kotlinx-coroutines-core/src/main/kotlin/kotlinx/coroutines/experimental/CoroutineContext.kt

package kotlinx.coroutines.experimental

import java.util.concurrent.atomic.AtomicLong
import kotlin.coroutines.AbstractCoroutineContextElement
import kotlin.coroutines.CoroutineContext

private const val DEBUG_PROPERTY_NAME = "kotlinx.coroutines.debug"

private val DEBUG = run {
    val value = System.getProperty(DEBUG_PROPERTY_NAME)
    when (value) {
        "auto", null -> CoroutineId::class.java.desiredAssertionStatus()
        "on", "" -> true
        "off" -> false
        else -> error("System property '$DEBUG_PROPERTY_NAME' has unrecognized value '$value'")
    }
}

private val COROUTINE_ID = AtomicLong()

/**
 * A coroutine dispatcher that executes initial continuation of the coroutine _right here_ in the current call-frame
 * and let the coroutine resume in whatever thread that is used by the corresponding suspending function, without
 * mandating any specific threading policy.
 */
public object Here : CoroutineDispatcher() {
    override fun isDispatchNeeded(context: CoroutineContext): Boolean = false
    override fun dispatch(context: CoroutineContext, block: Runnable) { throw UnsupportedOperationException() }
}

/**
 * Creates context for the new coroutine with optional support for debugging facilities (when turned on).
 *
 * **Debugging facilities:** In debug mode every coroutine is assigned a unique consecutive identifier.
 * Every thread that executes a coroutine has its name modified to include the name and identifier of the
 * currently currently running coroutine.
 * When one coroutine is suspended and resumes another coroutine that is dispatched in the same thread,
 * then the thread name displays
 * the whole stack of coroutine descriptions that are being executed on this thread.
 *
 * Enable debugging facilities with "`kotlinx.coroutines.debug`" system property, use the following values:
 * * "`auto`" (default mode) -- enabled when assertions are enabled with "`-ea`" JVM option.
 * * "`on`" or empty string -- enabled.
 * * "`off`" -- disabled.
 *
 * Coroutine name can be explicitly assigned using [CoroutineName] context element.
 * The string "coroutine" is used as a default name.
 */
public fun newCoroutineContext(context: CoroutineContext): CoroutineContext =
    if (DEBUG) context + CoroutineId(COROUTINE_ID.incrementAndGet()) else context

/**
 * Executes a block using a given coroutine context.
 */
internal inline fun <T> withCoroutineContext(context: CoroutineContext, block: () -> T): T {
    val oldName = updateContext(context)
    try {
        return block()
    } finally {
        restoreContext(oldName)
    }
}

@PublishedApi
internal fun updateContext(context: CoroutineContext): String? {
    if (!DEBUG) return null
    val newId = context[CoroutineId] ?: return null
    val currentThread = Thread.currentThread()
    val oldName = currentThread.name
    val coroutineName = context[CoroutineName]?.name ?: "coroutine"
    currentThread.name = buildString(oldName.length + coroutineName.length + 10) {
        append(oldName)
        append(" @")
        append(coroutineName)
        append('#')
        append(newId.id)
    }
    return oldName
}

@PublishedApi
internal fun restoreContext(oldName: String?) {
    if (oldName != null) Thread.currentThread().name = oldName
}

private class CoroutineId(val id: Long) : AbstractCoroutineContextElement(CoroutineId) {
    companion object Key : CoroutineContext.Key<CoroutineId>
    override fun toString(): String = "CoroutineId($id)"
}