⚡ Bolt: optimize answer fragment reassembly to single-pass O(N)

.jules/bolt.md

@@ -0,0 +1,3 @@
+## 2026-07-20 - O(N) single-pass bucket-sort fragment reassembly optimization
+**Learning:** Fragmented Pkarr packet records can cause O(N^2) CPU overhead if we probe each index sequentially with a full linear scan over all resource records in the packet. We can optimize this by doing a single pass over `all_resource_records()` and bucket-sorting the extracted indexes.
+**Action:** Use single-pass loops over resource records with index-based array lookups to avoid sequential DNS record probes.

crates/openhost-pkarr/src/offer.rs

@@ -1097,18 +1097,51 @@ pub fn decode_answer_fragments_from_packet(
         "{ANSWER_TXT_PREFIX}{}",
         zbase32::encode_full_bytes(&client_hash)
     );
+    let prefix = format!("{}-", base);
+    let prefix_bytes = prefix.as_bytes();
 
-    // TODO(perf): replace the per-fragment `collect_single_txt` probes
-    // with a single pass over `packet.all_resource_records()` that
-    // bucket-sorts matching names by their numeric `-<idx>` suffix.
-    // Today this walks the packet's RR list `chunk_total` times — fine
-    // for the 1–3 fragments we see in practice, O(N²) in the
-    // pathological MAX_FRAGMENT_TOTAL=255 case. Not a hotpath (one
-    // reassembly per dial attempt) so the refactor is deferred.
+    let mut buckets: [Option<String>; 256] = {
+        const NONE: Option<String> = None;
+        [NONE; 256]
+    };
+
+    // Single pass over packet's all resource records to bucket-sort them by numeric idx suffix.
+    for rr in packet.all_resource_records() {
+        let first_label = rr.name.get_labels().first();
+        if let Some(label) = first_label {
+            let label_bytes = label.as_ref();
+            if label_bytes.starts_with(prefix_bytes) {
+                let suffix = &label_bytes[prefix.len()..];
+                if let Ok(suffix_str) = std::str::from_utf8(suffix) {
+                    if let Ok(idx) = suffix_str.parse::<u8>() {
+                        if let RData::TXT(txt) = &rr.rdata {
+                            if buckets[idx as usize].is_some() {
+                                return Err(PkarrError::MultipleOpenhostRecords);
+                            }
+                            let mut out = String::new();
+                            for (key, value) in txt.iter_raw() {
+                                out.push_str(
+                                    core::str::from_utf8(key)
+                                        .map_err(|_| PkarrError::InvalidUtf8)?,
+                                );
+                                if let Some(v) = value {
+                                    out.push('=');
+                                    out.push_str(
+                                        core::str::from_utf8(v)
+                                            .map_err(|_| PkarrError::InvalidUtf8)?,
+                                    );
+                                }
+                            }
+                            buckets[idx as usize] = Some(out);
+                        }
+                    }
+                }
+            }
+        }
+    }
 
     // Probe idx = 0 first. Missing zero-fragment ⇒ no answer for us.
-    let first_name = format!("{base}-0");
-    let Some(first_text) = collect_single_txt(packet, &first_name)? else {
+    let Some(first_text) = &buckets[0] else {
         return Ok(None);
     };
     let first_bytes = URL_SAFE_NO_PAD.decode(first_text.as_bytes())?;
@@ -1123,10 +1156,11 @@ pub fn decode_answer_fragments_from_packet(
     let mut fragments: Vec<DecodedFragment> = Vec::with_capacity(total as usize);
     fragments.push(first);
     for i in 1..total {
-        let name = format!("{base}-{i}");
-        let text = collect_single_txt(packet, &name)?.ok_or(PkarrError::MalformedCanonical(
-            "answer fragment set is missing an idx",
-        ))?;
+        let text = buckets[i as usize]
+            .as_ref()
+            .ok_or(PkarrError::MalformedCanonical(
+                "answer fragment set is missing an idx",
+            ))?;
         let bytes = URL_SAFE_NO_PAD.decode(text.as_bytes())?;
         let frag = decode_fragment(&bytes)?;
         if frag.total != total {