Swift: Generate Roman numerals

roman.swift

private let onesSymbols     = ["", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX"]
private let tensSymbols     = ["", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC"]
private let hundredsSymbols = ["", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM"]

/// Generate Roman numeral representation of a value.
///
/// - parameter value: Numeric value to be represented.
/// - returns: `String` (possibly empty)
public func romanNumeralForValue(value: Int) -> String {
    if value <= 0 {
        return ""
    }

    let ones = value % 10
    let tens = (value / 10) % 10
    let hundreds = (value / 100) % 10
    let thousands = value / 1000

    let thousandsSymbols = String(count: thousands, repeatedValue: Character("M"))

    return thousandsSymbols + hundredsSymbols[hundreds] + tensSymbols[tens] + onesSymbols[ones]
}

// Tests
assert(romanNumeralForValue(0) == "")
assert(romanNumeralForValue(1) == "I")
assert(romanNumeralForValue(999) == "CMXCIX")
assert(romanNumeralForValue(1967) == "MCMLXVII")
assert(romanNumeralForValue(2016) == "MMXVI")