Make Parser.expression parse comparisons

lib/liquid.rb

@@ -62,6 +62,7 @@ require 'liquid/interrupts'
 require 'liquid/tags'
 require "liquid/environment"
 require 'liquid/lexer'
+require 'liquid/binary_expression'
 require 'liquid/parser'
 require 'liquid/i18n'
 require 'liquid/drop'

lib/liquid/binary_expression.rb

@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+module Liquid
+  class BinaryExpression
+    attr_reader :left, :operator, :right
+
+    def initialize(left, operator, right)
+      @left = left
+      @operator = operator
+      @right = right
+    end
+
+    def evaluate(context)
+      left_value = value(left, context)
+      right_value = value(@right, context)
+
+      case operator
+      when '>'
+        left_value > right_value
+      when '>='
+        left_value >= right_value
+      when '<'
+        left_value < right_value
+      when '<='
+        left_value <= right_value
+      when '=='
+        left_value == right_value
+      when '!=', '<>'
+        left_value != right_value
+      when 'contains'
+        if left_value && right_value && left_value.respond_to?(:include?)
+          right_value = right_value.to_s if left_value.is_a?(String)
+          left_value.include?(right_value)
+        else
+          false
+        end
+      end
+    end
+
+    private
+
+    def value(expr, context)
+      Utils.to_liquid_value(context.evaluate(expr))
+    end
+  end
+end

lib/liquid/parser.rb

@@ -47,7 +47,24 @@ module Liquid
       tok[0] == type
     end
 
+    # expression := comparison
+    # comparison := primary ((">=" | ">" | "<" | "<=" | ... ) primary)*
+    # primary    := string | number | variable_lookup | range | boolean
     def expression
+      comparison
+    end
+
+    # comparison := primary ((">=" | ">" | "<" | "<=" | ... ) primary)*
+    def comparison
+      expr = primary
+      while look(:comparison)
+        operator = consume
+        expr = BinaryExpression.new(expr, operator, primary)
+      end
+      expr
+    end
+
+    def primary
       token = @tokens[@p]
       case token[0]
       when :id

test/unit/parser_unit_test.rb

@@ -77,6 +77,30 @@ class ParserUnitTest < Minitest::Test
     assert_equal(0..5, p.expression)
   end
 
+  def test_comparison
+    p = new_parser("a > b")
+    expr = p.expression
+    assert(expr.is_a?(BinaryExpression))
+    assert_equal('>', expr.operator)
+    assert(expr.left.is_a?(VariableLookup))
+    assert_equal('a', expr.left.name)
+    assert(expr.right.is_a?(VariableLookup))
+    assert_equal('b', expr.right.name)
+
+    # BinaryExpression(>=)
+    #   left: BinaryExpression(>)
+    #     left: 10
+    #     right: 5
+    #   right: 4
+    p = new_parser("10 > 5 >= 4")
+    expr = p.expression
+    assert(expr.is_a?(BinaryExpression))
+    assert_equal('>=', expr.operator)
+    assert_equal(10, expr.left.left)
+    assert_equal(5, expr.left.right)
+    assert_equal(4, expr.right)
+  end
+
   def test_number
     p = new_parser('-1 0 1 2.0')
     assert_equal(-1, p.number)