#include <stdio.h>
#include <libxml/parser.h>
#include <libxml/tree.h>
#include <libxml/xpath.h>
#include <libxml/xpointer.h>

static xmlXPathObjectPtr matchXPathExpression(
	char *filename, xmlDocPtr doc, xmlChar *expression
);

static xmlXPathObjectPtr matchXPointerExpression(
    char *filename, xmlDocPtr doc, xmlChar *expression
);

static char *commandname = "parse";

int main(int argc, char *argv[])
{
    xmlDocPtr myTree;
    char *filename;
    int xpointer = 0; /* 1 to use xpointer rather than path */

    commandname = argv[0];

    if (argc == 4) {
	if (!strcmp(argv[1], "-xpointer")) {
	    xpointer = 1;
	    --argc;
	    ++argv;
	}
    }

    if (argc != 3) {
	fprintf(stderr, "usage: %s [-xpointer] filename xpath-expr\n", commandname);
	exit(1);
    }

    filename = argv[1];

    myTree = xmlParseFile(filename);

    if (!myTree) {
	fprintf(stderr, "%s: %s: the parse failed. Alas, I must leave you.\n",
	    commandname, filename
	);
	exit(1);
    }

    /* now extract the given xpath expression */
    void *result = (xpointer) ?
	matchXPointerExpression(filename, myTree, (xmlChar *) argv[2]) :
	matchXPathExpression(filename, myTree, (xmlChar *) argv[2])
    ;

    if (!result) {
	fprintf(stderr, "%s: %s: no match for %s\n",
	    commandname, filename, argv[2]
	);
	exit(1);
    }
    exit(0);
}

static xmlXPathObjectPtr matchXPathExpression(
    char *filename, xmlDocPtr doc, xmlChar *expression
)
{
    xmlXPathContextPtr context;
    xmlXPathObjectPtr result;
    xmlChar *resultString;
    xmlXPathInit();

    context = xmlXPathNewContext(doc);

    if (!context) {
	return NULL;
    }

    result = xmlXPathEval(expression, context);
    if (!result) {
	return NULL;
    }

    /* convert to string for printing */
    resultString = xmlXPathCastToString(result);
    if (resultString) {
	printf("%s\n", resultString); /* utf8 */
    } else {
	fprintf(stderr, "%s: %s: could not convert result of XPath expression %s to string", commandname, filename, expression);
    }
    return result;
}

static xmlXPathObjectPtr matchXPointerExpression(
    char *filename, xmlDocPtr doc, xmlChar *expression
)
{
    xmlXPathContextPtr context;
    xmlXPathObjectPtr result;
    xmlChar *resultString;

    context = xmlXPtrNewContext(doc,
	    NULL, NULL /* nodeptr, origin */
    );
    if (!context) {
	return NULL;
    }

    result = xmlXPtrEval(expression, context);
    if (!result) {
	return NULL;
    }

    /* convert to string for printing */
    resultString = xmlXPathCastToString(result);
    if (resultString) {
	printf("%s\n", resultString); /* utf8 */
    } else {
	fprintf(stderr, "%s: %s: could not convert result of XPath expression %s to string", commandname, filename, expression);
    }
    return result;
}