/**
 * Unfinished assistant text recovered when a child ends abnormally.
 *
 * Final output is built from completed `message_end` messages, so text that was
 * still streaming when a child timed out or its session threw is otherwise lost.
 * The tracker keeps one in-memory reference to the latest unfinished assistant
 * message. Nothing is persisted.
 */
import type { ChildSessionEvent } from "./child-session.ts";
import { extractTextFromContent } from "../../shared/utils.ts";

export type PartialOutputCause = "timeout" | "child error";

export interface PartialOutputTracker {
	observe(event: ChildSessionEvent): void;
	/** Latest unfinished assistant text that is newer than the last completed reply. */
	text(): string | undefined;
}

function isAssistant(message: unknown): boolean {
	return !!message && typeof message === "object" && (message as { role?: unknown }).role === "assistant";
}

function isErroredAssistant(message: unknown): boolean {
	const { stopReason, errorMessage } = message as { stopReason?: unknown; errorMessage?: unknown };
	return stopReason === "error" || (typeof errorMessage === "string" && errorMessage.length > 0);
}

export function createPartialOutputTracker(): PartialOutputTracker {
	// Only a reference is kept per event; text is extracted once, when a child ends abnormally.
	let latest: { content?: unknown } | undefined;
	return {
		observe(event) {
			if ((event.type !== "message_update" && event.type !== "message_end") || !isAssistant(event.message)) return;
			// A completed reply, including a tool-only one, is already part of the final output.
			// A provider-error message is skipped there, so its text is the newest unfinished text.
			latest = event.type === "message_update" || isErroredAssistant(event.message) ? event.message as { content?: unknown } : undefined;
		},
		text() {
			const text = extractTextFromContent(latest?.content);
			return text.trim() ? text : undefined;
		},
	};
}

export function formatPartialOutput(text: string, cause: PartialOutputCause): string {
	return `Partial output before ${cause}:\n${text}`;
}