Introduce the AST Optimizer

... that can only optimize one thing: combine `| append: 'a' | append: 'b'` into a single `| append_all: ['a', 'b']` filter call.

This avoids all the intermediate allocations caused by several `append`.

Also introduce a new filter `append_all: [*items]`.

lib/liquid.rb

@@ -84,6 +84,7 @@ require 'liquid/usage'
 require 'liquid/register'
 require 'liquid/static_registers'
 require 'liquid/template_factory'
+require 'liquid/optimizer'
 
 # Load all the tags of the standard library
 #

lib/liquid/optimizer.rb

@@ -0,0 +1,55 @@
+# frozen_string_literal: true
+
+module Liquid
+  class Optimizer
+    def optimize(node)
+      case node
+      when Liquid::Template       then optimize(node.root.body)
+      when Liquid::Document       then optimize(node.body)
+      when Liquid::BlockBody      then optimize_block(node)
+      when Liquid::Variable       then optimize_variable(node)
+      when Liquid::Assign         then optimize_assign(node)
+      when Liquid::For            then optimize_for(node)
+      when Liquid::If             then optimize_if(node)
+      end
+      node
+    end
+
+    def optimize_block(block)
+      block.nodelist.each { |node| optimize(node) }
+    end
+
+    def optimize_variable(node)
+      # Turn chained `| append: "..."| append: "..."`, into a single `append_all: [...]`
+      if node.filters.size > 1 && node.filters.all? { |f, _| f == "append" }
+        node.filters = [["append_all", node.filters.map { |f, (arg)| arg }]]
+      end
+    end
+
+    def optimize_assign(node)
+      optimize(node.from)
+    end
+
+    def optimize_for(node)
+      optimize(node.collection_name)
+      optimize_block(node)
+    end
+
+    def optimize_if(node)
+      node.blocks.each do |block|
+        optimize_condition(block)
+        optimize(block.attachment)
+      end
+    end
+
+    def optimize_condition(node)
+      case node
+      when Liquid::ElseCondition
+        # noop
+      when Liquid::Condition
+        optimize(node.left)
+        optimize(node.right) if node.right
+      end
+    end
+  end
+end

lib/liquid/standardfilters.rb

@@ -281,6 +281,10 @@ module Liquid
       input.to_s + string.to_s
     end
 
+    def append_all(input, *items)
+      input.to_s + items.join
+    end
+
     def concat(input, array)
       unless array.respond_to?(:to_ary)
         raise ArgumentError, "concat filter requires an array argument"

test/unit/optimizer_test.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+
+class OptimizerUnitTest < Minitest::Test
+  include Liquid
+
+  def test_combines_append_filters
+    optimizer = Optimizer.new
+    var = Variable.new('hello | append: "a" | append: b', ParseContext.new)
+    var = optimizer.optimize(var)
+    assert_equal([
+      ['append_all', ["a", VariableLookup.new("b")]]
+    ], var.filters)
+  end
+end