package roomserver

import (
	"context"
	"strings"

	"github.com/dariuszjaniak/warp/server/internal/matrix/event"
	"github.com/dariuszjaniak/warp/server/internal/matrix/roomversion"
	"github.com/dariuszjaniak/warp/server/internal/storage"
)

// RoomSyncRequest asks what one user sees of one room between two
// positions.
//
// It is the read model's largest operation and the one /sync is built on.
// It is a single call rather than a timeline read plus two state reads
// because the three answers have to agree with each other: which state the
// response reports depends on where the timeline turned out to start, and
// working that out in the caller would mean the caller deciding what the
// timeline was — which is the question it asked.
type RoomSyncRequest struct {
	RoomID string

	// User is the full user ID syncing.
	User string

	// Since is the position the user has already seen, exclusive. Zero is
	// an initial sync.
	Since int64

	// To is the top of this sync, inclusive. It is fixed by the caller for
	// every room in one response, so that a token names one moment rather
	// than a different one per room.
	To int64

	// TimelineLimit caps the events in the timeline.
	TimelineLimit int

	// TimelineFilter and StateFilter narrow the two halves of the
	// response. The state filter is applied in this package rather than in
	// SQL: a state read is a set of tuples, not a window of a stream.
	TimelineFilter storage.TimelineFilter
	StateFilter    storage.TimelineFilter

	// FullState asks for the room's whole state even on an incremental
	// sync, which is what ?full_state=true means.
	FullState bool

	// LazyLoadMembers leaves the member events out of the state except for
	// the senders who appear in the timeline.
	LazyLoadMembers bool
}

// RoomSync is what one user sees of one room between two positions.
type RoomSync struct {
	// Membership is the user's current membership in the room.
	Membership string

	Timeline Timeline

	// State is the room's state as of the start of the timeline on an
	// initial sync, and what changed between Since and the start of the
	// timeline on an incremental one. For an incremental sync with no gap
	// it is empty: every state change in the window is in the timeline
	// itself.
	State []ClientEvent

	// Summary is the room's member counts and heroes.
	Summary RoomSyncSummary

	// Visible is the span of the stream the user may read, which the
	// caller needs in order to bound the ephemeral streams the same way.
	Visible VisibleRange
}

// RoomSync reads one room for one user.
func (r *API) RoomSync(ctx context.Context, req RoomSyncRequest) (RoomSync, error) {
	room, rules, err := r.roomAndRules(ctx, req.RoomID)
	if err != nil {
		return RoomSync{}, err
	}
	summary, err := r.store.RoomSummary(ctx, room.ID)
	if err != nil {
		return RoomSync{}, err
	}
	visible, err := r.visibleRange(ctx, room.ID, rules, req.User, summary.HistoryVisibility)
	if err != nil {
		return RoomSync{}, err
	}
	if err := visible.refusal(room.ID, req.User); err != nil {
		return RoomSync{}, err
	}

	out := RoomSync{Membership: visible.Membership, Visible: visible}

	// The timeline is read backwards even though it is reported forwards.
	// A client wants the *newest* events of the window it has missed — the
	// bottom of the room, which is what it is looking at — and a forward
	// read with a limit would hand it the oldest instead and then call the
	// rest a gap.
	out.Timeline, err = r.timeline(ctx, rules, visible, TimelineRequest{
		RoomID:    req.RoomID,
		User:      req.User,
		Low:       req.Since,
		High:      req.To,
		Direction: storage.DirectionBackward,
		Limit:     req.TimelineLimit,
		Filter:    req.TimelineFilter,
	})
	if err != nil {
		return RoomSync{}, err
	}

	if out.State, err = r.syncState(ctx, rules, visible, req, out.Timeline); err != nil {
		return RoomSync{}, err
	}
	if out.Summary, err = r.RoomSummaryFor(ctx, req.RoomID, req.User); err != nil {
		return RoomSync{}, err
	}
	return out, nil
}

// syncState decides which state a room's half of a sync response reports.
//
// The rule the spec sets is that a client which applies `state` and then
// `timeline` ends up with the room's state at the end of the timeline. That
// gives three cases:
//
//   - An initial sync, or ?full_state=true: the whole state as of the start
//     of the timeline.
//   - An incremental sync whose timeline reaches back to `since`: nothing.
//     Every state change in between is a timeline event the client is
//     already being given.
//   - An incremental sync whose timeline was cut short — a gap: what
//     changed between `since` and where the timeline starts. Those changes
//     are not in the timeline and the client would otherwise never see
//     them.
func (r *API) syncState(ctx context.Context, rules roomversion.Rules, visible VisibleRange, req RoomSyncRequest, timeline Timeline) ([]ClientEvent, error) {
	statePos, haveState := statePosition(timeline, req.To, visible)

	switch {
	case !haveState:
		// The timeline reaches back to the room's create event, so there
		// is nothing before it to report.
		return nil, nil

	case req.FullState || req.Since == 0:
		state, err := r.stateAt(ctx, req.RoomID, rules, statePos)
		if err != nil {
			return nil, err
		}
		return r.shapeState(ctx, rules, req, timeline, state)

	case !timeline.Limited:
		return nil, nil

	default:
		before, err := r.stateAt(ctx, req.RoomID, rules, req.Since)
		if err != nil {
			return nil, err
		}
		after, err := r.stateAt(ctx, req.RoomID, rules, statePos)
		if err != nil {
			return nil, err
		}
		return r.shapeState(ctx, rules, req, timeline, stateDelta(before, after))
	}
}

// statePosition is the position a room's state is reported as of, and
// whether there is any state to report at all.
//
// The state belongs at the start of the timeline, so that applying it and
// then the timeline lands on the room's current state. A room with no
// timeline events in this window is reported as of the top of the sync
// instead: the state may still have been read for the first time.
func statePosition(timeline Timeline, to int64, visible VisibleRange) (int64, bool) {
	if len(timeline.Events) > 0 {
		return timeline.Start, timeline.Start > 0
	}
	if visible.To != 0 && (to == 0 || to > visible.To) {
		to = visible.To
	}
	return to, to > 0
}

// shapeState applies the state filter and lazy loading to a state set.
func (r *API) shapeState(ctx context.Context, rules roomversion.Rules, req RoomSyncRequest, timeline Timeline, state []ClientEvent) ([]ClientEvent, error) {
	state = filterClientEvents(state, req.StateFilter)
	if !req.LazyLoadMembers {
		return state, nil
	}

	// Lazy loading: drop the member events and put back only the ones the
	// client needs to render the timeline it is being given.
	//
	// The spec also lets a server leave out members it has already sent
	// this device, which would need per-device bookkeeping of what each
	// one has seen. Warp sends them every time instead: it is the
	// behaviour `include_redundant_members: true` asks for, so no client
	// can be surprised by it, and it costs one state read rather than a
	// table.
	kept := make([]ClientEvent, 0, len(state))
	for _, e := range state {
		if e.Type != event.TypeMember {
			kept = append(kept, e)
		}
	}

	senders := timelineSenders(timeline)
	members, err := r.memberEvents(ctx, req.RoomID, rules, senders)
	if err != nil {
		return nil, err
	}
	return append(kept, filterClientEvents(members, req.StateFilter)...), nil
}

// timelineSenders lists the distinct senders of a timeline, in the order
// they first appear.
func timelineSenders(timeline Timeline) []string {
	seen := make(map[string]struct{}, len(timeline.Events))
	out := make([]string, 0, len(timeline.Events))
	for _, e := range timeline.Events {
		if _, dup := seen[e.Sender]; dup {
			continue
		}
		seen[e.Sender] = struct{}{}
		out = append(out, e.Sender)
	}
	return out
}

// stateKey identifies one piece of state.
type stateKeyTuple struct {
	eventType string
	stateKey  string
}

// tupleOf is the state tuple of a client event, and whether it has one.
func tupleOf(e ClientEvent) (stateKeyTuple, bool) {
	if e.StateKey == nil {
		return stateKeyTuple{}, false
	}
	return stateKeyTuple{eventType: e.Type, stateKey: *e.StateKey}, true
}

// stateDelta is what changed between two state sets: every tuple of `after`
// whose event is not the one `before` had.
//
// A tuple that `before` had and `after` does not is left out. State is only
// ever replaced, never removed — there is no way to unset a piece of room
// state — so such a tuple can only come from state resolution dropping an
// event, and there is nothing a client could be sent that would express it.
func stateDelta(before, after []ClientEvent) []ClientEvent {
	had := make(map[stateKeyTuple]string, len(before))
	for _, e := range before {
		if tuple, ok := tupleOf(e); ok {
			had[tuple] = e.EventID
		}
	}

	out := make([]ClientEvent, 0, len(after))
	for _, e := range after {
		tuple, ok := tupleOf(e)
		if !ok {
			continue
		}
		if had[tuple] == e.EventID {
			continue
		}
		out = append(out, e)
	}
	return out
}

// filterClientEvents applies the part of an event filter that is a
// comparison on the event itself.
//
// It is the same filter the store applies to a timeline read, applied in Go
// because a state read is a set of tuples rather than a window of a stream:
// there is no index to push it down to.
func filterClientEvents(events []ClientEvent, filter storage.TimelineFilter) []ClientEvent {
	if filter.IsEmpty() {
		return events
	}
	out := make([]ClientEvent, 0, len(events))
	for _, e := range events {
		if matchesFilter(e, filter) {
			out = append(out, e)
		}
	}
	return out
}

// matchesFilter reports whether one event passes a filter. Exclusions win
// over inclusions, which is what the spec requires.
func matchesFilter(e ClientEvent, filter storage.TimelineFilter) bool {
	switch {
	case matchesAny(e.Type, filter.NotTypes):
		return false
	case matchesAny(e.Sender, filter.NotSenders):
		return false
	case len(filter.Types) > 0 && !matchesAny(e.Type, filter.Types):
		return false
	case len(filter.Senders) > 0 && !matchesAny(e.Sender, filter.Senders):
		return false
	}
	return true
}

// matchesAny reports whether value matches one of the filter patterns, each
// of which may end in '*' as the spec's wildcard.
func matchesAny(value string, patterns []string) bool {
	for _, pattern := range patterns {
		if prefix, isWildcard := strings.CutSuffix(pattern, "*"); isWildcard {
			if strings.HasPrefix(value, prefix) {
				return true
			}
			continue
		}
		if value == pattern {
			return true
		}
	}
	return false
}