Optimize markdownify allocations with in-place gsub! and combined regex (#4805)

* Optimize markdownify allocations with in-place gsub and combined regex

Avoid chaining multiple full-string copies during markdown parsing. Duplicate
the input string once in ApplicationHelper#markdownify and modify it in-place
using gsub! in GrowstuffMarkdown#expand_crops! and #expand_members!.
Combine MEMBER_REGEX and MEMBER_AT_REGEX with Regexp.union to parse member
mentions in a single pass.

Co-authored-by: CloCkWeRX <365751+CloCkWeRX@users.noreply.github.com>

* Apply suggestion from @CloCkWeRX

* Apply suggestion from @CloCkWeRX

* Optimize markdownify allocations with in-place gsub and combined regex

Avoid chaining multiple full-string copies during markdown parsing. Duplicate
the input string once in ApplicationHelper#markdownify and modify it in-place
using gsub! in GrowstuffMarkdown#expand_crops! and #expand_members!.
Combine MEMBER_REGEX and MEMBER_AT_REGEX with Regexp.union to parse member
mentions in a single pass.

Co-authored-by: CloCkWeRX <365751+CloCkWeRX@users.noreply.github.com>

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

app/helpers/application_helper.rb

@@ -59,7 +59,10 @@ module ApplicationHelper
   # of HAML, Tilt, and dynamic compilation with interpolated ruby.
   def markdownify(text)
     translator = Haml::Filters::GrowstuffMarkdown.new
-    translator.expand_members!(translator.expand_crops!(text.to_s))
+    text = text.to_s.dup
+    translator.expand_crops!(text)
+    translator.expand_members!(text)
+    text
   end
 
   #

lib/haml/filters/growstuff_markdown.rb

@@ -6,31 +6,32 @@ class Haml::Filters
     CROP_REGEX = /(?<!\\)\[([^\[\]]+?)\]\(crop\)/
     MEMBER_REGEX = /(?<!\\)\[([^\[\]]+?)\]\(member\)/
     MEMBER_AT_REGEX = /(?<!\\)(@\w+)/
+    MEMBER_COMBINED_REGEX = Regexp.union(MEMBER_REGEX, MEMBER_AT_REGEX)
     MEMBER_ESCAPE_AT_REGEX = /(?<!\\)\\(?=@\w+)/
     HOST = Rails.application.config.host
 
     def expand_crops!(text)
       # turn [tomato](crop) into [tomato](http://growstuff.org/crops/tomato)
-      text.gsub(CROP_REGEX) do
+      text.gsub!(CROP_REGEX) do
         crop_str = Regexp.last_match(1)
         # find crop case-insensitively
         crop = Crop.where('lower(name) = ?', crop_str.downcase).first
         crop_link crop, crop_str
       end
+      text
     end
 
     def expand_members!(text)
       # turn [jane](member) into [jane](http://growstuff.org/members/jane)
       # turn @jane into [@jane](http://growstuff.org/members/jane)
-      [MEMBER_REGEX, MEMBER_AT_REGEX].each do |re|
-        text = text.gsub(re) do
-          member_str = Regexp.last_match(1)
-          member = find_member(member_str)
-          member_link(member, member_str)
-        end
+      text.gsub!(MEMBER_COMBINED_REGEX) do
+        member_str = Regexp.last_match(1) || Regexp.last_match(2)
+        member = find_member(member_str)
+        member_link(member, member_str)
       end
 
-      text.gsub(MEMBER_ESCAPE_AT_REGEX, '')
+      text.gsub!(MEMBER_ESCAPE_AT_REGEX, '')
+      text
     end
 
     def member_link(member, link_text)