simple implementation of the mod 7 graph walking algorithm

mod7.go

// simple implementation of the mod 7 algorithm described here:
//   http://blog.tanyakhovanova.com/?p=262

// compile with "go build mod7.go"
// then:
// 
//    $ echo 1098712349087123409871234908712340917823490187234091827340918273490128734 | ./mod7
//    1
//    
//    time echo 1098712349087123409871234908712340917823490187234091827340918273490128734 | ./mod7
//    1
//    echo 109871234908712340987123490871234091782349018723409182734091827349012873  0.00s user 0.00s system 0% cpu 0.001 total
//    ./mod7  0.00s user 0.00s system 0% cpu 0.004 total

package main

import (
	"fmt"
	"strconv"
	"strings"
)

func mod7(numStr string) int {
	var state int = 0
	// the white edges on the graph
	white := []int{0, 3, 6, 2, 5, 1, 4}
	// black edges on the graph 
	// (ie, mod 7 as a lookup table for small values)
	black := []int{0, 1, 2, 3, 4, 5, 6, 0, 1, 2, 3, 4, 5, 6, 0, 1, 2, 3, 4, 5, 6}
	chars := strings.Split(numStr, "")
	for c := range chars {
		if current, err := strconv.Atoi(chars[c]); err == nil {
			state = black[white[state]+current]
		} // just skip non-digit input
	}
	return state
}

func main() {
	var num string
	fmt.Scanf("%s",&num)
	fmt.Println(mod7(num))
}