/**
 * @file Reads what an append-only JSONL journal gained since a byte offset —
 * the incremental read a session-journal poller does (omp-usage.js). Only
 * complete lines are returned: one still being written
 * stays for the next pass. Every open goes through `openSafeAsync`, so a
 * member's journal (S3) is read only when it is a regular file they own,
 * never through a link.
 * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
 */

const CHUNK = 8 * 1024 * 1024; // read per file per pass; the rest waits for the next

/**
 * The complete lines written between `offset` and `size`: `{lines, offset}`.
 * A chunk with no newline at all (one line over CHUNK) is skipped whole.
 */
async function readNew(file, offset, size, { chunk = CHUNK } = {}) {
  const fh = await require("./run-as").openSafeAsync(file);
  try {
    const len = Math.min(size - offset, chunk);
    if (len <= 0) return { lines: [], offset };
    const buf = Buffer.alloc(len);
    let got = 0;
    for (let n; got < len; got += n) {
      ({ bytesRead: n } = await fh.read(buf, got, len - got, offset + got));
      if (!n) break;
    }
    const end = got ? buf.lastIndexOf(0x0a, got - 1) : -1;
    if (end === -1) return { lines: [], offset: got === chunk ? offset + got : offset };
    return { lines: buf.toString("utf8", 0, end).split("\n"), offset: offset + end + 1 };
  } finally {
    await fh.close();
  }
}

module.exports = { readNew };