XPath support for nested pipe operators

Basically this will process the left-hand side first, assign the result
to a variable and then append this set with the nodes from the
right-hand side.

Fixes #149

lib/oga/xpath/compiler.rb

@@ -591,14 +591,18 @@ module Oga
         left, right = *ast
 
         union = unique_literal(:union)
-        conversion = literal(Conversion)
 
+        # Expressions such as "a | b | c"
+        if left.type == :pipe
+          union.assign(process(left, input))
+            .followed_by(process(right, input) { |node| union << node })
+            .followed_by(union)
+        # Expressions such as "a | b"
+        else
           union.assign(literal(XML::NodeSet).new)
             .followed_by(process(left, input) { |node| union << node })
             .followed_by(process(right, input) { |node| union << node })
-          .followed_by do
-            # block present means we're in a predicate
-            block ? conversion.to_boolean(union).if_true(&block) : union
+            .followed_by(union)
         end
       end
 

spec/oga/xpath/compiler/operators/pipe_spec.rb

@@ -3,10 +3,12 @@ require 'spec_helper'
 describe Oga::XPath::Compiler do
   describe 'pipe operator' do
     before do
-      @document = parse('<root><a></a><b></b></root>')
+      @document = parse('<root><a></a><b></b><c></c></root>')
 
-      @a1 = @document.children[0].children[0]
-      @b1 = @document.children[0].children[1]
+      @root = @document.children[0]
+      @a1   = @root.children[0]
+      @b1   = @root.children[1]
+      @c1   = @root.children[2]
     end
 
     it 'merges two node sets' do
@@ -24,5 +26,31 @@ describe Oga::XPath::Compiler do
     it 'merges two identical sets' do
       evaluate_xpath(@document, 'root/a | root/a').should == node_set(@a1)
     end
+
+    it 'merges three sets' do
+      evaluate_xpath(@document, 'root/a | root/b | root/c')
+        .should == node_set(@a1, @b1, @c1)
+    end
+
+    it 'merges three identical sets' do
+      evaluate_xpath(@document, 'root/a | root/a | root/a')
+        .should == node_set(@a1)
+    end
+
+    it 'merges two non-empty sets in a predicate' do
+      evaluate_xpath(@document, 'root[a | b]').should == node_set(@root)
+    end
+
+    it 'merges three non-empty sets in a predicate' do
+      evaluate_xpath(@document, 'root[a | b | c]').should == node_set(@root)
+    end
+
+    it 'merges two empty sets in a predicate' do
+      evaluate_xpath(@document, 'root[x | y]').should == node_set
+    end
+
+    it 'merges three empty sets in a predicate' do
+      evaluate_xpath(@document, 'root[x | y | z]').should == node_set
+    end
   end
 end