import type { AiProvider, ChatMessage, ToolSpec } from "@stc/contracts";

/** SSE body from a list of [event, data] pairs, split into awkward chunks to exercise buffering. */
export function sseBody(events: [string | null, unknown][], chunk = 7): ReadableStream<Uint8Array> {
  const text = events.map(([e, d]) => `${e ? `event: ${e}\n` : ""}data: ${typeof d === "string" ? d : JSON.stringify(d)}\n\n`).join("");
  const bytes = new TextEncoder().encode(text);
  let i = 0;
  return new ReadableStream({
    pull(c) {
      if (i >= bytes.length) return c.close();
      c.enqueue(bytes.slice(i, i + chunk));
      i += chunk;
    },
  });
}

export type Captured = { url: string; init: RequestInit; body: any };
/** A fake fetch that records requests and answers from a queue of responses. */
export function fakeFetch(responses: (() => Response)[]) {
  const calls: Captured[] = [];
  const f = (async (url: string | URL | Request, init?: RequestInit) => {
    calls.push({ url: String(url), init: init ?? {}, body: init?.body ? JSON.parse(String(init.body)) : null });
    const next = responses.shift();
    if (!next) throw new Error("fakeFetch: no response queued");
    return next();
  }) as unknown as typeof fetch;
  return { fetch: f, calls };
}

export async function collect(it: AsyncIterable<{ text?: string; toolCall?: { id: string; name: string; input: unknown } }>) {
  let text = "";
  const toolCalls: { id: string; name: string; input: unknown }[] = [];
  for await (const ev of it) {
    if (ev.text) text += ev.text;
    if (ev.toolCall) toolCalls.push(ev.toolCall);
  }
  return { text, toolCalls };
}

/** Scripted fake provider: each call to chat() plays the next turn and records the messages it got. */
export function fakeProvider(turns: ((messages: ChatMessage[], tools?: ToolSpec[]) => { text?: string; toolCalls?: { id: string; name: string; input: unknown }[] })[]) {
  const seen: { messages: ChatMessage[]; tools?: ToolSpec[] }[] = [];
  const provider: AiProvider = {
    id: "fake",
    async *chat(messages, opts) {
      seen.push({ messages: structuredClone(messages), tools: opts.tools });
      const turn = turns[Math.min(seen.length - 1, turns.length - 1)]!(messages, opts.tools);
      if (turn.text) for (const part of turn.text.match(/.{1,8}/gs) ?? []) yield { text: part };
      for (const tc of turn.toolCalls ?? []) yield { toolCall: tc };
    },
  };
  return { provider, seen };
}