diff --git a/.github/workflows/core-tests.yml b/.github/workflows/core-tests.yml
new file mode 100644
index 00000000..b9263bb7
--- /dev/null
+++ b/.github/workflows/core-tests.yml
@@ -0,0 +1,19 @@
+name: VisuStructCore tests
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  swift-test:
+    runs-on: macos-15
+    defaults:
+      run:
+        working-directory: VisuStruct-iOS/VisuStructCore
+    steps:
+      - uses: actions/checkout@v4
+      - name: Swift version
+        run: swift --version
+      - name: Build and test
+        run: swift test
diff --git a/VisuStruct-iOS/VisuStructCore/Sources/VisuStructCore/CodeGeneration/VisuStructCodeGenerator+JavaClassFile.swift b/VisuStruct-iOS/VisuStructCore/Sources/VisuStructCore/CodeGeneration/VisuStructCodeGenerator+JavaClassFile.swift
index 40d72a84..72f2c96c 100644
--- a/VisuStruct-iOS/VisuStructCore/Sources/VisuStructCore/CodeGeneration/VisuStructCodeGenerator+JavaClassFile.swift
+++ b/VisuStruct-iOS/VisuStructCore/Sources/VisuStructCore/CodeGeneration/VisuStructCodeGenerator+JavaClassFile.swift
@@ -30,6 +30,10 @@ extension VisuStructCodeGenerator {
         }
         result = result.trimmingCharacters(in: CharacterSet(charactersIn: "_"))
         if result.isEmpty { return "Main" }
+        // Das Trimmen oben entfernt auch den `_` vor einer führenden Ziffer.
+        if let first = result.unicodeScalars.first, CharacterSet.decimalDigits.contains(first) {
+            result = "_" + result
+        }
         let reserved: Set<String> = [
             "abstract", "assert", "boolean", "break", "byte", "case", "catch", "char", "class", "const",
             "continue", "default", "do", "double", "else", "enum", "extends", "final", "finally",
diff --git a/VisuStruct-iOS/VisuStructCore/Tests/VisuStructCoreTests/CodeGeneratorTests.swift b/VisuStruct-iOS/VisuStructCore/Tests/VisuStructCoreTests/CodeGeneratorTests.swift
new file mode 100644
index 00000000..579a7a20
--- /dev/null
+++ b/VisuStruct-iOS/VisuStructCore/Tests/VisuStructCoreTests/CodeGeneratorTests.swift
@@ -0,0 +1,50 @@
+import XCTest
+import VisuStructCore
+
+final class CodeGeneratorTests: XCTestCase {
+    /// Die mitgelieferten `.java`-Dateien sind der erwartete Export der zugehörigen Diagramme.
+    func testJavaClassFileMatchesExamples() throws {
+        for name in ["Collatz", "Fibonacci"] {
+            let document = try ExampleFiles.document(named: "\(name).visustruct")
+            let generated = VisuStructCodeGenerator.generateJavaClassFile(document: document, className: name)
+            XCTAssertEqual(generated, try ExampleFiles.text(named: "\(name).java"), name)
+        }
+    }
+
+    func testJavaClassFileWithInputImportsScannerOnce() throws {
+        let document = try ExampleFiles.document(named: "2026-04-23-NumberEven.visustruct")
+        let generated = VisuStructCodeGenerator.generateJavaClassFile(document: document, className: "NumberEven")
+        XCTAssertTrue(generated.hasPrefix("import java.util.Scanner;\n\npublic class NumberEven {\n"))
+        XCTAssertEqual(generated.components(separatedBy: "new Scanner(System.in)").count - 1, 1)
+        XCTAssertTrue(generated.contains("if (number % 2 == 0) {"))
+    }
+
+    func testCollatzInPythonAndJavaScript() throws {
+        let document = try ExampleFiles.document(named: "Collatz.visustruct")
+
+        let python = VisuStructCodeGenerator.generate(document: document, target: .python)
+        XCTAssertTrue(python.contains("while number != 1:"), python)
+        XCTAssertTrue(python.contains("print(number)"), python)
+        XCTAssertFalse(python.contains("{"), python)
+
+        let javaScript = VisuStructCodeGenerator.generate(document: document, target: .javascript)
+        XCTAssertTrue(javaScript.hasPrefix("\"use strict\";"), javaScript)
+        XCTAssertTrue(javaScript.contains("while (number != 1) {"), javaScript)
+        XCTAssertTrue(javaScript.contains("console.log(number);"), javaScript)
+    }
+
+    func testSanitizedJavaClassName() {
+        XCTAssertEqual(VisuStructCodeGenerator.sanitizedJavaClassName("Fibonacci"), "Fibonacci")
+        let sanitized = VisuStructCodeGenerator.sanitizedJavaClassName("2 Summe bilden!")
+        XCTAssertFalse(sanitized.isEmpty)
+        XCTAssertFalse(sanitized.contains(" "))
+        XCTAssertFalse(sanitized.first?.isNumber ?? true)
+    }
+
+    func testEmptyDocumentProducesNonEmptyPlaceholder() {
+        for target in [VisuStructCodeGenerator.Target.java, .python, .javascript] {
+            let code = VisuStructCodeGenerator.generate(document: VisuStructDocument(), target: target)
+            XCTAssertFalse(code.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty, target.rawValue)
+        }
+    }
+}
diff --git a/VisuStruct-iOS/VisuStructCore/Tests/VisuStructCoreTests/ExampleFilesTests.swift b/VisuStruct-iOS/VisuStructCore/Tests/VisuStructCoreTests/ExampleFilesTests.swift
new file mode 100644
index 00000000..e028bde9
--- /dev/null
+++ b/VisuStruct-iOS/VisuStructCore/Tests/VisuStructCoreTests/ExampleFilesTests.swift
@@ -0,0 +1,36 @@
+import XCTest
+import VisuStructCore
+
+/// Jede Beispieldatei muss sich laden, verlustfrei speichern und in alle Zielsprachen übersetzen lassen.
+final class ExampleFilesTests: XCTestCase {
+    func testExampleDirectoryContainsDiagrams() throws {
+        XCTAssertFalse(try ExampleFiles.allDiagramURLs().isEmpty, "Keine Beispieldateien gefunden in \(ExampleFiles.directory.path)")
+    }
+
+    func testAllExamplesDecodeAndRoundTrip() throws {
+        for url in try ExampleFiles.allDiagramURLs() {
+            let name = url.lastPathComponent
+            let decoded = try VisuStructXMLCodec.decode(url: url)
+            XCTAssertFalse(decoded.elements.isEmpty, "\(name): keine Elemente")
+
+            let reencoded = VisuStructXMLCodec.encode(decoded)
+            let decodedAgain = try VisuStructXMLCodec.decode(data: reencoded)
+            XCTAssertEqual(decodedAgain, decoded, "\(name): Speichern und erneutes Laden verändert das Diagramm")
+        }
+    }
+
+    func testAllExamplesGenerateCodeForEveryTarget() throws {
+        let targets: [VisuStructCodeGenerator.Target] = [.java, .python, .javascript]
+        for url in try ExampleFiles.allDiagramURLs() {
+            let name = url.lastPathComponent
+            let document = try VisuStructXMLCodec.decode(url: url)
+            for target in targets {
+                let code = VisuStructCodeGenerator.generate(document: document, target: target)
+                XCTAssertFalse(
+                    code.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty,
+                    "\(name): leerer Code für \(target.rawValue)"
+                )
+            }
+        }
+    }
+}
diff --git a/VisuStruct-iOS/VisuStructCore/Tests/VisuStructCoreTests/SimulationEngineTests.swift b/VisuStruct-iOS/VisuStructCore/Tests/VisuStructCoreTests/SimulationEngineTests.swift
new file mode 100644
index 00000000..a1b4cd10
--- /dev/null
+++ b/VisuStruct-iOS/VisuStructCore/Tests/VisuStructCoreTests/SimulationEngineTests.swift
@@ -0,0 +1,113 @@
+import XCTest
+import VisuStructCore
+
+final class SimulationEngineTests: XCTestCase {
+    func testCollatzExampleOutputsSequence() throws {
+        let state = runSimulation(try ExampleFiles.document(named: "Collatz.visustruct"))
+        XCTAssertEqual(
+            state.outputLines,
+            ["58", "29", "88", "44", "22", "11", "34", "17", "52", "26", "13", "40", "20", "10", "5", "16", "8", "4", "2", "1"]
+        )
+        XCTAssertEqual(state.variables["number"], .int(1))
+    }
+
+    func testFibonacciExampleOutputsFirstTenNumbers() throws {
+        let state = runSimulation(try ExampleFiles.document(named: "Fibonacci.visustruct"))
+        XCTAssertEqual(state.outputLines, ["1", "1", "2", "3", "5", "8", "13", "21", "34", "55"])
+    }
+
+    func testNumberEvenExampleUsesInput() throws {
+        let document = try ExampleFiles.document(named: "2026-04-23-NumberEven.visustruct")
+        XCTAssertEqual(runSimulation(document, inputs: ["4"]).outputLines, ["number is even"])
+        XCTAssertEqual(runSimulation(document, inputs: ["7"]).outputLines, ["number is odd"])
+    }
+
+    func testInvalidIntegerInputIsRejectedAndCanBeRetried() {
+        let document = VisuStructDocument(elements: [
+            VisuStructElement(type: .anweisung, textLines: ["input: int n"]),
+            VisuStructElement(type: .anweisung, textLines: ["print: n * 2"]),
+        ])
+        var engine = VisuStructSimulationEngine(document: document)
+        engine.step()
+        XCTAssertEqual(engine.state.inputRequest?.variableName, "n")
+
+        engine.provideInput("abc")
+        XCTAssertNotNil(engine.state.inputError)
+        XCTAssertNotNil(engine.state.inputRequest)
+
+        engine.provideInput("21")
+        XCTAssertNil(engine.state.inputRequest)
+        engine.step()
+        XCTAssertEqual(engine.state.outputLines, ["42"])
+    }
+
+    func testCountingLoops() {
+        let forLoop = VisuStructDocument(elements: [
+            VisuStructElement(type: .anweisung, textLines: ["int sum = 0"]),
+            VisuStructElement(
+                type: .forSchleife,
+                textLines: ["int i = 1", "i <= 4", "i++"],
+                loopBody: [VisuStructElement(type: .anweisung, textLines: ["sum += i"])]
+            ),
+        ])
+        XCTAssertEqual(runSimulation(forLoop).variables["sum"], .int(10))
+
+        let doUntil = VisuStructDocument(elements: [
+            VisuStructElement(type: .anweisung, textLines: ["int k = 0"]),
+            VisuStructElement(
+                type: .doUntilSchleife,
+                textLines: ["k < 3"],
+                loopBody: [VisuStructElement(type: .anweisung, textLines: ["k++"])]
+            ),
+        ])
+        XCTAssertEqual(runSimulation(doUntil).variables["k"], .int(3))
+    }
+
+    func testSwitchFallsBackToDefaultCase() {
+        func document(day: Int) -> VisuStructDocument {
+            VisuStructDocument(elements: [
+                VisuStructElement(type: .anweisung, textLines: ["int day = \(day)"]),
+                VisuStructElement(
+                    type: .fallauswahl,
+                    textLines: ["day"],
+                    cases: [
+                        VisuStructCase(name: "1", elements: [VisuStructElement(type: .anweisung, textLines: ["print: \"Montag\""])]),
+                        VisuStructCase(name: "2", elements: [VisuStructElement(type: .anweisung, textLines: ["print: \"Dienstag\""])]),
+                        VisuStructCase(name: "default", elements: [VisuStructElement(type: .anweisung, textLines: ["print: \"anderer Tag\""])]),
+                    ]
+                ),
+            ])
+        }
+        XCTAssertEqual(runSimulation(document(day: 2)).outputLines, ["Dienstag"])
+        XCTAssertEqual(runSimulation(document(day: 5)).outputLines, ["anderer Tag"])
+    }
+
+    func testBooleanOperatorsInConditions() {
+        func document(_ condition: String) -> VisuStructDocument {
+            VisuStructDocument(elements: [
+                VisuStructElement(type: .anweisung, textLines: ["int a = 5"]),
+                VisuStructElement(
+                    type: .verzweigung,
+                    textLines: [condition],
+                    cases: [
+                        VisuStructCase(name: "true", elements: [VisuStructElement(type: .anweisung, textLines: ["print: \"ja\""])]),
+                        VisuStructCase(name: "false", elements: [VisuStructElement(type: .anweisung, textLines: ["print: \"nein\""])]),
+                    ]
+                ),
+            ])
+        }
+        XCTAssertEqual(runSimulation(document("a > 3 && a < 10")).outputLines, ["ja"])
+        XCTAssertEqual(runSimulation(document("a < 3 || a == 5")).outputLines, ["ja"])
+        XCTAssertEqual(runSimulation(document("a != 5")).outputLines, ["nein"])
+    }
+
+    func testDivisionByZeroReportsError() {
+        let document = VisuStructDocument(elements: [
+            VisuStructElement(type: .anweisung, textLines: ["int x = 1 / 0"]),
+        ])
+        var engine = VisuStructSimulationEngine(document: document)
+        engine.step()
+        XCTAssertEqual(engine.state.message, "Division by zero.")
+        XCTAssertNil(engine.state.variables["x"])
+    }
+}
diff --git a/VisuStruct-iOS/VisuStructCore/Tests/VisuStructCoreTests/TestSupport.swift b/VisuStruct-iOS/VisuStructCore/Tests/VisuStructCoreTests/TestSupport.swift
new file mode 100644
index 00000000..22d44a35
--- /dev/null
+++ b/VisuStruct-iOS/VisuStructCore/Tests/VisuStructCoreTests/TestSupport.swift
@@ -0,0 +1,61 @@
+import Foundation
+import XCTest
+import VisuStructCore
+
+/// Beispieldiagramme im Repo-Ordner `VisuStruct-Dateien` (relativ zu dieser Datei aufgelöst).
+enum ExampleFiles {
+    static let directory: URL = URL(fileURLWithPath: #filePath)
+        .deletingLastPathComponent() // VisuStructCoreTests
+        .deletingLastPathComponent() // Tests
+        .deletingLastPathComponent() // VisuStructCore
+        .deletingLastPathComponent() // VisuStruct-iOS
+        .deletingLastPathComponent() // Repo-Wurzel
+        .appendingPathComponent("VisuStruct-Dateien", isDirectory: true)
+
+    /// Alle `.visustruct`-Dateien, rekursiv, sortiert.
+    static func allDiagramURLs() throws -> [URL] {
+        guard let enumerator = FileManager.default.enumerator(at: directory, includingPropertiesForKeys: nil) else {
+            return []
+        }
+        return enumerator
+            .compactMap { $0 as? URL }
+            .filter { $0.pathExtension == "visustruct" }
+            .sorted { $0.path < $1.path }
+    }
+
+    static func document(named name: String) throws -> VisuStructDocument {
+        try VisuStructXMLCodec.decode(url: directory.appendingPathComponent(name))
+    }
+
+    static func text(named name: String) throws -> String {
+        try String(contentsOf: directory.appendingPathComponent(name), encoding: .utf8)
+    }
+}
+
+/// Lässt die Simulation bis zum Ende laufen und beantwortet `input:`-Anfragen der Reihe nach.
+func runSimulation(
+    _ document: VisuStructDocument,
+    inputs: [String] = [],
+    maxSteps: Int = 10_000,
+    file: StaticString = #filePath,
+    line: UInt = #line
+) -> VisuStructSimulationState {
+    var engine = VisuStructSimulationEngine(document: document)
+    var remainingInputs = inputs[...]
+    var stepsTaken = 0
+    while stepsTaken < maxSteps {
+        if engine.state.inputRequest != nil {
+            guard let next = remainingInputs.popFirst() else {
+                XCTFail("Simulation wartet auf Eingabe, aber es sind keine Eingaben mehr übrig.", file: file, line: line)
+                return engine.state
+            }
+            engine.provideInput(next)
+            continue
+        }
+        guard engine.canStep else { break }
+        engine.step()
+        stepsTaken += 1
+    }
+    XCTAssertLessThan(stepsTaken, maxSteps, "Simulation endet nicht.", file: file, line: line)
+    return engine.state
+}
diff --git a/VisuStruct-iOS/VisuStructCore/Tests/VisuStructCoreTests/VisuStructXMLCodecTests.swift b/VisuStruct-iOS/VisuStructCore/Tests/VisuStructCoreTests/VisuStructXMLCodecTests.swift
index f59b3741..c3de3747 100644
--- a/VisuStruct-iOS/VisuStructCore/Tests/VisuStructCoreTests/VisuStructXMLCodecTests.swift
+++ b/VisuStruct-iOS/VisuStructCore/Tests/VisuStructCoreTests/VisuStructXMLCodecTests.swift
@@ -2,12 +2,13 @@ import XCTest
 import VisuStructCore
 
 final class VisuStructXMLCodecTests: XCTestCase {
+    /// Texte stehen wie im Java-Editor als `;`-getrennte Unicode-Codepoints im XML.
     func testDecodesLegacyStruktogrammeditorXML() throws {
         let xml = """
         <?xml version="1.0" encoding="UTF-8"?>
-        <struktogramm caption="Beispiel">
+        <struktogramm caption="66;101;105;115;112;105;101;108;">
           <strelem typ="0" zx="0" zy="0">
-            <text>x = 1</text>
+            <text>120;32;61;32;49;</text>
           </strelem>
         </struktogramm>
         """