struct RawServerResponse : Codable {
enum Keys : String, CodingKey {
    case data = "data"
}
let data : [String:Base]
}

struct Base : Codable {
enum CodingKeys : String, CodingKey {
    case id = "id"
    case name = "name"
    case symbol = "symbol"
}

let id : Int64
let name : String
let symbol : String
}

struct Quote : Codable {
enum CodingKeys : String, CodingKey {
    case price = "price"
    case marketCap = "market_cap"
}

let price :  Double
let marketCap : Double
}

extension RawServerResponse {
enum BaseKeys : String {
    case btc = "1"
    case eth = "1027"
}
var btc : Base? { return data[BaseKeys.btc.rawValue] }
var eth : Base? { return data[BaseKeys.eth.rawValue] }

}

extension Base {
enum Currencies : String {
    case usd = "USD"
}
var usd : Quote? { return quotes[Currencies.usd.rawValue]}
}

struct ServerResponse: Codable {
let btcName: String?
let btcSymbol: String?

init(from decoder: Decoder) throws {
    let rawResponse = try RawServerResponse(from: decoder)

    btcSymbol = rawResponse.btc?.symbol

{
"data": {
    "1": {
        "id": 1,
        "name": "Bitcoin",
        "symbol": "BTC",
        "website_slug": "bitcoin",
        "rank": 1,
        "circulating_supply": 17041575.0,
        "total_supply": 17041575.0,
        "max_supply": 21000000.0,
        "quotes": {
            "USD": {
                "price": 8214.7,
                "volume_24h": 5473430000.0,
                "market_cap": 139991426153.0,
                "percent_change_1h": 0.09,
                "percent_change_24h": 2.29,
                "percent_change_7d": -2.44
            }
        },
        "last_updated": 1526699671
    },
    "1027": {
        "id": 1027,
        "name": "Ethereum",
        "symbol": "ETH",
        "website_slug": "ethereum",
        "rank": 2,
        "circulating_supply": 99524121.0,
        "total_supply": 99524121.0,
        "max_supply": null,
        "quotes": {
            "USD": {
                "price": 689.891,
                "volume_24h": 2166100000.0,
                "market_cap": 68660795252.0,
                "percent_change_1h": 0.13,
                "percent_change_24h": 2.51,
                "percent_change_7d": 2.54
            }
        }

struct RawServerResponse : Codable {
    var data = [String:Base]()

    private enum CodingKeys: String, CodingKey { case data }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        let baseDictionary = try container.decode([String:Base].self, forKey: .data)
        baseDictionary.forEach { data[$0.1.symbol] = $0.1 }
    }
}

struct Base : Codable {
    let id : Int64
    let name : String
    let symbol : String
}


do {
    let decoder = JSONDecoder()
    decoder.keyDecodingStrategy = .convertFromSnakeCase
    let rawResponse = try decoder.decode(RawServerResponse.self, from: data)
    for (symbol, base) in rawResponse.data {
        print(symbol, base.quotes["USD"]?.marketCap)
        // ETH Optional(68660795252.0)
        // BTC Optional(139991426153.0)
    }
} catch { print(error) }