[P0] Make handling of warm keys deterministic

subnet/host/finality.go

@@ -0,0 +1,61 @@
+package host
+
+import (
+	"subnet/storage"
+	"subnet/types"
+)
+
+// computeFinalizedNonce returns the highest nonce F such that >=2/3 of the
+// session group (by slot count) has signed at nonce >= F.
+//
+// A slot that signed at nonce n is considered to have implicitly confirmed all
+// nonces <= n by building on top of them.
+func computeFinalizedNonce(store storage.Storage, escrowID string, latestNonce uint64, group []types.SlotAssignment) uint64 {
+	// confirmedBy[n] = bitmap of slots that have signed at nonce >= n.
+	// Bitmap128 is a value type (16 bytes); max group size is 128.
+	confirmedBy := make(map[uint64]types.Bitmap128)
+
+	for n := uint64(1); n <= latestNonce; n++ {
+		sigs, err := store.GetSignatures(escrowID, n)
+		if err != nil {
+			continue
+		}
+		for slotID := range sigs {
+			// This slot signed at n, confirming all nonces 1..n.
+			for prev := uint64(1); prev <= n; prev++ {
+				bm := confirmedBy[prev] // zero value if absent
+				bm.Set(slotID)
+				confirmedBy[prev] = bm
+			}
+		}
+	}
+
+	threshold := twoThirdsWeight(group)
+	for f := latestNonce; f > 0; f-- {
+		if bitmapSlotWeight(confirmedBy[f], group) >= threshold {
+			return f
+		}
+	}
+	return 0 // warm-up period: not yet finalized
+}
+
+// bitmapSlotWeight counts the number of group slots whose bit is set in bm.
+func bitmapSlotWeight(bm types.Bitmap128, group []types.SlotAssignment) uint32 {
+	var total uint32
+	for _, sa := range group {
+		if bm.IsSet(sa.SlotID) {
+			total++
+		}
+	}
+	return total
+}
+
+// twoThirdsWeight returns ceil(2/3 * totalSlots).
+func twoThirdsWeight(group []types.SlotAssignment) uint32 {
+	total := uint32(len(group))
+	if total == 0 {
+		return 0
+	}
+	return (total*2 + 2) / 3
+}
+