// Copyright 2017 Vector Creations Ltd
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// Package state provides functions for reading state from the database.
// The functions for writing state to the database are the input package.
package state

import (
	"context"
	"fmt"
	"sort"
	"time"

	"github.com/matrix-org/dendrite/roomserver/types"
	"github.com/matrix-org/gomatrixserverlib"
	"github.com/matrix-org/util"
	"github.com/prometheus/client_golang/prometheus"
)

// A RoomStateDatabase has the storage APIs needed to load state from the database
type RoomStateDatabase interface {
	// Store the room state at an event in the database
	AddState(
		ctx context.Context,
		roomNID types.RoomNID,
		stateBlockNIDs []types.StateBlockNID,
		state []types.StateEntry,
	) (types.StateSnapshotNID, error)
	// Look up the state of a room at each event for a list of string event IDs.
	// Returns an error if there is an error talking to the database
	// Returns a types.MissingEventError if the room state for the event IDs aren't in the database
	StateAtEventIDs(ctx context.Context, eventIDs []string) ([]types.StateAtEvent, error)
	// Look up the numeric IDs for a list of string event types.
	// Returns a map from string event type to numeric ID for the event type.
	EventTypeNIDs(ctx context.Context, eventTypes []string) (map[string]types.EventTypeNID, error)
	// Look up the numeric IDs for a list of string event state keys.
	// Returns a map from string state key to numeric ID for the state key.
	EventStateKeyNIDs(ctx context.Context, eventStateKeys []string) (map[string]types.EventStateKeyNID, error)
	// Look up the numeric state data IDs for each numeric state snapshot ID
	// The returned slice is sorted by numeric state snapshot ID.
	StateBlockNIDs(ctx context.Context, stateNIDs []types.StateSnapshotNID) ([]types.StateBlockNIDList, error)
	// Look up the state data for each numeric state data ID
	// The returned slice is sorted by numeric state data ID.
	StateEntries(ctx context.Context, stateBlockNIDs []types.StateBlockNID) ([]types.StateEntryList, error)
	// Look up the state data for the state key tuples for each numeric state block ID
	// This is used to fetch a subset of the room state at a snapshot.
	// If a block doesn't contain any of the requested tuples then it can be discarded from the result.
	// The returned slice is sorted by numeric state block ID.
	StateEntriesForTuples(
		ctx context.Context,
		stateBlockNIDs []types.StateBlockNID,
		stateKeyTuples []types.StateKeyTuple,
	) ([]types.StateEntryList, error)
	// Look up the Events for a list of numeric event IDs.
	// Returns a sorted list of events.
	Events(ctx context.Context, eventNIDs []types.EventNID) ([]types.Event, error)
	// Look up snapshot NID for an event ID string
	SnapshotNIDFromEventID(ctx context.Context, eventID string) (types.StateSnapshotNID, error)
}

// LoadStateAtSnapshot loads the full state of a room at a particular snapshot.
// This is typically the state before an event or the current state of a room.
// Returns a sorted list of state entries or an error if there was a problem talking to the database.
func LoadStateAtSnapshot(
	ctx context.Context, db RoomStateDatabase, stateNID types.StateSnapshotNID,
) ([]types.StateEntry, error) {
	stateBlockNIDLists, err := db.StateBlockNIDs(ctx, []types.StateSnapshotNID{stateNID})
	if err != nil {
		return nil, err
	}
	// We've asked for exactly one snapshot from the db so we should have exactly one entry in the result.
	stateBlockNIDList := stateBlockNIDLists[0]

	stateEntryLists, err := db.StateEntries(ctx, stateBlockNIDList.StateBlockNIDs)
	if err != nil {
		return nil, err
	}
	stateEntriesMap := stateEntryListMap(stateEntryLists)

	// Combine all the state entries for this snapshot.
	// The order of state block NIDs in the list tells us the order to combine them in.
	var fullState []types.StateEntry
	for _, stateBlockNID := range stateBlockNIDList.StateBlockNIDs {
		entries, ok := stateEntriesMap.lookup(stateBlockNID)
		if !ok {
			// This should only get hit if the database is corrupt.
			// It should be impossible for an event to reference a NID that doesn't exist
			panic(fmt.Errorf("Corrupt DB: Missing state block numeric ID %d", stateBlockNID))
		}
		fullState = append(fullState, entries...)
	}

	// Stable sort so that the most recent entry for each state key stays
	// remains later in the list than the older entries for the same state key.
	sort.Stable(stateEntryByStateKeySorter(fullState))
	// Unique returns the last entry and hence the most recent entry for each state key.
	fullState = fullState[:util.Unique(stateEntryByStateKeySorter(fullState))]
	return fullState, nil
}

// LoadStateAtEvent loads the full state of a room at a particular event.
func LoadStateAtEvent(
	ctx context.Context, db RoomStateDatabase, eventID string,
) ([]types.StateEntry, error) {
	snapshotNID, err := db.SnapshotNIDFromEventID(ctx, eventID)
	if err != nil {
		return nil, err
	}

	stateEntries, err := LoadStateAtSnapshot(ctx, db, snapshotNID)
	if err != nil {
		return nil, err
	}

	return stateEntries, nil
}

// LoadCombinedStateAfterEvents loads a snapshot of the state after each of the events
// and combines those snapshots together into a single list.
func LoadCombinedStateAfterEvents(
	ctx context.Context, db RoomStateDatabase, prevStates []types.StateAtEvent,
) ([]types.StateEntry, error) {
	stateNIDs := make([]types.StateSnapshotNID, len(prevStates))
	for i, state := range prevStates {
		stateNIDs[i] = state.BeforeStateSnapshotNID
	}
	// Fetch the state snapshots for the state before the each prev event from the database.
	// Deduplicate the IDs before passing them to the database.
	// There could be duplicates because the events could be state events where
	// the snapshot of the room state before them was the same.
	stateBlockNIDLists, err := db.StateBlockNIDs(ctx, uniqueStateSnapshotNIDs(stateNIDs))
	if err != nil {
		return nil, err
	}

	var stateBlockNIDs []types.StateBlockNID
	for _, list := range stateBlockNIDLists {
		stateBlockNIDs = append(stateBlockNIDs, list.StateBlockNIDs...)
	}
	// Fetch the state entries that will be combined to create the snapshots.
	// Deduplicate the IDs before passing them to the database.
	// There could be duplicates because a block of state entries could be reused by
	// multiple snapshots.
	stateEntryLists, err := db.StateEntries(ctx, uniqueStateBlockNIDs(stateBlockNIDs))
	if err != nil {
		return nil, err
	}
	stateBlockNIDsMap := stateBlockNIDListMap(stateBlockNIDLists)
	stateEntriesMap := stateEntryListMap(stateEntryLists)

	// Combine the entries from all the snapshots of state after each prev event into a single list.
	var combined []types.StateEntry
	for _, prevState := range prevStates {
		// Grab the list of state data NIDs for this snapshot.
		stateBlockNIDs, ok := stateBlockNIDsMap.lookup(prevState.BeforeStateSnapshotNID)
		if !ok {
			// This should only get hit if the database is corrupt.
			// It should be impossible for an event to reference a NID that doesn't exist
			panic(fmt.Errorf("Corrupt DB: Missing state snapshot numeric ID %d", prevState.BeforeStateSnapshotNID))
		}

		// Combine all the state entries for this snapshot.
		// The order of state block NIDs in the list tells us the order to combine them in.
		var fullState []types.StateEntry
		for _, stateBlockNID := range stateBlockNIDs {
			entries, ok := stateEntriesMap.lookup(stateBlockNID)
			if !ok {
				// This should only get hit if the database is corrupt.
				// It should be impossible for an event to reference a NID that doesn't exist
				panic(fmt.Errorf("Corrupt DB: Missing state block numeric ID %d", stateBlockNID))
			}
			fullState = append(fullState, entries...)
		}
		if prevState.IsStateEvent() {
			// If the prev event was a state event then add an entry for the event itself
			// so that we get the state after the event rather than the state before.
			fullState = append(fullState, prevState.StateEntry)
		}

		// Stable sort so that the most recent entry for each state key stays
		// remains later in the list than the older entries for the same state key.
		sort.Stable(stateEntryByStateKeySorter(fullState))
		// Unique returns the last entry and hence the most recent entry for each state key.
		fullState = fullState[:util.Unique(stateEntryByStateKeySorter(fullState))]
		// Add the full state for this StateSnapshotNID.
		combined = append(combined, fullState...)
	}
	return combined, nil
}

// DifferenceBetweeenStateSnapshots works out which state entries have been added and removed between two snapshots.
func DifferenceBetweeenStateSnapshots(
	ctx context.Context, db RoomStateDatabase, oldStateNID, newStateNID types.StateSnapshotNID,
) (removed, added []types.StateEntry, err error) {
	if oldStateNID == newStateNID {
		// If the snapshot NIDs are the same then nothing has changed
		return nil, nil, nil
	}

	var oldEntries []types.StateEntry
	var newEntries []types.StateEntry
	if oldStateNID != 0 {
		oldEntries, err = LoadStateAtSnapshot(ctx, db, oldStateNID)
		if err != nil {
			return nil, nil, err
		}
	}
	if newStateNID != 0 {
		newEntries, err = LoadStateAtSnapshot(ctx, db, newStateNID)
		if err != nil {
			return nil, nil, err
		}
	}

	var oldI int
	var newI int
	for {
		switch {
		case oldI == len(oldEntries):
			// We've reached the end of the old entries.
			// The rest of the new list must have been newly added.
			added = append(added, newEntries[newI:]...)
			return
		case newI == len(newEntries):
			// We've reached the end of the new entries.
			// The rest of the old list must be have been removed.
			removed = append(removed, oldEntries[oldI:]...)
			return
		case oldEntries[oldI] == newEntries[newI]:
			// The entry is in both lists so skip over it.
			oldI++
			newI++
		case oldEntries[oldI].LessThan(newEntries[newI]):
			// The lists are sorted so the old entry being less than the new entry means that it only appears in the old list.
			removed = append(removed, oldEntries[oldI])
			oldI++
		default:
			// Reaching the default case implies that the new entry is less than the old entry.
			// Since the lists are sorted this means that it only appears in the new list.
			added = append(added, newEntries[newI])
			newI++
		}
	}
}

// LoadStateAtSnapshotForStringTuples loads the state for a list of event type and state key pairs at a snapshot.
// This is used when we only want to load a subset of the room state at a snapshot.
// If there is no entry for a given event type and state key pair then it will be discarded.
// This is typically the state before an event or the current state of a room.
// Returns a sorted list of state entries or an error if there was a problem talking to the database.
func LoadStateAtSnapshotForStringTuples(
	ctx context.Context,
	db RoomStateDatabase,
	stateNID types.StateSnapshotNID,
	stateKeyTuples []gomatrixserverlib.StateKeyTuple,
) ([]types.StateEntry, error) {
	numericTuples, err := stringTuplesToNumericTuples(ctx, db, stateKeyTuples)
	if err != nil {
		return nil, err
	}
	return loadStateAtSnapshotForNumericTuples(ctx, db, stateNID, numericTuples)
}

// stringTuplesToNumericTuples converts the string state key tuples into numeric IDs
// If there isn't a numeric ID for either the event type or the event state key then the tuple is discarded.
// Returns an error if there was a problem talking to the database.
func stringTuplesToNumericTuples(
	ctx context.Context,
	db RoomStateDatabase,
	stringTuples []gomatrixserverlib.StateKeyTuple,
) ([]types.StateKeyTuple, error) {
	eventTypes := make([]string, len(stringTuples))
	stateKeys := make([]string, len(stringTuples))
	for i := range stringTuples {
		eventTypes[i] = stringTuples[i].EventType
		stateKeys[i] = stringTuples[i].StateKey
	}
	eventTypes = util.UniqueStrings(eventTypes)
	eventTypeMap, err := db.EventTypeNIDs(ctx, eventTypes)
	if err != nil {
		return nil, err
	}
	stateKeys = util.UniqueStrings(stateKeys)
	stateKeyMap, err := db.EventStateKeyNIDs(ctx, stateKeys)
	if err != nil {
		return nil, err
	}

	var result []types.StateKeyTuple
	for _, stringTuple := range stringTuples {
		var numericTuple types.StateKeyTuple
		var ok1, ok2 bool
		numericTuple.EventTypeNID, ok1 = eventTypeMap[stringTuple.EventType]
		numericTuple.EventStateKeyNID, ok2 = stateKeyMap[stringTuple.StateKey]
		// Discard the tuple if there wasn't a numeric ID for either the event type or the state key.
		if ok1 && ok2 {
			result = append(result, numericTuple)
		}
	}

	return result, nil
}

// loadStateAtSnapshotForNumericTuples loads the state for a list of event type and state key pairs at a snapshot.
// This is used when we only want to load a subset of the room state at a snapshot.
// If there is no entry for a given event type and state key pair then it will be discarded.
// This is typically the state before an event or the current state of a room.
// Returns a sorted list of state entries or an error if there was a problem talking to the database.
func loadStateAtSnapshotForNumericTuples(
	ctx context.Context,
	db RoomStateDatabase,
	stateNID types.StateSnapshotNID,
	stateKeyTuples []types.StateKeyTuple,
) ([]types.StateEntry, error) {
	stateBlockNIDLists, err := db.StateBlockNIDs(ctx, []types.StateSnapshotNID{stateNID})
	if err != nil {
		return nil, err
	}
	// We've asked for exactly one snapshot from the db so we should have exactly one entry in the result.
	stateBlockNIDList := stateBlockNIDLists[0]

	stateEntryLists, err := db.StateEntriesForTuples(
		ctx, stateBlockNIDList.StateBlockNIDs, stateKeyTuples,
	)
	if err != nil {
		return nil, err
	}
	stateEntriesMap := stateEntryListMap(stateEntryLists)

	// Combine all the state entries for this snapshot.
	// The order of state block NIDs in the list tells us the order to combine them in.
	var fullState []types.StateEntry
	for _, stateBlockNID := range stateBlockNIDList.StateBlockNIDs {
		entries, ok := stateEntriesMap.lookup(stateBlockNID)
		if !ok {
			// If the block is missing from the map it means that none of its entries matched a requested tuple.
			// This can happen if the block doesn't contain an update for one of the requested tuples.
			// If none of the requested tuples are in the block then it can be safely skipped.
			continue
		}
		fullState = append(fullState, entries...)
	}

	// Stable sort so that the most recent entry for each state key stays
	// remains later in the list than the older entries for the same state key.
	sort.Stable(stateEntryByStateKeySorter(fullState))
	// Unique returns the last entry and hence the most recent entry for each state key.
	fullState = fullState[:util.Unique(stateEntryByStateKeySorter(fullState))]
	return fullState, nil
}

// LoadStateAfterEventsForStringTuples loads the state for a list of event type
// and state key pairs after list of events.
// This is used when we only want to load a subset of the room state after a list of events.
// If there is no entry for a given event type and state key pair then it will be discarded.
// This is typically the state before an event.
// Returns a sorted list of state entries or an error if there was a problem talking to the database.
func LoadStateAfterEventsForStringTuples(
	ctx context.Context,
	db RoomStateDatabase,
	prevStates []types.StateAtEvent,
	stateKeyTuples []gomatrixserverlib.StateKeyTuple,
) ([]types.StateEntry, error) {
	numericTuples, err := stringTuplesToNumericTuples(ctx, db, stateKeyTuples)
	if err != nil {
		return nil, err
	}
	return loadStateAfterEventsForNumericTuples(ctx, db, prevStates, numericTuples)
}

func loadStateAfterEventsForNumericTuples(
	ctx context.Context,
	db RoomStateDatabase,
	prevStates []types.StateAtEvent,
	stateKeyTuples []types.StateKeyTuple,
) ([]types.StateEntry, error) {
	if len(prevStates) == 1 {
		// Fast path for a single event.
		prevState := prevStates[0]
		result, err := loadStateAtSnapshotForNumericTuples(
			ctx, db, prevState.BeforeStateSnapshotNID, stateKeyTuples,
		)
		if err != nil {
			return nil, err
		}
		if prevState.IsStateEvent() {
			// The result is current the state before the requested event.
			// We want the state after the requested event.
			// If the requested event was a state event then we need to
			// update that key in the result.
			// If the requested event wasn't a state event then the state after
			// it is the same as the state before it.
			for i := range result {
				if result[i].StateKeyTuple == prevState.StateKeyTuple {
					result[i] = prevState.StateEntry
				}
			}
		}
		return result, nil
	}

	// Slow path for more that one event.
	// Load the entire state so that we can do conflict resolution if we need to.
	// TODO: The are some optimistations we could do here:
	//    1) We only need to do conflict resolution if there is a conflict in the
	//       requested tuples so we might try loading just those tuples and then
	//       checking for conflicts.
	//    2) When there is a conflict we still only need to load the state
	//       needed to do conflict resolution which would save us having to load
	//       the full state.

	// TODO: Add metrics for this as it could take a long time for big rooms
	// with large conflicts.
	fullState, _, _, err := calculateStateAfterManyEvents(ctx, db, prevStates)
	if err != nil {
		return nil, err
	}

	// Sort the full state so we can use it as a map.
	sort.Sort(stateEntrySorter(fullState))

	// Filter the full state down to the required tuples.
	var result []types.StateEntry
	for _, tuple := range stateKeyTuples {
		eventNID, ok := stateEntryMap(fullState).lookup(tuple)
		if ok {
			result = append(result, types.StateEntry{
				StateKeyTuple: tuple,
				EventNID:      eventNID,
			})
		}
	}
	sort.Sort(stateEntrySorter(result))
	return result, nil
}

var calculateStateDurations = prometheus.NewSummaryVec(
	prometheus.SummaryOpts{
		Namespace: "dendrite",
		Subsystem: "roomserver",
		Name:      "calculate_state_duration_microseconds",
		Help:      "How long it takes to calculate the state after a list of events",
	},
	// Takes two labels:
	//   algorithm:
	//      The algorithm used to calculate the state or the step it failed on if it failed.
	//      Labels starting with "_" are used to indicate when the algorithm fails halfway.
	//  outcome:
	//      Whether the state was successfully calculated.
	//
	// The possible values for algorithm are:
	//    empty_state -> The list of events was empty so the state is empty.
	//    no_change -> The state hasn't changed.
	//    single_delta -> There was a single event added to the state in a way that can be encoded as a single delta
	//    full_state_no_conflicts -> We created a new copy of the full room state, but didn't enounter any conflicts
	//                               while doing so.
	//    full_state_with_conflicts -> We created a new copy of the full room state and had to resolve conflicts to do so.
	//    _load_state_block_nids -> Failed loading the state block nids for a single previous state.
	//    _load_combined_state -> Failed to load the combined state.
	//    _resolve_conflicts -> Failed to resolve conflicts.
	[]string{"algorithm", "outcome"},
)

var calculateStatePrevEventLength = prometheus.NewSummaryVec(
	prometheus.SummaryOpts{
		Namespace: "dendrite",
		Subsystem: "roomserver",
		Name:      "calculate_state_prev_event_length",
		Help:      "The length of the list of events to calculate the state after",
	},
	[]string{"algorithm", "outcome"},
)

var calculateStateFullStateLength = prometheus.NewSummaryVec(
	prometheus.SummaryOpts{
		Namespace: "dendrite",
		Subsystem: "roomserver",
		Name:      "calculate_state_full_state_length",
		Help:      "The length of the full room state.",
	},
	[]string{"algorithm", "outcome"},
)

var calculateStateConflictLength = prometheus.NewSummaryVec(
	prometheus.SummaryOpts{
		Namespace: "dendrite",
		Subsystem: "roomserver",
		Name:      "calculate_state_conflict_state_length",
		Help:      "The length of the conflicted room state.",
	},
	[]string{"algorithm", "outcome"},
)

type calculateStateMetrics struct {
	algorithm       string
	startTime       time.Time
	prevEventLength int
	fullStateLength int
	conflictLength  int
}

func (c *calculateStateMetrics) stop(stateNID types.StateSnapshotNID, err error) (types.StateSnapshotNID, error) {
	var outcome string
	if err == nil {
		outcome = "success"
	} else {
		outcome = "failure"
	}
	endTime := time.Now()
	calculateStateDurations.WithLabelValues(c.algorithm, outcome).Observe(
		float64(endTime.Sub(c.startTime).Nanoseconds()) / 1000.,
	)
	calculateStatePrevEventLength.WithLabelValues(c.algorithm, outcome).Observe(
		float64(c.prevEventLength),
	)
	calculateStateFullStateLength.WithLabelValues(c.algorithm, outcome).Observe(
		float64(c.fullStateLength),
	)
	calculateStateConflictLength.WithLabelValues(c.algorithm, outcome).Observe(
		float64(c.conflictLength),
	)
	return stateNID, err
}

func init() {
	prometheus.MustRegister(
		calculateStateDurations, calculateStatePrevEventLength,
		calculateStateFullStateLength, calculateStateConflictLength,
	)
}

// CalculateAndStoreStateBeforeEvent calculates a snapshot of the state of a room before an event.
// Stores the snapshot of the state in the database.
// Returns a numeric ID for the snapshot of the state before the event.
func CalculateAndStoreStateBeforeEvent(
	ctx context.Context,
	db RoomStateDatabase,
	event gomatrixserverlib.Event,
	roomNID types.RoomNID,
) (types.StateSnapshotNID, error) {
	// Load the state at the prev events.
	prevEventRefs := event.PrevEvents()
	prevEventIDs := make([]string, len(prevEventRefs))
	for i := range prevEventRefs {
		prevEventIDs[i] = prevEventRefs[i].EventID
	}

	prevStates, err := db.StateAtEventIDs(ctx, prevEventIDs)
	if err != nil {
		return 0, err
	}

	// The state before this event will be the state after the events that came before it.
	return CalculateAndStoreStateAfterEvents(ctx, db, roomNID, prevStates)
}

// CalculateAndStoreStateAfterEvents finds the room state after the given events.
// Stores the resulting state in the database and returns a numeric ID for that snapshot.
func CalculateAndStoreStateAfterEvents(
	ctx context.Context,
	db RoomStateDatabase,
	roomNID types.RoomNID,
	prevStates []types.StateAtEvent,
) (types.StateSnapshotNID, error) {
	metrics := calculateStateMetrics{startTime: time.Now(), prevEventLength: len(prevStates)}

	if len(prevStates) == 0 {
		// 2) There weren't any prev_events for this event so the state is
		// empty.
		metrics.algorithm = "empty_state"
		return metrics.stop(db.AddState(ctx, roomNID, nil, nil))
	}

	if len(prevStates) == 1 {
		prevState := prevStates[0]
		if prevState.EventStateKeyNID == 0 {
			// 3) None of the previous events were state events and they all
			// have the same state, so this event has exactly the same state
			// as the previous events.
			// This should be the common case.
			metrics.algorithm = "no_change"
			return metrics.stop(prevState.BeforeStateSnapshotNID, nil)
		}
		// The previous event was a state event so we need to store a copy
		// of the previous state updated with that event.
		stateBlockNIDLists, err := db.StateBlockNIDs(
			ctx, []types.StateSnapshotNID{prevState.BeforeStateSnapshotNID},
		)
		if err != nil {
			metrics.algorithm = "_load_state_blocks"
			return metrics.stop(0, err)
		}
		stateBlockNIDs := stateBlockNIDLists[0].StateBlockNIDs
		if len(stateBlockNIDs) < maxStateBlockNIDs {
			// 4) The number of state data blocks is small enough that we can just
			// add the state event as a block of size one to the end of the blocks.
			metrics.algorithm = "single_delta"
			return metrics.stop(db.AddState(
				ctx, roomNID, stateBlockNIDs, []types.StateEntry{prevState.StateEntry},
			))
		}
		// If there are too many deltas then we need to calculate the full state
		// So fall through to calculateAndStoreStateAfterManyEvents
	}

	return calculateAndStoreStateAfterManyEvents(ctx, db, roomNID, prevStates, metrics)
}

// maxStateBlockNIDs is the maximum number of state data blocks to use to encode a snapshot of room state.
// Increasing this number means that we can encode more of the state changes as simple deltas which means that
// we need fewer entries in the state data table. However making this number bigger will increase the size of
// the rows in the state table itself and will require more index lookups when retrieving a snapshot.
// TODO: Tune this to get the right balance between size and lookup performance.
const maxStateBlockNIDs = 64

// calculateAndStoreStateAfterManyEvents finds the room state after the given events.
// This handles the slow path of calculateAndStoreStateAfterEvents for when there is more than one event.
// Stores the resulting state and returns a numeric ID for the snapshot.
func calculateAndStoreStateAfterManyEvents(
	ctx context.Context,
	db RoomStateDatabase,
	roomNID types.RoomNID,
	prevStates []types.StateAtEvent,
	metrics calculateStateMetrics,
) (types.StateSnapshotNID, error) {

	state, algorithm, conflictLength, err :=
		calculateStateAfterManyEvents(ctx, db, prevStates)
	metrics.algorithm = algorithm
	if err != nil {
		return metrics.stop(0, err)
	}

	// TODO: Check if we can encode the new state as a delta against the
	// previous state.
	metrics.conflictLength = conflictLength
	metrics.fullStateLength = len(state)
	return metrics.stop(db.AddState(ctx, roomNID, nil, state))
}

func calculateStateAfterManyEvents(
	ctx context.Context, db RoomStateDatabase, prevStates []types.StateAtEvent,
) (state []types.StateEntry, algorithm string, conflictLength int, err error) {
	var combined []types.StateEntry
	// Conflict resolution.
	// First stage: load the state after each of the prev events.
	combined, err = LoadCombinedStateAfterEvents(ctx, db, prevStates)
	if err != nil {
		algorithm = "_load_combined_state"
		return
	}

	// Collect all the entries with the same type and key together.
	// We don't care about the order here because the conflict resolution
	// algorithm doesn't depend on the order of the prev events.
	// Remove duplicate entires.
	combined = combined[:util.SortAndUnique(stateEntrySorter(combined))]

	// Find the conflicts
	conflicts := findDuplicateStateKeys(combined)

	if len(conflicts) > 0 {
		conflictLength = len(conflicts)

		// 5) There are conflicting state events, for each conflict workout
		// what the appropriate state event is.

		// Work out which entries aren't conflicted.
		var notConflicted []types.StateEntry
		for _, entry := range combined {
			if _, ok := stateEntryMap(conflicts).lookup(entry.StateKeyTuple); !ok {
				notConflicted = append(notConflicted, entry)
			}
		}

		var resolved []types.StateEntry
		resolved, err = resolveConflicts(ctx, db, notConflicted, conflicts)
		if err != nil {
			algorithm = "_resolve_conflicts"
			return
		}
		algorithm = "full_state_with_conflicts"
		state = resolved
	} else {
		algorithm = "full_state_no_conflicts"
		// 6) There weren't any conflicts
		state = combined
	}
	return
}

// resolveConflicts resolves a list of conflicted state entries. It takes two lists.
// The first is a list of all state entries that are not conflicted.
// The second is a list of all state entries that are conflicted
// A state entry is conflicted when there is more than one numeric event ID for the same state key tuple.
// Returns a list that combines the entries without conflicts with the result of state resolution for the entries with conflicts.
// The returned list is sorted by state key tuple.
// Returns an error if there was a problem talking to the database.
func resolveConflicts(
	ctx context.Context,
	db RoomStateDatabase,
	notConflicted, conflicted []types.StateEntry,
) ([]types.StateEntry, error) {

	// Load the conflicted events
	conflictedEvents, eventIDMap, err := loadStateEvents(ctx, db, conflicted)
	if err != nil {
		return nil, err
	}

	// Work out which auth events we need to load.
	needed := gomatrixserverlib.StateNeededForAuth(conflictedEvents)

	// Find the numeric IDs for the necessary state keys.
	var neededStateKeys []string
	neededStateKeys = append(neededStateKeys, needed.Member...)
	neededStateKeys = append(neededStateKeys, needed.ThirdPartyInvite...)
	stateKeyNIDMap, err := db.EventStateKeyNIDs(ctx, neededStateKeys)
	if err != nil {
		return nil, err
	}

	// Load the necessary auth events.
	tuplesNeeded := stateKeyTuplesNeeded(stateKeyNIDMap, needed)
	var authEntries []types.StateEntry
	for _, tuple := range tuplesNeeded {
		if eventNID, ok := stateEntryMap(notConflicted).lookup(tuple); ok {
			authEntries = append(authEntries, types.StateEntry{
				StateKeyTuple: tuple,
				EventNID:      eventNID,
			})
		}
	}
	authEvents, _, err := loadStateEvents(ctx, db, authEntries)
	if err != nil {
		return nil, err
	}

	// Resolve the conflicts.
	resolvedEvents := gomatrixserverlib.ResolveStateConflicts(conflictedEvents, authEvents)

	// Map from the full events back to numeric state entries.
	for _, resolvedEvent := range resolvedEvents {
		entry, ok := eventIDMap[resolvedEvent.EventID()]
		if !ok {
			panic(fmt.Errorf("Missing state entry for event ID %q", resolvedEvent.EventID()))
		}
		notConflicted = append(notConflicted, entry)
	}

	// Sort the result so it can be searched.
	sort.Sort(stateEntrySorter(notConflicted))
	return notConflicted, nil
}

// stateKeyTuplesNeeded works out which numeric state key tuples we need to authenticate some events.
func stateKeyTuplesNeeded(stateKeyNIDMap map[string]types.EventStateKeyNID, stateNeeded gomatrixserverlib.StateNeeded) []types.StateKeyTuple {
	var keyTuples []types.StateKeyTuple
	if stateNeeded.Create {
		keyTuples = append(keyTuples, types.StateKeyTuple{
			EventTypeNID:     types.MRoomCreateNID,
			EventStateKeyNID: types.EmptyStateKeyNID,
		})
	}
	if stateNeeded.PowerLevels {
		keyTuples = append(keyTuples, types.StateKeyTuple{
			EventTypeNID:     types.MRoomPowerLevelsNID,
			EventStateKeyNID: types.EmptyStateKeyNID,
		})
	}
	if stateNeeded.JoinRules {
		keyTuples = append(keyTuples, types.StateKeyTuple{
			EventTypeNID:     types.MRoomJoinRulesNID,
			EventStateKeyNID: types.EmptyStateKeyNID,
		})
	}
	for _, member := range stateNeeded.Member {
		stateKeyNID, ok := stateKeyNIDMap[member]
		if ok {
			keyTuples = append(keyTuples, types.StateKeyTuple{
				EventTypeNID:     types.MRoomMemberNID,
				EventStateKeyNID: stateKeyNID,
			})
		}
	}
	for _, token := range stateNeeded.ThirdPartyInvite {
		stateKeyNID, ok := stateKeyNIDMap[token]
		if ok {
			keyTuples = append(keyTuples, types.StateKeyTuple{
				EventTypeNID:     types.MRoomThirdPartyInviteNID,
				EventStateKeyNID: stateKeyNID,
			})
		}
	}
	return keyTuples
}

// loadStateEvents loads the matrix events for a list of state entries.
// Returns a list of state events in no particular order and a map from string event ID back to state entry.
// The map can be used to recover which numeric state entry a given event is for.
// Returns an error if there was a problem talking to the database.
func loadStateEvents(
	ctx context.Context, db RoomStateDatabase, entries []types.StateEntry,
) ([]gomatrixserverlib.Event, map[string]types.StateEntry, error) {
	eventNIDs := make([]types.EventNID, len(entries))
	for i := range entries {
		eventNIDs[i] = entries[i].EventNID
	}
	events, err := db.Events(ctx, eventNIDs)
	if err != nil {
		return nil, nil, err
	}
	eventIDMap := map[string]types.StateEntry{}
	result := make([]gomatrixserverlib.Event, len(entries))
	for i := range entries {
		event, ok := eventMap(events).lookup(entries[i].EventNID)
		if !ok {
			panic(fmt.Errorf("Corrupt DB: Missing event numeric ID %d", entries[i].EventNID))
		}
		result[i] = event.Event
		eventIDMap[event.Event.EventID()] = entries[i]
	}
	return result, eventIDMap, nil
}

// findDuplicateStateKeys finds the state entries where the state key tuple appears more than once in a sorted list.
// Returns a sorted list of those state entries.
func findDuplicateStateKeys(a []types.StateEntry) []types.StateEntry {
	var result []types.StateEntry
	// j is the starting index of a block of entries with the same state key tuple.
	j := 0
	for i := 1; i < len(a); i++ {
		// Check if the state key tuple matches the start of the block
		if a[j].StateKeyTuple != a[i].StateKeyTuple {
			// If the state key tuple is different then we've reached the end of a block of duplicates.
			// Check if the size of the block is bigger than one.
			// If the size is one then there was only a single entry with that state key tuple so we don't add it to the result
			if j+1 != i {
				// Add the block to the result.
				result = append(result, a[j:i]...)
			}
			// Start a new block for the next state key tuple.
			j = i
		}
	}
	// Check if the last block with the same state key tuple had more than one event in it.
	if j+1 != len(a) {
		result = append(result, a[j:]...)
	}
	return result
}

type stateEntrySorter []types.StateEntry

func (s stateEntrySorter) Len() int           { return len(s) }
func (s stateEntrySorter) Less(i, j int) bool { return s[i].LessThan(s[j]) }
func (s stateEntrySorter) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }

type stateBlockNIDListMap []types.StateBlockNIDList

func (m stateBlockNIDListMap) lookup(stateNID types.StateSnapshotNID) (stateBlockNIDs []types.StateBlockNID, ok bool) {
	list := []types.StateBlockNIDList(m)
	i := sort.Search(len(list), func(i int) bool {
		return list[i].StateSnapshotNID >= stateNID
	})
	if i < len(list) && list[i].StateSnapshotNID == stateNID {
		ok = true
		stateBlockNIDs = list[i].StateBlockNIDs
	}
	return
}

type stateEntryListMap []types.StateEntryList

func (m stateEntryListMap) lookup(stateBlockNID types.StateBlockNID) (stateEntries []types.StateEntry, ok bool) {
	list := []types.StateEntryList(m)
	i := sort.Search(len(list), func(i int) bool {
		return list[i].StateBlockNID >= stateBlockNID
	})
	if i < len(list) && list[i].StateBlockNID == stateBlockNID {
		ok = true
		stateEntries = list[i].StateEntries
	}
	return
}

type stateEntryByStateKeySorter []types.StateEntry

func (s stateEntryByStateKeySorter) Len() int { return len(s) }
func (s stateEntryByStateKeySorter) Less(i, j int) bool {
	return s[i].StateKeyTuple.LessThan(s[j].StateKeyTuple)
}
func (s stateEntryByStateKeySorter) Swap(i, j int) { s[i], s[j] = s[j], s[i] }

type stateNIDSorter []types.StateSnapshotNID

func (s stateNIDSorter) Len() int           { return len(s) }
func (s stateNIDSorter) Less(i, j int) bool { return s[i] < s[j] }
func (s stateNIDSorter) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }

func uniqueStateSnapshotNIDs(nids []types.StateSnapshotNID) []types.StateSnapshotNID {
	return nids[:util.SortAndUnique(stateNIDSorter(nids))]
}

type stateBlockNIDSorter []types.StateBlockNID

func (s stateBlockNIDSorter) Len() int           { return len(s) }
func (s stateBlockNIDSorter) Less(i, j int) bool { return s[i] < s[j] }
func (s stateBlockNIDSorter) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }

func uniqueStateBlockNIDs(nids []types.StateBlockNID) []types.StateBlockNID {
	return nids[:util.SortAndUnique(stateBlockNIDSorter(nids))]
}

// Map from event type, state key tuple to numeric event ID.
// Implemented using binary search on a sorted array.
type stateEntryMap []types.StateEntry

// lookup an entry in the event map.
func (m stateEntryMap) lookup(stateKey types.StateKeyTuple) (eventNID types.EventNID, ok bool) {
	// Since the list is sorted we can implement this using binary search.
	// This is faster than using a hash map.
	// We don't have to worry about pathological cases because the keys are fixed
	// size and are controlled by us.
	list := []types.StateEntry(m)
	i := sort.Search(len(list), func(i int) bool {
		return !list[i].StateKeyTuple.LessThan(stateKey)
	})
	if i < len(list) && list[i].StateKeyTuple == stateKey {
		ok = true
		eventNID = list[i].EventNID
	}
	return
}

// Map from numeric event ID to event.
// Implemented using binary search on a sorted array.
type eventMap []types.Event

// lookup an entry in the event map.
func (m eventMap) lookup(eventNID types.EventNID) (event *types.Event, ok bool) {
	// Since the list is sorted we can implement this using binary search.
	// This is faster than using a hash map.
	// We don't have to worry about pathological cases because the keys are fixed
	// size are controlled by us.
	list := []types.Event(m)
	i := sort.Search(len(list), func(i int) bool {
		return list[i].EventNID >= eventNID
	})
	if i < len(list) && list[i].EventNID == eventNID {
		ok = true
		event = &list[i]
	}
	return
}
