gistfile1.swift

func romanify(n: Int) -> String {
    let pivotRomans = [
        1: "I",
        4: "IV",
        5: "V",
        9: "IX",
        10: "X",
        40: "XL",
        50: "L",
        90: "XC",
        100: "C",
        400: "CD",
        500: "D",
        900: "CM",
        1000: "M"
    ]
    var roman = ""
    var remaining = n
    
    while remaining > 0 {
        let pivotValues = Array(pivotRomans.keys).sorted(>)
        var i = 0
        for ; remaining < pivotValues[i]; i++ {}
        let highestPivot = pivotValues[i]
        roman += pivotRomans[highestPivot]!
        remaining -= highestPivot
    }
    return roman
}

assert(romanify(1) == "I", "Did not convert 1 to roman properly")
assert(romanify(2) == "II", "Did not convert 2 to roman properly")
assert(romanify(3) == "III", "Did not convert 3 to roman properly")
assert(romanify(4) == "IV", "Did not convert 4 to roman properly")
assert(romanify(5) == "V", "Did not convert 5 to roman properly")
assert(romanify(6) == "VI", "Did not convert 6 to roman properly")
assert(romanify(8) == "VIII", "")
assert(romanify(9) == "IX", "")
assert(romanify(10) == "X", "")
assert(romanify(14) == "XIV", "")
assert(romanify(15) == "XV", "")
assert(romanify(19) == "XIX", "")
assert(romanify(20) == "XX", "")
assert(romanify(40) == "XL", "")
assert(romanify(66) == "LXVI", "")
assert(romanify(99) == "XCIX", "")

Also consider not using a map, since it makes you need to sort which isn't great