/**
 * Content-Length framing for MCP stdio transport.
 *
 * Each JSON-RPC message is framed as:
 *   Content-Length: <byte-length>\r\n\r\n<JSON bytes>
 *
 * Same framing as LSP — the MCP spec inherited this from the LSP base protocol.
 *
 * PURE: no I/O. Operates on bytes (Uint8Array) so multi-byte UTF-8 content is
 * handled correctly — `Content-Length` is a *byte* count, not a character count.
 */

const HEADER_SEP = "\r\n\r\n";
const CONTENT_LENGTH_RE = /Content-Length:\s*(\d+)/i;

const SEP_BYTES = new TextEncoder().encode(HEADER_SEP);

/**
 * Encode a JSON string into a single Content-Length-framed message.
 * Returns the full frame (header + blank line + body) as bytes.
 */
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 `needle` in `haystack` at or after `from`. -1 if absent. */
function indexOfBytes(haystack: Uint8Array, needle: Uint8Array, from: number): number {
	if (needle.length === 0) return from;
	const max = haystack.length - needle.length;
	for (let i = from; i <= max; i++) {
		let match = true;
		for (let j = 0; j < needle.length; j++) {
			if (haystack[i + j] !== needle[j]) {
				match = false;
				break;
			}
		}
		if (match) return i;
	}
	return -1;
}

/**
 * Feed raw bytes into the decoder. Returns all complete JSON messages that can
 * be extracted from the accumulated buffer. Buffers partial frames across calls.
 */
export class FrameDecoder {
	private buf: Uint8Array = new Uint8Array(0);
	private readonly decoder = new TextDecoder();

	decode(chunk: Uint8Array): string[] {
		// Append the incoming chunk to the internal byte buffer.
		const next = new Uint8Array(this.buf.length + chunk.length);
		next.set(this.buf);
		next.set(chunk, this.buf.length);
		this.buf = next;

		const messages: string[] = [];

		while (true) {
			const sepIdx = indexOfBytes(this.buf, SEP_BYTES, 0);
			if (sepIdx === -1) break;

			// The header block is everything before the separator; parse
			// Content-Length from it (ASCII, so decoding the slice is safe).
			const headerText = this.decoder.decode(this.buf.subarray(0, sepIdx));
			const match = CONTENT_LENGTH_RE.exec(headerText);
			const bodyStart = sepIdx + SEP_BYTES.length;

			if (!match?.[1]) {
				// No usable Content-Length — drop this header and continue scanning.
				this.buf = this.buf.subarray(bodyStart);
				continue;
			}

			const length = Number.parseInt(match[1], 10);
			if (length < 0) {
				this.buf = this.buf.subarray(bodyStart);
				continue;
			}

			if (this.buf.length - bodyStart < length) {
				// Body not fully received yet; wait for more bytes.
				break;
			}

			// Decode exactly `length` body bytes (preserves multi-byte UTF-8).
			messages.push(this.decoder.decode(this.buf.subarray(bodyStart, bodyStart + length)));
			this.buf = this.buf.subarray(bodyStart + length);
		}

		return messages;
	}
}
