Function computing a hash value of a string

StringHash.swift

let prime = 1231247
let alphabetSize = 26
let A: UInt32 = 96

func stringHash(inout input: String, fromIndex: String.UnicodeScalarView.Index, length: Int) -> Int {
    
    var hash = 0
    var index = fromIndex
    for i in 0..<length {
        let char = input.unicodeScalars[index]
        hash += Int(char.value - A) * Int(pow(Float(alphabetSize), Float(length - 1 - i))) % prime
        index = index.successor()
    }
    return hash
}