path: root/ethvm/vm.go

package ethvm

import (
    "fmt"
    "math"
    "math/big"

    "github.com/ethereum/eth-go/ethcrypto"
    "github.com/ethereum/eth-go/ethstate"
    "github.com/ethereum/eth-go/ethutil"
)

type Debugger interface {
    BreakHook(step int, op OpCode, mem *Memory, stack *Stack, object *ethstate.StateObject) bool
    StepHook(step int, op OpCode, mem *Memory, stack *Stack, object *ethstate.StateObject) bool
    BreakPoints() []int64
    SetCode(byteCode []byte)
}

type Vm struct {
    // Stack for processing contracts
    stack *Stack
    // non-persistent key/value memory storage
    mem map[string]*big.Int

    env Environment

    Verbose bool

    logTy  byte
    logStr string

    err error

    // Debugging
    Dbg Debugger

    BreakPoints []int64
    Stepping    bool
    Fn          string

    Recoverable bool
}

type Environment interface {
    State() *ethstate.State

    Origin() []byte
    BlockNumber() *big.Int
    PrevHash() []byte
    Coinbase() []byte
    Time() int64
    Difficulty() *big.Int
    Value() *big.Int
    BlockHash() []byte
}

type Object interface {
    GetStorage(key *big.Int) *ethutil.Value
    SetStorage(key *big.Int, value *ethutil.Value)
}

func New(env Environment) *Vm {
    lt := LogTyPretty
    if ethutil.Config.Diff {
        lt = LogTyDiff
    }

    return &Vm{env: env, logTy: lt, Recoverable: true}
}

func (self *Vm) RunClosure(closure *Closure) (ret []byte, err error) {
    if self.Recoverable {
        // Recover from any require exception
        defer func() {
            if r := recover(); r != nil {
                ret = closure.Return(nil)
                err = fmt.Errorf("%v", r)
                vmlogger.Errorln("vm err", err)
            }
        }()
    }

    // Debug hook
    if self.Dbg != nil {
        self.Dbg.SetCode(closure.Code)
    }

    // Don't bother with the execution if there's no code.
    if len(closure.Code) == 0 {
        return closure.Return(nil), nil
    }

    vmlogger.Debugf("(%s) %x gas: %v (d) %x\n", self.Fn, closure.Address(), closure.Gas, closure.Args)

    var (
        op OpCode

        mem      = &Memory{}
        stack    = NewStack()
        pc       = big.NewInt(0)
        step     = 0
        prevStep = 0
        require  = func(m int) {
            if stack.Len() < m {
                panic(fmt.Sprintf("%04v (%v) stack err size = %d, required = %d", pc, op, stack.Len(), m))
            }
        }
    )

    for {
        prevStep = step
        // The base for all big integer arithmetic
        base := new(big.Int)

        step++
        // Get the memory location of pc
        val := closure.Get(pc)
        // Get the opcode (it must be an opcode!)
        op = OpCode(val.Uint())

        // XXX Leave this Println intact. Don't change this to the log system.
        // Used for creating diffs between implementations
        if self.logTy == LogTyDiff {
            /*
                switch op {
                case STOP, RETURN, SUICIDE:
                    closure.object.EachStorage(func(key string, value *ethutil.Value) {
                        value.Decode()
                        fmt.Printf("%x %x\n", new(big.Int).SetBytes([]byte(key)).Bytes(), value.Bytes())
                    })
                }
            */

            b := pc.Bytes()
            if len(b) == 0 {
                b = []byte{0}
            }

            fmt.Printf("%x %x %x %x\n", closure.Address(), b, []byte{byte(op)}, closure.Gas.Bytes())
        }

        gas := new(big.Int)
        addStepGasUsage := func(amount *big.Int) {
            if amount.Cmp(ethutil.Big0) >= 0 {
                gas.Add(gas, amount)
            }
        }

        addStepGasUsage(GasStep)

        var newMemSize uint64 = 0
        switch op {
        case STOP:
            gas.Set(ethutil.Big0)
        case SUICIDE:
            gas.Set(ethutil.Big0)
        case SLOAD:
            gas.Set(GasSLoad)
        case SSTORE:
            var mult *big.Int
            y, x := stack.Peekn()
            val := closure.GetStorage(x)
            if val.BigInt().Cmp(ethutil.Big0) == 0 && len(y.Bytes()) > 0 {
                mult = ethutil.Big2
            } else if val.BigInt().Cmp(ethutil.Big0) != 0 && len(y.Bytes()) == 0 {
                mult = ethutil.Big0
            } else {
                mult = ethutil.Big1
            }
            gas = new(big.Int).Mul(mult, GasSStore)
        case BALANCE:
            gas.Set(GasBalance)
        case MSTORE:
            require(2)
            newMemSize = stack.Peek().Uint64() + 32
        case MLOAD:
            require(1)

            newMemSize = stack.Peek().Uint64() + 32
        case MSTORE8:
            require(2)
            newMemSize = stack.Peek().Uint64() + 1
        case RETURN:
            require(2)

            newMemSize = stack.Peek().Uint64() + stack.data[stack.Len()-2].Uint64()
        case SHA3:
            require(2)

            gas.Set(GasSha)

            newMemSize = stack.Peek().Uint64() + stack.data[stack.Len()-2].Uint64()
        case CALLDATACOPY:
            require(3)

            newMemSize = stack.Peek().Uint64() + stack.data[stack.Len()-3].Uint64()
        case CODECOPY:
            require(3)

            newMemSize = stack.Peek().Uint64() + stack.data[stack.Len()-3].Uint64()
        case CALL:
            require(7)
            gas.Set(GasCall)
            addStepGasUsage(stack.data[stack.Len()-1])

            x := stack.data[stack.Len()-6].Uint64() + stack.data[stack.Len()-7].Uint64()
            y := stack.data[stack.Len()-4].Uint64() + stack.data[stack.Len()-5].Uint64()

            newMemSize = uint64(math.Max(float64(x), float64(y)))
        case CREATE:
            require(3)
            gas.Set(GasCreate)

            newMemSize = stack.data[stack.Len()-2].Uint64() + stack.data[stack.Len()-3].Uint64()
        }

        newMemSize = (newMemSize + 31) / 32 * 32
        if newMemSize > uint64(mem.Len()) {
            m := GasMemory.Uint64() * (newMemSize - uint64(mem.Len())) / 32
            addStepGasUsage(big.NewInt(int64(m)))
        }

        if !closure.UseGas(gas) {
            err := fmt.Errorf("Insufficient gas for %v. req %v has %v", op, gas, closure.Gas)

            closure.UseGas(closure.Gas)

            return closure.Return(nil), err
        }

        self.Printf("(pc) %-3d -o- %-14s", pc, op.String())
        self.Printf(" (g) %-3v (%v)", gas, closure.Gas)

        mem.Resize(newMemSize)

        switch op {
        case LOG:
            stack.Print()
            mem.Print()
            // 0x20 range
        case ADD:
            require(2)
            x, y := stack.Popn()
            self.Printf(" %v + %v", y, x)

            base.Add(y, x)

            ensure256(base)

            self.Printf(" = %v", base)
            // Pop result back on the stack
            stack.Push(base)
        case SUB:
            require(2)
            x, y := stack.Popn()
            self.Printf(" %v - %v", y, x)

            base.Sub(y, x)

            ensure256(base)

            self.Printf(" = %v", base)
            // Pop result back on the stack
            stack.Push(base)
        case MUL:
            require(2)
            x, y := stack.Popn()
            self.Printf(" %v * %v", y, x)

            base.Mul(y, x)

            ensure256(base)

            self.Printf(" = %v", base)
            // Pop result back on the stack
            stack.Push(base)
        case DIV:
            require(2)
            x, y := stack.Popn()
            self.Printf(" %v / %v", y, x)

            if x.Cmp(ethutil.Big0) != 0 {
                base.Div(y, x)
            }

            ensure256(base)

            self.Printf(" = %v", base)
            // Pop result back on the stack
            stack.Push(base)
        case SDIV:
            require(2)
            x, y := stack.Popn()
            self.Printf(" %v / %v", y, x)

            if x.Cmp(ethutil.Big0) != 0 {
                base.Div(y, x)
            }

            ensure256(base)

            self.Printf(" = %v", base)
            // Pop result back on the stack
            stack.Push(base)
        case MOD:
            require(2)
            x, y := stack.Popn()

            self.Printf(" %v %% %v", y, x)

            base.Mod(y, x)

            ensure256(base)

            self.Printf(" = %v", base)
            stack.Push(base)
        case SMOD:
            require(2)
            x, y := stack.Popn()

            self.Printf(" %v %% %v", y, x)

            base.Mod(y, x)

            ensure256(base)

            self.Printf(" = %v", base)
            stack.Push(base)

        case EXP:
            require(2)
            x, y := stack.Popn()

            self.Printf(" %v ** %v", y, x)

            base.Exp(y, x, Pow256)

            ensure256(base)

            self.Printf(" = %v", base)

            stack.Push(base)
        case NEG:
            require(1)
            base.Sub(Pow256, stack.Pop())
            stack.Push(base)
        case LT:
            require(2)
            x, y := stack.Popn()
            self.Printf(" %v < %v", y, x)
            // x < y
            if y.Cmp(x) < 0 {
                stack.Push(ethutil.BigTrue)
            } else {
                stack.Push(ethutil.BigFalse)
            }
        case GT:
            require(2)
            x, y := stack.Popn()
            self.Printf(" %v > %v", y, x)

            // x > y
            if y.Cmp(x) > 0 {
                stack.Push(ethutil.BigTrue)
            } else {
                stack.Push(ethutil.BigFalse)
            }

        case SLT:
            require(2)
            x, y := stack.Popn()
            self.Printf(" %v < %v", y, x)
            // x < y
            if y.Cmp(x) < 0 {
                stack.Push(ethutil.BigTrue)
            } else {
                stack.Push(ethutil.BigFalse)
            }
        case SGT:
            require(2)
            x, y := stack.Popn()
            self.Printf(" %v > %v", y, x)

            // x > y
            if y.Cmp(x) > 0 {
                stack.Push(ethutil.BigTrue)
            } else {
                stack.Push(ethutil.BigFalse)
            }

        case EQ:
            require(2)
            x, y := stack.Popn()
            self.Printf(" %v == %v", y, x)

            // x == y
            if x.Cmp(y) == 0 {
                stack.Push(ethutil.BigTrue)
            } else {
                stack.Push(ethutil.BigFalse)
            }
        case NOT:
            require(1)
            x := stack.Pop()
            if x.Cmp(ethutil.BigFalse) > 0 {
                stack.Push(ethutil.BigFalse)
            } else {
                stack.Push(ethutil.BigTrue)
            }

            // 0x10 range
        case AND:
            require(2)
            x, y := stack.Popn()
            self.Printf(" %v & %v", y, x)

            stack.Push(base.And(y, x))
        case OR:
            require(2)
            x, y := stack.Popn()
            self.Printf(" %v | %v", y, x)

            stack.Push(base.Or(y, x))
        case XOR:
            require(2)
            x, y := stack.Popn()
            self.Printf(" %v ^ %v", y, x)

            stack.Push(base.Xor(y, x))
        case BYTE:
            require(2)
            val, th := stack.Popn()
            if th.Cmp(big.NewInt(32)) < 0 && th.Cmp(big.NewInt(int64(len(val.Bytes())))) < 0 {
                byt := big.NewInt(int64(ethutil.LeftPadBytes(val.Bytes(), 32)[th.Int64()]))
                stack.Push(byt)

                self.Printf(" => 0x%x", byt.Bytes())
            } else {
                stack.Push(ethutil.BigFalse)
            }

            // 0x20 range
        case SHA3:
            require(2)
            size, offset := stack.Popn()
            data := ethcrypto.Sha3Bin(mem.Get(offset.Int64(), size.Int64()))

            stack.Push(ethutil.BigD(data))

            self.Printf(" => %x", data)
            // 0x30 range
        case ADDRESS:
            stack.Push(ethutil.BigD(closure.Address()))

            self.Printf(" => %x", closure.Address())
        case BALANCE:
            require(1)

            addr := stack.Pop().Bytes()
            balance := self.env.State().GetBalance(addr)

            stack.Push(balance)

            self.Printf(" => %v (%x)", balance, addr)
        case ORIGIN:
            origin := self.env.Origin()

            stack.Push(ethutil.BigD(origin))

            self.Printf(" => %x", origin)
        case CALLER:
            caller := closure.caller.Address()
            stack.Push(ethutil.BigD(caller))

            self.Printf(" => %x", caller)
        case CALLVALUE:
            value := self.env.Value()

            stack.Push(value)

            self.Printf(" => %v", value)
        case CALLDATALOAD:
            require(1)
            var (
                offset  = stack.Pop()
                data    = make([]byte, 32)
                lenData = big.NewInt(int64(len(closure.Args)))
            )

            if lenData.Cmp(offset) >= 0 {
                length := new(big.Int).Add(offset, ethutil.Big32)
                length = ethutil.BigMin(length, lenData)

                copy(data, closure.Args[offset.Int64():length.Int64()])
            }

            self.Printf(" => 0x%x", data)

            stack.Push(ethutil.BigD(data))
        case CALLDATASIZE:
            l := int64(len(closure.Args))
            stack.Push(big.NewInt(l))

            self.Printf(" => %d", l)
        case CALLDATACOPY:
            var (
                size = int64(len(closure.Args))
                mOff = stack.Pop().Int64()
                cOff = stack.Pop().Int64()
                l    = stack.Pop().Int64()
            )

            if cOff > size {
                cOff = 0
                l = 0
            } else if cOff+l > size {
                l = 0
            }

            code := closure.Args[cOff : cOff+l]

            mem.Set(mOff, l, code)
        case CODESIZE:
            l := big.NewInt(int64(len(closure.Code)))
            stack.Push(l)

            self.Printf(" => %d", l)
        case CODECOPY:
            var (
                size = int64(len(closure.Code))
                mOff = stack.Pop().Int64()
                cOff = stack.Pop().Int64()
                l    = stack.Pop().Int64()
            )

            if cOff > size {
                cOff = 0
                l = 0
            } else if cOff+l > size {
                l = 0
            }

            code := closure.Code[cOff : cOff+l]

            mem.Set(mOff, l, code)
        case GASPRICE:
            stack.Push(closure.Price)

            self.Printf(" => %v", closure.Price)

            // 0x40 range
        case PREVHASH:
            prevHash := self.env.PrevHash()

            stack.Push(ethutil.BigD(prevHash))

            self.Printf(" => 0x%x", prevHash)
        case COINBASE:
            coinbase := self.env.Coinbase()

            stack.Push(ethutil.BigD(coinbase))

            self.Printf(" => 0x%x", coinbase)
        case TIMESTAMP:
            time := self.env.Time()

            stack.Push(big.NewInt(time))

            self.Printf(" => 0x%x", time)
        case NUMBER:
            number := self.env.BlockNumber()

            stack.Push(number)

            self.Printf(" => 0x%x", number.Bytes())
        case DIFFICULTY:
            difficulty := self.env.Difficulty()

            stack.Push(difficulty)

            self.Printf(" => 0x%x", difficulty.Bytes())
        case GASLIMIT:
            // TODO
            stack.Push(big.NewInt(0))

            // 0x50 range
        case PUSH1, PUSH2, PUSH3, PUSH4, PUSH5, PUSH6, PUSH7, PUSH8, PUSH9, PUSH10, PUSH11, PUSH12, PUSH13, PUSH14, PUSH15, PUSH16, PUSH17, PUSH18, PUSH19, PUSH20, PUSH21, PUSH22, PUSH23, PUSH24, PUSH25, PUSH26, PUSH27, PUSH28, PUSH29, PUSH30, PUSH31, PUSH32:
            a := big.NewInt(int64(op) - int64(PUSH1) + 1)
            pc.Add(pc, ethutil.Big1)
            data := closure.Gets(pc, a)
            val := ethutil.BigD(data.Bytes())
            // Push value to stack
            stack.Push(val)
            pc.Add(pc, a.Sub(a, big.NewInt(1)))

            step += int(op) - int(PUSH1) + 1

            self.Printf(" => 0x%x", data.Bytes())
        case POP:
            require(1)
            stack.Pop()
        case DUP1, DUP2, DUP3, DUP4, DUP5, DUP6, DUP7, DUP8, DUP9, DUP10, DUP11, DUP12, DUP13, DUP14, DUP15, DUP16:
            n := int(op - DUP1 + 1)
            stack.Dupn(n)

            self.Printf(" => [%d] 0x%x", n, stack.Peek().Bytes())
        case SWAP1, SWAP2, SWAP3, SWAP4, SWAP5, SWAP6, SWAP7, SWAP8, SWAP9, SWAP10, SWAP11, SWAP12, SWAP13, SWAP14, SWAP15, SWAP16:
            n := int(op - SWAP1 + 1)
            x, y := stack.Swapn(n)

            self.Printf(" => [%d] %x [0] %x", n, x.Bytes(), y.Bytes())
        case DUP:
            // NOP
        case SWAP:
            // NOP
        case MLOAD:
            require(1)
            offset := stack.Pop()
            val := ethutil.BigD(mem.Get(offset.Int64(), 32))
            stack.Push(val)

            self.Printf(" => 0x%x", val.Bytes())
        case MSTORE: // Store the value at stack top-1 in to memory at location stack top
            require(2)
            // Pop value of the stack
            val, mStart := stack.Popn()
            mem.Set(mStart.Int64(), 32, ethutil.BigToBytes(val, 256))

            self.Printf(" => 0x%x", val)
        case MSTORE8:
            require(2)
            val, mStart := stack.Popn()
            //base.And(val, new(big.Int).SetInt64(0xff))
            //mem.Set(mStart.Int64(), 32, ethutil.BigToBytes(base, 256))
            mem.store[mStart.Int64()] = byte(val.Int64() & 0xff)

            self.Printf(" => 0x%x", val)
        case SLOAD:
            require(1)
            loc := stack.Pop()
            val := closure.GetStorage(loc)

            stack.Push(val.BigInt())

            self.Printf(" {0x%x : 0x%x}", loc.Bytes(), val.Bytes())
        case SSTORE:
            require(2)
            val, loc := stack.Popn()
            closure.SetStorage(loc, ethutil.NewValue(val))

            closure.message.AddStorageChange(loc.Bytes())

            self.Printf(" {0x%x : 0x%x}", loc.Bytes(), val.Bytes())
        case JUMP:
            require(1)
            pc = stack.Pop()
            // Reduce pc by one because of the increment that's at the end of this for loop
            self.Printf(" ~> %v", pc).Endl()

            continue
        case JUMPI:
            require(2)
            cond, pos := stack.Popn()
            if cond.Cmp(ethutil.BigTrue) >= 0 {
                pc = pos

                self.Printf(" ~> %v (t)", pc).Endl()

                continue
            } else {
                self.Printf(" (f)")
            }
        case PC:
            stack.Push(pc)
        case MSIZE:
            stack.Push(big.NewInt(int64(mem.Len())))
        case GAS:
            stack.Push(closure.Gas)
            // 0x60 range
        case CREATE:
            require(3)

            var (
                err          error
                value        = stack.Pop()
                size, offset = stack.Popn()

                // Snapshot the current stack so we are able to
                // revert back to it later.
                snapshot = self.env.State().Copy()
            )

            // Generate a new address
            addr := ethcrypto.CreateAddress(closure.Address(), closure.object.Nonce)
            for i := uint64(0); self.env.State().GetStateObject(addr) != nil; i++ {
                ethcrypto.CreateAddress(closure.Address(), closure.object.Nonce+i)
            }
            closure.object.Nonce++

            self.Printf(" (*) %x", addr).Endl()

            msg := self.env.State().Manifest().AddMessage(&ethstate.Message{
                To: addr, From: closure.Address(),
                Origin: self.env.Origin(),
                Block:  self.env.BlockHash(), Timestamp: self.env.Time(), Coinbase: self.env.Coinbase(), Number: self.env.BlockNumber(),
                Value: value,
            })

            // Create a new contract
            contract := self.env.State().NewStateObject(addr)
            if contract.Balance.Cmp(value) >= 0 {
                closure.object.SubAmount(value)
                contract.AddAmount(value)

                // Set the init script
                initCode := mem.Get(offset.Int64(), size.Int64())
                msg.Input = initCode

                // Transfer all remaining gas to the new
                // contract so it may run the init script
                gas := new(big.Int).Set(closure.Gas)
                closure.UseGas(closure.Gas)

                // Create the closure
                c := NewClosure(msg, closure, contract, initCode, gas, closure.Price)
                // Call the closure and set the return value as
                // main script.
                contract.Code, _, err = c.Call(self, nil)
            } else {
                err = fmt.Errorf("Insufficient funds to transfer value. Req %v, has %v", value, closure.object.Balance)
            }

            if err != nil {
                stack.Push(ethutil.BigFalse)

                // Revert the state as it was before.
                self.env.State().Set(snapshot)

                self.Printf("CREATE err %v", err)
            } else {
                stack.Push(ethutil.BigD(addr))

                msg.Output = contract.Code
            }
            self.Endl()

            // Debug hook
            if self.Dbg != nil {
                self.Dbg.SetCode(closure.Code)
            }
        case CALL:
            require(7)

            self.Endl()

            gas := stack.Pop()
            // Pop gas and value of the stack.
            value, addr := stack.Popn()
            // Pop input size and offset
            inSize, inOffset := stack.Popn()
            // Pop return size and offset
            retSize, retOffset := stack.Popn()

            // Get the arguments from the memory
            args := mem.Get(inOffset.Int64(), inSize.Int64())

            msg := self.env.State().Manifest().AddMessage(&ethstate.Message{
                To: addr.Bytes(), From: closure.Address(),
                Input:  args,
                Origin: self.env.Origin(),
                Block:  self.env.BlockHash(), Timestamp: self.env.Time(), Coinbase: self.env.Coinbase(), Number: self.env.BlockNumber(),
                Value: value,
            })

            if closure.object.Balance.Cmp(value) < 0 {
                vmlogger.Debugf("Insufficient funds to transfer value. Req %v, has %v", value, closure.object.Balance)

                closure.ReturnGas(gas, nil)

                stack.Push(ethutil.BigFalse)
            } else {
                snapshot := self.env.State().Copy()

                stateObject := self.env.State().GetOrNewStateObject(addr.Bytes())

                closure.object.SubAmount(value)
                stateObject.AddAmount(value)

                // Create a new callable closure
                c := NewClosure(msg, closure, stateObject, stateObject.Code, gas, closure.Price)
                // Executer the closure and get the return value (if any)
                ret, _, err := c.Call(self, args)
                if err != nil {
                    stack.Push(ethutil.BigFalse)

                    vmlogger.Debugf("Closure execution failed. %v\n", err)

                    self.env.State().Set(snapshot)
                } else {
                    stack.Push(ethutil.BigTrue)

                    mem.Set(retOffset.Int64(), retSize.Int64(), ret)
                }

                msg.Output = ret

                // Debug hook
                if self.Dbg != nil {
                    self.Dbg.SetCode(closure.Code)
                }
            }
        case RETURN:
            require(2)
            size, offset := stack.Popn()
            ret := mem.Get(offset.Int64(), size.Int64())

            self.Printf(" => (%d) 0x%x", len(ret), ret).Endl()

            return closure.Return(ret), nil
        case SUICIDE:
            require(1)

            receiver := self.env.State().GetOrNewStateObject(stack.Pop().Bytes())

            receiver.AddAmount(closure.object.Balance)

            closure.object.MarkForDeletion()

            fallthrough
        case STOP: // Stop the closure
            self.Endl()

            return closure.Return(nil), nil
        default:
            vmlogger.Debugf("(pc) %-3v Invalid opcode %x\n", pc, op)

            return closure.Return(nil), fmt.Errorf("Invalid opcode %x", op)
        }

        pc.Add(pc, ethutil.Big1)

        self.Endl()

        if self.Dbg != nil {
            for _, instrNo := range self.Dbg.BreakPoints() {
                if pc.Cmp(big.NewInt(instrNo)) == 0 {
                    self.Stepping = true

                    if !self.Dbg.BreakHook(prevStep, op, mem, stack, closure.Object()) {
                        return nil, nil
                    }
                } else if self.Stepping {
                    if !self.Dbg.StepHook(prevStep, op, mem, stack, closure.Object()) {
                        return nil, nil
                    }
                }
            }
        }

    }
}

func (self *Vm) Printf(format string, v ...interface{}) *Vm {
    if self.Verbose && self.logTy == LogTyPretty {
        self.logStr += fmt.Sprintf(format, v...)
    }

    return self
}

func (self *Vm) Endl() *Vm {
    if self.Verbose && self.logTy == LogTyPretty {
        vmlogger.Debugln(self.logStr)
        self.logStr = ""
    }

    return self
}

func ensure256(x *big.Int) {
    //max, _ := big.NewInt(0).SetString("115792089237316195423570985008687907853269984665640564039457584007913129639936", 0)
    //if x.Cmp(max) >= 0 {
    d := big.NewInt(1)
    d.Lsh(d, 256).Sub(d, big.NewInt(1))
    x.And(x, d)
    //}

    // Could have done this with an OR, but big ints are costly.

    if x.Cmp(new(big.Int)) < 0 {
        x.SetInt64(0)
    }
}