil)
+					super(value&.downcase)
+				end
+
+				# Add a value to the priority header.
+				#
+				# @parameter value [String] the directive to add to the header.
+				def << value
+					super(value.downcase)
+				end
+
+				# The default urgency level if not specified.
+				DEFAULT_URGENCY = 3
+
+				# Returns the urgency level if specified. 0 is the highest priority, and 7 is the lowest.
+				#
+				# @returns [Integer | Nil] the urgency level if specified, or `nil` if not present.
+				def urgency(default = DEFAULT_URGENCY)
+					if value = self.find { |value| value.start_with?("u=") }
+						_, level = value.split("=", 2)
+						return level.to_i
+					end
+
+					return default
+				end
+
+				# Checks if the response should be delivered incrementally.
+				#
+				# The `i` directive, when present, indicates that the response can be delivered incrementally as data becomes available.
+				#
+				# @returns [Boolean] whether the request should be delivered incrementally.
+				def incremental?
+					self.include?("i")
+				end
+			end
+		end
+	end
+end

lib/protocol/http/headers.rb

@@ -13,6 +13,7 @@
 require_relative "header/vary"
 require_relative "header/authorization"
 require_relative "header/date"
+require_relative "header/priority"
 
 module Protocol
 	module HTTP
@@ -246,6 +247,7 @@ def []= key, value
 				"connection" => Header::Connection,
 				"cache-control" => Header::CacheControl,
 				"vary" => Header::Vary,
+				"priority" => Header::Priority,
 
 				# Headers specifically for proxies:
 				"via" => Split,

test/protocol/http/header/priority.rb

@@ -0,0 +1,83 @@
+# frozen_string_literal: true
+
+# Released under the MIT License.
+# Copyright, 2024, by Samuel Williams.
+
+require "protocol/http/header/priority"
+
+describe Protocol::HTTP::Header::Priority do
+	let(:header) { subject.new(description) }
+
+	with "u=1, i" do
+		it "correctly parses priority header" do
+			expect(header).to have_attributes(
+				urgency: be == 1,
+				incremental?: be == true,
+			)
+		end
+	end
+
+	with "u=0" do
+		it "correctly parses priority header" do
+			expect(header).to have_attributes(
+				urgency: be == 0,
+				incremental?: be == false,
+			)
+		end
+	end
+
+	with "i" do
+		it "correctly parses incremental flag" do
+			expect(header).to have_attributes(
+				# Default urgency level is used:
+				urgency: be == 3,
+				incremental?: be == true,
+			)
+		end
+	end
+
+	with "u=6" do
+		it "correctly parses urgency level" do
+			expect(header).to have_attributes(
+				urgency: be == 6,
+			)
+		end
+	end
+
+	with "u=9, i" do
+		it "gracefully handles non-standard urgency levels" do
+			expect(header).to have_attributes(
+				# Non-standard value is preserved
+				urgency: be == 9,
+				incremental?: be == true,
+			)
+		end
+	end
+
+	with "u=2, u=5" do
+		it "prioritizes the first urgency directive" do
+			expect(header).to have_attributes(
+				# First occurrence takes precedence
+				urgency: be == 2,
+			)
+		end
+	end
+
+	with "#<<" do
+		let(:header) { subject.new }
+
+		it "can append values" do
+			header << "u=4"
+			expect(header).to have_attributes(
+				urgency: be == 4,
+			)
+		end
+
+		it "can append incremental flag" do
+			header << "i"
+			expect(header).to have_attributes(
+				incremental?: be == true,
+			)
+		end
+	end
+end