import type { ChatSseEvent } from "./public-contracts.js";

import { parseChatSseEvent } from "./contracts.js";
import { WidgetError } from "./errors.js";

/**
 * Parses arbitrary byte chunks into validated SSE data events. Neither UTF-8
 * characters nor SSE frame boundaries are assumed to align with stream reads.
 */
export async function* parseSseStream(
  stream: ReadableStream<Uint8Array>,
): AsyncGenerator<ChatSseEvent> {
  const reader = stream.getReader();
  const decoder = new TextDecoder();
  let buffer = "";

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

      buffer += decoder.decode(reading.value, { stream: true });
      // The final section is retained because its terminating blank line may
      // arrive in a later read.
      const extraction = extractCompleteFrames(buffer);
      buffer = extraction.remainder;
      for (const frame of extraction.frames) {
        const event = parseFrame(frame);
        if (event !== undefined) yield event;
      }
    }

    // Accept a valid final frame without a trailing blank line, while the API
    // client still requires a terminal `done` event for stream completeness.
    if (buffer.trim().length > 0) {
      const event = parseFrame(buffer);
      if (event !== undefined) yield event;
    }
  } finally {
    reader.releaseLock();
  }
}

function extractCompleteFrames(buffer: string): {
  frames: string[];
  remainder: string;
} {
  const sections = buffer.split(/\r?\n\r?\n/);
  return {
    frames: sections.slice(0, -1),
    remainder: sections.at(-1) ?? "",
  };
}

function parseFrame(frame: string): ChatSseEvent | undefined {
  // Multiple data lines are legal SSE and are joined according to the protocol.
  // Comments and unrelated SSE fields are intentionally ignored.
  const payload = frame
    .split(/\r?\n/)
    .filter((line) => line.startsWith("data:"))
    .map((line) => line.slice(5).trimStart())
    .join("\n");
  if (payload.length === 0) return undefined;

  let value: unknown;
  try {
    value = JSON.parse(payload) as unknown;
  } catch {
    throw new WidgetError("stream_error");
  }
  return parseChatSseEvent(value);
}