Page chat history at turn boundaries

src/server/sessions/messagePaging.ts

@@ -0,0 +1,55 @@
+const DEFAULT_LIMIT = 100;
+const MAX_LIMIT = 500;
+
+export interface MessagePageRequest {
+  before?: number;
+  limit?: number;
+}
+
+export interface MessagePageResult<T> {
+  messages: T[];
+  start: number;
+  total: number;
+}
+
+export function pageMessagesAtSafeBoundary<T>(messages: T[], page?: MessagePageRequest): T[] | MessagePageResult<T> {
+  if (page?.before === undefined && page?.limit === undefined) return messages;
+  const total = messages.length;
+  const before = clampInteger(page.before ?? total, 0, total);
+  const limit = clampInteger(page.limit ?? DEFAULT_LIMIT, 1, MAX_LIMIT);
+  const requestedStart = Math.max(0, before - limit);
+  const start = expandStartToSafeBoundary(messages, requestedStart);
+  return { messages: messages.slice(start, before), start, total };
+}
+
+export function expandStartToSafeBoundary(messages: unknown[], requestedStart: number): number {
+  const start = clampInteger(requestedStart, 0, messages.length);
+  if (start === 0 || isTurnBoundary(messages[start])) return start;
+  for (let index = start - 1; index >= 0; index -= 1) {
+    if (isTurnBoundary(messages[index])) return index;
+  }
+  return 0;
+}
+
+function isTurnBoundary(message: unknown): boolean {
+  return getString(message, "role") === "user";
+}
+
+function getProperty(value: unknown, key: string): unknown {
+  if (!isRecord(value)) return undefined;
+  return value[key];
+}
+
+function isRecord(value: unknown): value is Record<string, unknown> {
+  return typeof value === "object" && value !== null;
+}
+
+function getString(value: unknown, key: string): string | undefined {
+  const property = getProperty(value, key);
+  return typeof property === "string" ? property : undefined;
+}
+
+function clampInteger(value: number, min: number, max: number): number {
+  if (!Number.isFinite(value)) return max;
+  return Math.max(min, Math.min(max, Math.floor(value)));
+}