package paychmgr

import (
	"context"
	"errors"
	"fmt"

	"github.com/ipfs/go-cid"
	"golang.org/x/xerrors"

	"github.com/filecoin-project/go-address"
	cborutil "github.com/filecoin-project/go-cbor-util"
	actorstypes "github.com/filecoin-project/go-state-types/actors"
	"github.com/filecoin-project/go-state-types/big"
	"github.com/filecoin-project/go-state-types/builtin/v8/paych"

	"github.com/filecoin-project/lotus/api"
	lpaych "github.com/filecoin-project/lotus/chain/actors/builtin/paych"
	"github.com/filecoin-project/lotus/chain/types"
	"github.com/filecoin-project/lotus/lib/sigs"
)

// insufficientFundsErr indicates that there are not enough funds in the
// channel to create a voucher
type insufficientFundsErr interface {
	Shortfall() types.BigInt
}

type ErrInsufficientFunds struct {
	shortfall types.BigInt
}

func newErrInsufficientFunds(shortfall types.BigInt) *ErrInsufficientFunds {
	return &ErrInsufficientFunds{shortfall: shortfall}
}

func (e *ErrInsufficientFunds) Error() string {
	return fmt.Sprintf("not enough funds in channel to cover voucher - shortfall: %d", e.shortfall)
}

func (e *ErrInsufficientFunds) Shortfall() types.BigInt {
	return e.shortfall
}

type laneState struct {
	redeemed big.Int
	nonce    uint64
}

func (ls laneState) Redeemed() (big.Int, error) {
	return ls.redeemed, nil
}

func (ls laneState) Nonce() (uint64, error) {
	return ls.nonce, nil
}

// channelAccessor is used to simplify locking when accessing a channel
type channelAccessor struct {
	from address.Address
	to   address.Address

	// chctx is used by background processes (eg when waiting for things to be
	// confirmed on chain)
	chctx         context.Context
	sa            *stateAccessor
	api           managerAPI
	store         *Store
	lk            *channelLock
	fundsReqQueue []*fundsReq
	msgListeners  msgListeners
}

func newChannelAccessor(pm *Manager, from address.Address, to address.Address) *channelAccessor {
	return &channelAccessor{
		from:         from,
		to:           to,
		chctx:        pm.ctx,
		sa:           pm.sa,
		api:          pm.pchapi,
		store:        pm.store,
		lk:           &channelLock{globalLock: &pm.lk},
		msgListeners: newMsgListeners(),
	}
}

func (ca *channelAccessor) messageBuilder(ctx context.Context, from address.Address) (lpaych.MessageBuilder, error) {
	nwVersion, err := ca.api.StateNetworkVersion(ctx, types.EmptyTSK)
	if err != nil {
		return nil, err
	}

	av, err := actorstypes.VersionForNetwork(nwVersion)
	if err != nil {
		return nil, err
	}
	return lpaych.Message(av, from), nil
}

func (ca *channelAccessor) getChannelInfo(ctx context.Context, addr address.Address) (*ChannelInfo, error) {
	ca.lk.Lock()
	defer ca.lk.Unlock()

	return ca.store.ByAddress(ctx, addr)
}

func (ca *channelAccessor) outboundActiveByFromTo(ctx context.Context, from, to address.Address) (*ChannelInfo, error) {
	ca.lk.Lock()
	defer ca.lk.Unlock()

	return ca.store.OutboundActiveByFromTo(ctx, ca.api, from, to)
}

// createVoucher creates a voucher with the given specification, setting its
// nonce, signing the voucher and storing it in the local datastore.
// If there are not enough funds in the channel to create the voucher, returns
// the shortfall in funds.
func (ca *channelAccessor) createVoucher(ctx context.Context, ch address.Address, voucher paych.SignedVoucher) (*api.VoucherCreateResult, error) {
	ca.lk.Lock()
	defer ca.lk.Unlock()

	// Find the channel for the voucher
	ci, err := ca.store.ByAddress(ctx, ch)
	if err != nil {
		return nil, xerrors.Errorf("failed to get channel info by address: %w", err)
	}

	// Set the voucher channel
	sv := &voucher
	sv.ChannelAddr = ch

	// Get the next nonce on the given lane
	sv.Nonce = ca.nextNonceForLane(ci, voucher.Lane)

	// Sign the voucher
	vb, err := sv.SigningBytes()
	if err != nil {
		return nil, xerrors.Errorf("failed to get voucher signing bytes: %w", err)
	}

	sig, err := ca.api.WalletSign(ctx, ci.Control, vb)
	if err != nil {
		return nil, xerrors.Errorf("failed to sign voucher: %w", err)
	}
	sv.Signature = sig

	// Store the voucher
	if _, err := ca.addVoucherUnlocked(ctx, ch, sv, types.NewInt(0)); err != nil {
		// If there are not enough funds in the channel to cover the voucher,
		// return a voucher create result with the shortfall
		var ife insufficientFundsErr
		if errors.As(err, &ife) {
			return &api.VoucherCreateResult{
				Shortfall: ife.Shortfall(),
			}, nil
		}

		return nil, xerrors.Errorf("failed to persist voucher: %w", err)
	}

	return &api.VoucherCreateResult{Voucher: sv, Shortfall: types.NewInt(0)}, nil
}

func (ca *channelAccessor) nextNonceForLane(ci *ChannelInfo, lane uint64) uint64 {
	var maxnonce uint64
	for _, v := range ci.Vouchers {
		if v.Voucher.Lane == lane {
			if v.Voucher.Nonce > maxnonce {
				maxnonce = v.Voucher.Nonce
			}
		}
	}

	return maxnonce + 1
}

func (ca *channelAccessor) checkVoucherValid(ctx context.Context, ch address.Address, sv *paych.SignedVoucher) (map[uint64]lpaych.LaneState, error) {
	ca.lk.Lock()
	defer ca.lk.Unlock()

	return ca.checkVoucherValidUnlocked(ctx, ch, sv)
}

func (ca *channelAccessor) checkVoucherValidUnlocked(ctx context.Context, ch address.Address, sv *paych.SignedVoucher) (map[uint64]lpaych.LaneState, error) {
	if sv.ChannelAddr != ch {
		return nil, xerrors.Errorf("voucher ChannelAddr doesn't match channel address, got %s, expected %s", sv.ChannelAddr, ch)
	}

	// check voucher is unlocked
	if sv.Extra != nil {
		return nil, xerrors.Errorf("voucher is Message Locked")
	}
	if sv.TimeLockMax != 0 {
		return nil, xerrors.Errorf("voucher is Max Time Locked")
	}
	if sv.TimeLockMin != 0 {
		return nil, xerrors.Errorf("voucher is Min Time Locked")
	}
	if len(sv.SecretHash) != 0 {
		return nil, xerrors.Errorf("voucher is Hash Locked")
	}

	// Load payment channel actor state
	act, pchState, err := ca.sa.loadPaychActorState(ctx, ch)
	if err != nil {
		return nil, err
	}

	// Load channel "From" account actor state
	f, err := pchState.From()
	if err != nil {
		return nil, err
	}

	from, err := ca.api.ResolveToDeterministicAddress(ctx, f, nil)
	if err != nil {
		return nil, err
	}

	// verify voucher signature
	vb, err := sv.SigningBytes()
	if err != nil {
		return nil, err
	}

	// TODO: technically, either party may create and sign a voucher.
	// However, for now, we only accept them from the channel creator.
	// More complex handling logic can be added later
	if err := sigs.Verify(sv.Signature, from, vb); err != nil {
		return nil, err
	}

	// Check the voucher against the highest known voucher nonce / value
	laneStates, err := ca.laneState(ctx, pchState, ch)
	if err != nil {
		return nil, err
	}

	// If the new voucher nonce value is less than the highest known
	// nonce for the lane
	ls, lsExists := laneStates[sv.Lane]
	if lsExists {
		n, err := ls.Nonce()
		if err != nil {
			return nil, err
		}

		if sv.Nonce <= n {
			return nil, fmt.Errorf("nonce too low")
		}

		// If the voucher amount is less than the highest known voucher amount
		r, err := ls.Redeemed()
		if err != nil {
			return nil, err
		}
		if sv.Amount.LessThanEqual(r) {
			return nil, fmt.Errorf("voucher amount is lower than amount for voucher with lower nonce")
		}
	}

	// Total redeemed is the total redeemed amount for all lanes, including
	// the new voucher
	// eg
	//
	// lane 1 redeemed:            3
	// lane 2 redeemed:            2
	// voucher for lane 1:         5
	//
	// Voucher supersedes lane 1 redeemed, therefore
	// effective lane 1 redeemed:  5
	//
	// lane 1:  5
	// lane 2:  2
	//          -
	// total:   7
	totalRedeemed, err := ca.totalRedeemedWithVoucher(laneStates, sv)
	if err != nil {
		return nil, err
	}

	// Total required balance must not exceed actor balance
	if act.Balance.LessThan(totalRedeemed) {
		return nil, newErrInsufficientFunds(types.BigSub(totalRedeemed, act.Balance))
	}

	if len(sv.Merges) != 0 {
		return nil, fmt.Errorf("dont currently support paych lane merges")
	}

	return laneStates, nil
}

func (ca *channelAccessor) checkVoucherSpendable(ctx context.Context, ch address.Address, sv *paych.SignedVoucher, secret []byte) (bool, error) {
	ca.lk.Lock()
	defer ca.lk.Unlock()

	recipient, err := ca.getPaychRecipient(ctx, ch)
	if err != nil {
		return false, err
	}

	ci, err := ca.store.ByAddress(ctx, ch)
	if err != nil {
		return false, err
	}

	// Check if voucher has already been submitted
	submitted, err := ci.wasVoucherSubmitted(sv)
	if err != nil {
		return false, err
	}
	if submitted {
		return false, nil
	}

	mb, err := ca.messageBuilder(ctx, recipient)
	if err != nil {
		return false, err
	}

	mes, err := mb.Update(ch, sv, secret)
	if err != nil {
		return false, err
	}

	ret, err := ca.api.Call(ctx, mes, nil)
	if err != nil {
		return false, err
	}

	if ret.MsgRct.ExitCode != 0 {
		return false, nil
	}

	return true, nil
}

func (ca *channelAccessor) getPaychRecipient(ctx context.Context, ch address.Address) (address.Address, error) {
	_, state, err := ca.api.GetPaychState(ctx, ch, nil)
	if err != nil {
		return address.Address{}, err
	}

	return state.To()
}

func (ca *channelAccessor) addVoucher(ctx context.Context, ch address.Address, sv *paych.SignedVoucher, minDelta types.BigInt) (types.BigInt, error) {
	ca.lk.Lock()
	defer ca.lk.Unlock()

	return ca.addVoucherUnlocked(ctx, ch, sv, minDelta)
}

func (ca *channelAccessor) addVoucherUnlocked(ctx context.Context, ch address.Address, sv *paych.SignedVoucher, minDelta types.BigInt) (types.BigInt, error) {
	ci, err := ca.store.ByAddress(ctx, ch)
	if err != nil {
		return types.BigInt{}, err
	}

	// Check if the voucher has already been added
	for _, v := range ci.Vouchers {
		eq, err := cborutil.Equals(sv, v.Voucher)
		if err != nil {
			return types.BigInt{}, err
		}
		if eq {
			// Ignore the duplicate voucher.
			log.Warnf("AddVoucher: voucher re-added")
			return types.NewInt(0), nil
		}

	}

	// Check voucher validity
	laneStates, err := ca.checkVoucherValidUnlocked(ctx, ch, sv)
	if err != nil {
		return types.NewInt(0), err
	}

	// The change in value is the delta between the voucher amount and
	// the highest previous voucher amount for the lane
	laneState, exists := laneStates[sv.Lane]
	redeemed := big.NewInt(0)
	if exists {
		redeemed, err = laneState.Redeemed()
		if err != nil {
			return types.NewInt(0), err
		}
	}

	delta := types.BigSub(sv.Amount, redeemed)
	if minDelta.GreaterThan(delta) {
		return delta, xerrors.Errorf("addVoucher: supplied token amount too low; minD=%s, D=%s; laneAmt=%s; v.Amt=%s", minDelta, delta, redeemed, sv.Amount)
	}

	ci.Vouchers = append(ci.Vouchers, &VoucherInfo{
		Voucher: sv,
	})

	if ci.NextLane <= sv.Lane {
		ci.NextLane = sv.Lane + 1
	}

	return delta, ca.store.putChannelInfo(ctx, ci)
}

func (ca *channelAccessor) submitVoucher(ctx context.Context, ch address.Address, sv *paych.SignedVoucher, secret []byte) (cid.Cid, error) {
	ca.lk.Lock()
	defer ca.lk.Unlock()

	ci, err := ca.store.ByAddress(ctx, ch)
	if err != nil {
		return cid.Undef, err
	}

	has, err := ci.hasVoucher(sv)
	if err != nil {
		return cid.Undef, err
	}

	// If the channel has the voucher
	if has {
		// Check that the voucher hasn't already been submitted
		submitted, err := ci.wasVoucherSubmitted(sv)
		if err != nil {
			return cid.Undef, err
		}
		if submitted {
			return cid.Undef, xerrors.Errorf("cannot submit voucher that has already been submitted")
		}
	}

	mb, err := ca.messageBuilder(ctx, ci.Control)
	if err != nil {
		return cid.Undef, err
	}

	msg, err := mb.Update(ch, sv, secret)
	if err != nil {
		return cid.Undef, err
	}

	smsg, err := ca.api.MpoolPushMessage(ctx, msg, nil)
	if err != nil {
		return cid.Undef, err
	}

	// If the channel didn't already have the voucher
	if !has {
		// Add the voucher to the channel
		ci.Vouchers = append(ci.Vouchers, &VoucherInfo{
			Voucher: sv,
		})
	}

	// Mark the voucher and any lower-nonce vouchers as having been submitted
	err = ca.store.MarkVoucherSubmitted(ctx, ci, sv)
	if err != nil {
		return cid.Undef, err
	}

	return smsg.Cid(), nil
}

func (ca *channelAccessor) allocateLane(ctx context.Context, ch address.Address) (uint64, error) {
	ca.lk.Lock()
	defer ca.lk.Unlock()

	return ca.store.AllocateLane(ctx, ch)
}

func (ca *channelAccessor) listVouchers(ctx context.Context, ch address.Address) ([]*VoucherInfo, error) {
	ca.lk.Lock()
	defer ca.lk.Unlock()

	// TODO: just having a passthrough method like this feels odd. Seems like
	// there should be some filtering we're doing here
	return ca.store.VouchersForPaych(ctx, ch)
}

// laneState gets the LaneStates from chain, then applies all vouchers in
// the data store over the chain state
func (ca *channelAccessor) laneState(ctx context.Context, state lpaych.State, ch address.Address) (map[uint64]lpaych.LaneState, error) {
	// TODO: we probably want to call UpdateChannelState with all vouchers to be fully correct
	//  (but technically don't need to)

	laneCount, err := state.LaneCount()
	if err != nil {
		return nil, err
	}

	// Note: we use a map instead of an array to store laneStates because the
	// client sets the lane ID (the index) and potentially they could use a
	// very large index.
	laneStates := make(map[uint64]lpaych.LaneState, laneCount)
	err = state.ForEachLaneState(func(idx uint64, ls lpaych.LaneState) error {
		laneStates[idx] = ls
		return nil
	})
	if err != nil {
		return nil, err
	}

	// Apply locally stored vouchers
	vouchers, err := ca.store.VouchersForPaych(ctx, ch)
	if err != nil && err != ErrChannelNotTracked {
		return nil, err
	}

	for _, v := range vouchers {
		for range v.Voucher.Merges {
			return nil, xerrors.Errorf("paych merges not handled yet")
		}

		// Check if there is an existing laneState in the payment channel
		// for this voucher's lane
		ls, ok := laneStates[v.Voucher.Lane]

		// If the voucher does not have a higher nonce than the existing
		// laneState for this lane, ignore it
		if ok {
			n, err := ls.Nonce()
			if err != nil {
				return nil, err
			}
			if v.Voucher.Nonce < n {
				continue
			}
		}

		// Voucher has a higher nonce, so replace laneState with this voucher
		laneStates[v.Voucher.Lane] = laneState{v.Voucher.Amount, v.Voucher.Nonce}
	}

	return laneStates, nil
}

// Get the total redeemed amount across all lanes, after applying the voucher
func (ca *channelAccessor) totalRedeemedWithVoucher(laneStates map[uint64]lpaych.LaneState, sv *paych.SignedVoucher) (big.Int, error) {
	// TODO: merges
	if len(sv.Merges) != 0 {
		return big.Int{}, xerrors.Errorf("dont currently support paych lane merges")
	}

	total := big.NewInt(0)
	for _, ls := range laneStates {
		r, err := ls.Redeemed()
		if err != nil {
			return big.Int{}, err
		}
		total = big.Add(total, r)
	}

	lane, ok := laneStates[sv.Lane]
	if ok {
		// If the voucher is for an existing lane, and the voucher nonce
		// is higher than the lane nonce
		n, err := lane.Nonce()
		if err != nil {
			return big.Int{}, err
		}

		if sv.Nonce > n {
			// Add the delta between the redeemed amount and the voucher
			// amount to the total
			r, err := lane.Redeemed()
			if err != nil {
				return big.Int{}, err
			}

			delta := big.Sub(sv.Amount, r)
			total = big.Add(total, delta)
		}
	} else {
		// If the voucher is *not* for an existing lane, just add its
		// value (implicitly a new lane will be created for the voucher)
		total = big.Add(total, sv.Amount)
	}

	return total, nil
}

func (ca *channelAccessor) settle(ctx context.Context, ch address.Address) (cid.Cid, error) {
	ca.lk.Lock()
	defer ca.lk.Unlock()

	ci, err := ca.store.ByAddress(ctx, ch)
	if err != nil {
		return cid.Undef, err
	}

	mb, err := ca.messageBuilder(ctx, ci.Control)
	if err != nil {
		return cid.Undef, err
	}
	msg, err := mb.Settle(ch)
	if err != nil {
		return cid.Undef, err
	}
	smgs, err := ca.api.MpoolPushMessage(ctx, msg, nil)
	if err != nil {
		return cid.Undef, err
	}

	ci.Settling = true
	err = ca.store.putChannelInfo(ctx, ci)
	if err != nil {
		log.Errorf("Error marking channel as settled: %s", err)
	}

	return smgs.Cid(), err
}

func (ca *channelAccessor) collect(ctx context.Context, ch address.Address) (cid.Cid, error) {
	ca.lk.Lock()
	defer ca.lk.Unlock()

	ci, err := ca.store.ByAddress(ctx, ch)
	if err != nil {
		return cid.Undef, err
	}

	mb, err := ca.messageBuilder(ctx, ci.Control)
	if err != nil {
		return cid.Undef, err
	}

	msg, err := mb.Collect(ch)
	if err != nil {
		return cid.Undef, err
	}

	smsg, err := ca.api.MpoolPushMessage(ctx, msg, nil)
	if err != nil {
		return cid.Undef, err
	}

	return smsg.Cid(), nil
}

type SignedVoucher struct {
	// ChannelAddr is the address of the payment channel this signed voucher is valid for
	ChannelAddr addr.Address
	// TimeLockMin sets a min epoch before which the voucher cannot be redeemed
	TimeLockMin abi.ChainEpoch
	// TimeLockMax sets a max epoch beyond which the voucher cannot be redeemed
	// TimeLockMax set to 0 means no timeout
	TimeLockMax abi.ChainEpoch
	// (optional) The SecretPreImage is used by `To` to validate
	SecretPreimage []byte
	// (optional) Extra can be specified by `From` to add a verification method to the voucher
	Extra *ModVerifyParams
	// Specifies which lane the Voucher merges into (will be created if does not exist)
	Lane uint64
	// Nonce is set by `From` to prevent redemption of stale vouchers on a lane
	Nonce uint64
	// Amount voucher can be redeemed for
	Amount big.Int
	// (optional) MinSettleHeight can extend channel MinSettleHeight if needed
	MinSettleHeight abi.ChainEpoch

	// (optional) Set of lanes to be merged into `Lane`
	Merges []Merge

	// Sender's signature over the voucher
	Signature *crypto.Signature
}

package paych

import (
	"bytes"

	addr "github.com/filecoin-project/go-address"
	"github.com/filecoin-project/go-state-types/abi"
	"github.com/filecoin-project/go-state-types/big"
	"github.com/filecoin-project/go-state-types/cbor"
	"github.com/filecoin-project/go-state-types/exitcode"
	paych0 "github.com/filecoin-project/specs-actors/actors/builtin/paych"
	paych7 "github.com/filecoin-project/specs-actors/v7/actors/builtin/paych"

	"github.com/ipfs/go-cid"

	"github.com/filecoin-project/specs-actors/v8/actors/builtin"
	"github.com/filecoin-project/specs-actors/v8/actors/runtime"
	"github.com/filecoin-project/specs-actors/v8/actors/util/adt"
)

const (
	ErrChannelStateUpdateAfterSettled = exitcode.FirstActorSpecificExitCode + iota
)

type Actor struct{}

func (a Actor) Exports() []interface{} {
	return []interface{}{
		builtin.MethodConstructor: a.Constructor,
		2:                         a.UpdateChannelState,
		3:                         a.Settle,
		4:                         a.Collect,
	}
}

func (a Actor) Code() cid.Cid {
	return builtin.PaymentChannelActorCodeID
}

func (a Actor) State() cbor.Er {
	return new(State)
}

var _ runtime.VMActor = Actor{}

//type ConstructorParams struct {
//	From addr.Address // Payer
//	To   addr.Address // Payee
//}
type ConstructorParams = paych0.ConstructorParams

// Constructor creates a payment channel actor. See State for meaning of params.
func (pca *Actor) Constructor(rt runtime.Runtime, params *ConstructorParams) *abi.EmptyValue {
	// Only InitActor can create a payment channel actor. It creates the actor on
	// behalf of the payer/payee.
	rt.ValidateImmediateCallerType(builtin.InitActorCodeID)

	// check that both parties are capable of signing vouchers
	to, err := pca.resolveAccount(rt, params.To)
	builtin.RequireNoErr(rt, err, exitcode.Unwrap(err, exitcode.ErrIllegalState), "failed to resolve to address: %s", params.To)
	from, err := pca.resolveAccount(rt, params.From)
	builtin.RequireNoErr(rt, err, exitcode.Unwrap(err, exitcode.ErrIllegalState), "failed to resolve from address: %s", params.From)

	emptyArr, err := adt.MakeEmptyArray(adt.AsStore(rt), LaneStatesAmtBitwidth)
	builtin.RequireNoErr(rt, err, exitcode.ErrIllegalState, "failed to create empty array")
	emptyArrCid, err := emptyArr.Root()
	builtin.RequireNoErr(rt, err, exitcode.ErrIllegalState, "failed to persist empty array")

	st := ConstructState(from, to, emptyArrCid)
	rt.StateCreate(st)

	return nil
}

// Resolves an address to a canonical ID address and requires it to address an account actor.
func (pca *Actor) resolveAccount(rt runtime.Runtime, raw addr.Address) (addr.Address, error) {
	resolved, err := builtin.ResolveToIDAddr(rt, raw)
	if err != nil {
		return addr.Undef, exitcode.ErrIllegalState.Wrapf("failed to resolve address %v: %w", raw, err)
	}

	codeCID, ok := rt.GetActorCodeCID(resolved)
	if !ok {
		return addr.Undef, exitcode.ErrIllegalArgument.Wrapf("no code for address %v", resolved)
	}
	if codeCID != builtin.AccountActorCodeID {
		return addr.Undef, exitcode.ErrForbidden.Wrapf("actor %v must be an account (%v), was %v", raw,
			builtin.AccountActorCodeID, codeCID)
	}

	return resolved, nil
}

////////////////////////////////////////////////////////////////////////////////
// Payment Channel state operations
////////////////////////////////////////////////////////////////////////////////

type UpdateChannelStateParams = paych7.UpdateChannelStateParams
type SignedVoucher = paych7.SignedVoucher

func VoucherSigningBytes(t *SignedVoucher) ([]byte, error) {
	osv := *t
	osv.Signature = nil

	buf := new(bytes.Buffer)
	if err := osv.MarshalCBOR(buf); err != nil {
		return nil, err
	}

	return buf.Bytes(), nil
}

// Modular Verification method
//type ModVerifyParams struct {
//	// Actor on which to invoke the method.
//	Actor addr.Address
//	// Method to invoke.
//	Method abi.MethodNum
//	// Pre-serialized method parameters.
//	Params []byte
//}
type ModVerifyParams = paych0.ModVerifyParams

// Specifies which `Lane`s to be merged with what `Nonce` on channelUpdate
//type Merge struct {
//	Lane  uint64
//	Nonce uint64
//}
type Merge = paych0.Merge

func (pca Actor) UpdateChannelState(rt runtime.Runtime, params *UpdateChannelStateParams) *abi.EmptyValue {
	var st State
	rt.StateReadonly(&st)

	// both parties must sign voucher: one who submits it, the other explicitly signs it
	rt.ValidateImmediateCallerIs(st.From, st.To)
	var signer addr.Address
	if rt.Caller() == st.From {
		signer = st.To
	} else {
		signer = st.From
	}
	sv := params.Sv

	if sv.Signature == nil {
		rt.Abortf(exitcode.ErrIllegalArgument, "voucher has no signature")
	}

	if st.SettlingAt != 0 && rt.CurrEpoch() >= st.SettlingAt {
		rt.Abortf(ErrChannelStateUpdateAfterSettled, "no vouchers can be processed after SettlingAt epoch")
	}

	if len(params.Secret) > MaxSecretSize {
		rt.Abortf(exitcode.ErrIllegalArgument, "secret must be at most 256 bytes long")
	}

	vb, err := VoucherSigningBytes(&sv)
	builtin.RequireNoErr(rt, err, exitcode.ErrIllegalArgument, "failed to serialize signedvoucher")

	err = rt.VerifySignature(*sv.Signature, signer, vb)
	builtin.RequireNoErr(rt, err, exitcode.ErrIllegalArgument, "voucher signature invalid")

	pchAddr := rt.Receiver()
	svpchIDAddr, found := rt.ResolveAddress(sv.ChannelAddr)
	if !found {
		rt.Abortf(exitcode.ErrIllegalArgument, "voucher payment channel address %s does not resolve to an ID address", sv.ChannelAddr)
	}
	if pchAddr != svpchIDAddr {
		rt.Abortf(exitcode.ErrIllegalArgument, "voucher payment channel address %s does not match receiver %s", svpchIDAddr, pchAddr)
	}

	if rt.CurrEpoch() < sv.TimeLockMin {
		rt.Abortf(exitcode.ErrIllegalArgument, "cannot use this voucher yet!")
	}

	if sv.TimeLockMax != 0 && rt.CurrEpoch() > sv.TimeLockMax {
		rt.Abortf(exitcode.ErrIllegalArgument, "this voucher has expired!")
	}

	if sv.Amount.Sign() < 0 {
		rt.Abortf(exitcode.ErrIllegalArgument, "voucher amount must be non-negative, was %v", sv.Amount)
	}

	if len(sv.SecretHash) > 0 {
		hashedSecret := rt.HashBlake2b(params.Secret)
		if !bytes.Equal(hashedSecret[:], sv.SecretHash) {
			rt.Abortf(exitcode.ErrIllegalArgument, "incorrect secret!")
		}
	}

	if sv.Extra != nil {

		code := rt.Send(
			sv.Extra.Actor,
			sv.Extra.Method,
			builtin.CBORBytes(sv.Extra.Data),
			abi.NewTokenAmount(0),
			&builtin.Discard{},
		)
		builtin.RequireSuccess(rt, code, "spend voucher verification failed")
	}

	rt.StateTransaction(&st, func() {
		laneFound := true

		lstates, err := adt.AsArray(adt.AsStore(rt), st.LaneStates, LaneStatesAmtBitwidth)
		builtin.RequireNoErr(rt, err, exitcode.ErrIllegalState, "failed to load lanes")

		// Find the voucher lane, creating if necessary.
		laneId := sv.Lane
		laneState := findLane(rt, lstates, sv.Lane)

		if laneState == nil {
			laneState = &LaneState{
				Redeemed: big.Zero(),
				Nonce:    0,
			}
			laneFound = false
		}

		if laneFound {
			if laneState.Nonce >= sv.Nonce {
				rt.Abortf(exitcode.ErrIllegalArgument, "voucher has an outdated nonce, existing nonce: %d, voucher nonce: %d, cannot redeem",
					laneState.Nonce, sv.Nonce)
			}
		}

		// The next section actually calculates the payment amounts to update the payment channel state
		// 1. (optional) sum already redeemed value of all merging lanes
		redeemedFromOthers := big.Zero()
		for _, merge := range sv.Merges {
			if merge.Lane == sv.Lane {
				rt.Abortf(exitcode.ErrIllegalArgument, "voucher cannot merge lanes into its own lane")
			}

			otherls := findLane(rt, lstates, merge.Lane)
			if otherls == nil {
				rt.Abortf(exitcode.ErrIllegalArgument, "voucher specifies invalid merge lane %v", merge.Lane)
				return // makes linters happy
			}

			if otherls.Nonce >= merge.Nonce {
				rt.Abortf(exitcode.ErrIllegalArgument, "merged lane in voucher has outdated nonce, cannot redeem")
			}

			redeemedFromOthers = big.Add(redeemedFromOthers, otherls.Redeemed)
			otherls.Nonce = merge.Nonce
			err = lstates.Set(merge.Lane, otherls)
			builtin.RequireNoErr(rt, err, exitcode.ErrIllegalState, "failed to store lane %d", merge.Lane)
		}

		// 2. To prevent double counting, remove already redeemed amounts (from
		// voucher or other lanes) from the voucher amount
		laneState.Nonce = sv.Nonce
		balanceDelta := big.Sub(sv.Amount, big.Add(redeemedFromOthers, laneState.Redeemed))
		// 3. set new redeemed value for merged-into lane
		laneState.Redeemed = sv.Amount

		newSendBalance := big.Add(st.ToSend, balanceDelta)

		// 4. check operation validity
		if newSendBalance.LessThan(big.Zero()) {
			rt.Abortf(exitcode.ErrIllegalArgument, "voucher would leave channel balance negative")
		}
		if newSendBalance.GreaterThan(rt.CurrentBalance()) {
			rt.Abortf(exitcode.ErrIllegalArgument, "not enough funds in channel to cover voucher")
		}

		// 5. add new redemption ToSend
		st.ToSend = newSendBalance

		// update channel settlingAt and MinSettleHeight if delayed by voucher
		if sv.MinSettleHeight != 0 {
			if st.SettlingAt != 0 && st.SettlingAt < sv.MinSettleHeight {
				st.SettlingAt = sv.MinSettleHeight
			}
			if st.MinSettleHeight < sv.MinSettleHeight {
				st.MinSettleHeight = sv.MinSettleHeight
			}
		}

		err = lstates.Set(laneId, laneState)
		builtin.RequireNoErr(rt, err, exitcode.ErrIllegalState, "failed to store lane", laneId)

		st.LaneStates, err = lstates.Root()
		builtin.RequireNoErr(rt, err, exitcode.ErrIllegalState, "failed to save lanes")
	})
	return nil
}

func (pca Actor) Settle(rt runtime.Runtime, _ *abi.EmptyValue) *abi.EmptyValue {
	var st State
	rt.StateTransaction(&st, func() {
		rt.ValidateImmediateCallerIs(st.From, st.To)

		if st.SettlingAt != 0 {
			rt.Abortf(exitcode.ErrIllegalState, "channel already settling")
		}

		st.SettlingAt = rt.CurrEpoch() + SettleDelay
		if st.SettlingAt < st.MinSettleHeight {
			st.SettlingAt = st.MinSettleHeight
		}
	})
	return nil
}

func (pca Actor) Collect(rt runtime.Runtime, _ *abi.EmptyValue) *abi.EmptyValue {
	var st State
	rt.StateReadonly(&st)
	rt.ValidateImmediateCallerIs(st.From, st.To)

	if st.SettlingAt == 0 || rt.CurrEpoch() < st.SettlingAt {
		rt.Abortf(exitcode.ErrForbidden, "payment channel not settling or settled")
	}

	// send ToSend to "To"
	codeTo := rt.Send(
		st.To,
		builtin.MethodSend,
		nil,
		st.ToSend,
		&builtin.Discard{},
	)
	builtin.RequireSuccess(rt, codeTo, "Failed to send funds to `To`")

	// the remaining balance will be returned to "From" upon deletion.
	rt.DeleteActor(st.From)

	return nil
}

// Returns the insertion index for a lane ID, with the matching lane state if found, or nil.
func findLane(rt runtime.Runtime, ls *adt.Array, id uint64) *LaneState {
	if id > MaxLane {
		rt.Abortf(exitcode.ErrIllegalArgument, "maximum lane ID is 2^63-1")
	}

	var out LaneState
	found, err := ls.Get(id, &out)
	builtin.RequireNoErr(rt, err, exitcode.ErrIllegalState, "failed to load lane %d", id)

	if !found {
		return nil
	}

	return &out
}