splitStringByWidth.swift

extension Unicode.Scalar {
    var width: UInt32 {
        let code = self.value
        if (code >= 0 && code <= 128) {
            return 1
        } else {
            return 2
        }
    }
}

extension Character {
    var width: UInt32 {
        return self.unicodeScalars.reduce(0) { $0 + $1.width }
    }
}

extension String {
    var width: UInt32 {
        return self.unicodeScalars.reduce(0) { $0 + $1.width }
    }
}

let LINE_WIDTH_MAX = 30
let original = "あいうえおかきくけこさしすせそはひふへほ\nsasisusesoたちつてと"

let result: String = original
    .split(separator: "\n")
    .flatMap { line in
        return line.reduce(into: [""], { (splitted, char) in
            if var last = splitted.last, last.width + char.width <= LINE_WIDTH_MAX {
                last.append(char)
                splitted[splitted.count - 1] = last
            } else {
                splitted.append(String(char))
            }
        })
    }
    .prefix(3)
    .joined(separator: "\n")

print(result)