diff --git a/Sources/FirebladeECS/EntityIdentifierGenerator.swift b/Sources/FirebladeECS/EntityIdentifierGenerator.swift
index ef324a5..c371b10 100644
--- a/Sources/FirebladeECS/EntityIdentifierGenerator.swift
+++ b/Sources/FirebladeECS/EntityIdentifierGenerator.swift
@@ -17,7 +17,7 @@ public protocol EntityIdentifierGenerator {
     /// Entity ids provided should be passed to `nextId()` in last out order up until the collection is empty.
     /// The default is an empty collection.
     /// - Parameter initialEntityIds: The entity ids to start providing up until the collection is empty (in last out order).
-    init<EntityIds>(startProviding initialEntityIds: EntityIds) where EntityIds: BidirectionalCollection, EntityIds.Element == EntityIdentifier
+    init<EntityIds>(startProviding initialEntityIds: EntityIds) where EntityIds: BidirectionalCollection & MutableCollection, EntityIds.Element == EntityIdentifier
 
     /// Provides the next unused entity identifier.
     ///
@@ -41,8 +41,19 @@ public struct DefaultEntityIdGenerator: EntityIdentifierGenerator {
         @usableFromInline var count: Int { stack.count }
 
         @usableFromInline
-        init<EntityIds>(startProviding initialEntityIds: EntityIds) where EntityIds: BidirectionalCollection, EntityIds.Element == EntityIdentifier {
-            stack = initialEntityIds.map { $0.id }
+        init<EntityIds>(startProviding initialEntityIds: EntityIds) where EntityIds: BidirectionalCollection & MutableCollection, EntityIds.Element == EntityIdentifier {
+            let initialInUse: [EntityIdentifier.Identifier] = initialEntityIds.map { $0.id }
+            let maxInUseValue = initialInUse.max() ?? 0
+            let inUseSet = Set(initialInUse) // a set of all eIds in use
+            let allSet = Set(0...maxInUseValue) // all eIds from 0 to including maxInUseValue
+            let freeSet = allSet.subtracting(inUseSet) // all "holes" / unused / free eIds
+            let initialFree = Array(freeSet).sorted().reversed() // order them to provide them linear increasing after all initially used are provided.
+            stack = initialFree + initialInUse
+        }
+
+        @usableFromInline
+        init() {
+            stack = [0]
         }
 
         @usableFromInline
@@ -65,16 +76,16 @@ public struct DefaultEntityIdGenerator: EntityIdentifierGenerator {
     @usableFromInline var count: Int { storage.count }
 
     @inlinable
-    public init<EntityIds>(startProviding initialEntityIds: EntityIds) where EntityIds: BidirectionalCollection, EntityIds.Element == EntityIdentifier {
+    public init<EntityIds>(startProviding initialEntityIds: EntityIds) where EntityIds: BidirectionalCollection & MutableCollection, EntityIds.Element == EntityIdentifier {
         self.storage = Storage(startProviding: initialEntityIds)
     }
 
     @inlinable
     public init() {
-        self.init(startProviding: [EntityIdentifier(0)])
+        self.storage = Storage()
     }
 
-    @inline(__always)
+    //@inline(__always)
     public func nextId() -> EntityIdentifier {
         storage.nextId()
     }
diff --git a/Tests/FirebladeECSTests/EntityIdGenTests.swift b/Tests/FirebladeECSTests/EntityIdGenTests.swift
new file mode 100644
index 0000000..bab14de
--- /dev/null
+++ b/Tests/FirebladeECSTests/EntityIdGenTests.swift
@@ -0,0 +1,76 @@
+//
+//  EntityIdGenTests.swift
+//
+//
+//  Created by Christian Treffs on 21.08.20.
+//
+
+import FirebladeECS
+import XCTest
+
+final class EntityIdGenTests: XCTestCase {
+    var gen: EntityIdentifierGenerator!
+
+    override func setUp() {
+        super.setUp()
+        gen = DefaultEntityIdGenerator()
+    }
+
+    func testGeneratorDefaultInit() {
+        XCTAssertEqual(gen.nextId(), 0)
+    }
+
+    func testLinearIncrement() {
+        for i in 0..<1_000_000 {
+            XCTAssertEqual(gen.nextId(), EntityIdentifier(EntityIdentifier.Identifier(i)))
+        }
+    }
+
+    func testGenerateWithInitialIds() {
+        let initialIds: [EntityIdentifier] = [2, 4, 11, 3, 0, 304]
+        gen = DefaultEntityIdGenerator(startProviding: initialIds)
+
+        let generatedIds: [EntityIdentifier] = (0..<initialIds.count).map { _ in gen.nextId() }.reversed()
+        XCTAssertEqual(initialIds, generatedIds)
+        XCTAssertEqual(gen.nextId(), 1)
+        XCTAssertEqual(gen.nextId(), 5)
+        XCTAssertEqual(gen.nextId(), 6)
+        XCTAssertEqual(gen.nextId(), 7)
+        XCTAssertEqual(gen.nextId(), 8)
+        XCTAssertEqual(gen.nextId(), 9)
+        XCTAssertEqual(gen.nextId(), 10)
+        XCTAssertEqual(gen.nextId(), 12)
+
+        for i in 13...304 {
+            XCTAssertEqual(gen.nextId(), EntityIdentifier(EntityIdentifier.Identifier(i)))
+        }
+
+        XCTAssertEqual(gen.nextId(), 305)
+    }
+
+    func testGeneratorMarkUnused() {
+        XCTAssertEqual(gen.nextId(), 0)
+        XCTAssertEqual(gen.nextId(), 1)
+        XCTAssertEqual(gen.nextId(), 2)
+
+        gen.markUnused(entityId: EntityIdentifier(1))
+
+        XCTAssertEqual(gen.nextId(), 1)
+        XCTAssertEqual(gen.nextId(), 3)
+        XCTAssertEqual(gen.nextId(), 4)
+
+        gen.markUnused(entityId: 3)
+        gen.markUnused(entityId: 0)
+
+        XCTAssertEqual(gen.nextId(), 0)
+        XCTAssertEqual(gen.nextId(), 3)
+
+        gen.markUnused(entityId: 3)
+
+        XCTAssertEqual(gen.nextId(), 3)
+        XCTAssertEqual(gen.nextId(), 5)
+        XCTAssertEqual(gen.nextId(), 6)
+        XCTAssertEqual(gen.nextId(), 7)
+        XCTAssertEqual(gen.nextId(), 8)
+    }
+}
diff --git a/Tests/FirebladeECSTests/XCTestManifests.swift b/Tests/FirebladeECSTests/XCTestManifests.swift
index 749f078..dc8b4df 100644
--- a/Tests/FirebladeECSTests/XCTestManifests.swift
+++ b/Tests/FirebladeECSTests/XCTestManifests.swift
@@ -32,6 +32,18 @@ extension EntityCreationTests {
     ]
 }
 
+extension EntityIdGenTests {
+    // DO NOT MODIFY: This is autogenerated, use:
+    //   `swift test --generate-linuxmain`
+    // to regenerate.
+    static let __allTests__EntityIdGenTests = [
+        ("testGenerateWithInitialIds", testGenerateWithInitialIds),
+        ("testGeneratorDefaultInit", testGeneratorDefaultInit),
+        ("testGeneratorMarkUnused", testGeneratorMarkUnused),
+        ("testLinearIncrement", testLinearIncrement)
+    ]
+}
+
 extension EntityTests {
     // DO NOT MODIFY: This is autogenerated, use:
     //   `swift test --generate-linuxmain`
@@ -292,6 +304,7 @@ public func __allTests() -> [XCTestCaseEntry] {
         testCase(ComponentIdentifierTests.__allTests__ComponentIdentifierTests),
         testCase(ComponentTests.__allTests__ComponentTests),
         testCase(EntityCreationTests.__allTests__EntityCreationTests),
+        testCase(EntityIdGenTests.__allTests__EntityIdGenTests),
         testCase(EntityTests.__allTests__EntityTests),
         testCase(Family1Tests.__allTests__Family1Tests),
         testCase(Family2Tests.__allTests__Family2Tests),