엔진은 집에 있어: 앱은 얇은 클라이언트야

import Foundation

struct EngineLocation: Equatable, Sendable {
    enum Failure: Error, Equatable {
        case scheme, host, portMissing, portRange, extraParts
    }

    let scheme: String   // "http" or "https"
    let host: String     // IPv6 keeps its brackets: "[::1]"
    let port: Int        // always written: the twins sit on adjacent ports

    var baseURL: URL { URL(string: "\(scheme)://\(host):\(port)")! }

    /// Strict: a scheme, a host and a WRITTEN port, nothing else but a trailing "/".
    init(parsing text: String) throws(Failure) {
        let trimmed = text.trimmingCharacters(in: .whitespaces)
        guard let components = URLComponents(string: trimmed),
              let scheme = components.scheme?.lowercased(), scheme == "http" || scheme == "https"
        else { throw .scheme }
        guard components.user == nil, components.password == nil,
              components.query == nil, components.fragment == nil,
              components.path.isEmpty || components.path == "/"
        else { throw .extraParts }
        guard let rawHost = components.percentEncodedHost, !rawHost.isEmpty else { throw .host }
        guard let port = components.port else { throw .portMissing }
        guard (1...65_535).contains(port) else { throw .portRange }
        self.scheme = scheme
        self.host = rawHost.contains(":") && !rawHost.hasPrefix("[") ? "[\(rawHost)]" : rawHost
        self.port = port
    }

    /// Answered from the parsed host, never the typed string. "127.evil.com" is a name, not loopback.
    var isLoopback: Bool {
        let bare = host.trimmingCharacters(in: CharacterSet(charactersIn: "[]")).lowercased()
        let octets = bare.split(separator: ".", omittingEmptySubsequences: false)
        let ipv4Loopback = octets.count == 4 && octets.allSatisfy { UInt8($0) != nil } && octets[0] == "127"
        return bare == "localhost" || bare == "::1" || ipv4Loopback
    }
}

// "http://office.example.ts.net:8900"  -> ok
// "http://office.example.ts.net"       -> portMissing
// "http://127.0.0.1:8900/api"          -> extraParts
// "HTTP://[::1]:8900"                  -> ok, isLoopback == true