Add TxID calculation & New{Layer}Tx Type

Add TxID calculation & New{Layer}Tx Type

New{Layer}Tx methods that compute the `TxID` & `TxType` values from the
transaction values:
- NewL1Tx
- NewL2Tx
- NewPoolL2Tx

Add TxID Scanner & Valuer for database/sql

HistoryDB & L2DB & API packages tests will need to be addapted to the
TestTransaction generation once is done.

common/pooll2tx.go

@@ -13,6 +13,10 @@ import (
 // PoolL2Tx is a struct that represents a L2Tx sent by an account to the coordinator hat is waiting to be forged
 type PoolL2Tx struct {
 	// Stored in DB: mandatory fileds
+
+	// TxID (12 bytes) for L2Tx is:
+	// bytes:  |  1   |    6    |   5   |
+	// values: | type | FromIdx | Nonce |
 	TxID        TxID               `meddler:"tx_id"`
 	FromIdx     Idx                `meddler:"from_idx"` // FromIdx is used by L1Tx/Deposit to indicate the Idx receiver of the L1Tx.LoadAmount (deposit)
 	ToIdx       Idx                `meddler:"to_idx"`   // ToIdx is ignored in L1Tx/Deposit, but used in the L1Tx/DepositAndTransfer
@@ -45,6 +49,44 @@ type PoolL2Tx struct {
 	TokenSymbol        string `meddler:"token_symbol"`
 }
 
+// NewPoolL2Tx returns the given L2Tx with the TxId & Type parameters calculated
+// from the L2Tx values
+func NewPoolL2Tx(poolL2Tx *PoolL2Tx) (*PoolL2Tx, error) {
+	// calculate TxType
+	var txType TxType
+	if poolL2Tx.ToIdx == Idx(0) {
+		txType = TxTypeTransfer
+	} else if poolL2Tx.ToIdx == Idx(1) {
+		txType = TxTypeExit
+	} else if poolL2Tx.ToIdx >= IdxUserThreshold {
+		txType = TxTypeTransfer
+	} else {
+		return poolL2Tx, fmt.Errorf("Can not determine type of PoolL2Tx, invalid ToIdx value: %d", poolL2Tx.ToIdx)
+	}
+
+	// if TxType!=poolL2Tx.TxType return error
+	if poolL2Tx.Type != "" && poolL2Tx.Type != txType {
+		return poolL2Tx, fmt.Errorf("PoolL2Tx.Type: %s, should be: %s", poolL2Tx.Type, txType)
+	}
+	poolL2Tx.Type = txType
+
+	var txid [TxIDLen]byte
+	txid[0] = TxIDPrefixL2Tx
+	fromIdxBytes, err := poolL2Tx.FromIdx.Bytes()
+	if err != nil {
+		return poolL2Tx, err
+	}
+	copy(txid[1:7], fromIdxBytes[:])
+	nonceBytes, err := poolL2Tx.Nonce.Bytes()
+	if err != nil {
+		return poolL2Tx, err
+	}
+	copy(txid[7:12], nonceBytes[:])
+	poolL2Tx.TxID = TxID(txid)
+
+	return poolL2Tx, nil
+}
+
 // TxCompressedData spec:
 // [ 1 bits  ] toBJJSign // 1 byte
 // [ 8 bits  ] userFee // 1 byte