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void _AGGenerateAndProcessInternal(dispatch_group_t pullGroup, dispatch_group_t procGroup, id (^generateBlock)(void), void (^processBlock)(id))
{
    dispatch_group_wait(procGroup, DISPATCH_TIME_FOREVER);
    dispatch_group_async(pullGroup, dispatch_get_global_queue(0, 0), ^{
        id result = generateBlock();
        if (result) {
            _AGGenerateAndProcessInternal(pullGroup, procGroup, generateBlock, processBlock);

            dispatch_group_async(procGroup, dispatch_get_global_queue(0, 0), ^{
                processBlock(result);
            });
        }
    });
}


void AGGenerateAndProcess(id (^generateBlock)(void), void (^processBlock)(id))
{
    dispatch_group_t pullGroup = dispatch_group_create();
    dispatch_group_t procGroup = dispatch_group_create();

    _AGGenerateAndProcessInternal(pullGroup, procGroup, generateBlock, processBlock);

    dispatch_group_wait(pullGroup, DISPATCH_TIME_FOREVER);
    dispatch_group_wait(procGroup, DISPATCH_TIME_FOREVER);

    dispatch_release(pullGroup);
    dispatch_release(procGroup);
}


int main (int argc, const char * argv[])
{
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
    NSUInteger numChunks = 5;
    __block NSUInteger numPulled = 0;


    AGGenerateAndProcess(
        ^{
            if (numPulled == numChunks) return (id)nil;
            numPulled++;

            int i = 1 + rand() % 6;
            NSLog(@"Pull %d (%d)", numPulled, i);
            sleep(i);

            return (id)[NSNumber numberWithInteger:numPulled];
        },

        ^(id obj){
            int i = 1 + rand() % 6;
            NSLog(@"Process %@ (%d)", obj, i);
            sleep(i);
        }
    );


    NSLog(@"All Done!");


    [pool drain];
    return 0;
}