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path: root/go/largest-series-product/largest_series_product.go
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package lsproduct

import "errors"

const testVersion = 3

func check(s string) bool {
	for _, v := range s {
		if v < '0' || v > '9' {
			return false
		}
	}
	return true
}

func split(s string, n int) <-chan string {
	c := make(chan string)
	go func() {
		for i := 0; i <= len(s)-n; i++ {
			c <- s[i : i+n]
		}
		close(c)
	}()
	return c
}

func max(c <-chan string) int {
	prod := func(s string) int {
		p := 1
		for _, v := range s {
			p *= int(v - '0')
		}
		return p
	}
	var m int
	for v := range c {
		if p := prod(v); p > m {
			m = p
		}
	}
	return m
}

func LargestSeriesProduct(s string, n int) (int, error) {
	if ok := check(s); !ok {
		return 0, errors.New("invalid chars")
	}
	if n < 0 || n > len(s) {
		return 0, errors.New("invalid length")
	}
	if n == 0 {
		return 1, nil
	}
	return max(split(s, n)), nil
}