/**
 * Pure helpers shared by the chat route (server) and the chat UI (client):
 * token budgeting and the sliding history window. No directive, no deps, so
 * both sides can import it.
 */

/** Rough token estimate (≈4 chars per token) — good enough for budgeting. */
export function estimateTokens(text: string): number {
  return Math.ceil(text.length / 4);
}

/** How much conversation history the sliding window keeps (≈6k tokens). */
export const HISTORY_TOKEN_BUDGET = 6000;

/** Keeps the most recent turns that fit the budget (sliding window). */
export function trimHistory<T extends { role: "user" | "assistant"; content: string }>(
  messages: T[],
  budget = HISTORY_TOKEN_BUDGET,
): T[] {
  const kept: T[] = [];
  let used = 0;
  for (let index = messages.length - 1; index >= 0; index -= 1) {
    const message = messages[index];
    const cost = estimateTokens(message.content);
    if (kept.length > 0 && used + cost > budget) break;
    kept.unshift(message);
    used += cost;
  }
  return kept;
}