class CompositeConfig(
    val nonOptionalVar: String,
    val optionalVar: String?
)

internal object CompositeEnvironmentHandler {

    val config: CompositeConfig

    private val spec = BiDiLensSpec("location", ParamMeta.ObjectParam,
        LensGet { location: String, env: Environment ->
            listOf(env[location]!!)
        },
        LensSet { _: String, _: List<String>, env: Environment ->
            env
        })
    private val lens = spec.composite {
        CompositeConfig(
            nonOptionalVar = required("NON_OPTIONAL_VAR")(it),
            optionalVar = optional("OPTIONAL_VAR")(it)
        )
    }

    init {

        // Fallback default config.
        val defaultEnvironment = Environment.defaults(
            EnvironmentKey.k8s.SERVICE_PORT of 8000
        )

        // Environment overrides by priority JVM > System > File > Default.
        val environment = Environment.JVM_PROPERTIES overrides
                Environment.ENV overrides
                fileSystemEnvironment() overrides
                defaultEnvironment
        config = lens(environment)
    }

    /**
     * Fetches configuration file from file system.
     * Will search for file under [environmentVariable] in JVMProps then in ENV.
     * If not found will search for it in file system with [fallbackName].
     */
    private fun fileSystemEnvironment(
        environmentVariable: String = "APP_CONFIG",
        fallbackName: String = "testConfig.yml"
    ): Environment {

        // Fetch file from env environmentVariable JVM > ENV > Default
        return (Environment.JVM_PROPERTIES[environmentVariable]
            ?: Environment.ENV[environmentVariable])?.let {
            Environment.from(File(it))
        } ?: Environment.fromResource(fallbackName)
    }
}