diff --git a/src/hbkit/archive.py b/src/hbkit/archive.py
index 64d8a71..29ace13 100644
--- a/src/hbkit/archive.py
+++ b/src/hbkit/archive.py
@@ -136,12 +136,13 @@ def __init__(self, d: str):
                 found[n] = (gen, f)
         if not found:
             raise FileNotFoundError(f"no .idx shards in {d}")
-        self.files = [found[k][1] for k in sorted(found)]
-        n = len(self.files)
+        shard_ids = sorted(found)
+        self.files = [found[k][1] for k in shard_ids]
         last = os.path.getsize(os.path.join(d, self.files[-1]))
-        self.sizes = [SHARD_SIZE] * (n - 1) + [last]
-        self.starts = [i * SHARD_SIZE for i in range(n)]
-        self.total = (n - 1) * SHARD_SIZE + last
+        self.sizes = [SHARD_SIZE] * (len(self.files) - 1) + [last]
+        self.starts = [k * SHARD_SIZE for k in shard_ids]
+        self.total = self.starts[-1] + last
+        self._positions = dict(zip(shard_ids, range(len(shard_ids))))
         self._fh: dict[int, object] = {}
         # Shard 0 carries a 64-byte header that declares the record size, so readers do
         # not have to hardcode per-DSM-version layouts.
@@ -160,15 +161,18 @@ def read(self, off: int, n: int) -> bytes:
         """Read n bytes at logical offset off, spanning shards as needed."""
         out = b""
         i = off // SHARD_SIZE
-        while n > 0 and i < len(self.files):
-            st = self.starts[i]
-            take = min(n, st + self.sizes[i] - off)
+        while n > 0:
+            pos = self._positions.get(i)
+            if pos is None:
+                break
+            st = self.starts[pos]
+            take = min(n, st + self.sizes[pos] - off)
             if take <= 0:
                 break
-            fh = self._fh.get(i)
+            fh = self._fh.get(pos)
             if fh is None:
-                fh = self._fh[i] = open(os.path.join(self.d, self.files[i]), "rb",
-                                        buffering=INDEX_BUF)
+                fh = self._fh[pos] = open(os.path.join(self.d, self.files[pos]), "rb",
+                                          buffering=INDEX_BUF)
             fh.seek(off - st)
             got = fh.read(take)
             out += got
diff --git a/src/hbkit/cli.py b/src/hbkit/cli.py
index 8b5c10f..14bfab9 100644
--- a/src/hbkit/cli.py
+++ b/src/hbkit/cli.py
@@ -231,7 +231,23 @@ def main() -> int:
 
     if cmd == "doctor":
         from . import doctor
-        r = doctor.diagnose(archive, password=resolve_password(password))
+        sample = None
+        for flag in ("-s", "-n", "--sample"):
+            if flag in a[2:]:
+                i = a.index(flag, 2)
+                try:
+                    sample = int(a[i + 1])
+                except (IndexError, ValueError):
+                    print(f"{flag} requires a positive integer", file=sys.stderr)
+                    return 2
+                if sample < 1:
+                    print(f"{flag} requires a positive integer", file=sys.stderr)
+                    return 2
+                break
+        kw = {"password": resolve_password(password)}
+        if sample is not None:
+            kw["sample"] = sample
+        r = doctor.diagnose(archive, **kw)
         print(doctor.render(r))
         return 0 if (r.ok and not r.blockers) else 1
 
diff --git a/src/hbkit/doctor.py b/src/hbkit/doctor.py
index 4f4d5cd..161e66a 100644
--- a/src/hbkit/doctor.py
+++ b/src/hbkit/doctor.py
@@ -202,7 +202,7 @@ def main() -> int:
     ap = argparse.ArgumentParser(description=__doc__)
     ap.add_argument("archive")
     ap.add_argument("-p", "--password", help="for encrypted archives")
-    ap.add_argument("-n", "--sample", type=int, default=10,
+    ap.add_argument("-n", "-s", "--sample", type=int, default=10,
                     help="files to rebuild per share as proof (default 10)")
     a = ap.parse_args()
     r = diagnose(a.archive, a.sample, password=a.password)
diff --git a/tests/test_hbkit.py b/tests/test_hbkit.py
index a3003d8..d05ca12 100644
--- a/tests/test_hbkit.py
+++ b/tests/test_hbkit.py
@@ -287,6 +287,18 @@ def test_cat_single_shard_still_works(tmp_path, monkeypatch):
     c.close()
 
 
+def test_cat_preserves_sparse_shard_offsets(tmp_path, monkeypatch):
+    from hbkit import archive as A
+    blob = _make_shards(tmp_path, [128, 10], 128, monkeypatch)
+    (tmp_path / "1.idx.2").rename(tmp_path / "2.idx.2")
+    c = A.Cat(str(tmp_path))
+    assert c.starts == [0, 256]
+    assert c.total == 266
+    assert c.read(128, 1) == b""
+    assert c.read(256, 10) == blob[128:]
+    c.close()
+
+
 # --------------------------------------------------- rclone mount helper (no network)
 
 def test_mount_profiles_pick_the_right_cache_mode(monkeypatch):
@@ -480,6 +492,24 @@ class Result:
     assert seen["pw"] == want
 
 
+@pytest.mark.parametrize("flag", ["-s", "-n", "--sample"])
+def test_doctor_forwards_sample_count(monkeypatch, flag):
+    from hbkit import cli, doctor
+
+    seen = {}
+
+    class Result:
+        ok, blockers = True, []
+
+    monkeypatch.setattr(doctor, "diagnose",
+                        lambda a, sample=10, password=None:
+                        (seen.update(sample=sample), Result())[1])
+    monkeypatch.setattr(doctor, "render", lambda r: "")
+    monkeypatch.setattr(sys, "argv", ["hbk", "/some/archive", "doctor", flag, "2"])
+    assert cli.main() == 0
+    assert seen["sample"] == 2
+
+
 def test_lazycats_lists_shards_before_any_are_opened():
     """`doctor` describes an archive without extracting from it, so nothing has opened a
     file_chunk shard by the time it prints them. Iterating the dict reported an empty list