1 @file:OptIn(ExperimentalContracts::class)
2
3 package kotlinx.coroutines.selects
4
5 import kotlin.contracts.*
6 import kotlin.coroutines.*
7
8 /**
9 * Waits for the result of multiple suspending functions simultaneously like [select], but in an _unbiased_
10 * way when multiple clauses are selectable at the same time.
11 *
12 * This unbiased implementation of `select` expression randomly shuffles the clauses before checking
13 * if they are selectable, thus ensuring that there is no statistical bias to the selection of the first
14 * clauses.
15 *
16 * See [select] function description for all the other details.
17 */
18 @OptIn(ExperimentalContracts::class)
selectUnbiasednull19 public suspend inline fun <R> selectUnbiased(crossinline builder: SelectBuilder<R>.() -> Unit): R {
20 contract {
21 callsInPlace(builder, InvocationKind.EXACTLY_ONCE)
22 }
23 return UnbiasedSelectImplementation<R>(coroutineContext).run {
24 builder(this)
25 doSelect()
26 }
27 }
28
29 /**
30 * The unbiased `select` inherits the [standard one][SelectImplementation],
31 * but does not register clauses immediately. Instead, it stores all of them
32 * in [clausesToRegister] lists, shuffles and registers them in the beginning of [doSelect]
33 * (see [shuffleAndRegisterClauses]), and then delegates the rest
34 * to the parent's [doSelect] implementation.
35 */
36 @PublishedApi
37 internal open class UnbiasedSelectImplementation<R>(context: CoroutineContext) : SelectImplementation<R>(context) {
38 private val clausesToRegister: MutableList<ClauseData> = arrayListOf()
39
invokenull40 override fun SelectClause0.invoke(block: suspend () -> R) {
41 clausesToRegister += ClauseData(clauseObject, regFunc, processResFunc, PARAM_CLAUSE_0, block, onCancellationConstructor)
42 }
43
invokenull44 override fun <Q> SelectClause1<Q>.invoke(block: suspend (Q) -> R) {
45 clausesToRegister += ClauseData(clauseObject, regFunc, processResFunc, null, block, onCancellationConstructor)
46 }
47
invokenull48 override fun <P, Q> SelectClause2<P, Q>.invoke(param: P, block: suspend (Q) -> R) {
49 clausesToRegister += ClauseData(clauseObject, regFunc, processResFunc, param, block, onCancellationConstructor)
50 }
51
52 @PublishedApi
doSelectnull53 override suspend fun doSelect(): R {
54 shuffleAndRegisterClauses()
55 return super.doSelect()
56 }
57
shuffleAndRegisterClausesnull58 private fun shuffleAndRegisterClauses() = try {
59 clausesToRegister.shuffle()
60 clausesToRegister.forEach { it.register() }
61 } finally {
62 clausesToRegister.clear()
63 }
64 }
65