chore(txs): remove utils file (#6162)

## Motivation

Having a `utils` file is an anti-pattern. Removing it and putting the helper function next to its usage.

txs/conservative_state.go

@@ -8,6 +8,7 @@ import (
 	"time"
 
 	"go.uber.org/zap"
+	"go.uber.org/zap/zapcore"
 
 	"github.com/spacemeshos/go-spacemesh/common/types"
 	"github.com/spacemeshos/go-spacemesh/events"
@@ -221,3 +222,54 @@ func (cs *ConservativeState) GetTransactionsByAddress(
 ) ([]*types.MeshTransaction, error) {
 	return transactions.GetByAddress(cs.db, from, to, address)
 }
+
+// ShuffleWithNonceOrder perform a Fisher-Yates shuffle on the transactions.
+// note that after shuffling, the original list of transactions are no longer in nonce order
+// within the same principal. we simply check which principal occupies the spot after
+// the shuffle and retrieve their transactions in nonce order.
+func ShuffleWithNonceOrder(
+	logger *zap.Logger,
+	rng *rand.Rand,
+	numTXs int,
+	ntxs []*NanoTX,
+	byAddrAndNonce map[types.Address][]*NanoTX,
+) []types.TransactionID {
+	rng.Shuffle(len(ntxs), func(i, j int) { ntxs[i], ntxs[j] = ntxs[j], ntxs[i] })
+	total := min(len(ntxs), numTXs)
+	result := make([]types.TransactionID, 0, total)
+	packed := make(map[types.Address][]uint64)
+	for _, ntx := range ntxs[:total] {
+		// if a spot is taken by a principal, we add its TX for the next eligible nonce
+		p := ntx.Principal
+		if _, ok := byAddrAndNonce[p]; !ok {
+			logger.Fatal("principal missing", zap.Stringer("address", p))
+		}
+		if len(byAddrAndNonce[p]) == 0 {
+			logger.Fatal("txs missing", zap.Stringer("address", p))
+		}
+		toAdd := byAddrAndNonce[p][0]
+		result = append(result, toAdd.ID)
+		if _, ok := packed[p]; !ok {
+			packed[p] = []uint64{toAdd.Nonce, toAdd.Nonce}
+		} else {
+			packed[p][1] = toAdd.Nonce
+		}
+		if len(byAddrAndNonce[p]) == 1 {
+			delete(byAddrAndNonce, p)
+		} else {
+			byAddrAndNonce[p] = byAddrAndNonce[p][1:]
+		}
+	}
+	logger.Debug("packed txs", zap.Array("ranges", zapcore.ArrayMarshalerFunc(func(encoder zapcore.ArrayEncoder) error {
+		for addr, nonces := range packed {
+			_ = encoder.AppendObject(zapcore.ObjectMarshalerFunc(func(encoder zapcore.ObjectEncoder) error {
+				encoder.AddString("addr", addr.String())
+				encoder.AddUint64("from", nonces[0])
+				encoder.AddUint64("to", nonces[1])
+				return nil
+			}))
+		}
+		return nil
+	})))
+	return result
+}