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/**
* LSP Content-Length framing codec.
*
* The LSP base protocol uses Content-Length headers to frame JSON messages.
* `encode` wraps a JSON message with headers; `FrameDecoder` reassembles
* complete messages from streaming byte chunks (handles partial frames and
* multiple frames per chunk).
*
* The buffer is a `Uint8Array` (not a string) because `Content-Length` counts
* **bytes** — slicing by JavaScript character count corrupts messages whose
* JSON body contains multi-byte UTF-8 characters (byte length ≠ char length).
*/
const CR = 0x0d; // \r
const LF = 0x0a; // \n
const CONTENT_LENGTH_RE = /Content-Length:\s*(\d+)/i;
export function encode(msg: string): Uint8Array {
const body = new TextEncoder().encode(msg);
const header = `Content-Length: ${body.length}\r\n\r\n`;
const frame = new TextEncoder().encode(header);
const result = new Uint8Array(frame.length + body.length);
result.set(frame);
result.set(body, frame.length);
return result;
}
/**
* Find the first occurrence of the 4-byte sequence \r\n\r\n in `buf`,
* starting at offset `from`. Returns the index of the first byte, or -1.
*/
function findHeaderSep(buf: Uint8Array, from: number): number {
const limit = buf.length - 3;
for (let i = from; i < limit; i++) {
if (buf[i] === CR && buf[i + 1] === LF && buf[i + 2] === CR && buf[i + 3] === LF) {
return i;
}
}
return -1;
}
export class FrameDecoder {
private buffer: Uint8Array = new Uint8Array(0);
private expectedLength: number | null = null;
private headerEndByte = -1;
/**
* Feed raw bytes into the decoder. Returns all complete JSON messages
* that can be extracted from the accumulated buffer.
*/
decode(chunk: Uint8Array): string[] {
// Append the new chunk to the internal byte buffer.
const newBuf = new Uint8Array(this.buffer.length + chunk.length);
newBuf.set(this.buffer);
newBuf.set(chunk, this.buffer.length);
this.buffer = newBuf;
const messages: string[] = [];
while (true) {
if (this.expectedLength === null) {
const sepIdx = findHeaderSep(this.buffer, 0);
if (sepIdx === -1) break;
// Decode only the header bytes (always ASCII) to read Content-Length.
const headerStr = new TextDecoder().decode(this.buffer.slice(0, sepIdx));
const match = CONTENT_LENGTH_RE.exec(headerStr);
if (!match?.[1]) {
// Not a Content-Length header — skip past this separator and retry.
this.buffer = this.buffer.slice(sepIdx + 4);
continue;
}
this.expectedLength = Number.parseInt(match[1], 10);
this.headerEndByte = sepIdx + 4; // skip \r\n\r\n
}
const bodyStart = this.headerEndByte;
const available = this.buffer.length - bodyStart;
if (available >= this.expectedLength) {
// Extract exactly `expectedLength` bytes (Content-Length is in bytes).
const bodyBytes = this.buffer.slice(bodyStart, bodyStart + this.expectedLength);
messages.push(new TextDecoder().decode(bodyBytes));
this.buffer = this.buffer.slice(bodyStart + this.expectedLength);
this.expectedLength = null;
this.headerEndByte = -1;
} else {
break;
}
}
return messages;
}
}
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