LF52DE45Z23AT54TUKZNQHIQDTLX5LOD56WV4QVDDY4R2X25OD4QC
while (idx < in.len) : (idx += 1) {
for (in[idx - marker_len .. idx]) |ch| {
alphabet.set(ch - 'a');
}
if (alphabet.count() == marker_len) return idx;
alphabet.mask = 0;
for (in) |ch, idx| {
alphabet.toggle(ch - 'a');
if (idx >= marker_len) alphabet.toggle(in[idx - marker_len] - 'a');
if (alphabet.count() == marker_len) return idx + 1;