using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Globalization;
using System.IO;
using System.Linq;
using System.Security.Cryptography;
using System.Text;
using Newtonsoft.Json;
using Newtonsoft.Json.Linq;

namespace game.PlayerDataStorage
{
    public enum PlayerKVSyncStatus { NotLoaded, Ready, ReadFailed, Conflict, UnknownOutcome, Unauthorized, StorageError, Closed }
    public enum PlayerKVOpenError { None, Validation, Busy, StorageError, RecoveryRequired }
    public enum PlayerKVChangeError { None, Validation, Blocked, StorageError }

    public sealed class PlayerKVStateResult
    {
        public bool Success => Error == PlayerKVChangeError.None;
        public PlayerKVChangeError Error { get; }
        public long AttemptSequence { get; }
        public long Revision { get; }
        public bool LocalCommitMayHaveSucceeded { get; }
        internal PlayerKVStateResult(PlayerKVChangeError error, long sequence = 0, long revision = 0, bool uncertain = false)
        { Error = error; AttemptSequence = sequence; Revision = revision; LocalCommitMayHaveSucceeded = uncertain; }
    }

    public sealed class PlayerKVOpenResult
    {
        public bool Success => Error == PlayerKVOpenError.None;
        public PlayerKVOpenError Error { get; }
        public PlayerKVSyncState State { get; }
        internal PlayerKVOpenResult(PlayerKVOpenError error, PlayerKVSyncState state = null) { Error = error; State = state; }
    }

    // Existence is separate from value: an empty string is not a deletion.
    public sealed class PlayerKVValue : IEquatable<PlayerKVValue>
    {
        public static readonly PlayerKVValue Missing = new PlayerKVValue(false, null);
        public bool Exists { get; }
        public string Value { get; }
        PlayerKVValue(bool exists, string value) { Exists = exists; Value = value; }
        public static PlayerKVValue Present(string value) => new PlayerKVValue(true, value ?? throw new ArgumentNullException(nameof(value)));
        public bool Equals(PlayerKVValue other) => other != null && Exists == other.Exists && Value == other.Value;
        public override bool Equals(object obj) => Equals(obj as PlayerKVValue);
        public override int GetHashCode() => Exists ? (Value.GetHashCode() ^ 397) : 0;
        internal static PlayerKVValue At(IReadOnlyDictionary<string, string> data, string key) =>
            data.TryGetValue(key, out var value) ? Present(value) : Missing;
    }

    public sealed class PlayerKVChange
    {
        public string Key { get; }
        public long Sequence { get; }
        public PlayerKVValue BaseValue { get; }
        public PlayerKVValue Target { get; }
        internal PlayerKVChange(string key, long sequence, PlayerKVValue baseline, PlayerKVValue target)
        { Key = key; Sequence = sequence; BaseValue = baseline; Target = target; }
        internal PlayerKVChange WithBase(PlayerKVValue baseline) => new PlayerKVChange(Key, Sequence, baseline, Target);
    }

    public sealed class PlayerKVBatch
    {
        public string Id { get; }
        public IReadOnlyList<PlayerKVChange> Items { get; }
        public bool IsDelete => !Items[0].Target.Exists;
        public bool IsOrdinary
        {
            get;
        }
        internal PlayerKVBatch(string id, IEnumerable<PlayerKVChange> items, bool ordinary = false)
        {
            Id = id;
            Items = Array.AsReadOnly(items.ToArray());
            IsOrdinary = ordinary;
        }
    }

    public sealed class PlayerKVDiagnosticSnapshot
    {
        public long Revision { get; }
        public long NextSequence { get; }
        public bool HasBaseline { get; }
        public bool AuthenticationInvalid { get; }
        public IReadOnlyDictionary<string, string> Baseline { get; }
        public IReadOnlyDictionary<string, string> LocalOnly { get; }
        public IReadOnlyDictionary<string, PlayerKVChange> Pending { get; }
        public IReadOnlyDictionary<string, PlayerKVValue> ConflictObservations { get; }
        public PlayerKVBatch ActiveBatch { get; }
        public PlayerKVBatch UnknownBatch { get; }
        public IReadOnlyDictionary<string, string> LocalData
        {
            get;
        }
        public IReadOnlyDictionary<string, long> LocalUpdateTicks
        {
            get;
        }
        public IReadOnlyList<IReadOnlyDictionary<string, PlayerKVChange>> OrdinaryBuffers
        {
            get;
        }
        public IReadOnlyList<PlayerKVBatch> PreparedOrdinaryBatches
        {
            get;
        }
        public IReadOnlyList<PlayerKVBatch> OrdinaryUnknownBatches
        {
            get;
        }
        public IReadOnlyDictionary<string, PlayerKVChange> LegacyPending
        {
            get;
        }
        public IReadOnlyDictionary<string, PlayerKVChange> ExplicitDeletes
        {
            get;
        }
        internal PlayerKVDiagnosticSnapshot(KVSnapshot source)
        {
            Revision = source.Revision; NextSequence = source.NextSequence; HasBaseline = source.HasBaseline;
            AuthenticationInvalid = source.AuthenticationInvalid;
            Baseline = ReadOnly(source.Baseline); LocalOnly = ReadOnly(source.LocalOnly);
            Pending = ReadOnly(source.Pending); ConflictObservations = ReadOnly(source.Conflicts);
            ActiveBatch = source.Active; UnknownBatch = source.Unknown;
            LocalData = ReadOnly(source.LocalData);
            LocalUpdateTicks = ReadOnly(source.LocalUpdateTicks);
            OrdinaryBuffers = Array.AsReadOnly(source.OrdinaryBuffers.Select(bucket => ReadOnly(bucket)).ToArray());
            PreparedOrdinaryBatches = Array.AsReadOnly(source.PreparedOrdinaryBatches.ToArray());
            OrdinaryUnknownBatches = Array.AsReadOnly(source.OrdinaryUnknownBatches.ToArray());
            LegacyPending = ReadOnly(source.LegacyPending);
            ExplicitDeletes = ReadOnly(source.ExplicitDeletes);
        }
        static IReadOnlyDictionary<string, T> ReadOnly<T>(Dictionary<string, T> source) =>
            new ReadOnlyDictionary<string, T>(new Dictionary<string, T>(source, StringComparer.Ordinal));
    }

    internal sealed class KVIdentity
    {
        internal readonly string Api, PlayerId, DirectoryPath;
        internal KVIdentity(string root, Uri api, string playerId)
        {
            if (string.IsNullOrWhiteSpace(root) || !Path.IsPathFullyQualified(root) || api == null || !api.IsAbsoluteUri ||
                (api.Scheme != "http" && api.Scheme != "https") || api.UserInfo.Length != 0 || api.Query.Length != 0 || api.Fragment.Length != 0 ||
                !long.TryParse(playerId, NumberStyles.None, CultureInfo.InvariantCulture, out var id) || id <= 0)
                throw new ArgumentException("Invalid KV cache identity.");
            Api = api.AbsoluteUri.TrimEnd('/') + "/";
            PlayerId = id.ToString(CultureInfo.InvariantCulture);
            using (var hash = SHA256.Create())
            {
                var bytes = hash.ComputeHash(Encoding.UTF8.GetBytes(Api));
                var fingerprint = string.Concat(bytes.Select(b => b.ToString("x2", CultureInfo.InvariantCulture)));
                DirectoryPath = Path.Combine(Path.GetFullPath(root), "player-kv", "v1", fingerprint, PlayerId);
            }
        }
    }

    internal sealed class KVSnapshot
    {
        internal long Revision, NextSequence = 1;
        internal bool HasBaseline, AuthenticationInvalid;
        internal Dictionary<string, string> Baseline = new Dictionary<string, string>(StringComparer.Ordinal);
        internal Dictionary<string, string> LocalData = new Dictionary<string, string>(StringComparer.Ordinal);
        internal Dictionary<string, long> LocalUpdateTicks = new Dictionary<string, long>(StringComparer.Ordinal);
        // A local origin has no second copy of its value. Ordinary origins can outlive a sent batch.
        internal Dictionary<string, (bool LocalOnly, long Sequence)> LocalOrigins =
            new Dictionary<string, (bool LocalOnly, long Sequence)>(StringComparer.Ordinal);
        internal HashSet<string> ImmediatePreservedKeys = new HashSet<string>(StringComparer.Ordinal);
        // Only v1/v2 conversion can populate this lower layer; the first successful Get removes it.
        internal Dictionary<string, string> LegacyLocalOnlyUnderImmediate = new Dictionary<string, string>(StringComparer.Ordinal);
        internal List<Dictionary<string, PlayerKVChange>> OrdinaryBuffers = Enumerable.Range(0, 5)
            .Select(_ => new Dictionary<string, PlayerKVChange>(StringComparer.Ordinal)).ToList();
        internal List<PlayerKVBatch> PreparedOrdinaryBatches = new List<PlayerKVBatch>();
        internal List<PlayerKVBatch> OrdinaryUnknownBatches = new List<PlayerKVBatch>();
        internal Dictionary<string, PlayerKVChange> LegacyPending = new Dictionary<string, PlayerKVChange>(StringComparer.Ordinal);
        internal Dictionary<string, PlayerKVChange> ExplicitDeletes = new Dictionary<string, PlayerKVChange>(StringComparer.Ordinal);
        internal Dictionary<string, PlayerKVValue> Conflicts = new Dictionary<string, PlayerKVValue>(StringComparer.Ordinal);
        internal Dictionary<long, PlayerKVChange> RemoteOnlyWrites = new Dictionary<long, PlayerKVChange>();
        internal PlayerKVBatch Active, Unknown;

        // Compatibility diagnostics only. A key projection loses cross-bucket sequence history.
        internal Dictionary<string, PlayerKVChange> Pending => Unsent().GroupBy(item => item.Key, StringComparer.Ordinal)
            .ToDictionary(group => group.Key, group => group.OrderBy(item => item.Sequence).Last(), StringComparer.Ordinal);
        internal Dictionary<string, string> LocalOnly => LocalOrigins.Where(pair => pair.Value.LocalOnly && LocalData.ContainsKey(pair.Key))
            .ToDictionary(pair => pair.Key, pair => LocalData[pair.Key], StringComparer.Ordinal);
        internal IEnumerable<PlayerKVChange> Unsent()
        {
            return OrdinaryBuffers.SelectMany(bucket => bucket.Values).Concat(LegacyPending.Values).Concat(ExplicitDeletes.Values);
        }
        internal IEnumerable<PlayerKVChange> OrdinaryOutstanding()
        {
            return OrdinaryBuffers.SelectMany(bucket => bucket.Values)
                .Concat(PreparedOrdinaryBatches.SelectMany(batch => batch.Items))
                .Concat(Active != null && Active.IsOrdinary ? Active.Items : Enumerable.Empty<PlayerKVChange>());
        }
        internal KVSnapshot Clone()
        {
            return new KVSnapshot
            {
                Revision = Revision, NextSequence = NextSequence, HasBaseline = HasBaseline, AuthenticationInvalid = AuthenticationInvalid,
                Baseline = new Dictionary<string, string>(Baseline, StringComparer.Ordinal),
                LocalData = new Dictionary<string, string>(LocalData, StringComparer.Ordinal),
                LocalUpdateTicks = new Dictionary<string, long>(LocalUpdateTicks, StringComparer.Ordinal),
                LocalOrigins = new Dictionary<string, (bool LocalOnly, long Sequence)>(LocalOrigins, StringComparer.Ordinal),
                ImmediatePreservedKeys = new HashSet<string>(ImmediatePreservedKeys, StringComparer.Ordinal),
                LegacyLocalOnlyUnderImmediate = new Dictionary<string, string>(LegacyLocalOnlyUnderImmediate, StringComparer.Ordinal),
                OrdinaryBuffers = OrdinaryBuffers.Select(bucket => new Dictionary<string, PlayerKVChange>(bucket, StringComparer.Ordinal)).ToList(),
                PreparedOrdinaryBatches = new List<PlayerKVBatch>(PreparedOrdinaryBatches),
                OrdinaryUnknownBatches = new List<PlayerKVBatch>(OrdinaryUnknownBatches),
                LegacyPending = new Dictionary<string, PlayerKVChange>(LegacyPending, StringComparer.Ordinal),
                ExplicitDeletes = new Dictionary<string, PlayerKVChange>(ExplicitDeletes, StringComparer.Ordinal),
                Conflicts = new Dictionary<string, PlayerKVValue>(Conflicts, StringComparer.Ordinal),
                Active = Active, Unknown = Unknown,
                RemoteOnlyWrites = new Dictionary<long, PlayerKVChange>(RemoteOnlyWrites)
            };
        }
        internal static bool ValidKey(string key) => !string.IsNullOrEmpty(key) && key.Length <= 128;

        internal string Encode(KVIdentity identity)
        {
            var conflicts = new JObject();
            foreach (var pair in Conflicts)
            {
                conflicts.Add(pair.Key, ValueJson(pair.Value));
            }
            var origins = new JObject();
            foreach (var pair in LocalOrigins)
            {
                origins.Add(pair.Key, new JObject { ["localOnly"] = pair.Value.LocalOnly, ["sequence"] = pair.Value.Sequence });
            }
            return new JObject
            {
                ["formatVersion"] = 3, ["apiBaseUri"] = identity.Api, ["playerId"] = identity.PlayerId,
                ["revision"] = Revision, ["nextSequence"] = NextSequence, ["hasBaseline"] = HasBaseline,
                ["authenticationInvalid"] = AuthenticationInvalid, ["baseline"] = JObject.FromObject(Baseline),
                ["localData"] = JObject.FromObject(LocalData), ["localUpdateTicks"] = JObject.FromObject(LocalUpdateTicks),
                ["localOrigins"] = origins, ["immediatePreservedKeys"] = new JArray(ImmediatePreservedKeys),
                ["legacyLocalOnlyUnderImmediate"] = JObject.FromObject(LegacyLocalOnlyUnderImmediate),
                ["ordinaryBuffers"] = new JArray(OrdinaryBuffers.Select(bucket => (JToken)ChangesJson(bucket.Values))),
                ["preparedOrdinaryBatches"] = new JArray(PreparedOrdinaryBatches.Select(BatchJson)),
                ["ordinaryUnknownBatches"] = new JArray(OrdinaryUnknownBatches.Select(BatchJson)),
                ["legacyPending"] = ChangesJson(LegacyPending.Values), ["explicitDeletes"] = ChangesJson(ExplicitDeletes.Values),
                ["conflictObservations"] = conflicts, ["activeBatch"] = BatchJson(Active), ["unknownBatch"] = BatchJson(Unknown),
                ["remoteOnlyWrites"] = ChangesJson(RemoteOnlyWrites.Values)
            }.ToString(Formatting.None);
        }
        private static JArray ChangesJson(IEnumerable<PlayerKVChange> values)
        {
            return new JArray(values.OrderBy(item => item.Sequence).Select(ChangeJson));
        }
        private static JObject ValueJson(PlayerKVValue value)
        {
            return new JObject { ["exists"] = value.Exists, ["value"] = value.Value };
        }
        private static JObject ChangeJson(PlayerKVChange item)
        {
            return new JObject { ["key"] = item.Key, ["sequence"] = item.Sequence,
                ["base"] = ValueJson(item.BaseValue), ["target"] = ValueJson(item.Target) };
        }
        private static JToken BatchJson(PlayerKVBatch batch)
        {
            if (batch == null)
            {
                return JValue.CreateNull();
            }
            return new JObject { ["id"] = batch.Id, ["ordinary"] = batch.IsOrdinary, ["items"] = ChangesJson(batch.Items) };
        }

        internal static KVSnapshot Decode(string text, KVIdentity identity)
        {
            JObject root;
            using (var reader = new JsonTextReader(new StringReader(text)) { DateParseHandling = DateParseHandling.None })
            {
                root = JObject.Load(reader, new JsonLoadSettings { DuplicatePropertyNameHandling = DuplicatePropertyNameHandling.Error });
                if (reader.Read())
                {
                    throw Invalid();
                }
            }
            var version = Integer(root, "formatVersion");
            if (version < 1 || version > 3 || String(root, "apiBaseUri") != identity.Api || String(root, "playerId") != identity.PlayerId)
            {
                throw Invalid();
            }
            var result = new KVSnapshot
            {
                Revision = Integer(root, "revision"), NextSequence = Integer(root, "nextSequence"),
                HasBaseline = Boolean(root, "hasBaseline"), AuthenticationInvalid = Boolean(root, "authenticationInvalid"),
                Baseline = Strings(Field(root, "baseline", JTokenType.Object)),
                Active = Batch(root, "activeBatch", version == 3), Unknown = Batch(root, "unknownBatch", version == 3)
            };
            foreach (var property in ((JObject)Field(root, "conflictObservations", JTokenType.Object)).Properties())
            {
                if (!ValidKey(property.Name))
                {
                    throw Invalid();
                }
                result.Conflicts.Add(property.Name, Value(property.Value));
            }
            if (version >= 2)
            {
                foreach (var item in Field(root, "remoteOnlyWrites", JTokenType.Array))
                {
                    var change = Change(item);
                    if (!change.Target.Exists || result.RemoteOnlyWrites.ContainsKey(change.Sequence))
                    {
                        throw Invalid();
                    }
                    result.RemoteOnlyWrites.Add(change.Sequence, change);
                }
            }
            if (version < 3)
            {
                DecodeLegacy(root, version, result);
            }
            else
            {
                result.LocalData = Strings(Field(root, "localData", JTokenType.Object));
                result.LegacyLocalOnlyUnderImmediate = Strings(Field(root, "legacyLocalOnlyUnderImmediate", JTokenType.Object));
                foreach (var property in ((JObject)Field(root, "localUpdateTicks", JTokenType.Object)).Properties())
                {
                    if (!ValidKey(property.Name) || property.Value.Type != JTokenType.Integer)
                    {
                        throw Invalid();
                    }
                    result.LocalUpdateTicks.Add(property.Name, (long)property.Value);
                }
                foreach (var property in ((JObject)Field(root, "localOrigins", JTokenType.Object)).Properties())
                {
                    if (!ValidKey(property.Name))
                    {
                        throw Invalid();
                    }
                    result.LocalOrigins.Add(property.Name, (Boolean(property.Value, "localOnly"), Integer(property.Value, "sequence")));
                }
                foreach (var key in Field(root, "immediatePreservedKeys", JTokenType.Array))
                {
                    if (key.Type != JTokenType.String || !ValidKey((string)key) || !result.ImmediatePreservedKeys.Add((string)key))
                    {
                        throw Invalid();
                    }
                }
                result.OrdinaryBuffers = Field(root, "ordinaryBuffers", JTokenType.Array).Select(Changes).ToList();
                result.LegacyPending = Changes(Field(root, "legacyPending", JTokenType.Array));
                result.ExplicitDeletes = Changes(Field(root, "explicitDeletes", JTokenType.Array));
                result.PreparedOrdinaryBatches = ReadBatches(root, "preparedOrdinaryBatches");
                result.OrdinaryUnknownBatches = ReadBatches(root, "ordinaryUnknownBatches");
            }
            result.Validate();
            return result;
        }
        private static Dictionary<string, PlayerKVChange> Changes(JToken array)
        {
            if (array.Type != JTokenType.Array)
            {
                throw Invalid();
            }
            var values = new Dictionary<string, PlayerKVChange>(StringComparer.Ordinal);
            foreach (var token in array)
            {
                var item = Change(token);
                if (values.ContainsKey(item.Key))
                {
                    throw Invalid();
                }
                values.Add(item.Key, item);
            }
            return values;
        }
        private static List<PlayerKVBatch> ReadBatches(JToken root, string name)
        {
            var result = new List<PlayerKVBatch>();
            foreach (var item in Field(root, name, JTokenType.Array))
            {
                var wrapper = new JObject { ["batch"] = item.DeepClone() };
                var batch = Batch(wrapper, "batch", true);
                if (batch == null || !batch.IsOrdinary || batch.IsDelete)
                {
                    throw Invalid();
                }
                result.Add(batch);
            }
            return result;
        }

        private static void DecodeLegacy(JObject root, long version, KVSnapshot result)
        {
            var pending = Changes(Field(root, "pending", JTokenType.Array));
            var localOnly = Strings(Field(root, "localOnly", JTokenType.Object));
            var overrides = new Dictionary<string, PlayerKVValue>(StringComparer.Ordinal);
            if (version == 2)
            {
                foreach (var property in ((JObject)Field(root, "remoteLocalOverrides", JTokenType.Object)).Properties())
                {
                    if (!ValidKey(property.Name))
                    {
                        throw Invalid();
                    }
                    overrides.Add(property.Name, Value(property.Value));
                }
            }
            // Validate the old shape before converting; new format rules must not accept a corrupt old file.
            if (result.Revision < 1 || result.NextSequence < 1 || (result.Active != null && result.Unknown != null) ||
                (!result.HasBaseline && (result.Baseline.Count != 0 || localOnly.Count != 0 || pending.Count != 0 ||
                    result.Active != null || result.Unknown != null || overrides.Count != 0 || result.RemoteOnlyWrites.Count != 0)))
            {
                throw Invalid();
            }
            var sequences = new HashSet<long>();
            foreach (var item in pending.Values.Concat(result.RemoteOnlyWrites.Values))
            {
                if (item.Sequence < 1 || item.Sequence >= result.NextSequence || !sequences.Add(item.Sequence))
                {
                    throw Invalid();
                }
            }
            if (result.Conflicts.Keys.Any(key => !pending.ContainsKey(key)))
            {
                throw Invalid();
            }
            var oldBatch = result.Active ?? result.Unknown;
            if (oldBatch != null)
            {
                foreach (var item in oldBatch.Items)
                {
                    var hasPending = pending.TryGetValue(item.Key, out var latest);
                    var hasSource = result.RemoteOnlyWrites.Values.Any(source => source.Key == item.Key && source.Sequence > item.Sequence);
                    var same = hasPending && latest.Sequence == item.Sequence && latest.BaseValue.Equals(item.BaseValue) && latest.Target.Equals(item.Target);
                    if (item.Sequence < 1 || item.Sequence >= result.NextSequence || (sequences.Contains(item.Sequence) && !same) ||
                        (!hasPending && !hasSource) || (hasPending && (latest.Sequence < item.Sequence || !latest.BaseValue.Equals(item.BaseValue) ||
                            (latest.Sequence == item.Sequence && !same))))
                    {
                        throw Invalid();
                    }
                    sequences.Add(item.Sequence);
                }
            }
            result.LegacyPending = pending;
            result.LocalData = new Dictionary<string, string>(result.Baseline, StringComparer.Ordinal);
            foreach (var item in pending.Values)
            {
                ApplyValue(result.LocalData, item.Key, item.Target);
                result.LocalOrigins[item.Key] = (false, item.Sequence);
            }
            foreach (var pair in localOnly)
            {
                result.LocalData[pair.Key] = pair.Value;
                // Old local-only writes had no stored sequence; zero is permitted only for this converted origin.
                result.LocalOrigins[pair.Key] = (true, 0);
            }
            foreach (var pair in overrides)
            {
                ApplyValue(result.LocalData, pair.Key, pair.Value);
                result.ImmediatePreservedKeys.Add(pair.Key);
                if (localOnly.TryGetValue(pair.Key, out var lower))
                {
                    result.LegacyLocalOnlyUnderImmediate[pair.Key] = lower;
                }
            }
        }
        private static void ApplyValue(Dictionary<string, string> values, string key, PlayerKVValue value)
        {
            if (value.Exists)
            {
                values[key] = value.Value;
            }
            else
            {
                values.Remove(key);
            }
        }

        internal void Validate()
        {
            if (Revision < 1 || NextSequence < 1 || OrdinaryBuffers.Count != 5 ||
                (Active != null && Unknown != null) || (Unknown != null && Unknown.IsOrdinary) ||
                OrdinaryBuffers.Any(bucket => bucket.Count > 10 || bucket.Values.Any(item => !item.Target.Exists)) ||
                ExplicitDeletes.Values.Any(item => item.Target.Exists) ||
                (!HasBaseline && (Baseline.Count != 0 || LocalData.Count != 0 || Unsent().Any() || Active != null || Unknown != null ||
                    PreparedOrdinaryBatches.Count != 0 || OrdinaryUnknownBatches.Count != 0 || RemoteOnlyWrites.Count != 0 ||
                    LocalOrigins.Count != 0 || LocalUpdateTicks.Count != 0 || ImmediatePreservedKeys.Count != 0 || LegacyLocalOnlyUnderImmediate.Count != 0)))
            {
                throw Invalid();
            }
            var sequences = new Dictionary<long, PlayerKVChange>();
            void Add(PlayerKVChange item, bool sourceReference = false)
            {
                if (!ValidKey(item.Key) || item.Sequence < 1 || item.Sequence >= NextSequence)
                {
                    throw Invalid();
                }
                if (sequences.TryGetValue(item.Sequence, out var prior))
                {
                    if (!sourceReference || prior.Key != item.Key || !prior.BaseValue.Equals(item.BaseValue) || !prior.Target.Equals(item.Target))
                    {
                        throw Invalid();
                    }
                }
                else
                {
                    sequences.Add(item.Sequence, item);
                }
            }
            foreach (var item in Unsent().Concat(RemoteOnlyWrites.Values))
            {
                Add(item);
            }
            var ids = new HashSet<string>(StringComparer.Ordinal);
            foreach (var batch in PreparedOrdinaryBatches.Concat(OrdinaryUnknownBatches).Concat(new[] { Active, Unknown }).Where(batch => batch != null))
            {
                if (!Guid.TryParseExact(batch.Id, "N", out _) || !ids.Add(batch.Id) || batch.Items.Count < 1 || batch.Items.Count > 10 ||
                    batch.Items.Select(item => item.Key).Distinct(StringComparer.Ordinal).Count() != batch.Items.Count ||
                    batch.Items.Any(item => item.Target.Exists != batch.Items[0].Target.Exists) || (batch.IsOrdinary && batch.IsDelete) ||
                    !batch.Items.Select(item => item.Sequence).SequenceEqual(batch.Items.Select(item => item.Sequence).OrderBy(value => value)))
                {
                    throw Invalid();
                }
                foreach (var item in batch.Items)
                {
                    var reference = !batch.IsOrdinary && (LegacyPending.Values.Concat(ExplicitDeletes.Values)
                        .Any(source => source.Sequence == item.Sequence));
                    Add(item, reference);
                }
            }
            if (RemoteOnlyWrites.Any(pair => pair.Key != pair.Value.Sequence || !pair.Value.Target.Exists) ||
                PreparedOrdinaryBatches.Concat(OrdinaryUnknownBatches).Any(batch => !batch.IsOrdinary) ||
                LocalOrigins.Any(pair => !ValidKey(pair.Key) || pair.Value.Sequence < 0 || pair.Value.Sequence >= NextSequence ||
                    (!pair.Value.LocalOnly && pair.Value.Sequence == 0)) ||
                LocalUpdateTicks.Any(pair => !ValidKey(pair.Key) || pair.Value < 0 || pair.Value > DateTime.MaxValue.Ticks) ||
                ImmediatePreservedKeys.Any(key => !ValidKey(key)) ||
                LegacyLocalOnlyUnderImmediate.Any(pair => !ImmediatePreservedKeys.Contains(pair.Key) ||
                    !LocalOrigins.TryGetValue(pair.Key, out var origin) || !origin.LocalOnly) ||
                Conflicts.Keys.Any(key => !Unsent().Any(item => item.Key == key) && !PreparedOrdinaryBatches.Any(batch => batch.Items.Any(item => item.Key == key)) &&
                    Unknown == null && OrdinaryUnknownBatches.Count == 0))
            {
                throw Invalid();
            }
        }
        static Dictionary<string, string> Strings(JToken value)
        {
            var result = new Dictionary<string, string>(StringComparer.Ordinal);
            foreach (var property in ((JObject)value).Properties())
            {
                if (!ValidKey(property.Name) || property.Value.Type != JTokenType.String) throw Invalid();
                result.Add(property.Name, (string)property.Value);
            }
            return result;
        }
        static PlayerKVChange Change(JToken item)
        {
            var key = String(item, "key");
            if (!ValidKey(key)) throw Invalid();
            return new PlayerKVChange(key, Integer(item, "sequence"), Value(Field(item, "base", JTokenType.Object)), Value(Field(item, "target", JTokenType.Object)));
        }
        static PlayerKVValue Value(JToken item)
        {
            var exists = Boolean(item, "exists");
            var value = Field(item, "value", exists ? JTokenType.String : JTokenType.Null);
            return exists ? PlayerKVValue.Present((string)value) : PlayerKVValue.Missing;
        }
        static PlayerKVBatch Batch(JToken parent, string name, bool withKind = false)
        {
            var node = parent[name];
            if (node == null) throw Invalid();
            if (node.Type == JTokenType.Null) return null;
            var id = String(node, "id");
            if (!Guid.TryParseExact(id, "N", out _)) throw Invalid();
            var items = Field(node, "items", JTokenType.Array).Select(Change).ToArray();
            if (items.Length == 0 || items.Length > 10 || items.Select(i => i.Key).Distinct(StringComparer.Ordinal).Count() != items.Length ||
                items.Select(i => i.Sequence).Distinct().Count() != items.Length || items.Any(i => i.Target.Exists != items[0].Target.Exists) ||
                !items.Select(i => i.Sequence).SequenceEqual(items.Select(i => i.Sequence).OrderBy(s => s))) throw Invalid();
            return new PlayerKVBatch(id, items, withKind && Boolean(node, "ordinary"));
        }
        static JToken Field(JToken parent, string name, JTokenType type)
        {
            if (!(parent is JObject obj) || obj[name] == null || obj[name].Type != type) throw Invalid();
            return obj[name];
        }
        static string String(JToken parent, string name) => (string)Field(parent, name, JTokenType.String);
        static long Integer(JToken parent, string name) => (long)Field(parent, name, JTokenType.Integer);
        static bool Boolean(JToken parent, string name) => (bool)Field(parent, name, JTokenType.Boolean);
        internal static JsonException Invalid() => new JsonException("Invalid KV cache snapshot.");
    }
}