@sports-alliance/sports-lib/deep_parse.mjs

A Library to for importing / exporting and processing GPX, TCX, FIT and JSON files from services such as Strava, Movescount, Garmin, Polar etc

import fs from 'fs';

const file = '../fit-parser/examples/file-with-zones.fit';
const buffer = fs.readFileSync(file);

const seenGids = new Set();

function parse() {
  let pos = 14;
  const defs = {};

  while (pos < buffer.length - 2) {
    const header = buffer[pos++];

    if ((header & 0x40) === 0x40) {
      // Definition
      const localId = header & 0x0f;
      pos++; // reserved
      const arch = buffer[pos++];
      const gid = arch === 0 ? buffer.readUInt16LE(pos) : buffer.readUInt16BE(pos);
      pos += 2;
      const numFields = buffer[pos++];
      const fields = [];
      for (let i = 0; i < numFields; i++) {
        fields.push({ id: buffer[pos++], size: buffer[pos++], type: buffer[pos++] });
      }
      if ((header & 0x20) === 0x20) {
        const numDev = buffer[pos++];
        pos += numDev * 3;
      }
      defs[localId] = { gid, fields };

      if (!seenGids.has(gid)) {
        console.log(`FOUND GID: ${gid}`);
        seenGids.add(gid);
      }
    } else {
      // Data
      const localId = header & 0x0f;
      const def = defs[localId];
      if (!def) break;

      let totalSize = 0;
      def.fields.forEach(f => (totalSize += f.size));

      if (def.gid === 140 || def.gid === 142) {
        console.log(`\n--- GID ${def.gid} Data at ${pos - 1} ---`);
        let fieldPos = pos;
        def.fields.forEach(f => {
          const valBytes = buffer.slice(fieldPos, fieldPos + f.size);
          let displayVal = valBytes.toString('hex');
          if (f.size === 1) displayVal += ` (${valBytes.readUInt8(0)})`;
          else if (f.size === 2) displayVal += ` (${valBytes.readUInt16LE(0)})`;
          else if (f.size === 4) displayVal += ` (${valBytes.readUInt32LE(0)} / ${valBytes.readFloatLE(0)})`;
          console.log(`  ID: ${f.id.toString().padEnd(3)} Raw: ${displayVal}`);
          fieldPos += f.size;
        });
      }
      pos += totalSize;
    }
  }
}

parse();