gistfile1.m

- (void)taskTerminated:(NSNotification *)aNotification {
    [self.progressBar incrementBy:1.];

    spawned_tasks -= 1;
    if (_unprocessed_files.count > 0)
        [self spawnCompressor];
}

- (void) spawnCompressor {
    NSString *executable = [[NSBundle mainBundle] pathForAuxiliaryExecutable:@"compress"];
    assert(executable != nil);

    if (spawned_tasks >= cores) {
        NSLog(@"Not spawning task as there are %lu tasks already", spawned_tasks);
        return;
    }
    NSLog(@"Spawning task and there are %lu tasks already", spawned_tasks);
    spawned_tasks += 1;
    
    NSUInteger oldcount = _unprocessed_files.count;
    assert(oldcount > 0);
    NSString *fn = [_unprocessed_files lastObject];
    [_unprocessed_files removeObject:fn];
    assert(oldcount > _unprocessed_files.count);
    NSString *dest = [NSString stringWithFormat:@"%@.pvr", fn];
    NSArray *args = [NSArray arrayWithObjects:fn, dest, nil];
    NSTask *task = [[NSTask alloc] init];
    task.launchPath = executable;
    task.arguments = args;
    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(taskTerminated:)
                                                 name:NSTaskDidTerminateNotification
                                               object:task];
    [task launch];
}

- (IBAction)selectDataset:(id)sender {
    NSLog(@"Presenting picker");
    _files = [self presentPicker];
    _unprocessed_files = [_files mutableCopy];
    spawned_tasks = 0;
    NSLog(@"User picked %lu files.", _files.count);

    self.progressBar.maxValue = _files.count;
    
    cores = [[NSProcessInfo processInfo] activeProcessorCount];
    // For every .dcm file, create a process and convert it to a PVRTC texture
    // PVRTexLib is not thread-safe.
    for (NSUInteger i = 0; i < cores; i++) {
        [self spawnCompressor];
    }
}