fable wants to DIY a hash

overlay/batch/dispatch_bench_test.go

@@ -110,3 +110,48 @@ func BenchmarkDispatchSeedHeavy(b *testing.B) {
 	}
 	runDispatchBench(b, pkts, len(pkts))
 }
+
+// TestFlowKeyDigestDistinct pins digest quality: distinct flows must produce distinct keys across
+// a large sample, and the v4/v6 domains must not alias. Collisions are tolerated by construction
+// (headersMatch gates every merge), so this is a quality canary, not a correctness requirement.
+func TestFlowKeyDigestDistinct(t *testing.T) {
+	seen := make(map[flowKey]struct{}, 1<<17)
+	v4 := make([]byte, 40)
+	v4[0] = 0x45
+	v4[3] = 40 // total length
+	add := func(fk flowKey) {
+		if _, dup := seen[fk]; dup {
+			t.Fatal("flow digest collision in small sample")
+		}
+		seen[fk] = struct{}{}
+	}
+	var fk flowKey
+	for a := range 256 {
+		for b := range 128 {
+			v4[15] = byte(a) // src low byte
+			v4[19] = byte(b) // dst low byte
+			v4[21] = byte(a)
+			v4[23] = byte(b)
+			trimmed, ok := fk.parseIPv4Prologue(v4)
+			if !ok {
+				t.Fatal("v4 prologue rejected synthetic packet")
+			}
+			_ = trimmed
+			add(fk.withPorts(uint32(a)<<16 | uint32(b)))
+		}
+	}
+	v6 := make([]byte, 60)
+	v6[0] = 0x60
+	for a := range 256 {
+		for b := range 128 {
+			v6[23] = byte(a)
+			v6[39] = byte(b)
+			trimmed, ok := fk.parseIPv6Prologue(v6)
+			if !ok {
+				t.Fatal("v6 prologue rejected synthetic packet")
+			}
+			_ = trimmed
+			add(fk.withPorts(uint32(a)<<16 | uint32(b)))
+		}
+	}
+}