func getData(data []byte, start, size *big.Int) []byte {
	dlen := big.NewInt(int64(len(data)))

	s := common.BigMin(start, dlen)
	e := common.BigMin(new(big.Int).Add(s, size), dlen)
	return common.RightPadBytes(data[s.Uint64():e.Uint64()], int(size.Uint64()))
}

// CalcGasLimit computes the gas limit of the next block after parent.
// The result may be modified by the caller.
// This is miner strategy, not consensus protocol.
func CalcGasLimit(parent *types.Block) *big.Int {
	// contrib = (parentGasUsed * 3 / 2) / 1024
	contrib := new(big.Int).Mul(parent.GasUsed(), big.NewInt(3))
	contrib = contrib.Div(contrib, big.NewInt(2))
	contrib = contrib.Div(contrib, params.GasLimitBoundDivisor)

	// decay = parentGasLimit / 1024 -1
	decay := new(big.Int).Div(parent.GasLimit(), params.GasLimitBoundDivisor)
	decay.Sub(decay, big.NewInt(1))

	/*
		strategy: gasLimit of block-to-mine is set based on parent's
		gasUsed value.  if parentGasUsed > parentGasLimit * (2/3) then we
		increase it, otherwise lower it (or leave it unchanged if it's right
		at that usage) the amount increased/decreased depends on how far away
		from parentGasLimit * (2/3) parentGasUsed is.
	*/
	gl := new(big.Int).Sub(parent.GasLimit(), decay)
	gl = gl.Add(gl, contrib)
	gl.Set(common.BigMax(gl, params.MinGasLimit))

	// however, if we're now below the target (TargetGasLimit) we increase the
	// limit as much as we can (parentGasLimit / 1024 -1)
	if gl.Cmp(params.TargetGasLimit) < 0 {
		gl.Add(parent.GasLimit(), decay)
		gl.Set(common.BigMin(gl, params.TargetGasLimit))
	}
	return gl
}

func (self *StateTransition) refundGas() {
	sender, _ := self.From() // err already checked
	// Return remaining gas
	remaining := new(big.Int).Mul(self.gas, self.gasPrice)
	sender.AddBalance(remaining)

	uhalf := remaining.Div(self.gasUsed(), common.Big2)
	refund := common.BigMin(uhalf, self.state.Refunds())
	self.gas.Add(self.gas, refund)
	self.state.AddBalance(sender.Address(), refund.Mul(refund, self.gasPrice))

	self.gp.AddGas(self.gas, self.gasPrice)
}

func (self *StateTransition) refundGas() {
	coinbase, sender := self.Coinbase(), self.From()
	// Return remaining gas
	remaining := new(big.Int).Mul(self.gas, self.msg.GasPrice())
	sender.AddBalance(remaining)

	uhalf := new(big.Int).Div(self.gasUsed(), common.Big2)
	for addr, ref := range self.state.Refunds() {
		refund := common.BigMin(uhalf, ref)
		self.gas.Add(self.gas, refund)
		self.state.AddBalance(common.StringToAddress(addr), refund.Mul(refund, self.msg.GasPrice()))
	}

	coinbase.RefundGas(self.gas, self.msg.GasPrice())
}

func (self *StateTransition) refundGas() {
	// Return eth for remaining gas to the sender account,
	// exchanged at the original rate.
	sender, _ := self.from() // err already checked
	remaining := new(big.Int).Mul(self.gas, self.gasPrice)
	sender.AddBalance(remaining)

	// Apply refund counter, capped to half of the used gas.
	uhalf := remaining.Div(self.gasUsed(), common.Big2)
	refund := common.BigMin(uhalf, self.state.GetRefund())
	self.gas.Add(self.gas, refund)
	self.state.AddBalance(sender.Address(), refund.Mul(refund, self.gasPrice))

	// Also return remaining gas to the block gas counter so it is
	// available for the next transaction.
	self.gp.AddGas(self.gas)
}

// CalcGasLimit computes the gas limit of the next block after parent.
// The result may be modified by the caller.
func CalcGasLimit(parent *types.Block) *big.Int {
	decay := new(big.Int).Div(parent.GasLimit(), params.GasLimitBoundDivisor)
	contrib := new(big.Int).Mul(parent.GasUsed(), big.NewInt(3))
	contrib = contrib.Div(contrib, big.NewInt(2))
	contrib = contrib.Div(contrib, params.GasLimitBoundDivisor)

	gl := new(big.Int).Sub(parent.GasLimit(), decay)
	gl = gl.Add(gl, contrib)
	gl = gl.Add(gl, big.NewInt(1))
	gl.Set(common.BigMax(gl, params.MinGasLimit))

	if gl.Cmp(params.GenesisGasLimit) < 0 {
		gl.Add(parent.GasLimit(), decay)
		gl.Set(common.BigMin(gl, params.GenesisGasLimit))
	}
	return gl
}