var (
	// ErrRemoteStale is returned by the validator if a remote fork checksum is a
	// subset of our already applied forks, but the announced next fork block is
	// not on our already passed chain.
	ErrRemoteStale = errors.New("remote needs update")

	// ErrLocalIncompatibleOrStale is returned by the validator if a remote fork
	// checksum does not match any local checksum variation, signalling that the
	// two chains have diverged in the past at some point (possibly at genesis).
	ErrLocalIncompatibleOrStale = errors.New("local incompatible or needs update")
)

type Blockchain interface {
	// Config retrieves the chain's fork configuration.
	Config() *params.ChainConfig

	// Genesis retrieves the chain's genesis block.
	Genesis() *types.Block

	// CurrentHeader retrieves the current head header of the canonical chain.
	CurrentHeader() *types.Header
}

type Filter func(id ID) error

type ID struct {
	Hash [4]byte // CRC32 checksum of the genesis block and passed fork block numbers
	Next uint64  // Block number of the next upcoming fork, or 0 if no forks are known
}