// RabinKarp演算法實現

// RabinKarp演算法實現
const primeRK = 16777619

func hashStr(seq string) (uint32, uint32) {
    hash := uint32(0)
    for _, value := range seq {
        hash = hash * primeRK + uint32(value)
    }
    
    var pow, sq uint32 = 1, primeRK
    for i := len(seq); i > 0; i >>= 1 {
        if i & 1 != 0 {
            pow *= sq
        }
        sq *= sq
    }
    return hash, pow
}

func rabinKarp(s, substr string) int {
    hashss, pow := hashStr(substr)
    n := len(substr)
    var h uint32
    for i := 0; i < n; i++ {
		h = h*primeRK + uint32(s[i])
	}
	if h == hashss && s[:n] == substr {
		return 0
	}
    for i := n; i < len(s); {
		h *= primeRK
		h += uint32(s[i])
		h -= pow * uint32(s[i-n])
		i++
		if h == hashss && s[i-n:i] == substr {
			return i - n
		}
	}
	return -1
}

func strStr(haystack string, needle string) int {
    n := len(needle) 
    switch {
        case n == 0:
            return 0
        case n == len(haystack):
            if haystack == needle {
                return 0
            }
            return -1
        case n > len(haystack):
            return -1
    }
    return rabinKarp(haystack, needle)
}