use go module for dependencies (#594)

2 files changed, 1022 insertions, 0 deletions

diff --git a/roomserver/state/state.go b/roomserver/state/state.go
new file mode 100644
index 00000000..2a0b7f57
--- /dev/null
+++ b/roomserver/state/state.go
@@ -0,0 +1,966 @@
+// 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
+}
diff --git a/roomserver/state/state_test.go b/roomserver/state/state_test.go
new file mode 100644
index 00000000..67af1867
--- /dev/null
+++ b/roomserver/state/state_test.go
@@ -0,0 +1,56 @@
+// 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
+
+import (
+	"testing"
+
+	"github.com/matrix-org/dendrite/roomserver/types"
+)
+
+func TestFindDuplicateStateKeys(t *testing.T) {
+	testCases := []struct {
+		Input []types.StateEntry
+		Want  []types.StateEntry
+	}{{
+		Input: []types.StateEntry{
+			{StateKeyTuple: types.StateKeyTuple{EventTypeNID: 1, EventStateKeyNID: 1}, EventNID: 1},
+			{StateKeyTuple: types.StateKeyTuple{EventTypeNID: 1, EventStateKeyNID: 1}, EventNID: 2},
+			{StateKeyTuple: types.StateKeyTuple{EventTypeNID: 2, EventStateKeyNID: 2}, EventNID: 3},
+		},
+		Want: []types.StateEntry{
+			{StateKeyTuple: types.StateKeyTuple{EventTypeNID: 1, EventStateKeyNID: 1}, EventNID: 1},
+			{StateKeyTuple: types.StateKeyTuple{EventTypeNID: 1, EventStateKeyNID: 1}, EventNID: 2},
+		},
+	}, {
+		Input: []types.StateEntry{
+			{StateKeyTuple: types.StateKeyTuple{EventTypeNID: 1, EventStateKeyNID: 1}, EventNID: 1},
+			{StateKeyTuple: types.StateKeyTuple{EventTypeNID: 1, EventStateKeyNID: 2}, EventNID: 2},
+		},
+		Want: nil,
+	}}
+
+	for _, test := range testCases {
+		got := findDuplicateStateKeys(test.Input)
+		if len(got) != len(test.Want) {
+			t.Fatalf("Wanted %v, got %v", test.Want, got)
+		}
+		for i := range got {
+			if got[i] != test.Want[i] {
+				t.Fatalf("Wanted %v, got %v", test.Want, got)
+			}
+		}
+	}
+}