273. Integer to English Words

/*
 Convert a non-negative integer to its english words representation. Given input is guaranteed to be less than 2^31 - 1.
 
 For example,
 123 -> "One Hundred Twenty Three"
 12345 -> "Twelve Thousand Three Hundred Forty Five"
 1234567 -> "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven"
*/

/*
 Thinking:
 2^31 - 1 = 2147483647
 2,147,483,647
 1. 按照量级：Billion Million Thousand
 2. 每个分割：Zero One Two Three Four Five Six Seven Eight Night (个位)
            Ten Twenty Thirty Forty Fifty Sixty Seventy Eighty Ninety （十位)
            (十位如果是1，则需要匹配10~19，Ten，Eleven，twelve，thirteen，fourteen, fourteen, fifteen, sixteen, seventeen, eighteen, nineteen)
            个位+Hundred (百位)
 
 Step:
 1. 先把整个数按 3 位划分
 2. 然后每个分割遇到 0 则不解析
 3. 如果数值就是 0，返回 Zero
 
 按字符串来处理，如果按整除来处理更快，每次除以1000则可以按3为进行分割了,
 而且还可以过滤前面的0
 
*/

func convIntToEng(_ intNumber: Int) -> String {
    guard intNumber > 0, intNumber <= 2147483647 else {
        return "Zero"
    }
    
    var number = intNumber
    var intArray: [Int] = []
    
    while number > 0 {
        //把余数放入到数组
        let subInt = number % 1000
        intArray.append(subInt)
        number = number / 1000
    }
    
    //20~90
    let tenStrings = ["Zero", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"]
    //100
    let hundrendString = "Hundred"
    //0 ~ 19
    let tenthStrings = ["Zero", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine","Ten","Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"]
    
    func appendStrWithSpace(_ input: inout String, _ append: String){
        if (!append.isEmpty){
            input.append(" ")
            input.append(append)
        }
    }
    
    func subString(_ subInt: Int) -> String {
        var retString = ""
        //百位
        var value = subInt / 100
        if (value > 0) {
            appendStrWithSpace(&retString, tenthStrings[Int(value)])
            appendStrWithSpace(&retString, hundrendString)
        }
        
        //十位
        value = subInt % 100
        if (value > 0) {
            if (value >= 20) {
                let v = value / 10
                appendStrWithSpace(&retString, tenStrings[Int(v)])
            }
            else { //20一下单独处理
                appendStrWithSpace(&retString, tenthStrings[Int(value)])
                return retString
            }
        }

        //个位
        value = value % 10
        if (value > 0) {
            appendStrWithSpace(&retString, tenthStrings[Int(value)])
        }
        
        return retString
    }
    
    
    let sectionsStrings = ["", "Thousand", "Million", "Billion"]
    var retString = ""
    for i in stride(from: intArray.count-1, through: 0, by: -1) {
        //Error: 每个section内部的头部本来就合并了空格，所以这里不需要再增加空格
        let sub = subString(intArray[i])
        if (!sub.isEmpty) {
            retString.append(sub)
            appendStrWithSpace(&retString, sectionsStrings[i])
        }
    }
    
    retString = retString.trimmingCharacters(in: NSCharacterSet.whitespaces)

    return retString
}

convIntToEng(12345)