/**
 * Pure filter applied after context-mode's `context` handler: its upstream
 * build appends one `{ role: "user", content }` message whose parts are
 * `"\n\n"`-joined and whose first part is a routing anchor that duplicates —
 * and contradicts — the bundle-owned `config/prompt/routing.jsonc`. The anchor
 * part is stripped; remaining parts (active memory, resume snapshot) stay.
 * No other message is touched; when nothing changes the input array is
 * returned unchanged (same reference).
 */

export const CONTEXT_MODE_ANCHOR_PREFIX = "context-mode active. Hierarchy:";

type MessageLike = { role?: unknown; content?: unknown } & Record<string, unknown>;

export function filterContextModeMessages<T extends MessageLike>(
	before: readonly T[],
	after: readonly T[],
): T[] {
	if (after.length <= before.length) return after as T[];
	let result: T[] | null = null;
	for (let i = after.length - 1; i >= before.length; i--) {
		const message = after[i];
		if (message === null || typeof message !== "object") continue;
		if (message.role !== "user" || typeof message.content !== "string") continue;
		const parts = message.content.split("\n\n");
		const kept = parts.filter((part) => !part.startsWith(CONTEXT_MODE_ANCHOR_PREFIX));
		if (kept.length === parts.length) continue;
		result ??= after.slice();
		if (kept.length === 0) {
			result.splice(i, 1);
		} else {
			result[i] = { ...message, content: kept.join("\n\n") };
		}
	}
	return result ?? (after as T[]);
}