// Minimal SSE + UIMessageChunk parser. The backend writes
//   data: ${JSON.stringify(chunk)}\n\n
// for each AI SDK UIMessageChunk, plus a final `data: [DONE]\n\n`.
// We reconstruct events by buffering stream text and splitting on blank lines.

export type UIMessageChunk =
    | { type: 'start'; messageId?: string }
    | { type: 'start-step' }
    | { type: 'finish-step' }
    | { type: 'finish' }
    | { type: 'error'; errorText: string }
    | { type: 'text-start'; id: string }
    | { type: 'text-delta'; id: string; delta: string }
    | { type: 'text-end'; id: string }
    | { type: 'tool-input-start'; toolCallId: string; toolName: string }
    | { type: 'tool-input-delta'; toolCallId: string; inputTextDelta: string }
    | { type: 'tool-input-available'; toolCallId: string; toolName: string; input: any }
    | { type: 'tool-output-available'; toolCallId: string; output: any }
    | { type: 'tool-output-error'; toolCallId: string; errorText: string }
    | { type: string; [key: string]: any };

// Reads the SSE body byte stream and yields UIMessageChunk objects.
export async function* readChatStream(
    body: ReadableStream<Uint8Array>,
): AsyncGenerator<UIMessageChunk> {
    const reader = body.getReader();
    const decoder = new TextDecoder();
    let buffer = '';

    try {
        while (true) {
            const { done, value } = await reader.read();
            if (done) break;
            buffer += decoder.decode(value, { stream: true });

            // Each SSE event is terminated by a blank line. Split off complete
            // events and keep the trailing partial in buffer.
            let sep: number;
            while ((sep = buffer.indexOf('\n\n')) !== -1) {
                const rawEvent = buffer.slice(0, sep);
                buffer = buffer.slice(sep + 2);

                // Grab lines starting with "data:" (there may be comments or
                // event: lines too — we ignore them).
                const lines = rawEvent.split('\n');
                for (const line of lines) {
                    if (!line.startsWith('data:')) continue;
                    const payload = line.slice(5).trimStart();
                    if (!payload || payload === '[DONE]') continue;
                    try {
                        yield JSON.parse(payload) as UIMessageChunk;
                    } catch {
                        // Bad JSON — skip this event rather than crash the stream.
                    }
                }
            }
        }
    } finally {
        reader.releaseLock();
    }
}