bootjp
diff --git a/‎adapter/sqs_partition_resolver.go‎
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+package adapter
+
+import (
+	"bytes"
+	"encoding/binary"
+)
+
+// SQSPartitionResolver maps a partitioned-SQS key to the operator-
+// chosen Raft group for the (queue, partition) tuple. Implements
+// kv.PartitionResolver via duck typing — see the integration in
+// main.go where the resolver is installed on ShardedCoordinator.
+//
+// The byte-range engine cannot route partitioned queues because
+// adding per-partition routes would break its non-overlapping-cover
+// invariant (a partition route for partition K of one queue would
+// leave a gap for legacy keys that fall lexicographically between
+// partitions K and K+1). The resolver-first dispatch path avoids
+// this — it answers only for keys that match a partitioned family
+// prefix and otherwise lets the engine handle dispatch.
+type SQSPartitionResolver struct {
+	routes map[string][]uint64
+}
+
+// NewSQSPartitionResolver builds a resolver from the operator-
+// supplied partition map. routes[queue][k] is the Raft group ID
+// that owns partition k of queue, with len(routes[queue]) equal to
+// the queue's PartitionCount.
+//
+// Returns nil when routes is empty so callers can keep the resolver
+// out of the request path entirely on a non-partitioned cluster
+// (kv.ShardRouter.WithPartitionResolver(nil) is a documented no-op).
+//
+// The constructor takes a defensive copy so a later caller mutation
+// to the input map does not leak into the resolver's view at
+// runtime.
+func NewSQSPartitionResolver(routes map[string][]uint64) *SQSPartitionResolver {
+	if len(routes) == 0 {
+		return nil
+	}
+	cp := make(map[string][]uint64, len(routes))
+	for queue, groups := range routes {
+		ids := make([]uint64, len(groups))
+		copy(ids, groups)
+		cp[queue] = ids
+	}
+	return &SQSPartitionResolver{routes: cp}
+}
+
+// sqsResolverFamilyPrefixes is the set of partitioned-SQS family
+// prefixes ResolveGroup recognises. Pre-converted to []byte so the
+// hot-path bytes.HasPrefix call avoids an allocation per check
+// (gemini medium on PR #715). Kept package-internal so any future
+// renamed prefix touches both this list and the constant
+// declaration in sqs_keys.go — TestSQSPartitionResolver_PrefixesAlign
+// pins the alignment.
+var sqsResolverFamilyPrefixes = [][]byte{
+	[]byte(SqsPartitionedMsgDataPrefix),
+	[]byte(SqsPartitionedMsgVisPrefix),
+	[]byte(SqsPartitionedMsgDedupPrefix),
+	[]byte(SqsPartitionedMsgGroupPrefix),
+	[]byte(SqsPartitionedMsgByAgePrefix),
+}
+
+// ResolveGroup decodes the (queue, partition) embedded in a
+// partitioned-SQS key and returns the operator-chosen Raft group.
+//
+// Returns (0, false) for any key that does not match a partitioned
+// family prefix (legacy SQS, KV, S3, DynamoDB, queue-meta records,
+// …) so kv.ShardRouter falls through to its byte-range engine for
+// default routing.
+//
+// Returns (0, false) for a partitioned-shaped key whose queue is
+// not in the routes map or whose partition index is beyond
+// len(routes[queue]). The router pairs this with
+// RecognisesPartitionedKey to fail closed instead of falling
+// through — silently routing through the engine's
+// !sqs|route|global default would mis-route HT-FIFO traffic during
+// partition-map drift (codex P1 round 2 on PR #715).
+func (r *SQSPartitionResolver) ResolveGroup(key []byte) (uint64, bool) {
+	if r == nil || len(key) == 0 {
+		return 0, false
+	}
+	queue, partition, ok := parsePartitionedSQSKey(key)
+	if !ok {
+		return 0, false
+	}
+	groups, found := r.routes[queue]
+	if !found {
+		return 0, false
+	}
+	// Defensive: a partition value outside the slice is a config /
+	// upstream-bug signal, not a routable key. Returning false
+	// surfaces it as "no route" at the router boundary, which is
+	// the correct fail-closed behaviour.
+	if uint64(partition) >= uint64(len(groups)) {
+		return 0, false
+	}
+	return groups[partition], true
+}
+
+// RecognisesPartitionedKey reports whether key has the structural
+// shape of a partitioned-SQS key — i.e. starts with one of the
+// partitioned family prefixes. The check is PREFIX-ONLY, not a
+// full parse: a key with a partitioned prefix followed by a
+// malformed queue / partition segment still answers true, so the
+// router fails closed via kv.PartitionResolver semantics instead
+// of falling through to the engine and silently routing to the
+// SQS catalog default group via routeKey's !sqs|route|global
+// collapse (round 5 review nit on PR #715).
+//
+// A nil receiver returns false so kv.ShardRouter's typed-nil case
+// (ResolveGroup(nil) == (0, false)) pairs with an honest "I don't
+// recognise anything" answer instead of falsely claiming a shape.
+func (r *SQSPartitionResolver) RecognisesPartitionedKey(key []byte) bool {
+	if r == nil || len(key) == 0 {
+		return false
+	}
+	_, ok := stripPartitionedFamilyPrefix(key)
+	return ok
+}
+
+// parsePartitionedSQSKey extracts the (queue, partition) pair from
+// a partitioned-SQS key. Returns ok=false for any key that does not
+// match a partitioned family prefix or that has a malformed queue /
+// partition segment. Exposed at package-internal scope so the
+// adapter's reaper / fanout reader can share the same parser
+// (Phase 3.D PR 5).
+func parsePartitionedSQSKey(key []byte) (string, uint32, bool) {
+	rest, matched := stripPartitionedFamilyPrefix(key)
+	if !matched {
+		return "", 0, false
+	}
+	// After the family prefix, the variable-length encoded queue
+	// segment is terminated by '|' (sqsPartitionedQueueTerminator).
+	// base64.RawURLEncoding never emits '|', so the first '|' in
+	// rest is unambiguously the queue terminator.
+	pipeIdx := bytes.IndexByte(rest, sqsPartitionedQueueTerminator)
+	if pipeIdx <= 0 {
+		return "", 0, false
+	}
+	encQueue := rest[:pipeIdx]
+	rest = rest[pipeIdx+1:]
+	const partitionLen = 4
+	if len(rest) < partitionLen {
+		return "", 0, false
+	}
+	partition := binary.BigEndian.Uint32(rest[:partitionLen])
+	queue, err := decodeSQSSegment(string(encQueue))
+	if err != nil {
+		return "", 0, false
+	}
+	return queue, partition, true
+}
+
+// stripPartitionedFamilyPrefix returns the bytes after the matched
+// family prefix. matched=false if key has none of the known
+// partitioned family prefixes.
+func stripPartitionedFamilyPrefix(key []byte) ([]byte, bool) {
+	for _, prefix := range sqsResolverFamilyPrefixes {
+		if bytes.HasPrefix(key, prefix) {
+			return key[len(prefix):], true
+		}
+	}
+	return nil, false
+}