Chapter 12: References

Co-authored-by: Hsiang-Ying Fu <66062782+fuhsnn@users.noreply.github.com>
Co-authored-by: Dibyendu Majumdar <mobile@majumdar.org.uk>
Co-authored-by: Thalia Archibald <thalia@archibald.dev>
Co-authored-by: tannal <tannal2409@gmail.com>
Co-authored-by: hadha <helsgithub15@gmail.com>
Co-authored-by: fuhsnn <66062782+fuhsnn@users.noreply.github.com>
Co-authored-by: XmiliaH <45106915+XmiliaH@users.noreply.github.com>
Co-authored-by: Tamás Hadházy <85063808+Hels15@users.noreply.github.com>
Co-authored-by: Symious <14933944+symious@users.noreply.github.com>
Co-authored-by: RobertObkircher <robert.obkircher@gmail.com>
Co-authored-by: Robert Obkircher <robert.obkircher@gmail.com>
Co-authored-by: Jun Kawasaki <jun784@gmail.com>
Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

8 files changed, 325 insertions, 73 deletions

diff --git a/src/test/java/com/seaofnodes/simple/Chapter05Test.java b/src/test/java/com/seaofnodes/simple/Chapter05Test.java
index cf7bd6cf..97cd9940 100644
--- a/src/test/java/com/seaofnodes/simple/Chapter05Test.java
+++ b/src/test/java/com/seaofnodes/simple/Chapter05Test.java
@@ -37,7 +37,7 @@ if (arg == 1) {
 }
 return c;""", TypeInteger.BOT);
         StopNode ret = parser.parse().iterate();
-        assertEquals("return Phi(Region17,4,3);", ret.toString());
+        assertEquals("return Phi(Region18,4,3);", ret.toString());
     }
 
     @Test
@@ -116,7 +116,7 @@ if( arg==0 )
 return arg+a+b;
 """, TypeInteger.BOT);
         StopNode stop = parser.parse();
-        assertEquals("return ((arg+Phi(Region14,1,0))+Phi(Region29,2,0));", stop.toString());
+        assertEquals("return ((arg+Phi(Region16,1,0))+Phi(Region32,2,0));", stop.toString());
     }
 
     @Test
@@ -130,7 +130,7 @@ if( arg==1 )
 }
 return a;""");
         StopNode ret = parser.parse().iterate();
-        assertEquals("return (arg==Phi(Region17,3,2));", ret.toString());
+        assertEquals("return (arg==Phi(Region18,3,2));", ret.toString());
     }
 
     @Test
diff --git a/src/test/java/com/seaofnodes/simple/Chapter06Test.java b/src/test/java/com/seaofnodes/simple/Chapter06Test.java
index 920e26e3..9af11b68 100644
--- a/src/test/java/com/seaofnodes/simple/Chapter06Test.java
+++ b/src/test/java/com/seaofnodes/simple/Chapter06Test.java
@@ -30,7 +30,7 @@ if (arg)
 return (arg < a) < 3;
 """);
         StopNode stop = parser.parse().iterate();
-        assertEquals("return ((arg<Phi(Region13,2,1))<3);", stop.toString());
+        assertEquals("return ((arg<Phi(Region15,2,1))<3);", stop.toString());
     }
 
     @Test
@@ -200,7 +200,7 @@ if( arg ) {
 return a+b;
 """);
         StopNode ret = parser.parse().iterate();
-        assertEquals("return Phi(Region25,4,1);", ret.toString());
+        assertEquals("return Phi(Region27,4,1);", ret.toString());
     }
 
 
@@ -251,7 +251,7 @@ if( arg ) {
 return a+b+c;
 """);
         StopNode ret = parser.parse().iterate();
-        assertEquals("return (Phi(Region37,Phi(Region24,2,3),0)+Phi(Region,3,1));", ret.toString());
+        assertEquals("return (Phi(Region39,Phi(Region24,2,3),0)+Phi(Region,3,1));", ret.toString());
     }
 
 
diff --git a/src/test/java/com/seaofnodes/simple/Chapter08Test.java b/src/test/java/com/seaofnodes/simple/Chapter08Test.java
index 4fa08bd9..02c3a301 100644
--- a/src/test/java/com/seaofnodes/simple/Chapter08Test.java
+++ b/src/test/java/com/seaofnodes/simple/Chapter08Test.java
@@ -24,7 +24,7 @@ while(arg < 10) {
 return arg;
                 """);
         StopNode stop = parser.parse().iterate();
-        assertEquals("return Phi(Region37,Phi(Region26,Phi(Loop7,arg,(Phi_arg+1)),Add),Add);", stop.toString());
+        assertEquals("return Phi(Region38,Phi(Region26,Phi(Loop7,arg,(Phi_arg+1)),Add),Add);", stop.toString());
         assertTrue(stop.ret().ctrl() instanceof RegionNode);
         Assert.assertEquals(5L, Evaluator.evaluate(stop, 1));
         Assert.assertEquals(10L, Evaluator.evaluate(stop, 6));
@@ -47,7 +47,7 @@ while(arg < 10) {
 return a;
                 """);
         StopNode stop = parser.parse().iterate();
-        assertEquals("return Phi(Loop8,1,Phi(Region43,Phi_a,(Phi_a+1)));", stop.toString());
+        assertEquals("return Phi(Loop8,1,Phi(Region45,Phi_a,(Phi_a+1)));", stop.toString());
         assertTrue(stop.ret().ctrl() instanceof ProjNode);
     }
 
@@ -66,7 +66,7 @@ while(arg < 10) {
 return arg;
                 """);
         StopNode stop = parser.parse().iterate();
-        assertEquals("return Phi(Region35,Phi(Loop7,arg,(Phi_arg+1)),Add);", stop.toString());
+        assertEquals("return Phi(Region36,Phi(Loop7,arg,(Phi_arg+1)),Add);", stop.toString());
         assertTrue(stop.ret().ctrl() instanceof RegionNode);
     }
 
diff --git a/src/test/java/com/seaofnodes/simple/Chapter09Test.java b/src/test/java/com/seaofnodes/simple/Chapter09Test.java
index 1b1920e8..b1fdc459 100644
--- a/src/test/java/com/seaofnodes/simple/Chapter09Test.java
+++ b/src/test/java/com/seaofnodes/simple/Chapter09Test.java
@@ -134,14 +134,14 @@ while(v1+arg) {
 }
                 """);
         StopNode stop = parser.parse().iterate();
-        assertEquals("Stop[ ]", stop.toString());
+        assertEquals("return 0;", stop.toString());
     }
 
     @Test
     public void testWhile0() {
         Parser parser = new Parser("while(0) continue; if(0) arg=0;");
         StopNode stop = parser.parse().iterate();
-        assertEquals("Stop[ ]", stop.toString());
+        assertEquals("return 0;", stop.toString());
     }
 
     @Test
@@ -153,7 +153,7 @@ if(0) while(0) {
 }
 """);
         StopNode stop = parser.parse().iterate();
-        assertEquals("Stop[ ]", stop.toString());
+        assertEquals("return 0;", stop.toString());
     }
 
 
@@ -215,7 +215,7 @@ return -0+0+0;
     public void testFuzz3() {
         Parser parser = new Parser("int v0=0; while(0==69) while(v0) return 0;");
         StopNode stop = parser.parse().iterate();
-        assertEquals("Stop[ ]", stop.toString());
+        assertEquals("return 0;", stop.toString());
     }
 
     @Test
@@ -260,7 +260,7 @@ while(0==1) while(v0)
         v0=1+v0;
                                    """);
         StopNode stop = parser.parse().iterate();
-        assertEquals("Stop[ ]", stop.toString());
+        assertEquals("return 0;", stop.toString());
     }
 
     @Test
diff --git a/src/test/java/com/seaofnodes/simple/Chapter10Test.java b/src/test/java/com/seaofnodes/simple/Chapter10Test.java
index 9a5e4677..e5d9b882 100644
--- a/src/test/java/com/seaofnodes/simple/Chapter10Test.java
+++ b/src/test/java/com/seaofnodes/simple/Chapter10Test.java
@@ -221,7 +221,7 @@ if( bar ) bar.a = 1;
 return bar;
 """);
         StopNode stop = parser.parse().iterate();
-        assertEquals("return Phi(Region16,null,new Bar);", stop.toString());
+        assertEquals("return Phi(Region31,(*void)Phi(Region18,null,new Bar),null);", stop.toString());
     }
 
     @Test
@@ -237,7 +237,7 @@ else bar.a = 1;
 return rez;
 """);
         StopNode stop = parser.parse().iterate();
-        assertEquals("return Phi(Region34,4,3);", stop.toString());
+        assertEquals("return Phi(Region37,4,3);", stop.toString());
     }
 
     @Test
@@ -290,8 +290,7 @@ while( i.x < i.len ) {
 return sum;
 """);
         StopNode stop = parser.parse().iterate();
-        assertEquals("return Phi(Loop16,0,(Phi(Loop,0,(Phi_$2+1))+Phi_sum));", stop.toString());
-        assertEquals(10L,Evaluator.evaluate(stop,5));
+        assertEquals("return Phi(Loop17,0,(Phi(Loop,0,(Phi_x+1))+Phi_sum));", stop.toString());
     }
 
 
@@ -311,7 +310,7 @@ while(arg) {
 return ret;
 """);
         StopNode stop = parser.parse().iterate();
-        assertEquals("return Phi(Loop11,new s0,Phi(Region34,new s0,Phi_ret));", stop.toString());
+        assertEquals("return Phi(Loop11,new s0,Phi(Region35,new s0,Phi_ret));", stop.toString());
     }
 
     @Test
@@ -328,7 +327,7 @@ while(arg) {
 return ret;
 """);
         StopNode stop = parser.parse().iterate();
-        assertEquals("return Phi(Loop15,new s0,Phi(Region35,new s0,Phi_ret));", stop.toString());
+        assertEquals("return Phi(Loop15,new s0,Phi(Region36,new s0,Phi_ret));", stop.toString());
     }
 
 
@@ -345,7 +344,7 @@ while(arg < 10) {
 return ret;
 """);
         StopNode stop = parser.parse().iterate();
-        assertEquals("return Phi(Loop11,new s0,Phi(Region32,new s0,Phi_ret));", stop.toString());
+        assertEquals("return Phi(Loop11,new s0,Phi(Region33,new s0,Phi_ret));", stop.toString());
     }
 
     @Test
@@ -375,7 +374,7 @@ if(0) {
 }
    """);
         StopNode stop = parser.parse().iterate();
-        assertEquals("Stop[ ]", stop.toString());
+        assertEquals("return 0;", stop.toString());
     }
 
     @Test
@@ -449,7 +448,7 @@ while(0) {}
 }
 """);
         StopNode stop = parser.parse().iterate();
-        assertEquals("Stop[ ]", stop.toString());
+        assertEquals("return 0;", stop.toString());
     }
 
     @Test
diff --git a/src/test/java/com/seaofnodes/simple/Chapter12Test.java b/src/test/java/com/seaofnodes/simple/Chapter12Test.java
new file mode 100644
index 00000000..0d4f3c97
--- /dev/null
+++ b/src/test/java/com/seaofnodes/simple/Chapter12Test.java
@@ -0,0 +1,200 @@
+package com.seaofnodes.simple;
+
+import com.seaofnodes.simple.evaluator.Evaluator;
+import com.seaofnodes.simple.node.StopNode;
+import com.seaofnodes.simple.type.TypeStruct;
+import com.seaofnodes.simple.type.TypeMemPtr;
+import org.junit.Ignore;
+import org.junit.Test;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.fail;
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertSame;
+
+public class Chapter12Test {
+
+    @Test
+    public void testLinkedList0() {
+        Parser parser = new Parser(
+"""
+struct LLI { LLI? next; int i; }
+LLI? head = null;
+while( arg ) {
+    LLI x = new LLI;
+    x.next = head;
+    x.i = arg;
+    head = x;
+    arg = arg-1;
+}
+return head.next.i;
+""");
+        try { parser.parse().iterate(); fail(); }
+        catch( Exception e ) { assertEquals("Might be null accessing 'next'",e.getMessage()); }
+    }
+
+    @Test
+    public void testLinkedList1() {
+        Parser parser = new Parser(
+"""
+struct LLI { LLI? next; int i; }
+LLI? head = null;
+while( arg ) {
+    LLI x = new LLI;
+    x.next = head;
+    x.i = arg;
+    head = x;
+    arg = arg-1;
+}
+if( !head ) return 0;
+LLI? next = head.next;
+if( next==null ) return 1;
+return next.i;
+""");
+        StopNode stop = parser.parse().iterate();
+        assertEquals("Stop[ return 0; return 1; return .i; ]", stop.toString());
+        assertEquals(2L, Evaluator.evaluate(stop,  3));
+    }
+
+    @Test
+    public void testCoRecur() {
+        Parser parser = new Parser(
+"""
+struct Left { int i; Right? f; }
+struct Right { int f; Left? i; }
+Left left = new Left;
+left.i = 17;
+Right right = new Right;
+right.f = 314;
+left.f = right;
+right.i = left;
+return right.i.f.i.i;
+""");
+        StopNode stop = parser.parse().iterate();
+        assertEquals("return 17;", stop.toString());
+    }
+
+    @Test
+    public void testNullReright() {
+        Parser parser = new Parser(
+"""
+struct N { N next; int i; }
+N n = new N;
+return n.next;
+""");
+        StopNode stop = parser.parse().iterate();
+        assertEquals("return null;", stop.toString());
+    }
+
+    @Test
+    public void testNullRef1() {
+        Parser parser = new Parser(
+"""
+struct N { N next; int i; }
+N n = new N;
+n.next = new N;
+return n.next;
+""");
+        StopNode stop = parser.parse().iterate();
+        assertEquals("return new N;", stop.toString());
+    }
+
+    @Test
+    public void testNullRef2() {
+        Parser parser = new Parser(
+"""
+struct N { N next; int i; }
+N n = new N;
+n.next = null;
+return n.next;
+""");
+        try { parser.parse().iterate(); fail(); }
+        catch( Exception e ) { assertEquals("Cannot store null into field *N next",e.getMessage()); }
+    }
+
+
+    @Test
+    public void testNullRef4() {
+        Parser parser = new Parser("-null-5/null-5");
+        try { parser.parse().iterate(); fail(); }
+        catch( Exception e ) { assertEquals("Expected an identifier, found 'null'",e.getMessage()); }
+    }
+
+    @Test public void testNullRef5() {
+        Parser parser = new Parser("return null+42;");
+        try { parser.parse().iterate(); fail(); }
+        catch( Exception e ) { assertEquals("Cannot 'Add' null",e.getMessage()); }
+    }
+
+    @Test
+    public void testEmpty() {
+        Parser parser = new Parser(
+"""
+struct S{};
+""");
+        StopNode stop = parser.parse().iterate();
+        assertEquals("return 0;", stop.toString());
+        assertEquals(0L, Evaluator.evaluate(stop,  0));
+    }
+
+    @Test
+    public void testForwardReright() {
+        Parser parser = new Parser(
+"""
+struct S1 { S2 s; }
+return new S2;
+""");
+        try { parser.parse().iterate(); fail(); }
+        catch( Exception e ) { assertEquals("Unknown struct type 'S2'",e.getMessage()); }
+    }
+
+    @Test
+    public void testForwardRef1() {
+        Parser parser = new Parser(
+"""
+struct S1 { S2? s; }
+struct S2 { int x; }
+return new S1.s=new S2;
+""");
+        StopNode stop = parser.parse().iterate();
+        assertEquals("return new S1;", stop.toString());
+    }
+
+    @Test
+    public void testcheckNull() {
+        Parser parser = new Parser(
+"""
+struct I {int i;}
+struct P {I pi;}
+P p1 = new P;
+P p2 = new P;
+p2.pi = new I;
+p2.pi.i = 2;
+if (arg) p1 = new P;
+return p1.pi.i + 1;
+""");
+        try { parser.parse().iterate();  fail(); }
+        catch( Exception e ) {  assertEquals("Might be null accessing 'i'",e.getMessage());  }
+    }
+
+
+    @Test
+    public void testShallowRecursiveType() {
+        new Parser("struct LLI { LLI? next; int i; } return 0;").parse().iterate();
+        TypeStruct l1 = (TypeStruct)Parser.TYPES.get("LLI");
+        TypeStruct l0 = ((TypeMemPtr)l1._fields[0]._type)._obj;
+        assertEquals("LLI", l0._name);
+        assertNull(l0._fields);
+        assertEquals(2, l1._fields.length);
+        assertSame(l0, l1.meet(l0));
+        assertSame(l0, l0.dual());
+        assertSame(l0, l0.glb());
+    }
+
+    @Test
+    public void testUnusedForwardReference() {
+        StopNode stop = new Parser("struct Holder { Missing? ref; } Holder h = new Holder; return h.ref;")
+            .parse().iterate();
+        assertEquals("return null;", stop.toString());
+        assertNull(((TypeStruct)Parser.TYPES.get("Missing"))._fields);
+    }
+}
diff --git a/src/test/java/com/seaofnodes/simple/TypeTest.java b/src/test/java/com/seaofnodes/simple/TypeTest.java
index f9c580cc..298fc6f8 100644
--- a/src/test/java/com/seaofnodes/simple/TypeTest.java
+++ b/src/test/java/com/seaofnodes/simple/TypeTest.java
@@ -36,9 +36,9 @@ public class TypeTest {
         Assert.assertEquals(TypeMem   .BOT, m2.meet(m3));
         Assert.assertEquals(TypeMem   .BOT, m3.meet(m4));
 
-        Assert.assertEquals(TypeMem.BOT, m1.glb());
+        Assert.assertEquals(m1, m1.glb());
         Assert.assertEquals(m1, m1.dual());
-        Assert.assertEquals(TypeMem.TOP, m1.glb().dual());
+        Assert.assertEquals(m1, m1.glb().dual());
 
         TypeMemPtr ptr1 = TypeMemPtr.make(s1);
         Assert.assertEquals(s1, ptr1._obj);
diff --git a/src/test/java/com/seaofnodes/simple/fuzzer/ScriptGenerator.java b/src/test/java/com/seaofnodes/simple/fuzzer/ScriptGenerator.java
index 155063ab..14baadba 100644
--- a/src/test/java/com/seaofnodes/simple/fuzzer/ScriptGenerator.java
+++ b/src/test/java/com/seaofnodes/simple/fuzzer/ScriptGenerator.java
@@ -3,9 +3,7 @@ package com.seaofnodes.simple.fuzzer;
 import com.seaofnodes.simple.Parser;
 import com.seaofnodes.simple.node.ScopeNode;
 
-import java.util.ArrayList;
-import java.util.HashSet;
-import java.util.Random;
+import java.util.*;
 import java.util.function.Predicate;
 
 /**
@@ -22,6 +20,7 @@ public class ScriptGenerator {
     private static final int MAX_NAME_LENGTH = 10;
     private static final int MAX_BLOCK_DEPTH = 4;
     private static final int MAX_EXPRESSION_DEPTH = 10;
+    private static final int MAX_SUFFIX_RECURSIVE_DEPTH = 10;
 
     /**
      * Number of spaces per indentation
@@ -68,12 +67,11 @@ public class ScriptGenerator {
     }
 
     private static class TypeStruct extends Type {
-        record Field (String name, Type type) {}
-        final Field[] fields;
+        record Field (String name, Type type, TypeStruct struct) {}
+        Field[] fields;
         final TypeNullable nullable = new TypeNullable(this);
-        TypeStruct(String name, Field[] fields) {
+        TypeStruct(String name) {
             super(name);
-            this.fields = fields;
         }
         boolean isa(Type other) { return this == other || other == nullable; }
     }
@@ -144,6 +142,10 @@ public class ScriptGenerator {
      */
     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;
@@ -286,6 +288,7 @@ public class ScriptGenerator {
         for (var s:structs) {
             if (s.name.equals(name)) return s;
         }
+        for (var s:forwardStructs) if (s.name.equals(name)) return s;
         return null;
     }
 
@@ -318,12 +321,18 @@ public class ScriptGenerator {
         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 structs.get(random.nextInt(structs.size())).name;
+        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();
@@ -342,14 +351,26 @@ public class ScriptGenerator {
      * Get a random type
      * @return A random type
      */
-    private Type getType() {
-        if (structs.isEmpty() || random.nextBoolean()) return TYPE_INT;
-        var idx = random.nextInt(structs.size());
-        TypeStruct struct = structs.get(idx);
+    private Type getType(boolean allowForward) {
+        int t = random.nextInt(10);
+        if( t<5 || structs.isEmpty() ) return TYPE_INT;
+        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.
@@ -409,19 +430,29 @@ public class ScriptGenerator {
      * @return 0
      */
     public int genStruct() {
-        var name = getStructName();
+        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(name).append(" {\n");
+        sb.append("struct ").append(struct.name).append(" {\n");
         indentation += INDENTATION;
         for (int i=0; i<fields.length; i++) {
             var fieldName = getRandomName();
-            var type = generateInvalid() ? getType() : TYPE_INT;
+            var type = getType(true);
             printIndentation().append(type.name).append(" ").append(fieldName).append(";\n");
-            fields[i] = new TypeStruct.Field(fieldName.toString(), type);
+            fields[i] = new TypeStruct.Field(fieldName.toString(), type, struct);
         }
         indentation -= INDENTATION;
         printIndentation().append("}");
-        structs.add(new TypeStruct(name, fields));
+        struct.fields = fields;
+        forwardStructs.remove(struct);
+        structs.add(struct);
         return 0;
     }
 
@@ -515,7 +546,7 @@ public class ScriptGenerator {
      */
     public int genIf() {
         sb.append("if(");
-        genExpression(TYPE_INT);
+        genExpression(TYPE_INT, true);
         sb.append(") ");
         var stop = genStatementBlock();
         if ((stop & FLAG_IF_WITHOUT_ELSE) == 0 && random.nextInt(10) > 3) {
@@ -561,7 +592,7 @@ public class ScriptGenerator {
      */
     public int genWhile() {
         sb.append("while(");
-        genExpression(TYPE_INT);
+        genExpression(TYPE_INT, true);
         sb.append(") ");
         loopDepth++;
         genStatementBlock();
@@ -587,17 +618,17 @@ public class ScriptGenerator {
         indentation += INDENTATION;
         String name = getVarName();
         printIndentation().append("int ").append(name).append("=");
-        genExpression(TYPE_INT);
+        genExpression(TYPE_INT, true);
         sb.append(";\n");
         addVariable(name, TYPE_INT);
         printIndentation().append("while(").append(name).append("<");
-        genExpression(TYPE_INT);
+        genExpression(TYPE_INT, true);
         sb.append(") {\n");
         indentation += INDENTATION;
         depth--;
         loopDepth++;
         printIndentation().append(name).append("=").append(name).append("+");
-        genExpression(TYPE_INT);
+        genExpression(TYPE_INT, true);
         sb.append(";\n");
         genStatements();
         loopDepth--;
@@ -621,7 +652,7 @@ public class ScriptGenerator {
         sb.append(generateInvalid() ? getRandomName() : type.name).append(" ").append(name);
         if (!(type instanceof TypeNullable) || random.nextBoolean()) {
             sb.append("=");
-            genExpression(type);
+            genExpression(type, true);
         }
         sb.append(";");
         addVariable(name, type);
@@ -656,7 +687,7 @@ public class ScriptGenerator {
             declared = field.type;
         }
         sb.append("=");
-        genExpression(declared);
+        genExpression(declared, true);
         sb.append(";");
         return 0;
     }
@@ -668,7 +699,7 @@ public class ScriptGenerator {
     public int genReturn() {
         var type = getType();
         sb.append("return ");
-        genExpression(type);
+        genExpression(type, true);
         sb.append(";");
         return FLAG_STOP;
     }
@@ -677,70 +708,91 @@ public class ScriptGenerator {
      * Generate a binary expression.
      * This method does not care about operator precedence.
      */
-    public void genExpression(Type type) {
-        if (generateInvalid()) type = getType();
+    public void genExpression(Type type, boolean change) {
+        if (change && generateInvalid()) type = getType();
         if (type != TYPE_INT) {
-            genUnary(type);
+            genUnary(type, false);
             return;
         }
         var num = randLog(MAX_BINARY_EXPRESSIONS_PER_EXPRESSION);
         while(num-->0) {
-            genUnary(TYPE_INT);
+            genUnary(type, false);
             sb.append(BINARY_OP[random.nextInt(BINARY_OP.length)]);
         }
-        genUnary(TYPE_INT);
+        genUnary(type, false);
     }
 
     /**
      * Generate a unary expression.
      */
-    public void genUnary(Type type) {
-        if (generateInvalid()) type = getType();
+    public void genUnary(Type type, boolean change) {
+        if (change && generateInvalid()) type = getType();
         if (type != TYPE_INT) {
-            genSuffix(type);
+            genSuffix(type, false);
             return;
         }
         var num = randLog(MAX_UNARY_EXPRESSIONS_PER_EXPRESSION);
         while(num-->0) {
             sb.append(UNARY_OP[random.nextInt(UNARY_OP.length)]);
         }
-        genSuffix(TYPE_INT);
+        genSuffix(type, false);
+    }
+
+    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();
     }
 
     /**
      * Generate
      */
-    public void genSuffix(Type type) {
-        if (generateInvalid()) type = getType();
-        if (type == TYPE_INT && random.nextBoolean()) {
-            var t = getType();
+    public void genSuffix(Type type, boolean change) {
+        genSuffixRecursive(type, change, MAX_SUFFIX_RECURSIVE_DEPTH);
+    }
+
+    public void genSuffixRecursive(Type type, boolean change, int depth) {
+        if (change && generateInvalid()) type = getType();
+        if (depth > 0 && random.nextBoolean()) {
             if (generateInvalid()) {
                 var field = getRandomName();
-                genPrimary(t);
+                var t = getType();
+                genSuffixRecursive(t, true, depth-1);
                 sb.append(".").append(field);
                 return;
-            } else if (t instanceof TypeStruct s && s.fields.length > 0) {
-                var field = s.fields[random.nextInt(s.fields.length)];
-                if (field.type == TYPE_INT) {
-                    genPrimary(t);
+            } else {
+                var field = getField(type);
+                if (field != null) {
+                    genSuffixRecursive(field.struct, true, depth-1);
                     sb.append(".").append(field.name);
                     return;
                 }
             }
         }
-        genPrimary(type);
+        genPrimary(type, false);
     }
 
     /**
      * Generate a primary expression.
      */
-    public void genPrimary(Type type) {
-        if (generateInvalid()) type = getType();
+    public void genPrimary(Type type, boolean change) {
+        if (change && generateInvalid()) type = getType();
         var rand = random.nextInt(10);
         if (rand == 0 && exprDepth != 0) {
             sb.append("(");
             exprDepth--;
-            genExpression(type);
+            genExpression(type, false);
             exprDepth++;
             sb.append(")");
         } else if (rand < 6) {
@@ -762,8 +814,9 @@ public class ScriptGenerator {
                 case 1 -> sb.append("false");
                 default -> sb.append(random.nextInt(1<<(rand-2)));
             }
+
         } else if (type instanceof TypeNullable n) {
-            if (random.nextBoolean()) {
+            if ((n.base instanceof TypeStruct s && s.fields == null) || random.nextBoolean()) {
                 sb.append("null");
             } else {
                 sb.append("new ").append(generateInvalid() ? getRandomName() : n.base.name);