Add custom reader buffer to SerialSource

Author: kuba2k2

src/serial/source.ts

@@ -5,6 +5,9 @@ import { SerialTransport } from "./types"
 export class SerialSource implements UnderlyingSource<Uint8Array> {
 	type: undefined
 	controller: ReadableStreamController<Uint8Array> = null
+	buffer: Uint8Array = null
+	bufferUsed: number = 0
+	wantData: boolean = false
 
 	public constructor(
 		private transport_: SerialTransport,
@@ -24,9 +27,48 @@ export class SerialSource implements UnderlyingSource<Uint8Array> {
 	start(controller: ReadableStreamController<Uint8Array>) {
 		debugLog("STREAM", "source", "start()")
 		this.controller = controller
+
+		const bufferSize = controller.desiredSize
+		this.buffer = new Uint8Array(bufferSize)
+		this.bufferUsed = 0
+		this.wantData = false
+
 		this.transport_.sourceFeedData = (data) => {
-			controller.enqueue(data)
+			if (this.bufferUsed + data.length >= bufferSize) {
+				// the buffer would overflow
+				const newSize = bufferSize - this.bufferUsed
+				const newData = data.slice(0, newSize)
+				// cut from data whatever fits in the buffer
+				data = data.slice(newSize)
+				// write it at the end
+				this.buffer.set(newData, this.bufferUsed)
+				// reset the buffer usage
+				this.bufferUsed = 0
+				// pass the entire buffer to the controller
+				controller.enqueue(new Uint8Array(this.buffer))
+			}
+			if (data.length > 0) {
+				// the buffer will NOT overflow anymore, whatever's left in 'data' will fit
+				this.buffer.set(data, this.bufferUsed)
+				this.bufferUsed += data.length
+				// if reader is waiting for data, enqueue it using pull()
+				if (this.wantData) this.pull(controller)
+			}
+		}
+	}
+
+	pull(controller: ReadableStreamController<Uint8Array>) {
+		if (this.bufferUsed == 0) {
+			// nothing to read, but the reader is waiting for data
+			this.wantData = true
+			return
 		}
+		// consume the entire buffer
+		const newData = this.buffer.slice(0, this.bufferUsed)
+		this.bufferUsed = 0
+		// enqueue after consume, since it may unblock and call pull()
+		controller.enqueue(newData)
+		this.wantData = false
 	}
 
 	cancel(_reason?: any) {