Fork/Join example with GPars

Question

I found an example for fork/join in GPars here: Fork/Join

import static groovyx.gpars.GParsPool.runForkJoin
import static groovyx.gpars.GParsPool.withPool
withPool() {
    println """Number of files: ${
        runForkJoin(new File("./src")) {file ->
            long count = 0
            file.eachFile {
                if (it.isDirectory()) {
                    println "Forking a child task for $it"
                    forkOffChild(it)           //fork a child task
                } else {
                    count++
                }
            }
            return count + (childrenResults.sum(0))
            //use results of children tasks to calculate and store own result
        }
    }"""
}

It works and returns the correct number of files, but unfortunately I don't understand this line:

return count + (childrenResults.sum(0))

How exactly work count and childrenResult?
Why is a 0 passed as a parameter to sum()?

Answer 1

I'm not much familiar with GPars, but the link you provided says it is a Divide-and-Conquer algorithm and clarifies a bit more what's implicit later on, explaining that forkOffChild() does not wait -- instead getChildrenResults() does.

You may find easier to understand the provided alternative approach in the same page, that uses a more Java-ish style, if you're more familiar to that.

childrenResults results in calling the method getChildrenResults(), this is the "join" in "Fork/Join", it waits for all children to finish and then returns a list with the results of them (or re-throws any exception a children may have thrown).

0 is just the initial value for the sum. If childrenResult is empty, that's what gets summed to count:

groovy:000> [].sum(1)
===> 1
groovy:000> [1].sum(1)
===> 2