package engine_v1

import (
	"errors"

	"github.com/ethereum/go-ethereum/common"
	"github.com/ethereum/go-ethereum/consensus/XDPoS/utils"
	"github.com/ethereum/go-ethereum/core/types"
	"github.com/ethereum/go-ethereum/params"
)

// decodeMasternodesFromHeaderExtra extracts masternodes from checkpoint header extra data.
func decodeMasternodesFromHeaderExtra(checkpointHeader *types.Header) []common.Address {
	extraVanity := 32
	extraSeal := 65
	masternodes := make([]common.Address, (len(checkpointHeader.Extra)-extraVanity-extraSeal)/common.AddressLength)
	for i := 0; i < len(masternodes); i++ {
		copy(masternodes[i][:], checkpointHeader.Extra[extraVanity+i*common.AddressLength:])
	}
	return masternodes
}

// getM1M2FromCheckpointHeader extracts the M1->M2 mapping from a checkpoint header.
// It validates that the header is a checkpoint block and returns the mapping.
func getM1M2FromCheckpointHeader(checkpointHeader *types.Header, currentHeader *types.Header, config *params.ChainConfig) (map[common.Address]common.Address, error) {
	if checkpointHeader.Number.Uint64()%common.EpocBlockRandomize != 0 {
		return nil, errors.New("this block is not checkpoint block")
	}
	// Get signers from this block.
	masternodes := decodeMasternodesFromHeaderExtra(checkpointHeader)
	validators := utils.ExtractValidatorsFromBytes(checkpointHeader.Validators)
	m1m2, _, err := getM1M2(masternodes, validators, currentHeader, config)
	if err != nil {
		return map[common.Address]common.Address{}, err
	}
	return m1m2, nil
}

// getM1M2 calculates the M1->M2 mapping given masternodes and validator indices.
func getM1M2(masternodes []common.Address, validators []int64, currentHeader *types.Header, config *params.ChainConfig) (map[common.Address]common.Address, uint64, error) {
	m1m2 := map[common.Address]common.Address{}
	maxMNs := len(masternodes)
	moveM2 := uint64(0)
	if len(validators) < maxMNs {
		return nil, moveM2, errors.New("len(m2) is less than len(m1)")
	}
	if maxMNs > 0 {
		isForked := config.IsTIPRandomize(currentHeader.Number)
		if isForked {
			moveM2 = ((currentHeader.Number.Uint64() % config.XDPoS.Epoch) / uint64(maxMNs)) % uint64(maxMNs)
		}
		for i, m1 := range masternodes {
			m2Index := uint64(validators[i] % int64(maxMNs))
			m2Index = (m2Index + moveM2) % uint64(maxMNs)
			m1m2[m1] = masternodes[m2Index]
		}
	}
	return m1m2, moveM2, nil
}