diff --git a/src/heyfastqlib/seqs.py b/src/heyfastqlib/seqs.py
index f7a82cf..7dee4cd 100644
--- a/src/heyfastqlib/seqs.py
+++ b/src/heyfastqlib/seqs.py
@@ -1,8 +1,89 @@
+from typing import Dict, Optional
+
+
 def kmers(seq, k=4):
     n_kmers = len(seq) - k + 1
     for i in range(n_kmers):
         yield seq[i : (i + k)]
 
 
+_BASE_TO_BITS: Dict[str, int] = {
+    "A": 0,
+    "C": 1,
+    "G": 2,
+    "T": 3,
+    "a": 0,
+    "c": 1,
+    "g": 2,
+    "t": 3,
+}
+
+
+_BITSET_CACHE: Dict[int, bytearray] = {}
+_MAX_BITSET_SIZE = 1 << 22  # 4 Mi entries (4 MiB) per cached bitset
+
+
+def _kscore_fallback(seq: str, k: int) -> float:
+    seq_len = len(seq)
+    if seq_len == 0:
+        return 0.0
+    return len(set(kmers(seq, k=k))) / seq_len
+
+
+def _bitset_for_k(k: int) -> Optional[bytearray]:
+    size = 1 << (2 * k)
+    if size > _MAX_BITSET_SIZE:
+        return None
+
+    bitset = _BITSET_CACHE.get(k)
+    if bitset is None or len(bitset) != size:
+        bitset = bytearray(size)
+        _BITSET_CACHE[k] = bitset
+    return bitset
+
+
 def kscore(seq, k=4):
-    return len(set(kmers(seq))) / len(seq)
+    seq_len = len(seq)
+    if seq_len == 0:
+        return 0.0
+
+    if k <= 0:
+        return _kscore_fallback(seq, k)
+
+    if seq_len < k:
+        return 0.0
+
+    bitset = _bitset_for_k(k)
+    if bitset is None:
+        return _kscore_fallback(seq, k)
+
+    mask = (1 << (2 * k)) - 1
+    touched = []
+    unique = 0
+    rolling_value = 0
+    window_len = 0
+    use_fallback = False
+
+    for base in seq:
+        bits = _BASE_TO_BITS.get(base)
+        if bits is None:
+            use_fallback = True
+            break
+
+        rolling_value = ((rolling_value << 2) | bits) & mask
+        window_len += 1
+
+        if window_len >= k:
+            idx = rolling_value
+            if not bitset[idx]:
+                bitset[idx] = 1
+                touched.append(idx)
+                unique += 1
+
+    for idx in touched:
+        bitset[idx] = 0
+
+    if use_fallback:
+        return _kscore_fallback(seq, k)
+
+    return unique / seq_len
diff --git a/tests/test_seqs.py b/tests/test_seqs.py
index b4493b9..58ad831 100644
--- a/tests/test_seqs.py
+++ b/tests/test_seqs.py
@@ -10,3 +10,13 @@ def test_kmers():
 def test_kscore():
     assert kscore("AAAAA", k=4) == 1 / 5
     assert kscore("AAAATAAAAT", k=4) == 5 / 10
+
+
+def test_kscore_with_ambiguous_bases_matches_original_behavior():
+    seq = "ACNGT"
+    expected = len(set(kmers(seq, k=2))) / len(seq)
+    assert kscore(seq, k=2) == expected
+
+
+def test_kscore_returns_zero_for_short_sequences():
+    assert kscore("AT", k=4) == 0.0