Parse GPX file using libxml2

gpxparse.c

/* gpxparse.c

   Copyright (c) 2010, Jeremiah LaRocco jeremiah.larocco@gmail.com

   Permission to use, copy, modify, and/or distribute this software for any
   purpose with or without fee is hereby granted, provided that the above
   copyright notice and this permission notice appear in all copies.

   THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
   WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
   MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
   ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
   WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
   ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
   OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
*/

/*
   Use libxml2 to parse GPX files and display some simple statistics.
   Created for comparison with http://github.com/jl2/RubyGPS/blob/master/parsegpx.rb
*/

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>

#define __USE_XOPEN  // For strptime
#include <time.h>

#include <math.h>

#include <libxml/tree.h>
#include <libxml/xpath.h>
#include <libxml/xpathInternals.h>

#include "utmconvert/utmconvert.h"

struct gpx_point {
    double lat, lon;
    double x, y;
    double ele;
    bool valid;
    struct tm time;
};

// Compute duration, distance and speed between two points
void timeDistSpeed(struct gpx_point *pt1, struct gpx_point *pt2, double *tdiff, double *dist, double *speed) {

    if (!pt1->valid || !pt2->valid) return;
    if (tdiff==0 && dist == 0 && speed == 0) return;

    double dx = (pt2->x-pt1->x);
    double dy = (pt2->y-pt1->y);
    double dz = (pt2->ele-pt1->ele);

    double dc = sqrt(dx*dx + dy*dy + dz*dz);
    double dt = difftime(mktime(&pt1->time), mktime(&pt2->time));

    if (dist) *dist = dc;
    if (tdiff) *tdiff = dt;
    if (speed) *speed = dc/dt;
}

// Convenience function to create an XPath context at a particular node
xmlXPathContextPtr xmlXPathNewContextNode(xmlDocPtr doc, xmlNodePtr node) {
    xmlXPathContextPtr retval = xmlXPathNewContext(doc);
    xmlXPathRegisterNs(retval, (xmlChar*)"gpx", (xmlChar*)"http://www.topografix.com/GPX/1/0");
    retval->node = node;
    return retval;
}

int main(int argc, char *argv[]){

    if (argc<2) {
        printf("No file name given!\n");
        return 1;
    }

    // Initialize libxml
    xmlInitParser();

    // Load file
    xmlDocPtr doc = xmlParseFile(argv[1]);

    // Create initial xpath context
    xmlXPathContextPtr context = xmlXPathNewContext(doc);
    xmlXPathRegisterNs(context, (xmlChar*)"gpx", (xmlChar*)"http://www.topografix.com/GPX/1/0");

    // Find track segments
    xmlXPathObjectPtr tracks = xmlXPathEvalExpression((xmlChar*)"//gpx:trk/gpx:trkseg", context);
    if(tracks == NULL || xmlXPathNodeSetIsEmpty(tracks->nodesetval)){
        printf("No track segments found in \"%s\"\n", argv[1]);
        return 1;
    }

    xmlNodeSetPtr trackNodes = tracks->nodesetval;

    // Book keeping variables
    struct gpx_point curPt = {0};
    struct gpx_point prevPt = {0};

    double dist=0;
    double speed=0;
    double tdiff=0;

    double curMax = 0;
    double totDist = 0;
    double totTime = 0;

    xmlChar *end = 0;

    // Loop through tracks
    for(int i = 0; i<trackNodes->nodeNr; ++i) {

        // Skip non-elements
        if(trackNodes->nodeTab[i]->type != XML_ELEMENT_NODE) continue;

        // Find all points in the current track
        xmlXPathContextPtr ptContext = xmlXPathNewContextNode(doc, trackNodes->nodeTab[i]);
        xmlXPathObjectPtr points = xmlXPathEvalExpression((xmlChar*)"gpx:trkpt", ptContext);

        if (points != 0 && !xmlXPathNodeSetIsEmpty(points->nodesetval)) {
            // Reset variables for each track
            curMax = totDist = totTime = 0;
            prevPt.valid = false;

            // Loop through track points
            for (int j=0; j<points->nodesetval->nodeNr; ++j) {
                curPt.valid = true;

                // Current point
                xmlNodePtr ptNode = points->nodesetval->nodeTab[j];

                // Check for lat and lon attributes
                if (xmlHasProp(ptNode, (xmlChar*)"lat") && xmlHasProp(ptNode, (xmlChar*)"lon")) {

                    // Parse latitude
                    xmlChar *lat = xmlGetProp(ptNode, (xmlChar*)"lat");
                    curPt.lat = strtod((char*)lat, (char**)&end);
                    if (end == lat) {
                        curPt.valid = false;
                    }
                    if (lat != 0) xmlFree(lat);

                    // Parse longitude
                    xmlChar *lon = xmlGetProp(ptNode, (xmlChar*)"lon");
                    curPt.lon = strtod((char*)lon, (char**)&end);
                    if (end == lon) {
                        curPt.valid = false;
                    }
                    if (lon != 0) xmlFree(lon);

                    // If everything went okay, calculate the utm coordinates
                    if (curPt.valid)
                        to_utm(curPt.lat, curPt.lon, &curPt.x, &curPt.y);

                } else {
                    curPt.valid = false;
                    continue;
                }

                // Get the elevation
                xmlXPathContextPtr eleContext = xmlXPathNewContextNode(doc, ptNode);
                xmlXPathObjectPtr elevation = xmlXPathEvalExpression((xmlChar*)"gpx:ele/text()", eleContext);
                if (elevation != 0 &&  xmlXPathNodeSetIsEmpty(elevation->nodesetval)) {
                    curPt.valid = false;

                } else if (elevation != 0) {
                    xmlChar *eVal = elevation->nodesetval->nodeTab[0]->content;
                    curPt.ele = strtod((char*)eVal, (char**)&end);
                    if (end == eVal) {
                        curPt.valid = false;
                    }
                }
                if (elevation) xmlXPathFreeObject(elevation);
                xmlXPathFreeContext(eleContext);

                // Get the time
                xmlXPathContextPtr timeContext = xmlXPathNewContextNode(doc, ptNode);
                xmlXPathObjectPtr time = xmlXPathEvalExpression((xmlChar*)"gpx:time/text()", timeContext);
                if ( time != 0 && xmlXPathNodeSetIsEmpty(time->nodesetval)) {
                    curPt.valid = false;

                } else if (time != 0) {
                    xmlChar *tVal = time->nodesetval->nodeTab[0]->content;
                    if (strptime((char*)tVal, "%Y-%m-%dT%H:%M:%SZ", &(curPt.time)) ==NULL)
                        curPt.valid = false;
                }
                if (time) xmlXPathFreeObject(time);
                xmlXPathFreeContext(timeContext);


                // If the point is good then update the stats
                if (curPt.valid && prevPt.valid) {
                    timeDistSpeed(&curPt, &prevPt, &tdiff, &dist, &speed);
                    totTime += tdiff;
                    totDist += dist;
                    if (speed > curMax)
                        curMax = speed;
                }
                prevPt = curPt;
            }
            
            // Print track stats
            if (totDist > 0 && totTime > 0) 
                printf("Went %.3f meters in %.3f seconds, average speed was %.3f, maximum speed was %.3f\n", totDist, totTime, totDist/totTime, curMax); 
        }
        if (points) xmlXPathFreeObject(points);
        xmlXPathFreeContext(ptContext);
    }

    if (tracks) xmlXPathFreeObject(tracks);
    xmlXPathFreeContext(context);

    xmlFreeDoc(doc);
    xmlCleanupParser();

    return 0;
}