// Package lsproduct is for calculating largest series product.
package lsproduct
import (
	"fmt"
	"log"
	"strconv"
	"unicode"
)
// LargestSeriesProduct return the largest spoduct in input for a given span or an error.
func LargestSeriesProduct(input string, span int) (int, error) {
	if span == 0 {
		return 1, nil
	}
	if span > len(input) || span < 0 {
		return -1, fmt.Errorf("negative span %d, or longer than input %d", span, len(input))
	}
	product := 0
	for i := range input {
		if i > len(input)-span {
			break
		}
		cand := 1
		for j := i; j < i+span; j++ {
			if !unicode.IsDigit(rune(input[j])) {
				return -1, fmt.Errorf("input must be all digits")
			}
			tmp, err := strconv.Atoi(string(input[j]))
			if err != nil {
				log.Panicln(err)
			}
			cand *= tmp
		}
		if cand > product {
			product = cand
		}
	}
	return product, nil
}