Question

Is there a general algorithm that address playback speed of a log file having the following properties. It would be nice to not have to read the whole log file as well, if possible.

• Every line contains a timestamp
• Sampling rate is unknown

The playback would then be able to be played at any speed 2x for example.

thought of a few different ways but thought there might be a generalized algorithm out there.

1. read the first timestamp and use that to decide if the next line should be run yet, scheduler-esce.
2. read all lines assigning a schedule into memory, then each line will fire at correct time. Again you have to read the first line to get a reference point.

thanks

Answer 1

If the lines in the log file are in chronological order, then it’s pretty easy.

1. Read the first line, save its time stamp, and execute it.
2. Read the second line, compute the difference in times, adjust with the speed factor, and add that interval to the current time.
3. The result is the time to execute the next command.
4. Execute the command at the proper time and loop to step 2.

So if you want to run at 2x:

cmd = read_log
previous_time = cmd.time;
while (true)
{
    new_time = cmd.time;
    execute_time = current_time + (new_time - previous_time)/2.0;
    previous_time = new_time;
    // wait here until it's time to execute the command
    execute_command();
    cmd = read_log
}

That’s the single-threaded way to do it. I would suggest, though, using two threads: one to read the log and queue commands, and another to read the queue and execute commands.