kotlin/builder/src/io/bazel/kotlin/builder/Meta.kt

/*
 * Copyright 2018 The Bazel Authors. All rights reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package io.bazel.kotlin.builder

interface Meta<T : Any> {
    val id: String
        get() = this.javaClass.simpleName

    val defaultValue: T?
        get() = null

    /**
     * Gets a mandatory value.
     */
    fun mustGet(ctx: Context): T =
            ctx[this] ?: checkNotNull(defaultValue) { "mandatory meta parameter missing in context and does not have a default value" }

    /**
     * Gets an optional value, if it has not been bound the default value is used.
     */
    operator fun get(ctx: Context): T? {
        val res = ctx[this]
        return when {
            res != null -> res
            defaultValue != null -> defaultValue
            else -> null
        }
    }

    operator fun set(ctx: Context, value: T?) {
        checkNotNull(value ?: defaultValue) { "attampting to initialize ctx to a null value: (hasDefault: ${defaultValue != null}" }.also {
            check(ctx.putIfAbsent(this, it) == null) { "attempting to change bound meta variable: $id " }
        }
    }

    companion object {
        operator fun <T : Any> invoke(id: String): Meta<T> = object : Meta<T> {
            override val id: String = id
            override val defaultValue: T? = null
        }
    }
}

open class MandatoryMeta<T: Any>(
        override val id: String,
        override val defaultValue: T? = null
): Meta<T> {
    override fun get(ctx: Context): T = checkNotNull(super.get(ctx)) { "ctx missing mandatory meta ${this.id}" }
}