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import { AsyncQueue } from './queue.js'
import type {
EffectRuntime,
IngestHandle,
MachineEffect,
MachineEvent,
MachineRuntime,
RuntimeOptions,
} from './types.js'
// What HookRuntime needs from its owning Runtime; the Runtime class never leaves
// this module, so nothing external gains access.
type HookHost<S, E extends MachineEvent, O> = {
readonly state: S
readonly signal: AbortSignal
emit(output: O): void
dispatch(event: E): void
}
// The runtime handle passed to transitions and effect handlers. Hooks have no
// lifecycle access: a machine terminates by returning `final` from a transition,
// and an effect that hits an unrecoverable error throws (destroying the machine).
// Handing hooks the full runtime would invite `await runtime.close()` from inside
// the drain loop — a structural promise-cycle deadlock.
class HookRuntime<S, E extends MachineEvent, O> implements EffectRuntime<S, E, O> {
#host: HookHost<S, E, O>
constructor(host: HookHost<S, E, O>) {
this.#host = host
}
get state(): S {
return this.#host.state
}
get signal(): AbortSignal {
return this.#host.signal
}
emit(output: O): void {
this.#host.emit(output)
}
dispatch(event: E): void {
this.#host.dispatch(event)
}
}
class Runtime<
S,
LC extends object,
RC extends object,
E extends MachineEvent,
FX extends MachineEffect,
O,
>
// Deliberately NOT an EffectRuntime: hooks must receive the HookRuntime view,
// whose close() cannot deadlock the drain loop they run in.
implements MachineRuntime<S, LC, E, O>
{
#ac = new AbortController()
#state: S
#ctx: LC & RC
#effects: RuntimeOptions<S, LC, RC, E, FX, O>['effects']
#transition: RuntimeOptions<S, LC, RC, E, FX, O>['transition']
// Assigned synchronously BEFORE the drain loop body runs its first transition
// (via a pre-created deferred), so every observer — a nested dispatch, close(),
// an external drain — sees the in-flight drain and waits for it instead of
// starting a second loop against stale (pre-transition) state or sealing the
// output queue mid-drain.
#drainTask: Promise<void> | null = null
#eventQueue: E[] = []
#outputQueue = new AsyncQueue<O>()
#closing = false
#closed = false
#destroyed = false
// Set when the machine is torn down by an internal error (a transition, effect,
// or ingest source threw) rather than a graceful close/destroy. destroy() then
// fails the output queue instead of closing it, so the iterator throws the stop
// reason and consumers observe the failure instead of a silent end (which would
// strand a facade waiting on a terminal signal).
#faulted = false
#closeTask: Promise<void> | null = null
#destroyTask: Promise<void> | null = null
#stopReason: unknown = null
#hookRuntime: HookRuntime<S, E, O>
constructor(options: RuntimeOptions<S, LC, RC, E, FX, O>) {
this.#state = options.initialState
// Merge env into ctx once at construction so both LC and RC fields
// live on the same object reference. Mutations in effects are reflected immediately.
this.#ctx = Object.assign(options.ctx, options.env) as unknown as LC & RC
this.#effects = options.effects
this.#transition = options.transition
this.#hookRuntime = new HookRuntime(this)
}
get state(): S {
return this.#state
}
get ctx(): LC {
return this.#ctx
}
get signal(): AbortSignal {
return this.#ac.signal
}
// The runtime stops accepting input once shutdown has begun in any form.
get #stopped(): boolean {
return this.#closing || this.#closed || this.#destroyed
}
emit(output: O): void {
Iif (this.#outputQueue.isClosed) {
return
}
try {
this.#outputQueue.push(output)
} catch {
// Queue may close concurrently during shutdown — ignore.
}
}
ingest<T>(
source: AsyncIterable<T>,
map: (input: T) => E | null,
signal?: AbortSignal
): IngestHandle {
if (this.#stopped) {
throw new Error('Runtime is not accepting new ingests')
}
let ac = new AbortController()
let task = (async () => {
using combined = linkSignals(this.#ac.signal, ac.signal, signal)
try {
for await (let input of source) {
if (combined.signal.aborted) {
return
}
if (this.#stopped) {
return
}
let event = map(input)
if (event !== null) {
this.dispatch(event)
}
}
} catch (error) {
Iif (combined.signal.aborted) {
return
}
Iif (this.#stopped) {
return
}
await this.#handleError(error)
}
})()
let disposed = false
return {
async [Symbol.asyncDispose](): Promise<void> {
Iif (disposed) {
return
}
disposed = true
ac.abort(new Error('Ingest disposed'))
await task.catch(() => {
// Ingest task errors are handled by runtime error path.
})
},
}
}
dispatch(event: E): void {
if (this.#stopped) {
return
}
this.#eventQueue.push(event)
// Drain handles errors internally (via #handleError → destroy).
// Suppress the rejected promise so Node does not surface it as
// an unhandled rejection when dispatch is fire-and-forget.
this.#drain().catch(() => {})
}
async close(reason?: unknown): Promise<void> {
Iif (this.#closed || this.#destroyed) {
return
}
if (this.#closeTask) {
await this.#closeTask
return
}
this.#closeTask = (async () => {
this.#markClosing(reason)
// Drain first so all outputs emitted during pending transitions are
// delivered before the output queue is sealed. A drain may already be
// in flight — wait for that loop to finish, then drain any tail.
while (this.#drainTask) {
// oxlint-disable-next-line no-await-in-loop
await this.#drainTask
}
await this.#drain()
if (this.#destroyed) {
return
}
this.#seal()
})()
await this.#closeTask
}
// Set closing (first caller's reason wins) so no new events are enqueued past
// this point.
#markClosing(reason?: unknown): void {
if (this.#closing) {
return
}
this.#closing = true
this.#stopReason = reason ?? new Error('Runtime closed')
}
// Idempotent terminal step of a graceful close: seal the output stream, then
// abort the signal — ingest loops and signal listeners are notified only once
// all pending output has been delivered.
#seal(): void {
if (this.#closed || this.#destroyed) {
return
}
this.#closed = true
this.#outputQueue.close()
this.#ac.abort(this.#stopReason)
}
async destroy(reason?: unknown): Promise<void> {
Iif (this.#destroyed) {
return
}
Iif (this.#destroyTask) {
await this.#destroyTask
return
}
this.#destroyTask = (async () => {
this.#destroyed = true
this.#closing = false
this.#closed = true
this.#stopReason = reason ?? new Error('Runtime destroyed')
this.#ac.abort(this.#stopReason)
// Hard shutdown: drop queued events immediately.
this.#eventQueue.length = 0
// Release pending iterators. An internal fault makes the output iterator
// throw the stop reason (so consumers observe the failure instead of a
// silent end); a graceful teardown just ends the stream.
if (this.#faulted) {
this.#outputQueue.fail(this.#stopReason)
} else {
this.#outputQueue.close()
}
})()
await this.#destroyTask
}
[Symbol.asyncIterator](): AsyncIterator<O> {
return this.#outputQueue[Symbol.asyncIterator]()
}
// Graceful async disposal first, then hard finalization.
async [Symbol.asyncDispose](): Promise<void> {
await this.close(new Error('Runtime async disposed'))
await this.destroy(new Error('Runtime async disposed'))
}
async #drain(): Promise<void> {
// Loop rather than branch: multiple callers may be waiting on the same
// drainTask. When it resolves they all wake up simultaneously — the loop
// ensures each re-checks whether a new task was already started before
// creating another one. A drain re-entered from a transition or effect
// (via dispatch) also lands here: the event is already in the shared queue
// and the running loop will pick it up, so waiting is correct — starting a
// second loop would process it against stale (pre-transition) state.
while (this.#drainTask) {
// oxlint-disable-next-line no-await-in-loop
await this.#drainTask
}
if (this.#destroyed || this.#eventQueue.length === 0) {
return
}
// Assign the task via a pre-created deferred BEFORE the body runs its first
// (synchronous) transition, so a close()/dispatch issued from within that
// transition already observes the in-flight drain.
let { promise, resolve } = Promise.withResolvers<void>()
this.#drainTask = promise
try {
while (this.#eventQueue.length > 0 && !this.#destroyed) {
let event = this.#eventQueue.shift()!
let result = this.#transition(this.#ctx, event, this.#hookRuntime)
if (!result) {
continue
}
if (result.state !== undefined) {
this.#state = result.state
}
// A terminal transition: stop accepting events now; the seal happens in
// the finally below, after this transition's effects (cleanup) run and
// the already-queued tail drains (terminal states ignore it).
if (result.final) {
this.#markClosing(result.final.reason)
}
if (!result.effects || result.effects.length === 0) {
continue
}
for (let effect of result.effects) {
Iif (this.#destroyed) {
return
}
let handler = this.#effects[effect.type as FX['type']]
Iif (!handler) {
throw new Error(`Missing effect handler for effect type "${effect.type}"`)
}
// oxlint-disable-next-line no-await-in-loop
await handler(this.#ctx, effect as never, this.#hookRuntime)
}
}
} catch (error) {
await this.#handleError(error)
} finally {
this.#drainTask = null
resolve()
// A close requested from within this drain (hook close()) seals here,
// after every queued event's outputs made it into the queue. dispatch()
// rejects new events once #closing is set, so the queue is empty unless
// the loop bailed on destroy — where sealing is destroy()'s job.
if (this.#closing && !this.#destroyed && this.#eventQueue.length === 0) {
this.#seal()
}
}
}
async #handleError(error: unknown): Promise<void> {
Iif (this.#destroyed) {
return
}
this.#faulted = true
await this.destroy(error)
}
}
export function createMachineRuntime<
S,
LC extends object,
RC extends object,
E extends MachineEvent,
FX extends MachineEffect,
O,
>(options: RuntimeOptions<S, LC, RC, E, FX, O>): MachineRuntime<S, LC, E, O> {
return new Runtime(options)
}
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