Convert endomondo json tracking data to gpx

endomondo_json2gpx.php

<?php declare(strict_types=1);

/*

Usage: php endomondo_json2gpx.php <path_to_file.json> […<path_to_other_file.json>]

The endomondo json format parsed here; only the fields actually used by this converted:

[
    {"sport": "…"},
    {"points": [
        [
            {"location": [[
                {"latitude": …},
                {"longitude": …}
            ]]},
            {"altitude": …},
            {"timestamp": "…"}
        ],

which will be converted to

<?xml version="1.0" encoding="UTF-8"?>
<gpx creator="endomondo_json2gpx.php">
  <trk>
    <type>…</type>
    <trkseg>
      <trkpt lat="…" lon="…">
        <time>…</time>
        <ele>…</ele>
      </trkpt>

 */

$script = array_shift($argv);
$jsonFiles = $argv;

if (!$jsonFiles) {
    echo "ERROR: No *.json files given\n\n";
    echo "Usage: $script <1.json> ... <n.json>\n";
    exit(1);
}

foreach ($jsonFiles as $jsonFile) {
    echo "Loading $jsonFile ...\n";
    $data = json_decode(file_get_contents($jsonFile), true);

    $json = transformJson($data);
    $xml = convertJsonToGpx($json);

    if (!$xml) {
        echo "WARNING: no usable points found, skipping file\n";
        continue;
    }

    $gpxFile = str_replace('.json', '.gpx', $jsonFile);
    echo "Writing $gpxFile ...\n";
    $xml->save($gpxFile);
}

function convertJsonToGpx(array $json): ?DOMDocument
{
    $xml = new DOMDocument();
    $xml->encoding = 'UTF-8';
    $xml->formatOutput = true;

    $gpx = $xml->createElement('gpx');
    $xml->appendChild($gpx);
    $gpx->setAttribute('creator', 'endomondo_json2gpx.php');

    $trk = $xml->createElement('trk');
    $gpx->appendChild($trk);
    
    $type = $xml->createElement('type', strtolower($json['sport']));
    $trk->appendChild($type);
    
    $trkseg = $xml->createElement('trkseg');
    $trk->appendChild($trkseg);

    $pointsAdded = 0;

    foreach ($json['points'] as $point) {
        $point = transformJson($point);

        if (!isset($point['location'])) {
            continue;
        }
        $pointsAdded++;

        $location = transformJson($point['location'][0]);
        $altitude = $point['altitude'] ?? null;
        $timestamp = new DateTimeImmutable($point['timestamp']);

        $trkpt = $xml->createElement('trkpt');
        $trkseg->appendChild($trkpt);
        $trkpt->setAttribute('lat', (string) $location['latitude']);
        $trkpt->setAttribute('lon', (string) $location['longitude']);

        $time = $xml->createElement('time', $timestamp->format(DateTimeInterface::ATOM));
        $trkpt->appendChild($time);

        if ($altitude) {
            $ele = $xml->createElement('ele', (string) $altitude);
            $trkpt->appendChild($ele);
        }
    }

    if (!$pointsAdded) {
        return null;
    }

    return $xml;
}

function transformJson(array $data): array
{
    $json = [];

    foreach ($data as $datum) {
        $key = array_key_first($datum);
        $json[$key] = $datum[$key];
    }

    return $json;
}