// Copyright 2021 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.

package misc

import (
	"fmt"

	"github.com/autonity/autonity/params"
)

// VerifyGaslimit verifies the header gas limit according increase/decrease
// in relation to the parent gas limit.
func VerifyGaslimit(parentGasLimit, headerGasLimit, gasLimitBoundDivisor uint64) error {
	// Verify that the gas limit remains within allowed bounds
	diff := int64(parentGasLimit) - int64(headerGasLimit)
	if diff < 0 {
		diff *= -1
	}
	limit := parentGasLimit / gasLimitBoundDivisor

	// standard case, make sure that the diff is not >= limit
	if parentGasLimit >= gasLimitBoundDivisor && uint64(diff) >= limit {
		return fmt.Errorf("invalid gas limit: have %d, want %d +-= %d", headerGasLimit, parentGasLimit, limit-1)
	}

	// extreme edge case, should not happen but let's handle it gracefully
	// in this case we allow diff to be = limit (0)
	if parentGasLimit < gasLimitBoundDivisor && uint64(diff) != 0 {
		return fmt.Errorf("invalid gas limit: have %d, want %d", headerGasLimit, parentGasLimit)
	}

	// sanity check
	if headerGasLimit < params.MinGasLimit {
		return fmt.Errorf("invalid gas limit (%d) below %d", headerGasLimit, params.MinGasLimit)
	}
	return nil
}
