From c64d4820b142e6df93ccf612d0e4717159a36591 Mon Sep 17 00:00:00 2001
From: Arseny Kapoulkine <arseny.kapoulkine@gmail.com>
Date: Mon, 27 Oct 2014 18:50:09 -0700
Subject: XPath: Optimize [position()=expr] and [last()]

To get more benefits from constant predicate/filter optimization we rewrite
[position()=expr] predicates into [expr] for numeric expressions. Right now
the rewrite is only for entire expressions - it may be beneficial to split
complex expressions like [position()=constant and expr] into [constant][expr]
but that is more complicated.

last() does not depend on the node set contents so is "constant" as far as
our optimization is concerned so we can evaluate it once.
---
 src/pugixml.cpp            | 10 +++++++++-
 tests/test_xpath_paths.cpp |  8 ++++++++
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/src/pugixml.cpp b/src/pugixml.cpp
index 67591a3..f0bfc31 100644
--- a/src/pugixml.cpp
+++ b/src/pugixml.cpp
@@ -9765,6 +9765,14 @@ PUGI__NS_BEGIN
 			if (_right) _right->optimize(alloc);
 			if (_next) _next->optimize(alloc);
 
+			// Rewrite [position()=expr] with [expr]
+			// Note that this step has to go before classification to recognize [position()=1]
+			if ((_type == ast_filter || _type == ast_predicate) &&
+				_right->_type == ast_op_equal && _right->_left->_type == ast_func_position && _right->_right->_rettype == xpath_type_number)
+			{
+				_right = _right->_right;
+			}
+
 			// Classify filter/predicate ops to perform various optimizations during evaluation
 			if (_type == ast_filter || _type == ast_predicate)
 			{
@@ -9772,7 +9780,7 @@ PUGI__NS_BEGIN
 
 				if (_right->_type == ast_number_constant && _right->_data.number == 1.0)
 					_test = predicate_constant_one;
-				else if (_right->_rettype == xpath_type_number && (_right->_type == ast_number_constant || _right->_type == ast_variable))
+				else if (_right->_rettype == xpath_type_number && (_right->_type == ast_number_constant || _right->_type == ast_variable || _right->_type == ast_func_last))
 					_test = predicate_constant;
 				else if (_right->_rettype != xpath_type_number && _right->is_posinv_expr())
 					_test = predicate_posinv;
diff --git a/tests/test_xpath_paths.cpp b/tests/test_xpath_paths.cpp
index 046592a..df0dfa4 100644
--- a/tests/test_xpath_paths.cpp
+++ b/tests/test_xpath_paths.cpp
@@ -475,6 +475,14 @@ TEST_XML(xpath_paths_predicate_constant_boolean, "<node><chapter/><chapter/><cha
 	CHECK_XPATH_NODESET_VAR(n, STR("following-sibling::chapter[$true]"), &set) % 6 % 7;
 }
 
+TEST_XML(xpath_paths_predicate_position_eq, "<node><chapter/><chapter/><chapter>3</chapter><chapter/><chapter/></node>")
+{
+	CHECK_XPATH_NODESET(doc, STR("node/chapter[position()=1]")) % 3;
+	CHECK_XPATH_NODESET(doc, STR("node/chapter[position()=2+2]")) % 7;
+	CHECK_XPATH_NODESET(doc, STR("node/chapter[position()=last()]")) % 8;
+	CHECK_XPATH_NODESET(doc, STR("node/chapter[position()=string()]")) % 5;
+}
+
 TEST_XML(xpath_paths_predicate_several, "<node><employee/><employee secretary=''/><employee assistant=''/><employee secretary='' assistant=''/><employee assistant='' secretary=''/></node>")
 {
 	xml_node n = doc.child(STR("node"));
-- 
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