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# WillSewell/qsort.go Secret

Created Apr 6, 2017
quicksort in go
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 package main import ( "fmt" "math/rand" "os" "runtime" "runtime/trace" "sync" "time" ) func qSort(data []int) { if len(data) < 2 { return } pivot := data[0] left, right := 1, len(data)-1 for right >= left { if data[left] <= pivot { left++ } else { data[right], data[left] = data[left], data[right] right-- } } // swap pivot into middle data[left-1], data[0] = data[0], data[left-1] qSort(data[:left-1]) qSort(data[left:]) } const thresh = 1e3 func qSortPar(data []int, wg *sync.WaitGroup) { if len(data) < 2 { // should have bailed to qSort by now but still wg.Done() return } pivot := data[0] left, right := 1, len(data)-1 for right >= left { if data[left] <= pivot { left++ } else { data[right], data[left] = data[left], data[right] right-- } } // swap pivot into middle data[left-1], data[0] = data[0], data[left-1] // launch tasks for big subsorts if left-1 > thresh { wg.Add(1) go qSortPar(data[:left-1], wg) } if len(data)-right > thresh { wg.Add(1) go qSortPar(data[left:], wg) } // do small subsorts now if left-1 <= thresh { qSort(data[:left-1]) } if len(data)-right <= thresh { qSort(data[left:]) } // we're done wg.Done() } func quicksort(data []int) { wg := new(sync.WaitGroup) wg.Add(1) qSortPar(data, wg) wg.Wait() } func main() { runtime.GOMAXPROCS(4) data := make([]int, 1e5) for i := range data { data[i] = int(rand.Uint32() >> 1) } t := time.Now() f, err := os.Create("trace.out") if err != nil { panic(err.Error()) } defer f.Close() trace.Start(f) quicksort(data) for i := range data[1:] { if data[i] > data[i+1] { fmt.Println("not sorted at index", i) panic("not sorted") } } trace.Stop() }

### WillSewell commented Apr 7, 2017

Disclaimer: I found this code on SO, but I can't find the answer I copied it from! If this is your code, please reply here so you can be credited.

### dartkron commented Apr 14, 2017

This won't compile. Need to remove 10 and 90 lines.