Swift environment configuration loader with validation

import Foundation

struct AppConfig {
    let host: String
    let port: Int
    let debug: Bool

    static func load(
        _ env: [String: String] = ProcessInfo.processInfo.environment
    ) throws -> AppConfig {
        let portText = env["PORT"] ?? "8080"
        guard let port = Int(portText), (1...65535).contains(port) else {
            throw ConfigError.invalid("PORT must be 1-65535, got \(portText)")
        }
        let host = env["HOST"].flatMap { $0.isEmpty ? nil : $0 } ?? "127.0.0.1"
        let debug = ["1", "true", "yes"].contains(env["DEBUG"]?.lowercased() ?? "")
        return AppConfig(host: host, port: port, debug: debug)
    }
}

enum ConfigError: Error { case invalid(String) }