status-im/status-go

package bigint

import (
    "math/big"
    "reflect"

    "github.com/ethereum/go-ethereum/common/hexutil"
)

// Unmarshals a u256 as a fixed-length hex string with 0x prefix and leading zeros
type HexBigInt struct {
    *big.Int
}

const FixedLength = 32 // u256 -> 32 bytes

var (
    hexBigIntT = reflect.TypeOf(HexBigInt{})
)

func (b *HexBigInt) UnmarshalJSON(input []byte) error {
    var buf [FixedLength]byte
    err := hexutil.UnmarshalFixedJSON(hexBigIntT, input, buf[:])

    if err != nil {
        return err
    }

    z := new(big.Int)
    z.SetBytes(buf[:])
    b.Int = z
    return nil
}