package storage

import (
	"context"
	"errors"
)

// Errors the sync store returns for cases a caller has to tell apart.
var (
	// ErrNoSuchFilter is returned for a filter ID the user has not
	// uploaded.
	ErrNoSuchFilter = errors.New("no such filter")
)

// StreamPositions is the top of every stream a sync token points into, read
// in one call.
//
// A sync response is assembled from several independent streams, and the
// token it returns has to name a position in each. Reading them one at a
// time would let a stream move between two of the reads, and the client
// would then be handed a token that skips whatever landed in the gap.
type StreamPositions struct {
	// PDU is the room event stream: the highest stream ordering in
	// roomserver_events.
	PDU int64

	Receipt     int64
	ToDevice    int64
	AccountData int64
	Presence    int64

	// DeviceList is the keyserver's device list stream: the users whose
	// end-to-end keys a client should fetch again.
	DeviceList int64
}

// TimelineDirection is which way a timeline read walks the stream.
type TimelineDirection string

// The two directions, spelled as the Client-Server API spells them in
// /messages' `dir` parameter.
const (
	// DirectionForward reads oldest first, which is the order /sync hands
	// events to a client.
	DirectionForward TimelineDirection = "f"

	// DirectionBackward reads newest first, which is how /messages pages
	// back through a room's history.
	DirectionBackward TimelineDirection = "b"
)

// TimelineRequest is one window of a room's event stream.
//
// The window is named by its two ends rather than by a direction and a
// starting point, because the two are independent: which end the limit
// trims, and which order the rows come back in, is Direction's business,
// and nothing else about the request changes with it. Spelling the bounds
// as "from" and "to" instead would mean each of them being the low bound in
// one direction and the high bound in the other, which is exactly the
// off-by-one every pagination bug is made of.
//
// Low is exclusive and High inclusive, in both directions. That is what
// lets a token a client was handed be passed straight back without
// re-reading the event it names: a position is somewhere a client has got
// to, not an event it still wants.
//
// Positions can be negative: history backfilled from other servers is
// numbered downwards from -1, below everything that was already here (see
// RoomStore.WriteBackfill). Zero is never a position, which is what lets
// it go on meaning "unbounded" here — and why the position just below an
// event is PositionBefore and not a plain subtraction.
type TimelineRequest struct {
	RoomID string

	// Low is an exclusive lower bound. Zero means unbounded: the start of
	// the room, backfilled history included.
	Low int64

	// High is an inclusive upper bound. Zero means the top of the stream,
	// and goes on meaning that as the stream grows.
	High int64

	// Direction decides which end of the window the limit trims — the
	// bottom when reading backwards, the top when reading forwards — and
	// the order the rows come back in.
	Direction TimelineDirection

	// Limit caps the rows returned. It is required: an unbounded timeline
	// read is a way to ask the server to serialise a whole room.
	Limit int

	// Filter narrows which events come back. The store applies it in SQL
	// because a filter applied after the limit would return fewer events
	// than the client asked for and then look like the end of the room.
	Filter TimelineFilter
}

// PositionBefore is the position just below pos: the bound that names
// "everything up to, but not including, the event at pos".
//
// It is pos-1 except at 1, whose predecessor is -1. Zero is not a
// position — it is the "unbounded" bound of every window — so a token
// below the first event this server ever persisted has to skip it, or
// handing that token back as a High bound would read the whole room again
// instead of the backfilled history beneath it.
func PositionBefore(pos int64) int64 {
	if pos == 1 {
		return -1
	}
	return pos - 1
}

// TimelineFilter is the part of a Client-Server API event filter that the
// store can apply itself.
//
// Only the fields that are a plain comparison on an indexed or stored
// column are here. The rest of a filter — `contains_url`, lazy-loading,
// `include_redundant_members` — is about what an event means or about how a
// response is assembled, and is applied by the syncapi.
type TimelineFilter struct {
	// Types, when non-empty, admits only these event types. A trailing '*'
	// is a wildcard, as the spec defines for filter patterns.
	Types []string

	// NotTypes excludes types, and wins over Types for an event matching
	// both, which is what the spec requires.
	NotTypes []string

	// Senders, when non-empty, admits only these senders.
	Senders []string

	// NotSenders excludes senders, and likewise wins.
	NotSenders []string
}

// IsEmpty reports whether the filter admits everything.
func (f TimelineFilter) IsEmpty() bool {
	return len(f.Types) == 0 && len(f.NotTypes) == 0 &&
		len(f.Senders) == 0 && len(f.NotSenders) == 0
}

// SyncMembership is a user's current membership in one room, with the
// position of the event that set it.
//
// The position is what bounds what a user who has left may see: their view
// of the room ends at their own leave event, and a room they were kicked
// out of must not keep streaming to them.
type SyncMembership struct {
	RoomID     string
	Membership string
	EventID    string

	// StreamOrdering is the position of the membership event.
	StreamOrdering int64
}

// Receipt is one read receipt.
type Receipt struct {
	RoomID      string
	UserID      string
	ReceiptType string

	// ThreadID is the thread the receipt is for, or empty for the room's
	// main timeline.
	ThreadID string

	EventID string

	// TimestampMS is when the receipt was sent, which is what a client
	// shows as "read at".
	TimestampMS int64

	// StreamPos is the receipt's position in the receipt stream.
	StreamPos int64
}

// ToDeviceMessage is one message addressed to a single device.
type ToDeviceMessage struct {
	// StreamPos is the message's position in the to-device stream.
	StreamPos int64

	Localpart string
	DeviceID  string
	Sender    string
	Type      string

	// Content is the message body as the sender wrote it.
	Content []byte

	SentAtMS int64
}

// NewToDeviceMessage is one message to enqueue.
type NewToDeviceMessage struct {
	Localpart string

	// DeviceID is the device to deliver to. The caller has already expanded
	// a wildcard addressee into one of these per device: which devices a
	// user has is the userapi's business, not the store's.
	DeviceID string

	Sender   string
	Type     string
	Content  []byte
	SentAtMS int64
}

// PresenceState is a user's presence as the store holds it.
type PresenceState struct {
	UserID string

	// Presence is "online", "offline" or "unavailable".
	Presence string

	StatusMsg string

	// LastActiveMS is when the user was last seen doing something.
	LastActiveMS int64

	// StreamPos is the position of the last change to this row.
	StreamPos int64
}

// AccountDataEntry locates one account data blob that has changed. The
// content is not carried: a sync response needs to know which blobs to
// re-read, and reading them is a separate, cacheable call.
type AccountDataEntry struct {
	// RoomID is empty for global account data.
	RoomID   string
	DataType string

	StreamPos int64
}

// StoredFilter is an uploaded filter.
type StoredFilter struct {
	Localpart string
	FilterID  string

	// JSON is the filter exactly as the client uploaded it.
	JSON []byte

	CreatedAtMS int64
}

// SyncStore persists the streams a sync response carries that are not room
// events, plus the filters clients upload: the storage half of the syncapi
// component.
//
// Typing notifications are not here on purpose. They expire in seconds and
// are worthless across a restart, so the syncapi keeps them in memory.
type SyncStore interface {
	// StreamPositions reads the top of every stream in one call.
	StreamPositions(ctx context.Context) (StreamPositions, error)

	// SetReceipt records a read receipt, replacing any earlier one from the
	// same user for the same type and thread, and returns its stream
	// position.
	//
	// A receipt that would move backwards is not written and reports the
	// current position with moved false: receipts only ever advance, and a
	// client syncing on two devices would otherwise flap.
	SetReceipt(ctx context.Context, r Receipt) (pos int64, moved bool, err error)

	// Receipts returns the current receipts of the given rooms whose
	// position is after from and at or before to. A from of 0 returns every
	// current receipt, which is what an initial sync needs.
	Receipts(ctx context.Context, roomIDs []string, from, to int64) ([]Receipt, error)

	// ReceiptsForUser returns one user's own current receipts in the given
	// rooms, whatever their position. A client needs its own read markers
	// on an initial sync even when it set them long ago.
	ReceiptsForUser(ctx context.Context, userID string, roomIDs []string) ([]Receipt, error)

	// SendToDevice enqueues messages for delivery and returns the highest
	// position it wrote. An empty batch is not an error and returns the
	// current position.
	SendToDevice(ctx context.Context, msgs []NewToDeviceMessage) (int64, error)

	// ToDeviceMessages returns the messages queued for one device whose
	// position is after from and at or before to, oldest first.
	ToDeviceMessages(ctx context.Context, localpart, deviceID string, from, to int64, limit int) ([]ToDeviceMessage, error)

	// DeleteToDeviceMessages removes the messages for a device at or before
	// pos. A device acknowledges a batch by syncing past it, so this is
	// what the next sync does with the batch before it.
	DeleteToDeviceMessages(ctx context.Context, localpart, deviceID string, pos int64) error

	// SetPresence records a user's presence and returns its stream
	// position, reporting whether anything a peer would care about
	// changed.
	//
	// A write that only refreshes LastActiveMS does not move the stream.
	// A client says it is still online by syncing, so every sync is one of
	// these — and a stream that moved each time would wake every peer's
	// long poll, which would sync, which would move it again.
	SetPresence(ctx context.Context, p PresenceState) (pos int64, moved bool, err error)

	// Presence reads one user's presence, returning ErrNoSuchUser when
	// nothing has ever been recorded for them.
	Presence(ctx context.Context, userID string) (PresenceState, error)

	// PresenceForUsers returns the presence of the given users whose
	// position is after from and at or before to. A from of 0 returns every
	// current state.
	PresenceForUsers(ctx context.Context, userIDs []string, from, to int64) ([]PresenceState, error)

	// AccountDataChanges lists the account data of a user that changed
	// after from and at or before to.
	AccountDataChanges(ctx context.Context, localpart string, from, to int64) ([]AccountDataEntry, error)

	// AccountDataAt lists every account data blob a user has, as an initial
	// sync needs.
	AccountDataAt(ctx context.Context, localpart string, to int64) ([]AccountDataEntry, error)

	// SaveFilter stores a filter under a new ID and returns it.
	SaveFilter(ctx context.Context, localpart string, filter []byte, createdAtMS int64) (string, error)

	// Filter reads an uploaded filter, returning ErrNoSuchFilter when the
	// ID is unknown.
	Filter(ctx context.Context, localpart, filterID string) ([]byte, error)
}