[libxml2] Add xmlXPathSetContextNode and xmlXPathNodeEval

[Daniel Veillard <veillard src gnome org>]

commit 28876afb4e67f9f4292fc2a7134f6b97f0618b7d
Author: Alex Bligh <alex alex org uk>
Date:   Sat Mar 23 17:23:27 2013 +0000

    Add xmlXPathSetContextNode and xmlXPathNodeEval
    
    This patch adds xmlXPathSetContextNode and xmlXPathNodeEval,
    which make it easier to evaluation XPath expressions with a
    context node other than the document root without poking about
    inside the internals of the context.
    
    This patch is compile-tested only, and is my first libxml2
    contribution, so please go easy.
    
    Signed-off-by: Alex Bligh <alex alex org uk>

 include/libxml/xpath.h |    7 +++++++
 xpath.c                |   43 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 50 insertions(+), 0 deletions(-)
---
diff --git a/include/libxml/xpath.h b/include/libxml/xpath.h
index 8819a29..d96776c 100644
--- a/include/libxml/xpath.h
+++ b/include/libxml/xpath.h
@@ -508,6 +508,13 @@ XMLPUBFUN int XMLCALL
  */
 XMLPUBFUN long XMLCALL
                    xmlXPathOrderDocElems       (xmlDocPtr doc);
+XMLPUBFUN int XMLCALL
+                   xmlXPathSetContextNode      (xmlNodePtr node,
+                                                xmlXPathContextPtr ctx);
+XMLPUBFUN xmlXPathObjectPtr XMLCALL
+                   xmlXPathNodeEval            (xmlNodePtr node,
+                                                const xmlChar *str,
+                                                xmlXPathContextPtr ctx);
 XMLPUBFUN xmlXPathObjectPtr XMLCALL
                    xmlXPathEval                (const xmlChar *str,
                                                 xmlXPathContextPtr ctx);
diff --git a/xpath.c b/xpath.c
index c5b6aeb..97410e7 100644
--- a/xpath.c
+++ b/xpath.c
@@ -15079,6 +15079,49 @@ xmlXPathEval(const xmlChar *str, xmlXPathContextPtr ctx) {
 }
 
 /**
+ * xmlXPathSetContextNode:
+ * @node: the node to to use as the context node
+ * @ctx:  the XPath context
+ *
+ * Sets 'node' as the context node. The node must be in the same
+ * document as that associated with the context.
+ *
+ * Returns -1 in case of error or 0 if successful
+ */
+int
+xmlXPathSetContextNode(xmlNodePtr node, xmlXPathContextPtr ctx) {
+    if ((node == NULL) || (ctx == NULL))
+        return(-1);
+
+    if (node->doc == ctx->doc) {
+        ctx->node = node;
+       return(0);
+    }
+    return(-1);
+}
+
+/**
+ * xmlXPathNodeEval:
+ * @node: the node to to use as the context node
+ * @str:  the XPath expression
+ * @ctx:  the XPath context
+ *
+ * Evaluate the XPath Location Path in the given context. The node 'node'
+ * is set as the context node. The context node is not restored.
+ *
+ * Returns the xmlXPathObjectPtr resulting from the evaluation or NULL.
+ *         the caller has to free the object.
+ */
+xmlXPathObjectPtr
+xmlXPathNodeEval(xmlNodePtr node, const xmlChar *str, xmlXPathContextPtr ctx) {
+    if (str == NULL)
+        return(NULL);
+    if (xmlXPathSetContextNode(node, ctx) < 0)
+        return(NULL);
+    return(xmlXPathEval(str, ctx));
+}
+
+/**
  * xmlXPathEvalExpression:
  * @str:  the XPath expression
  * @ctxt:  the XPath context