/**
 * Pure polling decision for the AgentDrawer's incremental transcript reads.
 *
 * Mirrors the TUI guest's contract (agent-transcript-viewer): a `null` reply
 * (timeout / session end) is transient and retryable from the same cursor; a
 * frame-level host `error` is terminal — the host replies with an unchanged
 * cursor, so retrying would loop hot without ever surfacing the failure.
 */

import type { SessionEntry } from "@oh-my-pi/pi-wire";
import type { TranscriptResult } from "./client";
import { parseJsonl } from "./jsonl";

/** What one transcript poll round decided (pure; the drawer's effect executes it). */
export type TranscriptPollDecision =
	/** Transient failure (timeout / not yet available): keep the cursor, poll again. */
	| { action: "retry" }
	/** Terminal host error: stop polling and surface the message (prior rows stay). */
	| { action: "stop"; message: string }
	/** Rows read: advance the cursor and append the parsed entries. */
	| { action: "advance"; newSize: number; carry: string; fresh: readonly SessionEntry[] };

/** Maps one {@link TranscriptResult} reply to a polling decision. */
export function decideTranscriptPoll(reply: TranscriptResult | null, carry: string): TranscriptPollDecision {
	if (reply === null) return { action: "retry" };
	if (reply.kind === "error") return { action: "stop", message: reply.message };
	const parsed = parseJsonl(reply.text, carry);
	const fresh: SessionEntry[] = [];
	for (const item of parsed.items) {
		if (typeof item !== "object" || item === null) continue;
		if ("type" in item && item.type === "session") continue;
		fresh.push(item as SessionEntry);
	}
	return { action: "advance", newSize: reply.newSize, carry: parsed.carry, fresh };
}