go/test/bench/shootout/fannkuch.go

/*
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    * Redistributions of source code must retain the above copyright
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    * Redistributions in binary form must reproduce the above copyright
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    documentation and/or other materials provided with the distribution.

    * 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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AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
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*/

/*
 * The Computer Language Benchmarks Game
 * http://shootout.alioth.debian.org/
 *
 * contributed by The Go Authors.
 * Based on fannkuch.c by Heiner Marxen
 */

package main

import (
	"flag"
	"fmt"
)

var n = flag.Int("n", 7, "count")

func fannkuch(n int) int {
	if n < 1 {
		return 0
	}

	n1 := n - 1
	perm := make([]int, n)
	perm1 := make([]int, n)
	count := make([]int, n)

	for i := 0; i < n; i++ {
		perm1[i] = i // initial (trivial) permutation
	}

	r := n
	didpr := 0
	flipsMax := 0
	for {
		if didpr < 30 {
			for i := 0; i < n; i++ {
				fmt.Printf("%d", 1+perm1[i])
			}
			fmt.Printf("\n")
			didpr++
		}
		for ; r != 1; r-- {
			count[r-1] = r
		}

		if perm1[0] != 0 && perm1[n1] != n1 {
			flips := 0
			for i := 1; i < n; i++ { // perm = perm1
				perm[i] = perm1[i]
			}
			k := perm1[0] // cache perm[0] in k
			for {         // k!=0 ==> k>0
				for i, j := 1, k-1; i < j; i, j = i+1, j-1 {
					perm[i], perm[j] = perm[j], perm[i]
				}
				flips++
				// Now exchange k (caching perm[0]) and perm[k]... with care!
				j := perm[k]
				perm[k] = k
				k = j
				if k == 0 {
					break
				}
			}
			if flipsMax < flips {
				flipsMax = flips
			}
		}

		for ; r < n; r++ {
			// rotate down perm[0..r] by one
			perm0 := perm1[0]
			for i := 0; i < r; i++ {
				perm1[i] = perm1[i+1]
			}
			perm1[r] = perm0
			count[r]--
			if count[r] > 0 {
				break
			}
		}
		if r == n {
			return flipsMax
		}
	}
	return 0
}

func main() {
	flag.Parse()
	fmt.Printf("Pfannkuchen(%d) = %d\n", *n, fannkuch(*n))
}