/**
 * Messages appended to a JSONL bus file after byte `from`, each with the absolute byte offset just
 * past its own line. Shared by the send-time herdr path and the seat's own tail, so both agree on
 * exactly which bytes a delivery covers — a second reader would be a second truth about where a
 * seat's cursor may move.
 *
 * A partial trailing line is left for the next read rather than guessed at, and a line that is not
 * JSON is skipped with the offset moving past it.
 */
import { readFileSync } from "node:fs";

export type OffsetMessage = { msg: Record<string, unknown>; end: number };

export function newMessagesIn(file: string, from: number): OffsetMessage[] {
  let buf: Buffer;
  try { buf = readFileSync(file); } catch { return []; }
  const out: OffsetMessage[] = [];
  let pos = Math.min(Math.max(0, from), buf.length);
  while (pos < buf.length) {
    const nl = buf.indexOf(0x0a, pos);
    if (nl === -1) break;
    const line = buf.subarray(pos, nl).toString("utf8");
    const end = nl + 1;
    pos = end;
    if (!line.trim()) continue;
    try { out.push({ msg: JSON.parse(line), end }); } catch { /* not a message; the offset moves past it */ }
  }
  return out;
}