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* Serial Output Batcher
*
* Batches serial output with baudrate-based rate limiting to prevent overwhelming
* the WebSocket connection and simulate realistic Arduino serial behavior.
*
* Key features:
* - Ring-Buffer (Uint8Array) for O(1) writes without string allocation overhead
* - Tick-based batching (default 50ms = 20 batches/sec, like PinStateBatcher)
* - Baudrate-based byte budget per tick
* - FIFO drop strategy when buffer full (no unbounded growth)
* - Burst tolerance for short spikes (e.g., setup() output)
* - Telemetry tracking (intended/actual/dropped bytes)
* - Newline-aware cutting (prefer line boundaries)
*/
import { RingBuffer } from "@shared/utils/ring-buffer";
interface SerialOutputBatcherConfig {
/** Baudrate in bits per second (e.g., 115200) */
baudrate: number;
/** Tick interval in milliseconds (default: 50ms = 20 batches/sec) */
tickIntervalMs?: number;
/** Callback invoked with each batch. firstLineIncomplete=true if this chunk starts with a truncated line (due to drops). */
onChunk: (data: string, firstLineIncomplete?: boolean) => void;
/** Burst factor (default: 3 = 3× normal budget for short spikes) */
burstFactor?: number;
}
export interface SerialOutputTelemetry {
/** Total bytes intended to send since last reset */
intended: number;
/** Total bytes actually sent since last reset */
actual: number;
/** Total bytes dropped since last reset */
dropped: number;
/** Number of chunks sent since last reset */
chunks: number;
/** Cumulative bytes intended since batcher start (never resets) */
totalBytes: number;
}
export class SerialOutputBatcher {
private readonly config: Required<SerialOutputBatcherConfig>;
private readonly pendingBuffer: RingBuffer; // Ring-Buffer instead of string accumulation
private tickTimer: NodeJS.Timeout | null = null;
// paused flag prevents enqueue/start while paused (tests rely on this)
private paused = false;
// Telemetry counters (reset periodically)
private intendedBytes = 0;
private actualBytes = 0;
private droppedBytes = 0;
private chunks = 0;
// Total bytes counter (never reset, accumulates over lifetime)
private totalBytes = 0;
// Burst budget tracking
private currentBudget = 0;
private maxBudget = 0;
// Fractional byte accumulator for low baudrates
// At baud=1: normalBudget per tick = 0.005 bytes, which rounds to 0.
// The accumulator carries over the fractional part so that after 200 ticks
// (10 seconds), 1 byte gets through — correctly simulating 0.1 bytes/s.
private budgetAccumulator = 0;
// Maximum queue size before applying FIFO drops (for memory safety)
// This prevents unbounded buffering in pathological cases (e.g., data arriving
// much faster than baudrate allows). Typical value: 100KB.
private readonly MAX_QUEUE_BYTES = 100_000;
// Backpressure threshold (upper watermark in bytes). When the buffered
// data rises above this, the runner will pause the sketch. We'll add
// a lower 'resume' watermark inside isOverloaded() to prevent thrashing.
// Start with 512 bytes to give some headroom for slow baud rates.
private readonly BACKPRESSURE_THRESHOLD = 512;
// internal flag used by hysteresis logic
private overloadedState = false;
// Flag to prevent enqueue after destroy
private destroyed = false;
constructor(config: SerialOutputBatcherConfig) {
this.config = {
baudrate: config.baudrate,
tickIntervalMs: config.tickIntervalMs ?? 50,
onChunk: config.onChunk,
burstFactor: config.burstFactor ?? 3,
};
// Initialize ring buffer with capacity equal to MAX_QUEUE_BYTES
// This prevents unbounded memory growth while allowing burst handling
this.pendingBuffer = new RingBuffer(this.MAX_QUEUE_BYTES);
this.updateBudget();
}
/**
* Calculate and update byte budget based on baudrate
*/
private updateBudget(): void {
// Byte budget per tick = (baudrate / 10 bits per byte) × (tick interval in seconds)
const bytesPerSecond = this.config.baudrate / 10;
const bytesPerTick = bytesPerSecond * (this.config.tickIntervalMs / 1000);
const burstBudget = bytesPerTick * this.config.burstFactor;
// maxBudget: determines initial burst capacity and max accumulation.
// For high bauds (≥ ~3333): burstBudget (3 × bytesPerTick) dominates naturally.
// For low bauds (< 3333): a proportional floor ensures typical println() works.
// - The floor = min(50, ceil(bytesPerSecond × 0.5)) = "half a second of output, max 50"
// - At 300 baud: floor = min(50, 15) = 15 → covers "Hello World!\n" (14 bytes)
// - At 1200 baud: floor = min(50, 60) = 50 → same as old MIN_BUDGET
// - At baud=1: floor = min(50, 1) = 1 → nearly nothing (correct for 0.1 bytes/s)
// The old hardcoded MIN_BUDGET=50 gave baud=1 a 50-byte free pass, defeating rate limiting.
// This proportional approach fixes that while preserving setup() burst for standard bauds.
const proportionalFloor = Math.min(50, Math.ceil(bytesPerSecond * 0.5));
this.maxBudget = Math.max(1, Math.floor(burstBudget), proportionalFloor);
this.currentBudget = this.maxBudget; // Start with full burst budget
this.budgetAccumulator = 0;
}
/**
* Enqueue data for batching
*
* If queue size would exceed MAX_QUEUE_BYTES (memory safety limit),
* drop oldest bytes using FIFO strategy (not "tail wins").
*
* NOTE: We count ALL enqueued data as "intended" - even if it will later be dropped.
* The telemetry semantic is: actual + dropped = intended
*/
enqueue(data: string): void {
// After destroy(), enqueue is a no-op
if (this.destroyed) return;
// Count as intended (part of telemetry accounting)
this.intendedBytes += data.length;
this.totalBytes += data.length;
// Write to ring buffer - it handles overflow internally via FIFO
const bytesWritten = this.pendingBuffer.write(data);
// Track any bytes that couldn't fit (dropped due to ring buffer capacity)
Iif (bytesWritten < data.length) {
const bytesDropped = data.length - bytesWritten;
this.droppedBytes += bytesDropped;
}
// Ensure the ticking timer is running; do not create multiple intervals.
// However, if we're paused we must not start or restart the timer – data
// should remain buffered until resume().
if (!this.tickTimer && !this.paused) {
this.tickTimer = setInterval(() => this.tick(), this.config.tickIntervalMs);
}
}
/**
* Stop the timer and flush remaining data (without limit)
*/
stop(): void {
Eif (this.tickTimer) {
clearInterval(this.tickTimer);
this.tickTimer = null;
}
// Flush all remaining data without budget limit
const remaining = this.pendingBuffer.readAll();
if (remaining.length > 0) {
this.config.onChunk(remaining);
this.actualBytes += remaining.length;
this.chunks++;
}
}
/**
* Pause the timer (keeps pending data)
*/
pause(): void {
this.paused = true;
Eif (this.tickTimer) {
clearInterval(this.tickTimer);
this.tickTimer = null;
}
}
/**
* Resume the timer
*/
resume(): void {
this.paused = false;
this.start();
}
/**
* Destroy the batcher (stop timer, discard data, no callbacks)
*/
destroy(): void {
this.destroyed = true;
if (this.tickTimer) {
clearInterval(this.tickTimer);
this.tickTimer = null;
}
this.pendingBuffer.clear();
}
/**
* Start the batching timer.
*
* The timer is not automatically started in the constructor because the
* caller may choose to configure additional callbacks first (e.g. the
* runner sets output/error handlers after instantiation). The public
* `start()` method mirrors the semantics described in the design docs and
* allows the timer to be restarted after `pause()`.
*/
start(): void {
// don't start when paused or already running
Iif (this.tickTimer || this.paused) return;
this.tickTimer = setInterval(() => this.tick(), this.config.tickIntervalMs);
}
/**
* Return true when the current buffer has grown past the overload threshold.
* Used by SandboxRunner to implement backpressure. Simple getter keeps the
* knowledge encapsulated and allows the threshold to change easily.
*/
isOverloaded(): boolean {
// Special-case: when the baudrate is so low that the C++ side applies its
// txDelay cap (e.g. 300 baud → 10 ms max delay), we intentionally disable
// backpressure. Injecting 50 ms SIGSTOP pauses would defeat the cap and
// slow sketches instead of helping them.
if (this.config.baudrate <= 300) {
return false;
}
// Scale threshold for low-but-not-tiny bauds. The original hardcoded
// BACKPRESSURE_THRESHOLD=512 bytes is fine for high-speed sketches, but a
// 1200‑baud sketch can easily accumulate several hundred bytes while the
// mock's txDelay cap is in effect. Raising the limit to 1024 gives more
// headroom and avoids premature SIGSTOP storms.
const threshold = this.config.baudrate < 4800 ? 1024 : this.BACKPRESSURE_THRESHOLD;
// Hysteresis: once we've declared overloaded we stay in that state until
// the buffer falls below a low-watermark. We keep the watermark at 128
// bytes (≈1/4 of the original threshold) so throttling behaviour remains
// snappy without toggling around the boundary.
const lowWatermark = 128;
const bufferSize = this.pendingBuffer.getSize();
Iif (this.overloadedState) {
if (bufferSize < lowWatermark) {
this.overloadedState = false;
}
} else if (bufferSize > threshold) {
this.overloadedState = true;
}
return this.overloadedState;
}
/**
* Change baudrate and recalculate budget
*/
setBaudrate(baudrate: number): void {
this.config.baudrate = baudrate;
this.updateBudget();
}
/**
* Get telemetry and reset counters (except totalBytes)
*/
getTelemetryAndReset(): SerialOutputTelemetry {
const telemetry: SerialOutputTelemetry = {
intended: this.intendedBytes,
actual: this.actualBytes,
dropped: this.droppedBytes,
chunks: this.chunks,
totalBytes: this.totalBytes,
};
// Reset periodic counters
this.intendedBytes = 0;
this.actualBytes = 0;
this.droppedBytes = 0;
this.chunks = 0;
return telemetry;
}
/**
* Tick handler: process pending data with budget limit
*
* STRATEGY: No data is dropped. Instead, data is buffered and will be sent
* as the baudrate allows. This is correct for serial data where FIFO order matters
* and completeness is more important than timing.
*
* If bandwidth is insufficient for the data rate, output will be delayed but complete.
* Uses RingBuffer for O(1) operations without string allocation overhead.
*/
private tick(): void {
// Token bucket replenishment with fractional byte accumulation.
// At low baudrates, bytesPerTick < 1 (e.g., baud=1 → 0.005 bytes/tick).
// We accumulate the fractional part and only grant whole bytes.
const bytesPerSecond = this.config.baudrate / 10;
const rawBytesPerTick = bytesPerSecond * (this.config.tickIntervalMs / 1000);
this.budgetAccumulator += rawBytesPerTick;
const wholeBytesToAdd = Math.floor(this.budgetAccumulator);
this.budgetAccumulator -= wholeBytesToAdd;
this.currentBudget = Math.min(
this.currentBudget + wholeBytesToAdd,
this.maxBudget
);
if (this.pendingBuffer.isEmpty()) {
return;
}
// Use current accumulated budget (can be up to maxBudget for bursts)
const budget = this.currentBudget;
const bufferSize = this.pendingBuffer.getSize();
if (bufferSize <= budget) {
// All data fits in budget - extract and send everything
const dataToSend = this.pendingBuffer.readAll();
Eif (dataToSend.length > 0) {
this.config.onChunk(dataToSend, false); // no truncation
this.actualBytes += dataToSend.length;
this.chunks++;
this.currentBudget -= dataToSend.length;
}
} else {
// Data exceeds budget: send what fits, keep rest in buffer for next tick
// Read up to budget bytes
let dataToSend = this.pendingBuffer.read(budget);
// Try to cut at newline boundary to avoid sending truncated lines
const lastNewlineIndex = dataToSend.lastIndexOf("\n");
if (lastNewlineIndex !== -1 && lastNewlineIndex < dataToSend.length - 1) {
// There's a newline within what we read (not at very end)
// Keep everything up to and including that newline, put back the rest
const toRequeue = dataToSend.slice(lastNewlineIndex + 1);
dataToSend = dataToSend.slice(0, lastNewlineIndex + 1);
// Re-enqueue the bytes we're not using yet
this.pendingBuffer.write(toRequeue);
}
if (dataToSend.length > 0) {
this.config.onChunk(dataToSend, false); // data is complete, not truncated
this.actualBytes += dataToSend.length;
this.chunks++;
}
// Deduct what we sent from budget
this.currentBudget = Math.max(0, this.currentBudget - dataToSend.length);
}
}
}
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