Memoize absolute_url and relative_url filters

These two filters are quite slow, as Addressable::URI.parse is quite
slow. Fortunately, they are pure functions and so can be easily
memoized. This one change shaves 8s off the CockroachDB docs build
(github.com/cockroachdb/docs).

lib/jekyll/filters/url_filters.rb

@@ -9,17 +9,8 @@ module URLFilters
       #
       # Returns the absolute URL as a String.
       def absolute_url(input)
-        return if input.nil?
-
-        input = input.url if input.respond_to?(:url)
-        return input if Addressable::URI.parse(input.to_s).absolute?
-
-        site = @context.registers[:site]
-        return relative_url(input) if site.config["url"].nil?
-
-        Addressable::URI.parse(
-          site.config["url"].to_s + relative_url(input)
-        ).normalize.to_s
+        cache = (@context.registers[:cached_absolute_urls] ||= {})
+        cache[input] ||= compute_absolute_url(input)
       end
 
       # Produces a URL relative to the domain root based on site.baseurl
@@ -29,15 +20,8 @@ def absolute_url(input)
       #
       # Returns a URL relative to the domain root as a String.
       def relative_url(input)
-        return if input.nil?
-
-        input = input.url if input.respond_to?(:url)
-        return input if Addressable::URI.parse(input.to_s).absolute?
-
-        parts = [sanitized_baseurl, input]
-        Addressable::URI.parse(
-          parts.compact.map { |part| ensure_leading_slash(part.to_s) }.join
-        ).normalize.to_s
+        cache = (@context.registers[:cached_relative_urls] ||= {})
+        cache[input] ||= compute_relative_url(input)
       end
 
       # Strips trailing `/index.html` from URLs to create pretty permalinks
@@ -53,6 +37,32 @@ def strip_index(input)
 
       private
 
+      def compute_absolute_url(input)
+        return if input.nil?
+
+        input = input.url if input.respond_to?(:url)
+        return input if Addressable::URI.parse(input.to_s).absolute?
+
+        site = @context.registers[:site]
+        return relative_url(input) if site.config["url"].nil?
+
+        Addressable::URI.parse(
+          site.config["url"].to_s + relative_url(input)
+        ).normalize.to_s
+      end
+
+      def compute_relative_url(input)
+        return if input.nil?
+
+        input = input.url if input.respond_to?(:url)
+        return input if Addressable::URI.parse(input.to_s).absolute?
+
+        parts = [sanitized_baseurl, input]
+        Addressable::URI.parse(
+          parts.compact.map { |part| ensure_leading_slash(part.to_s) }.join
+        ).normalize.to_s
+      end
+
       def sanitized_baseurl
         site = @context.registers[:site]
         site.config["baseurl"].to_s.chomp("/")