Server Engine Abstraction

/**
 * Interface for server engine implementations
 */
interface ApplicationEngine {
    /** Application engine environment */
    val environment: ApplicationEngineEnvironment
    
    /** Start the server engine */
    fun start(wait: Boolean = true): ApplicationEngine
    /** Stop the server engine */
    fun stop(gracePeriodMillis: Long, timeoutMillis: Long)
    /** Add shutdown hook */
    fun addShutdownHook(hook: () -> Unit)
}

/**
 * Base implementation for application engines
 */
abstract class BaseApplicationEngine(
    final override val environment: ApplicationEngineEnvironment,
    val pipeline: EnginePipeline = defaultEnginePipeline(environment)
) : ApplicationEngine {
    /** Engine is running */
    val isRunning: Boolean
    /** Engine monitor for lifecycle events */
    val monitor: Events
    
    /** Execute engine pipeline */
    protected suspend fun executeEngine(call: ApplicationCall)
    /** Start engine implementation */
    protected abstract fun start(wait: Boolean): ApplicationEngine
    /** Stop engine implementation */
    protected abstract fun stop(gracePeriodMillis: Long, timeoutMillis: Long)
}

/**
 * Environment for application engines
 */
interface ApplicationEngineEnvironment {
    /** Application instance */
    val application: Application
    /** Environment logger */
    val log: Logger
    /** Environment configuration */
    val config: ApplicationConfig
    /** Event monitor */
    val monitor: Events
    /** Development mode flag */
    val developmentMode: Boolean
    /** Server connectors configuration */
    val connectors: List<EngineConnectorConfig>
    /** Application modules to load */
    val modules: List<Application.() -> Unit>
    /** Parent coroutine context */
    val parentCoroutineContext: CoroutineContext
    /** Watch paths for development mode */
    val watchPatterns: List<String>
    /** Root path for the application */
    val rootPath: String
}

/**
 * Builder for creating engine environments
 */
class ApplicationEngineEnvironmentBuilder {
    /** Set parent coroutine context */
    var parentCoroutineContext: CoroutineContext = EmptyCoroutineContext
    /** Set development mode */
    var developmentMode: Boolean = false
    /** Set root path */
    var rootPath: String = "/"
    /** Watch patterns for auto-reload */
    val watchPatterns: MutableList<String> = mutableListOf()
    /** Connector configurations */
    val connectors: MutableList<EngineConnectorConfig> = mutableListOf()
    /** Application modules */
    val modules: MutableList<Application.() -> Unit> = mutableListOf()
    
    /** Add HTTP connector */
    fun connector(block: EngineConnectorConfigBuilder.() -> Unit)
    /** Add application module */  
    fun module(body: Application.() -> Unit)
    /** Build environment */
    fun build(): ApplicationEngineEnvironment
}

/**
 * Engine connector configuration
 */
interface EngineConnectorConfig {
    /** Connector type (HTTP, HTTPS) */
    val type: ConnectorType
    /** Server host */
    val host: String
    /** Server port */
    val port: Int
}

/**
 * Builder for engine connector configuration
 */
class EngineConnectorConfigBuilder {
    /** Connector host (default: 0.0.0.0) */
    var host: String = "0.0.0.0"
    /** Connector port (default: 80) */
    var port: Int = 80
    
    /** Build connector configuration */
    fun build(): EngineConnectorConfig
}

/**
 * Connector types
 */
enum class ConnectorType {
    HTTP,
    HTTPS
}

/**
 * HTTP connector configuration
 */
class HttpConnectorConfig(
    override val host: String = "0.0.0.0",
    override val port: Int = 80
) : EngineConnectorConfig {
    override val type: ConnectorType = ConnectorType.HTTP
}

/**
 * HTTPS connector configuration  
 */
class HttpsConnectorConfig(
    override val host: String = "0.0.0.0",
    override val port: Int = 443,
    val keyStorePath: String,
    val keyStorePassword: String,
    val privateKeyPassword: String? = null
) : EngineConnectorConfig {
    override val type: ConnectorType = ConnectorType.HTTPS
}

/**
 * Engine pipeline for request processing
 */
class EnginePipeline : Pipeline<Unit, ApplicationCall> {
    companion object {
        /** Before phase for engine pre-processing */
        val Before: PipelinePhase = PipelinePhase("Before")
        /** Call phase for application call processing */  
        val Call: PipelinePhase = PipelinePhase("Call")
        /** After phase for engine post-processing */
        val After: PipelinePhase = PipelinePhase("After")
    }
}

/**
 * Create default engine pipeline
 * @param environment - Engine environment
 * @return Configured engine pipeline
 */
fun defaultEnginePipeline(environment: ApplicationEngineEnvironment): EnginePipeline

/**
 * Command line configuration parser
 */
class CommandLineConfig(args: Array<String>) {
    /** Parsed configuration */
    val config: ApplicationConfig
    /** Server host from command line */
    val host: String?
    /** Server port from command line */
    val port: Int?
    /** Development mode flag */
    val developmentMode: Boolean
    /** Watch patterns from command line */
    val watchPatterns: List<String>
    
    /** Create engine environment from command line */
    fun createEnvironment(): ApplicationEngineEnvironment
}

/**
 * Parse command line arguments
 * @param args - Command line arguments array
 * @return Parsed command line configuration
 */
fun parseCommandLineArgs(args: Array<String>): CommandLineConfig

/**
 * Create application engine environment
 * @param configure - Environment configuration block
 * @return Configured environment
 */
fun applicationEngineEnvironment(configure: ApplicationEngineEnvironmentBuilder.() -> Unit): ApplicationEngineEnvironment

/**
 * Create development environment with auto-reload
 * @param configure - Environment configuration block
 * @return Development environment
 */
fun developmentEnvironment(configure: ApplicationEngineEnvironmentBuilder.() -> Unit): ApplicationEngineEnvironment

/**
 * Create production environment  
 * @param configure - Environment configuration block
 * @return Production environment
 */
fun productionEnvironment(configure: ApplicationEngineEnvironmentBuilder.() -> Unit): ApplicationEngineEnvironment

/**
 * Engine lifecycle events
 */
object EngineEvents {
    /** Engine starting event */
    val EngineStarting: EventDefinition<ApplicationEngine>
    /** Engine started event */
    val EngineStarted: EventDefinition<ApplicationEngine>
    /** Engine stopping event */
    val EngineStopping: EventDefinition<ApplicationEngine>
    /** Engine stopped event */
    val EngineStopped: EventDefinition<ApplicationEngine>
}

/**
 * Event monitoring for engines
 */
interface Events {
    /** Subscribe to event */
    fun <T> subscribe(definition: EventDefinition<T>, handler: (T) -> Unit)
    /** Unsubscribe from event */
    fun <T> unsubscribe(definition: EventDefinition<T>, handler: (T) -> Unit)
    /** Raise event */
    fun <T> raise(definition: EventDefinition<T>, value: T)
}

/**
 * Event definition
 */
data class EventDefinition<T>(val name: String)

import io.ktor.server.application.*
import io.ktor.server.engine.*
import io.ktor.server.response.*
import io.ktor.server.routing.*

// Basic engine environment setup
fun createBasicEnvironment(): ApplicationEngineEnvironment {
    return applicationEngineEnvironment {
        // Add HTTP connector
        connector {
            host = "0.0.0.0"
            port = 8080
        }
        
        // Add application module
        module {
            routing {
                get("/") {
                    call.respondText("Hello from Ktor!")
                }
            }
        }
    }
}

// Development environment with auto-reload
fun createDevelopmentEnvironment(): ApplicationEngineEnvironment {
    return developmentEnvironment {
        developmentMode = true
        
        // Watch for changes
        watchPatterns += "src/main/kotlin/**/*.kt"
        watchPatterns += "src/main/resources/**/*"
        
        connector {
            host = "localhost"
            port = 8080
        }
        
        module(Application::developmentModule)
    }
}

// Production environment with HTTPS
fun createProductionEnvironment(): ApplicationEngineEnvironment {
    return productionEnvironment {
        developmentMode = false
        
        // HTTP connector (for redirects)
        connector {
            host = "0.0.0.0"
            port = 80
        }
        
        // HTTPS connector
        connector {
            host = "0.0.0.0"
            port = 443
            // Note: HTTPS configuration would be engine-specific
        }
        
        module(Application::productionModule)
    }
}

// Multiple connectors setup
fun createMultiConnectorEnvironment(): ApplicationEngineEnvironment {
    return applicationEngineEnvironment {
        // Public HTTP connector
        connector {
            host = "0.0.0.0"
            port = 8080
        }
        
        // Admin connector on different port
        connector {
            host = "127.0.0.1"
            port = 8081
        }
        
        module {
            routing {
                get("/") {
                    call.respondText("Public API")
                }
                
                get("/admin") {
                    // Check if request came from admin connector
                    val localPort = call.request.local.port
                    if (localPort == 8081) {
                        call.respondText("Admin Interface")
                    } else {
                        call.respond(HttpStatusCode.NotFound)
                    }
                }
            }
        }
    }
}

// Command line configuration
fun main(args: Array<String>) {
    val commandLineConfig = parseCommandLineArgs(args)
    val environment = commandLineConfig.createEnvironment()
    
    // Engine would be started here with the environment
    // Example: embeddedServer(Netty, environment).start(wait = true)
}

// Custom engine implementation example
abstract class CustomApplicationEngine(
    environment: ApplicationEngineEnvironment
) : BaseApplicationEngine(environment) {
    
    override fun start(wait: Boolean): ApplicationEngine {
        // Subscribe to engine events
        environment.monitor.subscribe(EngineEvents.EngineStarting) { engine ->
            environment.log.info("Starting custom engine...")
        }
        
        environment.monitor.subscribe(EngineEvents.EngineStarted) { engine ->
            environment.log.info("Custom engine started on ${environment.connectors.joinToString { "${it.host}:${it.port}" }}")
        }
        
        // Start server implementation
        startServer()
        
        if (wait) {
            // Wait for shutdown
            Runtime.getRuntime().addShutdownHook(Thread {
                stop(1000, 5000)
            })
        }
        
        return this
    }
    
    override fun stop(gracePeriodMillis: Long, timeoutMillis: Long) {
        environment.monitor.raise(EngineEvents.EngineStopping, this)
        
        // Stop server implementation
        stopServer(gracePeriodMillis, timeoutMillis)
        
        environment.monitor.raise(EngineEvents.EngineStopped, this)
    }
    
    protected abstract fun startServer()
    protected abstract fun stopServer(gracePeriodMillis: Long, timeoutMillis: Long)
}

// Environment with custom configuration
fun createConfigurableEnvironment(config: ApplicationConfig): ApplicationEngineEnvironment {
    return applicationEngineEnvironment {
        // Load configuration
        val serverConfig = config.config("server")
        val host = serverConfig.property("host").getString()
        val port = serverConfig.property("port").getInt()
        
        connector {
            this.host = host
            this.port = port
        }
        
        // Set development mode from config
        developmentMode = config.propertyOrNull("app.development")?.getBoolean() ?: false
        
        // Set root path from config
        rootPath = config.propertyOrNull("app.rootPath")?.getString() ?: "/"
        
        // Load modules based on configuration
        val enabledModules = config.propertyOrNull("app.modules")?.getList() ?: emptyList()
        
        if ("routing" in enabledModules) {
            module(Application::routingModule)
        }
        
        if ("authentication" in enabledModules) {
            module(Application::authenticationModule)  
        }
        
        if ("monitoring" in enabledModules) {
            module(Application::monitoringModule)
        }
    }
}

// Extension modules
fun Application.developmentModule() {
    routing {
        get("/") {
            call.respondText("Development Server")
        }
        
        get("/reload") {
            call.respondText("Auto-reload enabled")
        }
    }
}

fun Application.productionModule() {
    routing {
        get("/") {
            call.respondText("Production Server")
        }
        
        get("/health") {
            call.respond(HttpStatusCode.OK, mapOf("status" to "healthy"))
        }
    }
}

fun Application.routingModule() {
    routing {
        get("/api/status") {
            call.respond(mapOf("service" to "running"))
        }
    }
}

fun Application.authenticationModule() {
    // Authentication setup would go here
}

fun Application.monitoringModule() {
    // Monitoring and metrics setup would go here
}