trails/packages/gpx/src/parse.ts

import type { Waypoint } from "@trails-cool/types";
import type { GpxData, TrackPoint, ElevationProfile } from "./types";

/**
 * Parse a GPX XML string into structured data.
 */
export function parseGpx(xml: string): GpxData {
  const parser = new DOMParser();
  const doc = parser.parseFromString(xml, "application/xml");

  const parserError = doc.querySelector("parsererror");
  if (parserError) {
    throw new Error(`Invalid GPX XML: ${parserError.textContent}`);
  }

  const name = doc.querySelector("metadata > name")?.textContent ?? undefined;
  const waypoints = parseWaypoints(doc);
  const tracks = parseTracks(doc);
  const elevation = computeElevation(tracks);

  return { name, waypoints, tracks, elevation };
}

function parseWaypoints(doc: Document): Waypoint[] {
  const wpts = doc.querySelectorAll("wpt");
  return Array.from(wpts).map((wpt) => {
    const lat = parseFloat(wpt.getAttribute("lat") ?? "0");
    const lon = parseFloat(wpt.getAttribute("lon") ?? "0");
    const name = wpt.querySelector("name")?.textContent ?? undefined;
    return { lat, lon, name };
  });
}

function parseTracks(doc: Document): TrackPoint[][] {
  const tracks: TrackPoint[][] = [];
  const trksegs = doc.querySelectorAll("trk > trkseg");

  for (const seg of trksegs) {
    const points: TrackPoint[] = [];
    for (const pt of seg.querySelectorAll("trkpt")) {
      const lat = parseFloat(pt.getAttribute("lat") ?? "0");
      const lon = parseFloat(pt.getAttribute("lon") ?? "0");
      const eleText = pt.querySelector("ele")?.textContent;
      const time = pt.querySelector("time")?.textContent ?? undefined;
      points.push({
        lat,
        lon,
        ele: eleText ? parseFloat(eleText) : undefined,
        time,
      });
    }
    tracks.push(points);
  }

  return tracks;
}

function computeElevation(tracks: TrackPoint[][]): GpxData["elevation"] {
  let gain = 0;
  let loss = 0;
  const profile: ElevationProfile[] = [];
  let totalDistance = 0;

  for (const track of tracks) {
    for (let i = 0; i < track.length; i++) {
      const pt = track[i]!;

      if (i > 0) {
        const prev = track[i - 1]!;
        totalDistance += haversineDistance(prev.lat, prev.lon, pt.lat, pt.lon);

        if (pt.ele !== undefined && prev.ele !== undefined) {
          const diff = pt.ele - prev.ele;
          if (diff > 0) gain += diff;
          else loss += Math.abs(diff);
        }
      }

      if (pt.ele !== undefined) {
        profile.push({ distance: totalDistance, elevation: pt.ele });
      }
    }
  }

  return { gain: Math.round(gain), loss: Math.round(loss), profile };
}

/** Haversine distance between two points in meters */
function haversineDistance(
  lat1: number,
  lon1: number,
  lat2: number,
  lon2: number,
): number {
  const R = 6371000;
  const toRad = (deg: number) => (deg * Math.PI) / 180;
  const dLat = toRad(lat2 - lat1);
  const dLon = toRad(lon2 - lon1);
  const a =
    Math.sin(dLat / 2) ** 2 +
    Math.cos(toRad(lat1)) * Math.cos(toRad(lat2)) * Math.sin(dLon / 2) ** 2;
  return R * 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
}