rlp: support for uint256 (#26898)

This adds built-in support in package rlp for encoding, decoding and generating code dealing with uint256.Int.

---------

Co-authored-by: Felix Lange <fjl@twurst.com>

rlp/encbuffer.go

@@ -17,10 +17,13 @@
 package rlp
 
 import (
+	"encoding/binary"
 	"io"
 	"math/big"
 	"reflect"
 	"sync"
+
+	"github.com/holiman/uint256"
 )
 
 type encBuffer struct {
@@ -169,6 +172,23 @@ func (w *encBuffer) writeBigInt(i *big.Int) {
 	}
 }
 
+// writeUint256 writes z as an integer.
+func (w *encBuffer) writeUint256(z *uint256.Int) {
+	bitlen := z.BitLen()
+	if bitlen <= 64 {
+		w.writeUint64(z.Uint64())
+		return
+	}
+	nBytes := byte((bitlen + 7) / 8)
+	var b [33]byte
+	binary.BigEndian.PutUint64(b[1:9], z[3])
+	binary.BigEndian.PutUint64(b[9:17], z[2])
+	binary.BigEndian.PutUint64(b[17:25], z[1])
+	binary.BigEndian.PutUint64(b[25:33], z[0])
+	b[32-nBytes] = 0x80 + nBytes
+	w.str = append(w.str, b[32-nBytes:]...)
+}
+
 // list adds a new list header to the header stack. It returns the index of the header.
 // Call listEnd with this index after encoding the content of the list.
 func (buf *encBuffer) list() int {
@@ -376,6 +396,11 @@ func (w EncoderBuffer) WriteBigInt(i *big.Int) {
 	w.buf.writeBigInt(i)
 }
 
+// WriteUint256 encodes uint256.Int as an RLP string.
+func (w EncoderBuffer) WriteUint256(i *uint256.Int) {
+	w.buf.writeUint256(i)
+}
+
 // WriteBytes encodes b as an RLP string.
 func (w EncoderBuffer) WriteBytes(b []byte) {
 	w.buf.writeBytes(b)