Cache header normalization to reduce object allocation

lib/http/headers.rb

@@ -4,6 +4,7 @@
 
 require "http/errors"
 require "http/headers/mixin"
+require "http/headers/normalizer"
 require "http/headers/known"
 
 module HTTP
@@ -12,13 +13,6 @@ class Headers
     extend Forwardable
     include Enumerable
 
-    # Matches HTTP header names when in "Canonical-Http-Format"
-    CANONICAL_NAME_RE = /\A[A-Z][a-z]*(?:-[A-Z][a-z]*)*\z/
-
-    # Matches valid header field name according to RFC.
-    # @see http://tools.ietf.org/html/rfc7230#section-3.2
-    COMPLIANT_NAME_RE = /\A[A-Za-z0-9!#$%&'*+\-.^_`|~]+\z/
-
     # Class constructor.
     def initialize
       # The @pile stores each header value using a three element array:
@@ -219,20 +213,15 @@ def coerce(object)
 
     private
 
+    class << self
+      def header_normalizer
+        @header_normalizer ||= Headers::Normalizer.new
+      end
+    end
+
     # Transforms `name` to canonical HTTP header capitalization
-    #
-    # @param [String] name
-    # @raise [HeaderError] if normalized name does not
-    #   match {HEADER_NAME_RE}
-    # @return [String] canonical HTTP header name
     def normalize_header(name)
-      return name if CANONICAL_NAME_RE.match?(name)
-
-      normalized = name.split(/[\-_]/).each(&:capitalize!).join("-")
-
-      return normalized if COMPLIANT_NAME_RE.match?(normalized)
-
-      raise HeaderError, "Invalid HTTP header field name: #{name.inspect}"
+      self.class.header_normalizer.normalize(name)
     end
 
     # Ensures there is no new line character in the header value

lib/http/headers/normalizer.rb

@@ -0,0 +1,82 @@
+# frozen_string_literal: true
+
+module HTTP
+  class Headers
+    class Normalizer
+      # Matches HTTP header names when in "Canonical-Http-Format"
+      CANONICAL_NAME_RE = /\A[A-Z][a-z]*(?:-[A-Z][a-z]*)*\z/
+
+      # Matches valid header field name according to RFC.
+      # @see http://tools.ietf.org/html/rfc7230#section-3.2
+      COMPLIANT_NAME_RE = /\A[A-Za-z0-9!#$%&'*+\-.^_`|~]+\z/
+
+      MAX_CACHE_SIZE = 200
+
+      def initialize
+        @cache = Cache.new(MAX_CACHE_SIZE)
+      end
+
+      # Transforms `name` to canonical HTTP header capitalization
+      def normalize(name)
+        @cache[name] ||= normalize_header(name)
+      end
+
+      private
+
+      # Transforms `name` to canonical HTTP header capitalization
+      #
+      # @param [String] name
+      # @raise [HeaderError] if normalized name does not
+      #   match {COMPLIANT_NAME_RE}
+      # @return [String] canonical HTTP header name
+      def normalize_header(name)
+        return name if CANONICAL_NAME_RE.match?(name)
+
+        normalized = name.split(/[\-_]/).each(&:capitalize!).join("-")
+
+        return normalized if COMPLIANT_NAME_RE.match?(normalized)
+
+        raise HeaderError, "Invalid HTTP header field name: #{name.inspect}"
+      end
+
+      class Cache
+        def initialize(max_size)
+          @max_size = max_size
+          @cache = {}
+        end
+
+        def get(key)
+          @cache[key]
+        end
+
+        def set(key, value)
+          @cache[key] = value
+
+          # Maintain cache size
+          return unless @cache.size > @max_size
+
+          oldest_key = @cache.keys.first
+          @cache.delete(oldest_key)
+        end
+
+        def size
+          @cache.size
+        end
+
+        def key?(key)
+          @cache.key?(key)
+        end
+
+        def [](key)
+          get(key)
+        end
+
+        def []=(key, value)
+          set(key, value)
+        end
+      end
+
+      private_constant :Cache
+    end
+  end
+end

spec/lib/http/headers/normalizer_spec.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+RSpec.describe HTTP::Headers::Normalizer do
+  subject(:normalizer) { described_class.new }
+
+  describe "#normalize" do
+    it "normalizes the header" do
+      expect(normalizer.normalize("content_type")).to eq "Content-Type"
+    end
+
+    it "caches normalized headers" do
+      object_id = normalizer.normalize("content_type").object_id
+      expect(object_id).to eq normalizer.normalize("content_type").object_id
+    end
+
+    it "only caches up to MAX_CACHE_SIZE headers" do
+      (1..described_class::MAX_CACHE_SIZE + 1).each do |i|
+        normalizer.normalize("header#{i}")
+      end
+
+      expect(normalizer.instance_variable_get(:@cache).size).to eq described_class::MAX_CACHE_SIZE
+    end
+  end
+end