Refine Nexus serialization

Sources/FirebladeECS/Nexus+Codable.swift

@@ -15,8 +15,10 @@ extension Nexus: Encodable {
 
 extension Nexus: Decodable {
     public convenience init(from decoder: Decoder) throws {
+        self.init()
+
         let container = try decoder.singleValueContainer()
         let sNexus = try container.decode(SNexus.self)
-        try self.init(from: sNexus)
+        try deserialize(from: sNexus, into: self)
     }
 }

Sources/FirebladeECS/Nexus+Serialization.swift

@@ -10,8 +10,6 @@ import struct Foundation.Data
 
 extension Nexus {
     final func serialize() throws -> SNexus {
-        let version = Version(major: 1, minor: 0, patch: 0)
-
         var componentInstances: [ComponentIdentifier.StableId: SComponent<SNexus>] = [:]
         var entityComponentsMap: [EntityIdentifier: Set<ComponentIdentifier.StableId>] = [:]
 
@@ -32,21 +30,15 @@ extension Nexus {
                       components: componentInstances)
     }
 
-    convenience init(from sNexus: SNexus) throws {
-        let entityIds = sNexus.entities.map { $0.key }.reversed()
-
-        // FIXME: this does not respect the generator of the target nexus!
-        self.init(componentsByType: [:],
-                  componentsByEntity: [:],
-                  entityIdGenerator: DefaultEntityIdGenerator(startProviding: entityIds),
-                  familyMembersByTraits: [:],
-                  codingStrategy: DefaultCodingStrategy())
+    final func deserialize(from sNexus: SNexus, into nexus: Nexus) throws {
+        for freeId in sNexus.entities.map { $0.key }.reversed() {
+            nexus.entityIdGenerator.markUnused(entityId: freeId)
+        }
 
         for componentSet in sNexus.entities.values {
             let entity = self.createEntity()
             for sCompId in componentSet {
                 guard let sComp = sNexus.components[sCompId] else {
-                    // FIXME: we want a dedicated error
                     throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: [], debugDescription: "Could not find component instance for \(sCompId)."))
                 }
 

Sources/FirebladeECS/Nexus.swift

@@ -6,6 +6,11 @@
 //
 
 public final class Nexus {
+    /// The version of this Nexus implementation.
+    ///
+    /// Used for serialization.
+    final let version = Version(0, 18, 0)
+
     /// - Key: ComponentIdentifier aka component type.
     /// - Value: Array of component instances of same type (uniform).
     ///          New component instances are appended.