public static void SaveConstructionTime(string idxname, long elapsed_ticks, long numdistances)
{
var outname = idxname + ".construction-time.json";
var seconds = TimeSpan.FromTicks(elapsed_ticks).TotalSeconds;
var msg = JsonConvert.SerializeObject (new Dictionary<string, object> () {
{"Seconds", seconds},
{"Distances", numdistances}
}, Formatting.Indented);
File.WriteAllText (outname, msg);
}