rpc: implement CalculateInputs for RPC client

Using getunspents RPC call.

pkg/rpc/client.go

@@ -11,7 +11,10 @@ import (
 	"sync"
 	"time"
 
+	"github.com/CityOfZion/neo-go/pkg/core"
+	"github.com/CityOfZion/neo-go/pkg/core/transaction"
 	"github.com/CityOfZion/neo-go/pkg/crypto/keys"
+	"github.com/CityOfZion/neo-go/pkg/util"
 	"github.com/pkg/errors"
 )
 
@@ -152,6 +155,29 @@ func (c *Client) SetClient(cli *http.Client) {
 	}
 }
 
+// CalculateInputs creates input transactions for the specified amount of given
+// asset belonging to specified address. This implementation uses GetUnspents
+// JSON-RPC call internally, so make sure your RPC server suppors that.
+func (c *Client) CalculateInputs(address string, asset util.Uint256, cost util.Fixed8) ([]transaction.Input, util.Fixed8, error) {
+	var utxos core.UnspentBalances
+
+	resp, err := c.GetUnspents(address)
+	if err != nil || resp.Error != nil {
+		if err == nil {
+			err = fmt.Errorf("remote returned %d: %s", resp.Error.Code, resp.Error.Message)
+		}
+		return nil, util.Fixed8(0), errors.Wrapf(err, "cannot get balance for address %v", address)
+	}
+	for _, ubi := range resp.Result.Balance {
+		if asset.Equals(ubi.AssetHash) {
+			utxos = ubi.Unspents
+			break
+		}
+	}
+	return unspentsToInputs(utxos, cost)
+
+}
+
 func (c *Client) performRequest(method string, p params, v interface{}) error {
 	var (
 		r = request{