Split from ch25, SCCP and interprocedural SCCPmain

4 files changed, 1416 insertions, 0 deletions

diff --git a/chapter-sccp/src/test/java/com/seaofnodes/simple/fuzzer/Fuzzer.java b/chapter-sccp/src/test/java/com/seaofnodes/simple/fuzzer/Fuzzer.java
new file mode 100644
index 00000000..af4305c0
--- /dev/null
+++ b/chapter-sccp/src/test/java/com/seaofnodes/simple/fuzzer/Fuzzer.java
@@ -0,0 +1,188 @@
+package com.seaofnodes.simple.fuzzer;
+
+import com.seaofnodes.simple.Parser;
+import com.seaofnodes.simple.Eval2;
+import com.seaofnodes.simple.codegen.CodeGen;
+import com.seaofnodes.simple.node.Node;
+import java.util.ArrayList;
+import java.util.Objects;
+import java.util.Random;
+import java.util.function.Consumer;
+
+/**
+ * Implementation of the fuzzer.
+ * Random testcases are generated using the ScriptGenerator.
+ * This will generate a script similar to how the parser parses it but instead of parsing it will
+ * randomly choose a variation the parser would parse and generate it.
+ * These scripts are the parsed by the parser and all exceptions are caught and filtered
+ * to only show one occurrence of one problem. This includes script which generate different
+ * results when graphs optimized with different worklist seeds are executed.
+ * To aid debugging scripts that cause errors are then reduced by applying rules and checking
+ * that the same issue persists.
+ */
+public class Fuzzer {
+
+    private static final int EVAL_TIMEOUT = 10000;
+
+    /**
+     * List of exceptions already encountered.
+     * This is used to filter new exceptions and discard them if they are already found.
+     */
+    private final ArrayList<Throwable> exceptions = new ArrayList<>();
+
+    /**
+     * Filter and record an exception in a script.
+     * @param e The exception caused by the script
+     * @param script The script causing the exception
+     * @param reproducer Reproducer which should be used to reproduce the test case.
+     */
+    private void recordException(Throwable e, String script, Consumer<String> reproducer, long seed) {
+        for (var ex : exceptions) {
+            if (FuzzerUtils.isExceptionFromSameCause(ex, e)) return;
+        }
+        exceptions.add(e);
+        System.out.println("========== Stack ==========");
+        e.printStackTrace(System.out);
+        System.out.println("========== Seed ==========");
+        System.out.println(seed);
+        //System.out.println("========== Code ===========");
+        //System.out.println(script);
+        System.out.println("========= Reduced =========");
+        System.out.println(Reducer.reduce(script, e, reproducer));
+        System.out.println("===========================");
+        System.out.flush();
+    }
+
+    private static boolean neq(Object a, Object b) {
+        if (Objects.equals(a, b)) return false;
+        return true;
+    }
+
+    /**
+     * Check that two graphs result in the same output when supplied with a value
+     * @param e1 Evaluator for the first graph
+     * @param e2 Evaluator for the second graph
+     * @param in The input value to both graphs to test for an equal output
+     */
+    private static void checkGraphs(CodeGen code1, CodeGen code2, long in) {
+        var r1 = Eval2.eval(code1, in, EVAL_TIMEOUT);
+        var r2 = Eval2.eval(code2, in, EVAL_TIMEOUT);
+        if( r1 == null || r2 == null ) return; // Timeout test invalid
+        if( neq(r1, r2) )
+            throw new RuntimeException("Different calculations values " + r1 + " vs " + r2);
+    }
+
+    /**
+     * Run checks for script. Compile the script with two different optimizer worklist seeds.
+     * Check that exceptions raised in the parser by both methods are the same and only happens if the script may be invalid.
+     * If the script was successfully parsed check that both version behave the same.
+     * @param script The script to test
+     * @param valid If the script is definitely valid. If not some exceptions may be suppressed.
+     */
+    private static void runCheck(String script, boolean valid) {
+        CodeGen code1;
+        try {
+            code1 = FuzzerUtils.parse(script, 123, true);
+        } catch (RuntimeException e1) {
+            try {
+                FuzzerUtils.parse(script, 456, false);
+            } catch (RuntimeException e2) {
+                if (FuzzerUtils.isExceptionFromSameCause(e1, e2)) {
+                    if (!valid || e1.getClass() == Parser.ParseException.class) return;
+                } else {
+                    e1.addSuppressed(e2);
+                }
+            }
+            throw e1;
+        }
+        CodeGen code2 = FuzzerUtils.parse(script, 456, false);
+        checkGraphs(code1, code2, 0);
+        checkGraphs(code1, code2, 1);
+        checkGraphs(code1, code2, 10);
+    }
+
+    /**
+     * Check that the script does not result in exceptions.
+     * @param script The script to test.
+     * @param valid If the script is valid or if it might contain syntax errors.
+     */
+    private void check(String script, boolean valid, long seed) {
+        try {
+            runCheck(script, valid);
+        } catch (Throwable e) {
+            recordException(e, script, s->runCheck(s, valid), seed);
+        }
+    }
+
+    /**
+     * Run one test with the given seed, and check for peephole counts.
+     * @param seed The seed to use for generating this test case
+     */
+    public int fuzzPeepTiming(long seed, int max_nids) {
+        var rand = new Random(seed);
+        var sb = new StringBuilder();
+        new ScriptGenerator(rand, sb, true).genProgram();
+        try {
+            var code = new CodeGen(sb.toString());
+
+            code.parse();
+            int nids = code.UID();
+            if( nids <= max_nids ) return max_nids; // Smaller, looking for trends as we grow
+            int parse_peeps= code._iter_cnt;
+            int parse_nops = code._iter_nop_cnt;
+            double parse_nop_ratio = (double)parse_nops/parse_peeps;
+            double parse_peeps_per_node = (double)parse_peeps/nids;
+
+            code.iter().opto().iterPostOpto().finalizeIR();
+            int iter_peeps= code._iter_cnt;
+            int iter_nops = code._iter_nop_cnt;
+            double iter_nop_ratio = (double)iter_nops/iter_peeps;
+            double iter_peeps_per_node = (double)iter_peeps/nids;
+
+            System.out.printf("%6d | Parsing: peeps: %5d, nops: %5d, ratio: %5.3f, last UID: %5d ps/node: %5.3f  | Iter: peeps: %6d, nops: %6d, ratio: %5.3f, ps/node: %5.3f\n",
+                              seed,
+                              parse_peeps,parse_nops,parse_nop_ratio,nids,parse_peeps_per_node,
+                              iter_peeps , iter_nops, iter_nop_ratio,      iter_peeps_per_node);
+            return nids;
+        } catch (Throwable e) {
+            // ignore exceptions, let the normal fullPeeps catch and reduce,
+            // we're interested in valid program optimizations
+        }
+        return max_nids;
+    }
+
+
+    /**
+     * Run one test with the given seed.
+     * @param seed The seed to use for generating this test case
+     */
+    public void fuzzPeeps(long seed) {
+        var rand = new Random(seed);
+        var sb = new StringBuilder();
+        var valid = new ScriptGenerator(rand, sb, true).genProgram();
+        check(sb.toString(), valid, seed);
+    }
+
+
+    /**
+     * Check that no exceptions happened.
+     * @return true if no exception happened
+     */
+    public boolean noExceptions() {
+        return exceptions.isEmpty();
+    }
+
+    /** Run a fixed chapter-local seed without reducing or swallowing failures. */
+    public void fuzzPeepsRegression(long seed) {
+        var rand = new Random(seed);
+        var sb = new StringBuilder();
+        var valid = new ScriptGenerator(rand, sb, true).genProgram();
+        try {
+            runCheck(sb.toString(), valid);
+        } catch (Throwable e) {
+            AssertionError ae = new AssertionError("Fuzzer regression seed failed: " + seed);
+            ae.initCause(e);
+            throw ae;
+        }
+    }
+}
diff --git a/chapter-sccp/src/test/java/com/seaofnodes/simple/fuzzer/FuzzerUtils.java b/chapter-sccp/src/test/java/com/seaofnodes/simple/fuzzer/FuzzerUtils.java
new file mode 100644
index 00000000..45558541
--- /dev/null
+++ b/chapter-sccp/src/test/java/com/seaofnodes/simple/fuzzer/FuzzerUtils.java
@@ -0,0 +1,111 @@
+package com.seaofnodes.simple.fuzzer;
+
+import com.seaofnodes.simple.IterPeeps;
+import com.seaofnodes.simple.codegen.CodeGen;
+import com.seaofnodes.simple.node.Node;
+import com.seaofnodes.simple.node.StopNode;
+import com.seaofnodes.simple.type.TypeInteger;
+import java.io.OutputStream;
+import java.io.PrintStream;
+import java.lang.invoke.MethodHandle;
+import java.lang.invoke.MethodHandles;
+import java.lang.reflect.Field;
+import java.util.BitSet;
+
+/**
+ * Some utilities for the fuzzer.
+ */
+class FuzzerUtils {
+
+    /**
+     * Get the val of a private field
+     */
+    @SuppressWarnings("unchecked")
+    public static <T> T getFieldValue(Object obj, String name) throws NoSuchFieldException, IllegalAccessException {
+        return (T) (obj instanceof Class<?> clz ? getField(clz, name).get(null) : getField(obj.getClass(), name).get(obj));
+    }
+
+    /**
+     * Get access to a private field
+     */
+    public static Field getField(Class<?> clz, String name) throws NoSuchFieldException {
+        var field = clz.getDeclaredField(name);
+        try {
+            field.setAccessible(true);
+        } catch (Throwable e) {
+            // Intentionally left empty
+        }
+        return field;
+    }
+
+    /**
+     * Try to rethrow an exception or create a runtime exception and throw it.
+     */
+    public static RuntimeException rethrow(Throwable e) {
+        if (e instanceof RuntimeException re) return re;
+        if (e instanceof Error er) throw er;
+        return new RuntimeException(e);
+    }
+
+    /**
+     * Some problems might trigger debug output. Suppress this output with a null print stream during fuzzing.
+     */
+    public static final PrintStream NULL_PRINT_STREAM = new PrintStream(OutputStream.nullOutputStream());
+
+    /**
+     * Check that the messages are similar.
+     */
+    private static boolean similarMessages(String a, String b) {
+        if (a == null || b == null) return true;
+        a = a.replaceAll("'\\w+'", "'normalized'").replaceAll("variable \\w+", "variable normalized");
+        b = b.replaceAll("'\\w+'", "'normalized'").replaceAll("variable \\w+", "variable normalized");
+        return a.equals(b);
+    }
+
+    /**
+     * Check if both stack traces have the same first entry. Empty stack traces can happen with NPE optimization
+     * and allow the other to be any stack trace.
+     */
+    private static boolean sameStackTraceOrigin(StackTraceElement[] a, StackTraceElement[] b) {
+        if (a == null || a.length == 0 || b == null || b.length == 0) return true; // One or both stack traces are missing.
+        return a[0].equals(b[0]);
+    }
+
+    /**
+     * Checks if two exceptions are caused by the same problem.
+     */
+    public static boolean isExceptionFromSameCause(Throwable a, Throwable b) {
+        if (a == null || b == null) return a == b; // Handle the null case
+        if (a.getClass() != b.getClass()) return false; // Different classes
+        if (!similarMessages(a.getMessage(), b.getMessage())) return false; // Different messages. Null is allowed as wildcard
+        var s1 = a.getStackTrace();
+        var s2 = b.getStackTrace();
+        if (!sameStackTraceOrigin(s1, s2)) return false; // Different stack trace origins
+        if (!isExceptionFromSameCause(a.getCause(), b.getCause())) return false; // Different causes
+        var o1 = a.getSuppressed();
+        var o2 = b.getSuppressed();
+        if (o1 == null || o2 == null || o1.length != o2.length) return o1 == o2; // Different suppression
+        for (int i=0; i<o1.length; i++) {
+            if (!isExceptionFromSameCause(o1[i], o2[i])) return false; // Different suppression
+        }
+        return true;
+    }
+
+    /**
+     * Parse and optimize using the given worklist seed
+     */
+    public static CodeGen parse(String script, long workListSeed, boolean reset) {
+        var err = System.err;
+        var out = System.out;
+        try {
+            System.setErr(NULL_PRINT_STREAM);
+            System.setOut(NULL_PRINT_STREAM);
+            var code = new CodeGen(script, TypeInteger.BOT, workListSeed, reset);
+            return code.driver(CodeGen.Phase.LocalSched);
+        } finally {
+            System.setErr(err);
+            System.setOut(out);
+        }
+    }
+
+}
diff --git a/chapter-sccp/src/test/java/com/seaofnodes/simple/fuzzer/Reducer.java b/chapter-sccp/src/test/java/com/seaofnodes/simple/fuzzer/Reducer.java
new file mode 100644
index 00000000..f6e12e11
--- /dev/null
+++ b/chapter-sccp/src/test/java/com/seaofnodes/simple/fuzzer/Reducer.java
@@ -0,0 +1,177 @@
+package com.seaofnodes.simple.fuzzer;
+
+import java.util.Arrays;
+import java.util.function.BiConsumer;
+import java.util.function.Consumer;
+import java.util.function.Predicate;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+/**
+ * This class is used to reduce test cases created by the fuzzer.
+ * It applies rewrite rules to the test case and checks that the
+ * test still produces the same exception.
+ * This is done in a fixpoint iteration until every rewrite produces
+ * no or a different exception.
+ */
+public class Reducer {
+
+    private interface Replacer extends BiConsumer<StringBuilder, Matcher> {}
+
+    /**
+     * Holder for the rewrite rules
+     */
+    private record Replace(Pattern pattern, boolean iterate, Replacer... replacements) {
+        Replace(Pattern p, Object... replacements) {
+            this(p, replacements[0] == Boolean.TRUE, Arrays.stream(replacements).filter(o->!(o instanceof Boolean)).map(Replace::from).toArray(Replacer[]::new));
+        }
+        private static Replacer from(Object o) {
+            if (o instanceof Replacer r) return r;
+            if (o instanceof Integer i) return (s, m)->s.append(m.group(i));
+            if (o instanceof String r) return (s, m)->s.append(r);
+            throw new IllegalArgumentException();
+        }
+    }
+
+    /**
+     * List of rewrite rules.
+     */
+    private static final Replace[] REPLACEMENTS = {
+            new Replace(Pattern.compile("\\b(?:[^\\W0]|\\w\\w+)\\b"), "0", "1"),
+            new Replace(Pattern.compile("(?<=\\n|^)[^\\n]+(?=\\n|$)"), true, ""),
+            new Replace(Pattern.compile("(?<=[+\\-*/<>=(])--"), ""),
+            new Replace(Pattern.compile("\\b--"), "+"),
+            new Replace(Pattern.compile("[-!]"), ""),
+            new Replace(Pattern.compile("\\b\\+-"), "-"),
+            new Replace(Pattern.compile("(?<![+\\-*/<>=&|])(?:-|\\*|/|>=|<=|>>>|>>|>|<<|<|!=|==|&&|\\|\\||&|\\||^)(?![+*/<>=&|])"), "+"),
+            new Replace(Pattern.compile("\\?([^;]+):"), "+"),
+            new Replace(Pattern.compile("\\{((?:[^{}]|\\{})+)(?:\\n[ \\t]*)?}"), "", "{}", 1),
+            new Replace(Pattern.compile("(?<=\\n|^)([ \\t]*)\\{\\n(.*?)\\n\\1}", Pattern.DOTALL), 2),
+            new Replace(Pattern.compile("\\(([^()]+)\\)"), 1),
+            new Replace(Pattern.compile("\\d+ ?\\+ ?"), ""),
+            new Replace(Pattern.compile(" ?\\+ ?\\d+"), ""),
+            new Replace(Pattern.compile("\\n\\n+"), "\n"),
+            new Replace(Pattern.compile("^\\n+|\\n+$"), ""),
+            new Replace(Pattern.compile("else *"), ""),
+            new Replace(Pattern.compile("\\bif\\([^()]+\\) *"), ""),
+            new Replace(Pattern.compile("\\bwhile\\([^()]+\\) *"), ""),
+            new Replace(Pattern.compile("\\bstruct \\w+ \\{[^{}]*}"), "")
+    };
+
+    /**
+     * Predicate to test if the reduced script generates the same error.
+     */
+    private final Predicate<String> tester;
+
+    /**
+     * Create a reducer with a test function.
+     * @param tester Test function which returns true when the same issue is found in the script.
+     */
+    private Reducer(Predicate<String> tester) {
+        this.tester = tester;
+    }
+
+    /**
+     * Rewrites the script with one rewrite rule.
+     */
+    private String doReplacement(String script, Pattern pattern, Replacer[] replacements) {
+        var matcher = pattern.matcher(script);
+        var sb = new StringBuilder();
+        var last = 0;
+        var skip = 0;
+        outer:
+        while (matcher.find(last + skip)) {
+            sb.append(script, last, matcher.start());
+            last = matcher.end();
+            skip = last == matcher.start() ? 1 : 0;
+            var pos = sb.length();
+            for (var replacement : replacements) {
+                replacement.accept(sb, matcher);
+                var tail = sb.length();
+                sb.append(script, last, script.length());
+                if (tester.test(sb.toString())) {
+                    sb.setLength(tail);
+                    continue outer;
+                }
+                sb.setLength(pos);
+            }
+            last = matcher.start();
+            skip = 1;
+        }
+        sb.append(script, last, script.length());
+        return sb.toString();
+    }
+
+    /**
+     * Clean up variable names from the random ones generated by the fuzzer.
+     */
+    private String cleanVariables(String script) {
+        var matcher = Pattern.compile("\\b([0-9a-zA-Z_]+) !?([0-9a-zA-Z_]+)").matcher(script);
+        int[] num = {0, 0};
+        while (matcher.find()) {
+            var type = matcher.group(1);
+            var var = matcher.group(2);
+            if (type.equals("new")) continue;
+            String prefix = "v";
+            int t = 0;
+            if (type.equals("struct")) {
+                t = 1;
+                prefix = "s";
+            }
+            if (var.matches(prefix+"\\d+")) continue;
+            while (script.contains(prefix+num[t])) num[t]++;
+            var n = script.replaceAll("\\b"+var+"\\b", prefix + num[t]);
+            if (tester.test(n)) {
+                script = n;
+            }
+        }
+        return script;
+    }
+
+    /**
+     * Run all rewrite rules once.
+     */
+    private String doAllReplacements(String script) {
+        for (var replace: REPLACEMENTS) {
+            String old;
+            do {
+                old = script;
+                script = doReplacement(script, replace.pattern, replace.replacements);
+            } while (replace.iterate && !old.equals(script));
+        }
+        return script;
+    }
+
+    /**
+     * Reduce the script to a minimal version which produces the same problem with a test function.
+     * @param tester Test function which returns true when the same issue is found in the script.
+     */
+    public static String reduce(String script, Predicate<String> tester) {
+        var reducer = new Reducer(tester);
+        String old;
+        do {
+            old = script;
+            script = reducer.doAllReplacements(script);
+            script = reducer.cleanVariables(script);
+        } while (!script.equals(old));
+        return script;
+    }
+
+    /**
+     * Reduce the script to a minimal version which produces the same problem with a reproducer.
+     * @param ex The exception originally caused by the script
+     * @param reproducer Test function which throws an exception which will be checked against ex.
+     */
+    public static String reduce(String script, Throwable ex, Consumer<String> reproducer) {
+        return reduce(script, s->{
+            Throwable nex = null;
+            try {
+                reproducer.accept(s);
+            } catch (Throwable e) {
+                nex = e;
+            }
+            return FuzzerUtils.isExceptionFromSameCause(nex, ex);
+        });
+    }
+
+}
\ No newline at end of file
diff --git a/chapter-sccp/src/test/java/com/seaofnodes/simple/fuzzer/ScriptGenerator.java b/chapter-sccp/src/test/java/com/seaofnodes/simple/fuzzer/ScriptGenerator.java
new file mode 100644
index 00000000..20815d5d
--- /dev/null
+++ b/chapter-sccp/src/test/java/com/seaofnodes/simple/fuzzer/ScriptGenerator.java
@@ -0,0 +1,940 @@
+package com.seaofnodes.simple.fuzzer;
+
+import com.seaofnodes.simple.Parser;
+import com.seaofnodes.simple.node.ScopeNode;
+import com.seaofnodes.simple.type.TypeInteger;
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Random;
+import java.util.function.Predicate;
+import static com.seaofnodes.simple.Parser.KEYWORDS;
+
+
+/**
+ * Generate a pseudo random script.
+ * This generator will generate the same script for two random number generators initialized with the same seed.
+ * To generate valid code it implements the parser but instead of parsing it emits the statements and expressions.
+ * It is guaranteed to terminate.
+ */
+public class ScriptGenerator {
+
+    private static final int MAX_STATEMENTS_PER_BLOCK = 5;
+    private static final int MAX_NAME_LENGTH = 10;
+    private static final int MAX_BLOCK_DEPTH = 4;
+    private static final int MAX_EXPRESSION_DEPTH = 10;
+
+    /**
+     * Number of spaces per indentation
+     */
+    private static final int INDENTATION = 4;
+    /**
+     * Valid characters for identifiers. The last 10 are not valid for the first character.
+     */
+    private static final String VAR_CHARS = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_0123456789";
+
+    /**
+     * Flag for a statement that it does not pass on control flow to the next statement.
+     */
+    private static final int FLAG_STOP = 0x01;
+    /**
+     * Flag for a statement that it contains a final if statement without an else branch.
+     */
+    private static final int FLAG_IF_WITHOUT_ELSE = 0x02;
+
+    private static class Type {
+        final String name;
+        Type(String name) {this.name=name;}
+        boolean isa(Type other) { return this == other; }
+    }
+
+    private static class TypeStruct extends Type {
+        record Field (String name, Type type, TypeStruct struct) {}
+        Field[] fields;
+        final TypeNullable nullable = new TypeNullable(this);
+        TypeStruct(String name) {
+            super(name);
+        }
+        boolean isa(Type other) { return this == other || other == nullable; }
+    }
+
+    private static class TypeNullable extends Type {
+
+        final Type base;
+
+        TypeNullable(Type base) {
+            super(base.name+"?");
+            this.base = base;
+        }
+
+    }
+
+    private static final Type TYPE_FLT = new Type("flt");
+
+    private static class TypeInt extends Type {
+
+        TypeInt(String name) {
+            super(name);
+        }
+
+        @Override
+        boolean isa(Type other) {
+            return super.isa(other) || other == TYPE_FLT || other instanceof TypeInt;
+        }
+    }
+
+    private static final List<TypeInt> INTTYPES = new ArrayList<>();
+
+    static {
+        for( var e : Parser.TYPES.entrySet() ) {
+            if( e.getValue() instanceof TypeInteger ) {
+                INTTYPES.add(new TypeInt(e.getKey()));
+            }
+        }
+    }
+
+    private static final Type TYPE_INT = INTTYPES.stream().filter(t->t.name.equals("int")).findAny().get();
+    private static final Type TYPE_BOOL = INTTYPES.stream().filter(t->t.name.equals("bool")).findAny().get();
+
+    private static class Variable {
+        final String name;
+        final Type declared;
+        final Variable shadowing;
+        Type type;
+        boolean shadowed = false;
+
+        Variable(String name, Type type, Variable var) {
+            this.name = name;
+            this.declared = type;
+            this.type = type;
+            this.shadowing = var;
+        }
+    }
+
+    /**
+     * The random number generator used for random decisions.
+     */
+    private final Random random;
+    /**
+     * Indentation depth.
+     */
+    private int indentation = 0;
+    /**
+     * Output string builder
+     */
+    private final StringBuilder sb;
+    /**
+     * Number of nested loops
+     */
+    private int loopDepth = 0;
+    /**
+     * Current scope start in the variables list.
+     */
+    private int currScopeStart = 0;
+    /**
+     * The depth of blocks allowed.
+     */
+    private int depth = MAX_BLOCK_DEPTH;
+    /**
+     * Current variables in scope.
+     */
+    private final ArrayList<Variable> variables = new ArrayList<>();
+    /**
+     * All defined structs.
+     */
+    private final ArrayList<TypeStruct> structs = new ArrayList<>();
+    /**
+     * All forward declared structs
+     */
+    private final ArrayList<TypeStruct> forwardStructs = new ArrayList<>();
+    /**
+     * Allow to declare structs.
+     */
+    private boolean allowStructs = true;
+    /**
+     * If this needs to generate valid scripts or is also allowed to generate invalid ones.
+     */
+    private boolean generateValid;
+    /**
+     * If this script might be invalid.
+     */
+    private boolean maybeInvalid = false;
+
+
+    /**
+     *
+     * @param random the random number generator.
+     * @param sb the script will be written to this string builder.
+     * @param generateValid if the script should be guaranteed valid or if it is allowed to contain bugs.
+     */
+    public ScriptGenerator(Random random, StringBuilder sb, boolean generateValid) {
+        this.random = random;
+        this.sb = sb;
+        this.generateValid = generateValid;
+    }
+
+    /**
+     * Random number with log distribution
+     * @param max The maximum value
+     * @return A random number in the range [0, max) with higher numbers being more uncommon.
+     */
+    private int randLog(int max) {
+        max--;
+        int n = 1<<max;
+        var num = random.nextInt(n);
+        for (int i=max-1; i>=0; i--) {
+            if ((num & (1<<i))!=0) return max-i-1;
+        }
+        return max;
+    }
+
+    /**
+     * Helper function to print indentation.
+     * @return sb for chaining
+     */
+    private StringBuilder printIndentation() {
+        return sb.repeat(' ', indentation);
+    }
+
+    /**
+     * Generate a random name for variables. This might be invalid when it is a keyword.
+     * @return The random name generated
+     */
+    private StringBuilder getRandomName() {
+        int len = random.nextInt(MAX_NAME_LENGTH) + 1;
+        StringBuilder sb = new StringBuilder(len);
+        sb.append(VAR_CHARS.charAt(random.nextInt(VAR_CHARS.length()-10)));
+        for (int i=1; i<len; i++)
+            sb.append(VAR_CHARS.charAt(random.nextInt(VAR_CHARS.length())));
+        return sb;
+    }
+
+    /**
+     * Should invalid code be generated?
+     * @return True if invalid code should be generated
+     */
+    private boolean generateInvalid() {
+        if (generateValid || random.nextInt(1000)>2) return false;
+        maybeInvalid = true;
+        return true;
+    }
+
+    /**
+     * Lookup a variable by name
+     * @param name name of the variable
+     * @return The index of the variable in variables or -1
+     */
+    private int lookup(String name) {
+        for (int i=variables.size()-1; i>=0; i--) {
+            if (variables.get(i).name.equals(name)) return i;
+        }
+        return -1;
+    }
+
+    /**
+     * Lookup a variable by name
+     * @param name name of the variable
+     * @return THe variable of null if not found
+     */
+    private Variable lookupVar(String name) {
+        int idx = lookup(name);
+        return idx == -1 ? null : variables.get(idx);
+    }
+
+    /**
+     * Add a variable to the current scope
+     * @param name The name of the variable
+     * @param type The declared type of the variable
+     */
+    private void addVariable(String name, Type type) {
+        var old = lookupVar(name);
+        variables.add(new Variable(name, type, old));
+        if (old != null) old.shadowed = true;
+    }
+
+    /**
+     * Pop variables to level to
+     * @param to Level until which variables should be popped
+     */
+    private void popVariables(int to) {
+        while (variables.size() > to) {
+            var v = variables.removeLast();
+            if (v.shadowing != null) v.shadowing.shadowed = false;
+        }
+    }
+
+    /**
+     * Get a random visible variable patching pred
+     * @param pred The predicate the variable needs to match
+     * @return A random visible variable matching pred or null if no variable mached pred.
+     */
+    private Variable findVisibleVariablesMatching(Predicate<Variable> pred) {
+        int num=0;
+        for (var v:variables) {
+            if (!v.shadowed && pred.test(v)) num++;
+        }
+        if (num == 0) return null;
+        num = random.nextInt(num);
+        for (var v:variables) {
+            if (!v.shadowed && pred.test(v) && num-- == 0) return v;
+        }
+        throw new AssertionError();
+    }
+
+    /**
+     * Get a structure by name
+     * @param name Name of the structure
+     * @return The structure of null
+     */
+    private TypeStruct getStruct(String name) {
+        for (var s:structs) {
+            if (s.name.equals(name)) return s;
+        }
+        for (var s:forwardStructs) if (s.name.equals(name)) return s;
+        return null;
+    }
+
+    /**
+     * Generate a variable name. This might be new random name or a name from an outer scope.
+     * If the program is allowed to be invalid this can return keywords and already used variable names in the current scope.
+     * @return The variable name.
+     */
+    private String getVarName() {
+        if (currScopeStart > 0 && random.nextInt(10) > 7) {
+            // Use a variable outside the current scope.
+            var v = variables.get(random.nextInt(currScopeStart));
+            if (lookup(v.name) < currScopeStart) return v.name;
+            if (!generateValid) {
+                maybeInvalid = true;
+                return v.name;
+            }
+        }
+        // Generate a new random variable name
+        StringBuilder sb = getRandomName();
+        var v = sb.toString();
+        if (!generateValid && (KEYWORDS.contains(v) || lookup(v) >= currScopeStart)) {
+            maybeInvalid = true;
+            return v;
+        }
+        while (KEYWORDS.contains(v) || lookup(v) >= currScopeStart) {
+            sb.append(VAR_CHARS.charAt(random.nextInt(VAR_CHARS.length())));
+            v = sb.toString();
+        }
+        return v;
+    }
+
+    private <T> T randFromList(List<T> list) {
+        return list.get(random.nextInt(list.size()));
+    }
+
+    /**
+     * Get a random structure name
+     * @return A random structure name
+     */
+    private String getStructName() {
+        if (!structs.isEmpty() && generateInvalid()) return randFromList(structs).name;
+        if (!forwardStructs.isEmpty() && generateInvalid()) return randFromList(forwardStructs).name;
+
+        // Generate a new random struct name
+        StringBuilder sb = getRandomName();
+        var v = sb.toString();
+        if (!generateValid && (KEYWORDS.contains(v) || getStruct(v) != null)) {
+            maybeInvalid = true;
+            return v;
+        }
+        while (KEYWORDS.contains(v) || getStruct(v) != null) {
+            sb.append(VAR_CHARS.charAt(random.nextInt(VAR_CHARS.length())));
+            v = sb.toString();
+        }
+        return v;
+    }
+
+    public TypeInt getIntType() {
+        return randFromList(INTTYPES);
+    }
+
+    /**
+     * Get a random type
+     * @return A random type
+     */
+    private Type getType(boolean allowForward) {
+        int t = random.nextInt(10);
+        if( t<5 || structs.isEmpty() ) return getIntType();
+        if( t<7 ) return TYPE_FLT;
+        if (allowForward && random.nextInt(10)==0) {
+            int i=random.nextInt(forwardStructs.size()+1);
+            if (i>0) return forwardStructs.get(i-1).nullable;
+            var name = getStructName();
+            var struct = new TypeStruct(name);
+            forwardStructs.add(struct);
+            return struct.nullable;
+        }
+        TypeStruct struct = randFromList(structs);
+        if (random.nextBoolean()) return struct.nullable;
+        return struct;
+    }
+
+    private Type getType() {
+        return getType(false);
+    }
+
+    /**
+     * Generates a program and writes it into the string builder supplied in the constructor.
+     * @return If the program is allowed to be invalid returns if something potentially invalid was generated.
+     */
+    public boolean genProgram() {
+        var oldGenerateValid = generateValid;
+        if (!generateValid && random.nextBoolean()) generateValid = true;
+        this.maybeInvalid = false;
+        addVariable(ScopeNode.ARG0, TYPE_INT);
+        currScopeStart = variables.size();
+        if ((genStatements() & FLAG_STOP) == 0) {
+            if (generateValid || random.nextInt(10)<7) {
+                genReturn();
+            } else {
+                this.maybeInvalid = true;
+            }
+        }
+        generateValid = oldGenerateValid;
+        return !this.maybeInvalid;
+    }
+
+    /**
+     * Generate a list of statements.
+     * @return flags FLAG_STOP and FLAG_IF_WITHOUT_ELSE for the last statement generated
+     */
+    public int genStatements() {
+        var num = random.nextInt(MAX_STATEMENTS_PER_BLOCK * (depth+1));
+        for (int i=0; i<num; i++) {
+            printIndentation();
+            var stop = genStatement();
+            sb.append("\n");
+            if ((stop & FLAG_STOP) != 0) return stop;
+        }
+        return 0;
+    }
+
+    /**
+     * Generate a single statement
+     * @return flags FLAG_STOP and FLAG_IF_WITHOUT_ELSE for the generated statement
+     */
+    public int genStatement() {
+        return switch (random.nextInt(12)) {
+            case 0 -> allowStructs || generateInvalid() ? genStruct() : genAssignment();
+            case 1 -> genBlock();
+            case 2 -> genIf();
+            case 3 -> genCountedLoop();
+            case 4 -> genWhile();
+            case 5, 6 -> genDecl();
+            case 7, 8, 9 -> genAssignment();
+            case 10 -> genNullCheck();
+            default -> genExit();
+        };
+    }
+
+    /**
+     * Generate a new structure
+     * @return 0
+     */
+    public int genStruct() {
+        int idx = random.nextInt(forwardStructs.size()+10);
+        TypeStruct struct;
+        if (idx < 10) {
+            var name = getStructName();
+            struct = new TypeStruct(name);
+            forwardStructs.add(struct);
+        } else {
+            struct = forwardStructs.get(idx-10);
+        }
+        var fields = new TypeStruct.Field[generateInvalid() ? 0 : random.nextInt(10)];
+        sb.append("struct ").append(struct.name).append(" {\n");
+        indentation += INDENTATION;
+        for (int i=0; i<fields.length; i++) {
+            var fieldName = getRandomName();
+            var type = getType(true);
+            printIndentation().append(declType(type)).append(" !").append(fieldName).append(";\n");
+            fields[i] = new TypeStruct.Field(fieldName.toString(), type, struct);
+        }
+        indentation -= INDENTATION;
+        printIndentation().append("};");
+        struct.fields = fields;
+        forwardStructs.remove(struct);
+        structs.add(struct);
+        return 0;
+    }
+
+    /**
+     * Generate a statement for if, else and while blocks.
+     * This is special since it disallows declarations and prefers to generate blocks.
+     * @return flags FLAG_STOP and FLAG_IF_WITHOUT_ELSE for the generated statement
+     */
+    public int genStatementBlock() {
+        var oldAllowStructs = allowStructs;
+        allowStructs = false;
+        indentation += INDENTATION;
+        int res;
+        if (depth == 0) {
+            res = random.nextBoolean() ? genAssignment() : genExit();
+        } else {
+            depth--;
+            res = switch (random.nextInt(12)) {
+                case 1, 2, 3, 4, 5 -> {
+                    depth++;
+                    indentation -= INDENTATION;
+                    var ret = genBlock();
+                    indentation += INDENTATION;
+                    depth--;
+                    yield ret;
+                }
+                case 6 -> genIf();
+                case 7 -> genCountedLoop();
+                case 8 -> genWhile();
+                case 9 -> genAssignment();
+                case 10 -> genNullCheck();
+                default -> genExit();
+            };
+            depth++;
+        }
+        indentation -= INDENTATION;
+        allowStructs = oldAllowStructs;
+        return res;
+    }
+
+    /**
+     * Generate an exit. This can be return or break and continue when in a loop.
+     * @return FLAG_STOP
+     */
+    public int genExit() {
+        if (loopDepth == 0 && generateValid) return genReturn();
+        return switch (random.nextInt(7)) {
+            case 0,1,2 -> {
+                if (loopDepth == 0) maybeInvalid = true;
+                sb.append("continue;");
+                yield FLAG_STOP;
+            }
+            case 3,4,5 -> {
+                if (loopDepth == 0) maybeInvalid = true;
+                sb.append("break;");
+                yield FLAG_STOP;
+            }
+            default -> genReturn();
+        };
+    }
+
+    /**
+     * Generate a block statement. After a certain depth only empty blocks are generated to ensure termination of the generator.
+     * @return flag FLAG_STOP for the generated statement
+     */
+    public int genBlock() {
+        if (depth == 0) {
+            sb.append("{}");
+            return 0;
+        }
+        var oldAllowStructs = allowStructs;
+        var oldCSS = currScopeStart;
+        allowStructs = false;
+        currScopeStart = variables.size();
+        depth--;
+        sb.append("{\n");
+        indentation += INDENTATION;
+        var stop = genStatements();
+        indentation -= INDENTATION;
+        printIndentation().append("}");
+        popVariables(currScopeStart);
+        depth++;
+        currScopeStart = oldCSS;
+        allowStructs = oldAllowStructs;
+        return stop & ~FLAG_IF_WITHOUT_ELSE;
+    }
+
+    /**
+     * Generate an if statement with an optional else block.
+     * @return flags FLAG_STOP and FLAG_IF_WITHOUT_ELSE for the generated statement
+     */
+    public int genIf() {
+        sb.append("if(");
+        genExpression(TYPE_BOOL, true, true);
+        sb.append(") ");
+        var stop = genStatementBlock();
+        if ((stop & FLAG_IF_WITHOUT_ELSE) == 0 && random.nextInt(10) > 3) {
+            sb.append("\n");
+            printIndentation().append("else ");
+            stop &= genStatementBlock();
+        } else {
+            stop = FLAG_IF_WITHOUT_ELSE;
+        }
+        return stop;
+    }
+
+    /**
+     * Generate a block with a null check
+     * if (nullable) { handle_nullable_as_non_nullable; }
+     */
+    public int genNullCheck() {
+        var v = findVisibleVariablesMatching(va->va.declared instanceof TypeNullable);
+        if (v == null) return genIf();
+        var n = (TypeNullable)v.declared;
+        boolean negate = random.nextBoolean();
+        sb.append("if(");
+        if (negate) sb.append("!");
+        sb.append(v.name).append(")");
+        if (!negate) v.type = n.base;
+        var stop = genStatementBlock();
+        v.type = n;
+        if ((stop & FLAG_IF_WITHOUT_ELSE) == 0 && random.nextInt(10) > 3) {
+            sb.append("\n");
+            printIndentation().append("else ");
+            if (negate) v.type = n.base;
+            stop &= genStatementBlock();
+            v.type = n;
+        } else {
+            stop = FLAG_IF_WITHOUT_ELSE;
+        }
+        return stop;
+    }
+
+    /**
+     * Generate a while loop.
+     * @return 0
+     */
+    public int genWhile() {
+        sb.append("while(");
+        genExpression(TYPE_BOOL, true, true);
+        sb.append(") ");
+        loopDepth++;
+        genStatementBlock();
+        loopDepth--;
+        return 0;
+    }
+
+    /**
+     * Generate a counted loop of the form
+     * <code>
+     *     {
+     *         int varname = start_expr;
+     *         while (varname < end_expr) loop_body;
+     *     }
+     * </code>
+     * @return 0
+     */
+    public int genCountedLoop() {
+        if (depth == 0) return genWhile();
+        var oldCSS = currScopeStart;
+        currScopeStart = variables.size();
+        sb.append("{\n");
+        indentation += INDENTATION;
+        String name = getVarName();
+        printIndentation().append("int ").append(name).append("=");
+        genExpression(TYPE_INT, true, false);
+        sb.append(";\n");
+        addVariable(name, TYPE_INT);
+        printIndentation().append("while(").append(name).append("<");
+        genShift(TYPE_INT, true, false, MAX_EXPRESSION_DEPTH);
+        sb.append(") {\n");
+        indentation += INDENTATION;
+        depth--;
+        loopDepth++;
+        printIndentation().append(name).append("=").append(name).append("+");
+        genExpression(TYPE_INT, true, false);
+        sb.append(";\n");
+        genStatements();
+        loopDepth--;
+        depth++;
+        indentation -= INDENTATION;
+        printIndentation().append("}\n");
+        indentation -= INDENTATION;
+        printIndentation().append("}");
+        popVariables(currScopeStart);
+        currScopeStart = oldCSS;
+        return 0;
+    }
+
+    // This generator mutates references and fields; request both permissions.
+    private static String declType(Type type) {
+        return (type instanceof TypeStruct || type instanceof TypeNullable ? "!" : "") + type.name;
+    }
+
+    /** Generate a declaration statement. */
+    public int genDecl() {
+        var type = getType();
+        var name = getVarName();
+        // Always make them mutable
+        sb.append(generateInvalid() ? getRandomName() : declType(type)).append(" !").append(name);
+        if (!(type instanceof TypeNullable) || random.nextBoolean()) {
+            sb.append("=");
+            genExpression(type, true, type==TYPE_BOOL);
+        }
+        sb.append(";");
+        addVariable(name, type);
+        return 0;
+    }
+
+    /**
+     * Generate an assignment statement.
+     * @return 0
+     */
+    public int genAssignment() {
+        Type type;
+        Type declared;
+        if (generateInvalid()) {
+            sb.append(getRandomName());
+            type = getType();
+            declared = type;
+        } else {
+            var v = findVisibleVariablesMatching(va->true);
+            if (v == null) return genDecl();
+            sb.append(v.name);
+            type = v.type;
+            declared = v.declared;
+        }
+        if (generateInvalid()) {
+            var name = getRandomName();
+            sb.append(".").append(name);
+            declared = getType();
+        } else if (type instanceof TypeStruct s && s.fields.length > 0 && random.nextBoolean()) {
+            var field = s.fields[random.nextInt(s.fields.length)];
+            sb.append(".").append(field.name);
+            declared = field.type;
+        }
+        sb.append("=");
+        genExpression(declared, true, declared==TYPE_BOOL);
+        sb.append(";");
+        return 0;
+    }
+
+    /**
+     * Generate a return statement.
+     * @return FLAG_STOP
+     */
+    public int genReturn() {
+        var type = getType();
+        sb.append("return ");
+        genExpression(type, true, false);
+        sb.append(";");
+        return FLAG_STOP;
+    }
+
+    public void genExpression(Type type, boolean change, boolean preferLogical) {
+        genExpression(type, change, preferLogical, MAX_EXPRESSION_DEPTH);
+    }
+
+    public void genExpression(Type type, boolean change, boolean preferLogical, int d) {
+        if (change && generateInvalid()) type = getType();
+        if (d > 0 && random.nextInt(10)==0) {
+            genLogical(TYPE_INT, true, true, d);
+            sb.append("?");
+            genExpression(type, true, preferLogical, d-1);
+            sb.append(":");
+            genExpression(type, false, preferLogical, d-1);
+        } else {
+            genLogical(type, false, preferLogical, d);
+        }
+    }
+
+    public void genLogical(Type type, boolean change, boolean preferLogical, int d) {
+        if (change && generateInvalid()) type = getType();
+        genBitwise(type, false, preferLogical, d);
+        while (d-->0 && random.nextInt(preferLogical?5:50) == 0) {
+            sb.append(random.nextBoolean()?"&&":"||");
+            genBitwise(type, true, preferLogical, d);
+        }
+    }
+
+    public void genBitwise(Type type, boolean change, boolean preferLogical, int d) {
+        if (change && generateInvalid()) type = getType();
+        genComparison(type, false, preferLogical, d);
+        while (type instanceof TypeInt && d-->0 && random.nextInt(preferLogical?50:5)==0) {
+            sb.append("&|^".charAt(random.nextInt(3)));
+            genComparison(type, true, preferLogical, d);
+        }
+    }
+
+    public void genComparison(Type type, boolean change, boolean preferLogical, int d) {
+        if (change && generateInvalid()) type = getType();
+        if (d > 0 && type instanceof TypeInt && random.nextInt(preferLogical?5:50)==0) {
+            type = getType();
+            genComparison(type, true, preferLogical, d-1);
+            sb.append(random.nextBoolean()?"==":"!=");
+        }
+        genChaining(type, false, preferLogical, d);
+    }
+
+    public void genChaining(Type type, boolean change, boolean preferLogical, int d) {
+        if (change && generateInvalid()) type = getType();
+        if (d>0 && type instanceof TypeInt && random.nextInt(preferLogical?5:50)==0) {
+            genShift(TYPE_INT, true, false, d);
+            char c = random.nextBoolean()?'>':'<';
+            sb.append(c);
+            if (random.nextInt(4)==0) sb.append('=');
+            genShift(TYPE_INT, true, false, --d);
+            while (d-->0 && random.nextInt(preferLogical?5:50)==0) {
+                if (generateInvalid()) c = c=='>' ? '<' : '>';
+                sb.append(c);
+                if (random.nextInt(4)==0) sb.append('=');
+                genShift(TYPE_INT, true, false, d);
+            }
+        } else {
+            genShift(type, false, preferLogical, d);
+        }
+    }
+
+    private static final String[] SHIFTS = {">>", ">>>", "<<"};
+
+    public void genShift(Type type, boolean change, boolean preferLogical, int d) {
+        if (change && generateInvalid()) type = getType();
+        genAddition(type, false, preferLogical, d);
+        while (d-- > 0 && type instanceof TypeInt && random.nextInt(50)==0) {
+            sb.append(SHIFTS[random.nextInt(3)]);
+            genAddition(type, true, preferLogical, d);
+        }
+    }
+
+    public void genAddition(Type type, boolean change, boolean preferLogical, int d) {
+        if (change && generateInvalid()) type = getType();
+        genMultiplication(type, false, preferLogical, d);
+        while (d-- > 0 && (type instanceof TypeInt || type==TYPE_FLT) && random.nextInt(50)==0) {
+            sb.append(random.nextBoolean()?" + ":" - ");
+            genMultiplication(type, true, preferLogical, d);
+        }
+    }
+
+    public void genMultiplication(Type type, boolean change, boolean preferLogical, int d) {
+        if (change && generateInvalid()) type = getType();
+        genUnary(type, false, preferLogical, d);
+        while (d-- > 0 && (type instanceof TypeInt || type==TYPE_FLT) && random.nextInt(50)==0) {
+            sb.append(random.nextBoolean()?'*':'/');
+            genUnary(type, true, preferLogical, d);
+        }
+    }
+
+    /**
+     * Generate a unary expression.
+     */
+    public void genUnary(Type type, boolean change, boolean preferLogical, int d) {
+        if (change && generateInvalid()) type = getType();
+        if (!(type instanceof TypeInt) && type != TYPE_FLT) {
+            genSuffix(type, false, preferLogical, d);
+            return;
+        }
+        while (d>0 && random.nextInt(5)==0) {
+            d--;
+            boolean what = type instanceof TypeInt && random.nextInt(preferLogical?2:20) == 0;
+            sb.append(what?'!':'-');
+        }
+        if (random.nextInt(10)==0) {
+            var t = type;
+            var v = findVisibleVariablesMatching(va->va.type.isa(t));
+            if (v != null) {
+                sb.append(random.nextBoolean()?"--":"++").append(v.name);
+                return;
+            }
+        }
+        genSuffix(type, false, preferLogical, d);
+    }
+
+    public TypeStruct.Field getField(Type type) {
+        int n=0;
+        for (var s: structs) {
+            for (var f: s.fields) {
+                if (f.type.isa(type)) n++;
+            }
+        }
+        if (n==0) return null;
+        n = random.nextInt(n);
+        for (var s: structs) {
+            for (var f: s.fields) {
+                if (f.type.isa(type)) if (n--==0) return f;
+            }
+        }
+        throw new AssertionError();
+    }
+
+    public void genSuffix(Type type, boolean change, boolean preferLogical, int d) {
+        if (change && generateInvalid()) type = getType();
+        if (d > 0 && random.nextBoolean() && genField(type, d-1)) return;
+        if (d > 0 && random.nextInt(10) == 0) {
+            if (random.nextBoolean() && genField(type, d-1)) return;
+            var t = type;
+            var v = findVisibleVariablesMatching(va -> va.type.isa(t));
+            if (v != null) {
+                sb.append(v.name).append(random.nextBoolean() ? "--" : "++");
+                return;
+            }
+        }
+        genPrimary(type, false, preferLogical, d);
+    }
+
+    public boolean genField(Type type, int d) {
+        if (generateInvalid()) {
+            var field = getRandomName();
+            var t = getType();
+            genSuffix(t, true, false, d);
+            sb.append(".").append(field);
+        } else {
+            var field = getField(type);
+            if (field == null) return false;
+            genSuffix(field.struct, true, false, d);
+            sb.append(".").append(field.name);
+        }
+        return true;
+    }
+
+    /**
+     * Generate a primary expression.
+     */
+    public void genPrimary(Type type, boolean change, boolean preferLogical, int d) {
+        if (change && generateInvalid()) type = getType();
+        var rand = random.nextInt(10);
+        if (d > 0 && rand == 0) {
+            sb.append("(");
+            genExpression(type, false, preferLogical, d-1);
+            sb.append(")");
+        } else if (rand < 6) {
+            genVariable(type);
+        } else {
+            genConst(type);
+        }
+    }
+
+    /**
+     * Generate a constant.
+     */
+    public void genConst(Type type) {
+        if (generateInvalid()) type = getType();
+        if (type instanceof TypeInt || (type == TYPE_FLT && random.nextBoolean())) {
+            var rand = random.nextInt(10);
+            switch (rand) {
+                case 0 -> sb.append("true");
+                case 1 -> sb.append("false");
+                default -> sb.append(random.nextInt(1<<(rand-2)));
+            }
+        } else if (type == TYPE_FLT) {
+            sb.append(random.nextFloat());
+        } else if (type instanceof TypeNullable n) {
+            if ((n.base instanceof TypeStruct s && s.fields == null) || random.nextBoolean()) {
+                sb.append("null");
+            } else {
+                sb.append("new ").append(generateInvalid() ? getRandomName() : n.base.name);
+            }
+        } else {
+            sb.append("new ").append(generateInvalid() ? getRandomName() : type.name);
+        }
+    }
+
+    /**
+     * Generate a variable. If there are none generate a number instead.
+     * If the program is allowed to be invalid a random name can be generated in some cases.
+     */
+    public void genVariable(Type type) {
+        if (generateInvalid()) {
+            sb.append(getRandomName());
+            return;
+        }
+        if (generateInvalid()) type = getType();
+        var t = type;
+        var v = findVisibleVariablesMatching(va->va.type.isa(t));
+        if (v == null) genConst(type);
+        else sb.append(v.name);
+    }
+
+}
\ No newline at end of file