FlowScope/Packages/com.flowscope.gamecore/Runtime/Save/SaveMigrationRegistry.cs

using System;
using System.Collections.Generic;

namespace FlowScope.Save
{
    public sealed class SaveMigrationRegistry
    {
        private readonly Dictionary<MigrationKey, ISaveMigration> _migrations = new();

        public void Register(ISaveMigration migration)
        {
            if (migration == null)
            {
                throw new ArgumentNullException(nameof(migration));
            }

            _migrations[new MigrationKey(migration.Key, migration.FromVersion)] = migration;
        }

        public string Migrate(
            string key,
            int fromVersion,
            int currentVersion,
            string json)
        {
            var current = fromVersion;
            var payload = json;
            while (current < currentVersion)
            {
                if (!_migrations.TryGetValue(new MigrationKey(key, current), out var migration) ||
                    migration.ToVersion != current + 1)
                {
                    throw new InvalidOperationException(
                        $"Missing save migration for key '{key}' from version {current}.");
                }

                payload = migration.Migrate(payload);
                current = migration.ToVersion;
            }

            return payload;
        }

        private readonly struct MigrationKey : IEquatable<MigrationKey>
        {
            private readonly string _key;
            private readonly int _fromVersion;

            public MigrationKey(string key, int fromVersion)
            {
                _key = key ?? string.Empty;
                _fromVersion = fromVersion;
            }

            public bool Equals(MigrationKey other)
            {
                return _key == other._key && _fromVersion == other._fromVersion;
            }

            public override bool Equals(object obj)
            {
                return obj is MigrationKey other && Equals(other);
            }

            public override int GetHashCode()
            {
                unchecked
                {
                    return ((_key != null ? _key.GetHashCode() : 0) * 397) ^ _fromVersion;
                }
            }
        }
    }
}