Bangle.on("health", health => {
  // ensure we write health info for *last* block
  var d = new Date(Date.now() - 590000);

  const DB_RECORD_LEN = 4;
  const DB_RECORDS_PER_HR = 6;
  const DB_RECORDS_PER_DAY = DB_RECORDS_PER_HR*24;
  const DB_RECORDS_PER_MONTH = DB_RECORDS_PER_DAY*31;
  const DB_HEADER_LEN = 8;
  const DB_FILE_LEN = DB_HEADER_LEN + DB_RECORDS_PER_MONTH*DB_RECORD_LEN;

  function getRecordFN(d) {
    return "health-"+d.getFullYear()+"-"+d.getMonth()+".raw";
  }
  function getRecordIdx(d) {
    return (DB_RECORDS_PER_DAY*(d.getDate()-1)) +
           (DB_RECORDS_PER_HR*d.getHours()) +
           (0|(d.getMinutes()*DB_RECORDS_PER_HR/60));
  }

  var rec = getRecordIdx(d);
  var fn = getRecordFN(d);
  var f = require("Storage").read(fn);
  if (f) {
    var dt = f.substr(DB_HEADER_LEN+(rec*DB_RECORD_LEN), DB_RECORD_LEN);
    if (dt!="\xFF\xFF\xFF\xFF") {
      print("HEALTH ERR: Already written!");
      return;
    }
  } else {
    require("Storage").write(fn, "HEALTH1\0", 0, DB_FILE_LEN); // header
  }
  var recordData = String.fromCharCode(
    health.steps>>8,health.steps&255, // 16 bit steps
    health.bpm, // 8 bit bpm
    Math.min(health.movement / 8, 255)); // movement
  require("Storage").write(fn, recordData, DB_HEADER_LEN+(rec*DB_RECORD_LEN), DB_FILE_LEN);
});
