package helpers

import (
	"math"

	"github.com/pkg/errors"
	types "github.com/prysmaticlabs/eth2-types"
	iface "github.com/prysmaticlabs/prysm/beacon-chain/state/interface"
	"github.com/prysmaticlabs/prysm/shared/interfaces"
	"github.com/prysmaticlabs/prysm/shared/params"
)

// VerifyNilBeaconBlock checks if any composite field of input signed beacon block is nil.
// Access to these nil fields will result in run time panic,
// it is recommended to run these checks as first line of defense.
func VerifyNilBeaconBlock(b interfaces.SignedBeaconBlock) error {
	if b == nil || b.IsNil() {
		return errors.New("signed beacon block can't be nil")
	}
	if b.Block().IsNil() {
		return errors.New("beacon block can't be nil")
	}
	if b.Block().Body().IsNil() {
		return errors.New("beacon block body can't be nil")
	}
	return nil
}

// BlockRootAtSlot returns the block root stored in the BeaconState for a recent slot.
// It returns an error if the requested block root is not within the slot range.
//
// Spec pseudocode definition:
//  def get_block_root_at_slot(state: BeaconState, slot: Slot) -> Root:
//    """
//    Return the block root at a recent ``slot``.
//    """
//    assert slot < state.slot <= slot + SLOTS_PER_HISTORICAL_ROOT
//    return state.block_roots[slot % SLOTS_PER_HISTORICAL_ROOT]
func BlockRootAtSlot(state iface.ReadOnlyBeaconState, slot types.Slot) ([]byte, error) {
	if math.MaxUint64-slot < params.BeaconConfig().SlotsPerHistoricalRoot {
		return []byte{}, errors.New("slot overflows uint64")
	}
	if slot >= state.Slot() || state.Slot() > slot+params.BeaconConfig().SlotsPerHistoricalRoot {
		return []byte{}, errors.Errorf("slot %d out of bounds", slot)
	}
	return state.BlockRootAtIndex(uint64(slot % params.BeaconConfig().SlotsPerHistoricalRoot))
}

// StateRootAtSlot returns the cached state root at that particular slot. If no state
// root has been cached it will return a zero-hash.
func StateRootAtSlot(state iface.ReadOnlyBeaconState, slot types.Slot) ([]byte, error) {
	if slot >= state.Slot() || state.Slot() > slot+params.BeaconConfig().SlotsPerHistoricalRoot {
		return []byte{}, errors.Errorf("slot %d out of bounds", slot)
	}
	return state.StateRootAtIndex(uint64(slot % params.BeaconConfig().SlotsPerHistoricalRoot))
}

// BlockRoot returns the block root stored in the BeaconState for epoch start slot.
//
// Spec pseudocode definition:
//  def get_block_root(state: BeaconState, epoch: Epoch) -> Root:
//    """
//    Return the block root at the start of a recent ``epoch``.
//    """
//    return get_block_root_at_slot(state, compute_start_slot_at_epoch(epoch))
func BlockRoot(state iface.ReadOnlyBeaconState, epoch types.Epoch) ([]byte, error) {
	s, err := StartSlot(epoch)
	if err != nil {
		return nil, err
	}
	return BlockRootAtSlot(state, s)
}