package helpers

import (
	"errors"
	"fmt"
	"math"
	"math/bits"
	"time"

	stateTrie "github.com/prysmaticlabs/prysm/beacon-chain/state"
	"github.com/prysmaticlabs/prysm/shared/params"
	"github.com/prysmaticlabs/prysm/shared/roughtime"
)

// MaxSlotBuffer specifies the max buffer given to slots from
// incoming objects. (24 mins with mainnet spec)
const MaxSlotBuffer = uint64(1 << 7)

// SlotToEpoch returns the epoch number of the input slot.
//
// Spec pseudocode definition:
//  def compute_epoch_of_slot(slot: Slot) -> Epoch:
//    """
//    Return the epoch number of ``slot``.
//    """
//    return Epoch(slot // SLOTS_PER_EPOCH)
func SlotToEpoch(slot uint64) uint64 {
	return slot / params.BeaconConfig().SlotsPerEpoch
}

// CurrentEpoch returns the current epoch number calculated from
// the slot number stored in beacon state.
//
// Spec pseudocode definition:
//  def get_current_epoch(state: BeaconState) -> Epoch:
//    """
//    Return the current epoch.
//    """
//    return compute_epoch_of_slot(state.slot)
func CurrentEpoch(state *stateTrie.BeaconState) uint64 {
	return SlotToEpoch(state.Slot())
}

// PrevEpoch returns the previous epoch number calculated from
// the slot number stored in beacon state. It also checks for
// underflow condition.
//
// Spec pseudocode definition:
//  def get_previous_epoch(state: BeaconState) -> Epoch:
//    """`
//    Return the previous epoch (unless the current epoch is ``GENESIS_EPOCH``).
//    """
//    current_epoch = get_current_epoch(state)
//    return GENESIS_EPOCH if current_epoch == GENESIS_EPOCH else Epoch(current_epoch - 1)
func PrevEpoch(state *stateTrie.BeaconState) uint64 {
	currentEpoch := CurrentEpoch(state)
	if currentEpoch == 0 {
		return 0
	}
	return currentEpoch - 1
}

// NextEpoch returns the next epoch number calculated from
// the slot number stored in beacon state.
func NextEpoch(state *stateTrie.BeaconState) uint64 {
	return SlotToEpoch(state.Slot()) + 1
}

// StartSlot returns the first slot number of the
// current epoch.
//
// Spec pseudocode definition:
//  def compute_start_slot_at_epoch(epoch: Epoch) -> Slot:
//    """
//    Return the start slot of ``epoch``.
//    """
//    return Slot(epoch * SLOTS_PER_EPOCH)
func StartSlot(epoch uint64) (uint64, error) {
	overflows, slot := bits.Mul64(epoch, params.BeaconConfig().SlotsPerEpoch)
	if overflows > 0 {
		return slot, errors.New("start slot calculation overflows")
	}
	return slot, nil
}

// IsEpochStart returns true if the given slot number is an epoch starting slot
// number.
func IsEpochStart(slot uint64) bool {
	return slot%params.BeaconConfig().SlotsPerEpoch == 0
}

// IsEpochEnd returns true if the given slot number is an epoch ending slot
// number.
func IsEpochEnd(slot uint64) bool {
	return IsEpochStart(slot + 1)
}

// SlotsSinceEpochStarts returns number of slots since the start of the epoch.
func SlotsSinceEpochStarts(slot uint64) uint64 {
	return slot % params.BeaconConfig().SlotsPerEpoch
}

// VerifySlotTime validates the input slot is not from the future.
func VerifySlotTime(genesisTime uint64, slot uint64, timeTolerance time.Duration) error {
	slotTime, err := SlotToTime(genesisTime, slot)
	if err != nil {
		return err
	}

	maxPossibleSlot := CurrentSlot(genesisTime) + MaxSlotBuffer
	// Defensive check to ensure that we only process slots up to a hard limit
	// from our local clock.
	if slot > maxPossibleSlot {
		return fmt.Errorf("slot %d > %d which exceeds max allowed value relative to the local clock", slot, maxPossibleSlot)
	}

	currentTime := roughtime.Now()
	diff := slotTime.Sub(currentTime)

	if diff > timeTolerance {
		return fmt.Errorf("could not process slot from the future, slot time %s > current time %s", slotTime, currentTime)
	}
	return nil
}

// SlotToTime takes the given slot and genesis time to determine the start time of the slot.
func SlotToTime(genesisTimeSec uint64, slot uint64) (time.Time, error) {
	if slot >= math.MaxInt64/params.BeaconConfig().SecondsPerSlot {
		return time.Unix(0, 0), fmt.Errorf("slot (%d) is in the far distant future", slot)
	}
	timeSinceGenesis := slot * params.BeaconConfig().SecondsPerSlot
	return time.Unix(int64(genesisTimeSec+timeSinceGenesis), 0), nil
}

// SlotsSince computes the number of time slots that have occurred since the given timestamp.
func SlotsSince(time time.Time) uint64 {
	return uint64(roughtime.Since(time).Seconds()) / params.BeaconConfig().SecondsPerSlot
}

// CurrentSlot returns the current slot as determined by the local clock and
// provided genesis time.
func CurrentSlot(genesisTimeSec uint64) uint64 {
	now := roughtime.Now().Unix()
	genesis := int64(genesisTimeSec)
	if now < genesis {
		return 0
	}
	return uint64(now-genesis) / params.BeaconConfig().SecondsPerSlot
}

// GenesisTime prioritizes to return the genesis time in config unless the config
// genesis time is 0. Then it returns the genesis time in state.
func GenesisTime(state *stateTrie.BeaconState) uint64 {
	if params.BeaconConfig().GenesisTime == 0 {
		return state.GenesisTime()
	}
	return params.BeaconConfig().GenesisTime
}

// RoundUpToNearestEpoch rounds up the provided slot value to the nearest epoch.
func RoundUpToNearestEpoch(slot uint64) uint64 {
	if slot%params.BeaconConfig().SlotsPerEpoch != 0 {
		slot -= slot % params.BeaconConfig().SlotsPerEpoch
		slot += params.BeaconConfig().SlotsPerEpoch
	}
	return slot
}