your swift ride is here

ride.swift

func encode(name :String) -> Int {
    return reduce(map(name.unicodeScalars) { Int($0.value) - 64 }, 1, *) % 47
}

func ride(group: String, comet: String) -> String {
    return encode(group) == encode(comet) ? "GO" : "STAY"
}

let testData = [
    ("COMETQ","HVNGAT","GO"),
    ("STARAB","USACO","STAY"),
    ("EARTH","LEFTB","GO"),
    ("PULSAR","VENUS","STAY"),
    ("KANSAS","UTAH","STAY"),
    ("APPLE","URSA","GO"),
    ("MSFT","MARS","STAY"),
    ("PLUTO","BKHOLE","STAY"),
    ("COWSBC","RIGHT","GO"),
    ("DRKMTR","SNIKER","STAY")]

func testRide() -> String {
    for testCase in testData {
        if ride(testCase.0, testCase.1) != testCase.2 {
            return "Fail: Group \(testCase.0) and comet \(testCase.1) should \(testCase.2)"
        }
    }
    return "pass"
}

let testResult = testRide()