From 4c4b75ff77c7d26ff16e4427f9d4071c7c0020c2 Mon Sep 17 00:00:00 2001
From: Ali Ebrahim <aebrahim@users.noreply.github.com>
Date: Wed, 21 Sep 2022 00:36:30 -0500
Subject: [PATCH] Implement a threadpool for faster builds.

Fixes #225
---
 jekyll_picture_tag.gemspec              |  2 ++
 lib/jekyll_picture_tag/srcsets/basic.rb | 37 +++++++++++++++++++++++--
 2 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/jekyll_picture_tag.gemspec b/jekyll_picture_tag.gemspec
index d2ab9931..1be63fb6 100644
--- a/jekyll_picture_tag.gemspec
+++ b/jekyll_picture_tag.gemspec
@@ -29,6 +29,8 @@ Gem::Specification.new do |spec|
 
   # addressable is used to url-encode image filenames.
   spec.add_runtime_dependency 'addressable', '~> 2.6'
+  # Needed for parallel excution of builds.
+  spec.add_runtime_dependency 'concurrent-ruby', '~> 1.1'
   # Jekyll versions older than 4.0 are not supported.
   spec.add_runtime_dependency 'jekyll', '~> 4.0'
   # MIME types are needed for <source> tags' type= attributes.
diff --git a/lib/jekyll_picture_tag/srcsets/basic.rb b/lib/jekyll_picture_tag/srcsets/basic.rb
index 446ba57b..7c05ae9f 100644
--- a/lib/jekyll_picture_tag/srcsets/basic.rb
+++ b/lib/jekyll_picture_tag/srcsets/basic.rb
@@ -1,4 +1,7 @@
 require 'mime-types'
+require 'set'
+require 'concurrent-ruby'
+
 module PictureTag
   # Handles srcset generation, which also handles file generation.
   module Srcsets
@@ -62,16 +65,30 @@ def height_attribute
 
       private
 
+      # Defining a threadpool takes 5 lines, but isn't really that complex
+      # of an operation, so we disable the rubocop method length warning.
+      # rubocop:disable Metrics/MethodLength
       def build_files
         # By 'files', we mean the GeneratedImage class.
         return target_files if target_files.all?(&:exists?)
 
         # This triggers GeneratedImage to actually build an image file.
-        files = checked_targets
-        files.each(&:generate)
+        files = unique_checked_targets
+        pool = Concurrent::ThreadPoolExecutor.new(
+          min_threads: 1, max_threads: Concurrent.processor_count,
+          max_queue: 2 * Concurrent.processor_count
+        )
+        files.each do |file|
+          pool.post do
+            file.generate
+          end
+        end
+        pool.shutdown
+        pool.wait_for_termination
 
         files
       end
+      # rubocop:enable Metrics/MethodLength
 
       def checked_targets
         if target_files.any? { |f| f.width > source_width }
@@ -85,6 +102,22 @@ def checked_targets
         files || target_files
       end
 
+      def unique_checked_targets
+        # Because we're processing in parallel, we can't use existince
+        # in the filestystem to prevent duplicate handling of files without
+        # a race condition, so we manually track generated files here so we
+        # don't attempt to generate the same image twice.
+        seen_files = Set[]
+        unique_targets = []
+        checked_targets.each do |target|
+          unless seen_files.include?(target.name)
+            seen_files.add(target.name)
+            unique_targets.push(target)
+          end
+        end
+        unique_targets
+      end
+
       def source_width
         source_image.width
       end