fannkuch-parallel.go

/*
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    * Redistributions in binary form must reproduce the above copyright
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    * Neither the name of "The Computer Language Benchmarks Game" nor the
    name of "The Computer Language Shootout Benchmarks" nor the names of
    its contributors may be used to endorse or promote products derived
    from this software without specific prior written permission.

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*/

/*
 * The Computer Language Benchmarks Game
 * http://shootout.alioth.debian.org/
 *
 * contributed by The Go Authors.
 * Based on fannkuch.scala by Rex Kerr
 */

package main

import (
	"flag"
	"fmt"
	"runtime"
)

var n = flag.Int("n", 7, "count")
var nCPU = flag.Int("ncpu", 4, "number of cpus")

type Job struct {
	start []int
	n     int
}

type Found struct {
	who *Kucher
	k   int
}

type Kucher struct {
	perm []int
	temp []int
	flip []int
	in   chan Job
}

func NewKucher(length int) *Kucher {
	return &Kucher{
		perm: make([]int, length),
		temp: make([]int, length),
		flip: make([]int, length),
		in:   make(chan Job),
	}
}

func (k *Kucher) permute(n int) bool {
	i := 0
	for ; i < n-1 && k.flip[i] == 0; i++ {
		t := k.perm[0]
		j := 0
		for ; j <= i; j++ {
			k.perm[j] = k.perm[j+1]
		}
		k.perm[j] = t
	}
	k.flip[i]--
	for i > 0 {
		i--
		k.flip[i] = i
	}
	return k.flip[n-1] >= 0
}

func (k *Kucher) count() int {
	K := 0
	copy(k.temp, k.perm)
	for k.temp[0] != 0 {
		m := k.temp[0]
		for i := 0; i < m; i++ {
			k.temp[i], k.temp[m] = k.temp[m], k.temp[i]
			m--
		}
		K++
	}
	return K
}

func (k *Kucher) Run(foreman chan<- Found) {
	for job := range k.in {
		verbose := 30
		copy(k.perm, job.start)
		for i, v := range k.perm {
			if v != i {
				verbose = 0
			}
			k.flip[i] = i
		}
		K := 0
		for {
			if verbose > 0 {
				for _, p := range k.perm {
					fmt.Print(p + 1)
				}
				fmt.Println()
				verbose--
			}
			count := k.count()
			if count > K {
				K = count
			}
			if !k.permute(job.n) {
				break
			}
		}
		foreman <- Found{k, K}
	}
}

type Fanner struct {
	jobind   int
	jobsdone int
	k        int
	jobs     []Job
	workers  []*Kucher
	in       chan Found
	result   chan int
}

func NewFanner(jobs []Job, workers []*Kucher) *Fanner {
	return &Fanner{
		jobs: jobs, workers: workers,
		in:     make(chan Found),
		result: make(chan int),
	}
}

func (f *Fanner) Run(N int) {
	for msg := range f.in {
		if msg.k > f.k {
			f.k = msg.k
		}
		if msg.k >= 0 {
			f.jobsdone++
		}
		if f.jobind < len(f.jobs) {
			msg.who.in <- f.jobs[f.jobind]
			f.jobind++
		} else if f.jobsdone == len(f.jobs) {
			f.result <- f.k
			return
		}
	}
}

func swapped(a []int, i, j int) []int {
	b := make([]int, len(a))
	copy(b, a)
	b[i], b[j] = a[j], a[i]
	return b
}

func main() {
	flag.Parse()
	runtime.GOMAXPROCS(*nCPU)
	N := *n
	base := make([]int, N)
	for i := range base {
		base[i] = i
	}

	njobs := 1
	if N > 8 {
		njobs += (N*(N-1))/2 - 28 // njobs = 1 + sum(8..N-1) = 1 + sum(1..N-1) - sum(1..7)
	}
	jobs := make([]Job, njobs)
	jobsind := 0

	firstN := N
	if firstN > 8 {
		firstN = 8
	}
	jobs[jobsind] = Job{base, firstN}
	jobsind++
	for i := N - 1; i >= 8; i-- {
		for j := 0; j < i; j++ {
			jobs[jobsind] = Job{swapped(base, i, j), i}
			jobsind++
		}
	}

	nworkers := *nCPU
	if njobs < nworkers {
		nworkers = njobs
	}
	workers := make([]*Kucher, nworkers)
	foreman := NewFanner(jobs, workers)
	go foreman.Run(N)
	for i := range workers {
		k := NewKucher(N)
		workers[i] = k
		go k.Run(foreman.in)
		foreman.in <- Found{k, -1}
	}
	fmt.Printf("Pfannkuchen(%d) = %d\n", N, <-foreman.result)
}