Chapter 8: Lazy Phis, Break, Continue, and Evaluator

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

11 files changed, 1051 insertions, 17 deletions

diff --git a/src/test/java/com/seaofnodes/simple/Chapter01Test.java b/src/test/java/com/seaofnodes/simple/Chapter01Test.java
index 547c36f8..13b42f96 100644
--- a/src/test/java/com/seaofnodes/simple/Chapter01Test.java
+++ b/src/test/java/com/seaofnodes/simple/Chapter01Test.java
@@ -31,7 +31,7 @@ public class Chapter01Test {
         parser.parse();
         StartNode start = Parser.START;
         for( Node use : start._outputs )
-            if( use instanceof ConstantNode con )
+            if( use instanceof ConstantNode con && con._type instanceof TypeInteger )
                 assertEquals(TypeInteger.constant(0),con._type);
     }
 
diff --git a/src/test/java/com/seaofnodes/simple/Chapter03Test.java b/src/test/java/com/seaofnodes/simple/Chapter03Test.java
index 12201ca0..609dff76 100644
--- a/src/test/java/com/seaofnodes/simple/Chapter03Test.java
+++ b/src/test/java/com/seaofnodes/simple/Chapter03Test.java
@@ -31,7 +31,7 @@ public class Chapter03Test {
 
     @Test
     public void testVarDist() {
-        Parser parser = new Parser("int x0=1; int y0=2; int x1=3; int y1=4; return (x0-x1)*(x0-x1) + (y0-y1)*(y0-y1);");
+        Parser parser = new Parser("int x0=1; int y0=2; int x1=3; int y1=4; return (x0-x1)*(x0-x1) + (y0-y1)*(y0-y1); ");
         StopNode ret = parser.parse();
         assertEquals("return 8;", ret.print());
     }
@@ -45,7 +45,7 @@ public class Chapter03Test {
             assertEquals("Undefined name 'a'",e.getMessage());
         }
     }
-    
+
     @Test
     public void testRedeclareVar() {
         try {
diff --git a/src/test/java/com/seaofnodes/simple/Chapter04Test.java b/src/test/java/com/seaofnodes/simple/Chapter04Test.java
index e5840622..f1603cc4 100644
--- a/src/test/java/com/seaofnodes/simple/Chapter04Test.java
+++ b/src/test/java/com/seaofnodes/simple/Chapter04Test.java
@@ -10,7 +10,7 @@ public class Chapter04Test {
 
     @Test
     public void testPeephole() {
-        Parser parser = new Parser("return 1+arg+2;");
+        Parser parser = new Parser("return 1+arg+2; ");
         StopNode ret = parser.parse();
         assertEquals("return (arg+3);", ret.print());
     }
@@ -38,7 +38,7 @@ public class Chapter04Test {
 
     @Test
     public void testPeephole3() {
-        Parser parser = new Parser("return 1+arg+2+arg+3;");
+        Parser parser = new Parser("return 1+arg+2+arg+3; ");
         StopNode ret = parser.parse();
         assertEquals("return ((arg*2)+6);", ret.print());
     }
@@ -80,7 +80,7 @@ public class Chapter04Test {
 
     @Test
     public void testVarArg() {
-        Parser parser = new Parser("return arg;");
+        Parser parser = new Parser("return arg; ");
         StopNode stop = parser.parse();
         ReturnNode ret = stop.ret();
         assertTrue(ret.in(0) instanceof ProjNode);
@@ -89,49 +89,49 @@ public class Chapter04Test {
 
     @Test
     public void testConstantArg() {
-        Parser parser = new Parser("return arg;", TypeInteger.constant(2));
+        Parser parser = new Parser("return arg; ", TypeInteger.constant(2));
         StopNode ret = parser.parse();
         assertEquals("return 2;", ret.print());
     }
 
     @Test
     public void testCompEq() {
-        Parser parser = new Parser("return 3==3;");
+        Parser parser = new Parser("return 3==3; ");
         StopNode ret = parser.parse();
         assertEquals("return 1;", ret.print());
     }
 
     @Test
     public void testCompEq2() {
-        Parser parser = new Parser("return 3==4;");
+        Parser parser = new Parser("return 3==4; ");
         StopNode ret = parser.parse();
         assertEquals("return 0;", ret.print());
     }
 
     @Test
     public void testCompNEq() {
-        Parser parser = new Parser("return 3!=3;");
+        Parser parser = new Parser("return 3!=3; ");
         StopNode ret = parser.parse();
         assertEquals("return 0;", ret.print());
     }
 
     @Test
     public void testCompNEq2() {
-        Parser parser = new Parser("return 3!=4;");
+        Parser parser = new Parser("return 3!=4; ");
         StopNode ret = parser.parse();
         assertEquals("return 1;", ret.print());
     }
 
     @Test
     public void testBug1() {
-        Parser parser = new Parser("int a=arg+1; int b=a; b=1; return a+2;");
+        Parser parser = new Parser("int a=arg+1; int b=a; b=1; return a+2; ");
         StopNode ret = parser.parse();
         assertEquals("return (arg+3);", ret.print());
     }
 
     @Test
     public void testBug2() {
-        Parser parser = new Parser("int a=arg+1; a=a; return a;");
+        Parser parser = new Parser("int a=arg+1; a=a; return a; ");
         StopNode ret = parser.parse();
         assertEquals("return (arg+1);", ret.print());
     }
diff --git a/src/test/java/com/seaofnodes/simple/Chapter05Test.java b/src/test/java/com/seaofnodes/simple/Chapter05Test.java
index d6c91a4a..fa9a5e37 100644
--- a/src/test/java/com/seaofnodes/simple/Chapter05Test.java
+++ b/src/test/java/com/seaofnodes/simple/Chapter05Test.java
@@ -17,7 +17,9 @@ if (arg == 1)
     a = arg+2;
 else {
     a = arg-3;
+
 }
+
 return a;""");
         StopNode ret = parser.parse();
         assertEquals("return Phi(Region17,(arg+2),(arg-3));", ret.toString());
diff --git a/src/test/java/com/seaofnodes/simple/Chapter07Test.java b/src/test/java/com/seaofnodes/simple/Chapter07Test.java
index fb8dbcca..26b5c17d 100644
--- a/src/test/java/com/seaofnodes/simple/Chapter07Test.java
+++ b/src/test/java/com/seaofnodes/simple/Chapter07Test.java
@@ -35,7 +35,7 @@ if(arg){}else{
 return a;
 """);
         StopNode stop = parser.parse();
-        assertEquals("return Phi(Region23,1,Phi(Loop11,1,(Phi_a+1)));", stop.toString());
+        assertEquals("return Phi(Region22,1,Phi(Loop11,1,(Phi_a+1)));", stop.toString());
     }
 
     @Test
@@ -55,7 +55,7 @@ while(i < arg) {
 return sum;
 """);
         StopNode stop = parser.parse();
-        assertEquals("return Phi(Loop8,0,Phi(Loop21,Phi_sum,(Phi(Loop,0,(Phi_j+1))+Phi_sum)));", stop.toString());
+        assertEquals("return Phi(Loop8,0,Phi(Loop20,Phi_sum,(Phi_sum+Phi(Loop,0,(Phi_j+1)))));", stop.toString());
     }
 
     @Test
@@ -71,7 +71,7 @@ while(a < 10) {
 return b;
 """);
         StopNode stop = parser.parse();
-        assertEquals("return Phi(Loop8,2,Phi(Region27,Phi_b,4));", stop.toString());
+        assertEquals("return Phi(Loop8,2,Phi(Region26,Phi_b,4));", stop.toString());
         assertTrue(stop.ret().ctrl() instanceof ProjNode);
     }
 
@@ -90,7 +90,7 @@ while(a < 10) {
 return b;
 """);
         StopNode stop = parser.parse();
-        assertEquals("return Phi(Loop8,2,(Phi(Region27,Phi_b,4)+1));", stop.toString());
+        assertEquals("return Phi(Loop8,2,(Phi(Region26,Phi_b,4)+1));", stop.toString());
         assertTrue(stop.ret().ctrl() instanceof ProjNode);
     }
 
diff --git a/src/test/java/com/seaofnodes/simple/Chapter08Test.java b/src/test/java/com/seaofnodes/simple/Chapter08Test.java
new file mode 100644
index 00000000..5d9d1545
--- /dev/null
+++ b/src/test/java/com/seaofnodes/simple/Chapter08Test.java
@@ -0,0 +1,218 @@
+package com.seaofnodes.simple;
+
+import com.seaofnodes.simple.node.*;
+import com.seaofnodes.simple.type.TypeInteger;
+import org.junit.Assert;
+import org.junit.Test;
+import org.junit.Ignore;
+
+import static org.junit.Assert.*;
+
+public class Chapter08Test {
+
+    @Test
+    @Ignore
+    public void testJig() {
+        Parser parser = new Parser("""
+// Insert test case here
+""");
+        StopNode stop = parser.parse();
+    }
+
+    @Test
+    public void testEx6() {
+        Parser parser = new Parser(
+                """
+while(arg < 10) {
+    arg = arg + 1;
+    if (arg == 5)
+        break;
+    if (arg == 6)
+        break;
+}
+return arg;
+                """);
+        StopNode stop = parser.parse();
+        assertEquals("return Phi(Region36,Phi(Region25,Phi(Loop6,arg,(Phi_arg+1)),Add),Add);", stop.toString());
+        assertTrue(stop.ret().ctrl() instanceof RegionNode);
+        Assert.assertEquals(5, GraphEvaluator.evaluate(stop, 1));
+        Assert.assertEquals(10, GraphEvaluator.evaluate(stop, 6));
+    }
+
+
+    @Test
+    public void testEx5() {
+        Parser parser = new Parser(
+                """
+int a = 1;
+while(arg < 10) {
+    arg = arg + 1;
+    if (arg == 5)
+        continue;
+    if (arg == 7)
+        continue;
+    a = a + 1;
+}
+return a;
+                """);
+        StopNode stop = parser.parse();
+        assertEquals("return Phi(Loop7,1,Phi(Region42,Phi_a,(Phi_a+1)));", stop.toString());
+        assertTrue(stop.ret().ctrl() instanceof ProjNode);
+    }
+
+
+    @Test
+    public void testEx4() {
+        Parser parser = new Parser(
+                """
+while(arg < 10) {
+    arg = arg + 1;
+    if (arg == 5)
+        continue;
+    if (arg == 6)
+        break;
+}
+return arg;
+                """);
+        StopNode stop = parser.parse();
+        assertEquals("return Phi(Region34,Phi(Loop6,arg,(Phi_arg+1)),Add);", stop.toString());
+        assertTrue(stop.ret().ctrl() instanceof RegionNode);
+    }
+
+    @Test
+    public void testEx3() {
+        Parser parser = new Parser(
+                """
+while(arg < 10) {
+    arg = arg + 1;
+    if (arg == 6)
+        break;
+}
+return arg;
+                """);
+        StopNode stop = parser.parse();
+        assertEquals("return Phi(Region25,Phi(Loop6,arg,(Phi_arg+1)),Add);", stop.toString());
+        assertTrue(stop.ret().ctrl() instanceof RegionNode);
+    }
+
+
+    @Test
+    public void testEx2() {
+        Parser parser = new Parser(
+                """
+while(arg < 10) {
+    arg = arg + 1;
+    if (arg == 5)
+        continue;
+    if (arg == 6)
+        continue;
+}
+return arg;
+                """);
+        StopNode stop = parser.parse();
+        assertEquals("return Phi(Loop6,arg,(Phi_arg+1));", stop.toString());
+        assertTrue(stop.ret().ctrl() instanceof ProjNode);
+    }
+
+
+    @Test
+    public void testEx1() {
+        Parser parser = new Parser(
+                """
+while(arg < 10) {
+    arg = arg + 1;
+    if (arg == 5)
+        continue;
+}
+return arg;
+                """);
+        StopNode stop = parser.parse();
+        assertEquals("return Phi(Loop6,arg,(Phi_arg+1));", stop.toString());
+        assertTrue(stop.ret().ctrl() instanceof ProjNode);
+    }
+
+    @Test
+    public void testRegress1() {
+        Parser parser = new Parser(
+                """
+while( arg < 10 ) {
+    int a = arg+2;
+    if( a > 4 )
+        break;
+}
+return arg;
+                """);
+        StopNode stop = parser.parse();
+        assertEquals("return arg;", stop.toString());
+    }
+
+    @Test
+    public void testRegress2() {
+        Parser parser = new Parser("if(1) return 0;  else while(arg>--arg) arg=arg+1; return 0;");
+        StopNode stop = parser.parse();
+        assertEquals("Stop[ return 0; return 0; ]", stop.toString());
+    }
+
+    @Test
+    public void testBreakOutsideLoop() {
+        try {
+            new Parser("""
+if(arg <= 10) {
+    break;
+    arg = arg + 1;
+}
+return arg;
+""").parse();
+            fail();
+        } catch( RuntimeException e ) {
+            assertEquals("No active loop for a break or continue",e.getMessage());
+        }
+    }
+
+    @Test
+    public void testRegress3() {
+        Parser parser = new Parser("""
+while(arg < 10) {
+    break;
+}
+return arg;
+""");
+        StopNode stop = parser.parse();
+        assertEquals("return arg;", stop.toString());
+    }
+
+    @Test
+    public void testRegress4() {
+        Parser parser = new Parser("""
+int a = 1;
+while(arg < 10) {
+    a = a + 1;
+    if (arg > 2) {
+        int a = 17;
+        break;
+    }
+}
+return a;
+""");
+        StopNode stop = parser.parse();
+        assertEquals("return Phi(Region28,Phi(Loop7,1,(Phi_a+1)),Add);", stop.toString());
+        assertTrue(stop.ret().ctrl() instanceof RegionNode);
+    }
+
+    @Test
+    public void testRegress5() {
+        Parser parser = new Parser("""
+int a = 1;
+while(1) {
+    a = a + 1;
+    if (a<10) continue;
+    break;
+}
+return a;
+""");
+        StopNode stop = parser.parse();
+        assertEquals("return (Phi(Loop7,1,Add)+1);", stop.toString());
+        assertTrue(stop.ret().ctrl() instanceof RegionNode);
+    }
+
+}
diff --git a/src/test/java/com/seaofnodes/simple/FuzzerWrap.java b/src/test/java/com/seaofnodes/simple/FuzzerWrap.java
new file mode 100644
index 00000000..33434b6c
--- /dev/null
+++ b/src/test/java/com/seaofnodes/simple/FuzzerWrap.java
@@ -0,0 +1,41 @@
+package com.seaofnodes.simple;
+
+import com.seaofnodes.simple.fuzzer.Fuzzer;
+import org.junit.Ignore;
+import org.junit.Test;
+
+
+import static org.junit.Assert.assertTrue;
+
+/**
+ * Normal tests run chapter-local regression seeds. Open failures and exploratory
+ * fuzzing are opt-in (run directly in the IDE or temporarily remove Ignore).
+ * Seeds depend on this chapter's generator; do not copy them between chapters.
+ */
+public class FuzzerWrap {
+
+    private static final long[] REGRESSION_SEEDS = {
+    };
+
+    private static final long[] OPEN_FAILING_SEEDS = {
+    };
+
+    @Test         public void fuzzPeepsRegression  () { fuzzPeepsSeeds(REGRESSION_SEEDS); }
+    @Test @Ignore public void fuzzPeepsOpenFailures() { fuzzPeepsSeeds(OPEN_FAILING_SEEDS); }
+
+    private static void fuzzPeepsSeeds(long... seeds) {
+        var fuzzer = new Fuzzer();
+        for (long seed : seeds)
+            fuzzer.fuzzPeepsRegression(seed);
+    }
+
+    @Test
+    @Ignore
+    public void fuzzPeepsLarge() {
+        var fuzzer = new Fuzzer();
+        for (int i=0; i<100000; i++)
+            fuzzer.fuzzPeeps(i);
+        assertTrue(fuzzer.noExceptions());
+    }
+
+}
diff --git a/src/test/java/com/seaofnodes/simple/fuzzer/Fuzzer.java b/src/test/java/com/seaofnodes/simple/fuzzer/Fuzzer.java
new file mode 100644
index 00000000..ef73fa4b
--- /dev/null
+++ b/src/test/java/com/seaofnodes/simple/fuzzer/Fuzzer.java
@@ -0,0 +1,110 @@
+package com.seaofnodes.simple.fuzzer;
+
+import com.seaofnodes.simple.GraphEvaluator;
+import com.seaofnodes.simple.node.StopNode;
+
+import java.util.ArrayList;
+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. Generated graphs are evaluated
+ * to detect compiler/evaluator failures; this chapter has no differential oracle.
+ * 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 = 1000;
+
+    /**
+     * 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) {
+        for (var ex : exceptions) {
+            if (FuzzerUtils.isExceptionFromSameCause(ex, e)) return;
+        }
+        exceptions.add(e);
+        System.out.println("========== Stack ==========");
+        e.printStackTrace(System.out);
+        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();
+    }
+
+    /** Compile and evaluate generated programs with normal peepholes. */
+    private static void runCheck(String script, boolean valid) {
+        StopNode stop;
+        try {
+            stop = FuzzerUtils.parse(script);
+        } catch (RuntimeException e) {
+            if (!valid || e.getClass() == RuntimeException.class) return;
+            throw e;
+        }
+        for (int input : new int[] {0, 1, 10})
+            GraphEvaluator.evaluateWithResult(stop, input, EVAL_TIMEOUT);
+    }
+
+    /**
+     * 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) {
+        try {
+            runCheck(script, valid);
+        } catch (Throwable e) {
+            recordException(e, script, s->runCheck(s, valid));
+        }
+    }
+
+    /**
+     * 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, false).genProgram();
+        check(sb.toString(), valid);
+    }
+
+    /**
+     * 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, false).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/src/test/java/com/seaofnodes/simple/fuzzer/FuzzerUtils.java b/src/test/java/com/seaofnodes/simple/fuzzer/FuzzerUtils.java
new file mode 100644
index 00000000..0e83861d
--- /dev/null
+++ b/src/test/java/com/seaofnodes/simple/fuzzer/FuzzerUtils.java
@@ -0,0 +1,57 @@
+package com.seaofnodes.simple.fuzzer;
+
+import com.seaofnodes.simple.Parser;
+import com.seaofnodes.simple.node.Node;
+import com.seaofnodes.simple.node.StopNode;
+
+/**
+ * Some utilities for the fuzzer.
+ */
+class FuzzerUtils {
+
+    /**
+     * Check that eiter one of the objects is null or that both are equal.
+     */
+    private static boolean equalOrNull(Object a, Object b) {
+        if (a == null || b == null) return true;
+        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 (!equalOrNull(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 script with normal peephole optimization
+     */
+    public static StopNode parse(String script) {
+        var parser = new Parser(script);
+        return parser.parse();
+    }
+
+}
diff --git a/src/test/java/com/seaofnodes/simple/fuzzer/Reducer.java b/src/test/java/com/seaofnodes/simple/fuzzer/Reducer.java
new file mode 100644
index 00000000..b7fc18ec
--- /dev/null
+++ b/src/test/java/com/seaofnodes/simple/fuzzer/Reducer.java
@@ -0,0 +1,144 @@
+package com.seaofnodes.simple.fuzzer;
+
+import java.util.function.Consumer;
+import java.util.function.Predicate;
+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 {
+
+    /**
+     * Holder for the rewrite rules
+     */
+    private record Replace(Pattern p, String r) {}
+
+    /**
+     * List of rewrite rules.
+     */
+    private static final Replace[] REPLACEMENTS = {
+            new Replace(Pattern.compile("(?<=[+\\-*/<>=(])--"), ""),
+            new Replace(Pattern.compile("\\b--"), "+"),
+            new Replace(Pattern.compile("\\b\\+-"), "-"),
+            new Replace(Pattern.compile("\\bfalse\\b"), "0"),
+            new Replace(Pattern.compile("\\btrue\\b"), "1"),
+            new Replace(Pattern.compile("-"), ""),
+            new Replace(Pattern.compile("(?<![+\\-*/<>=])(?:-|\\*|/|>=|<=|>|<|!=|==)(?![+\\-*/<>=])"), "+"),
+            new Replace(Pattern.compile("\\{(?:[^{}]|\\{})+}"), "{}"),
+            new Replace(Pattern.compile("\\{((?:[^{}]|\\{})+)}"), "$1"),
+            new Replace(Pattern.compile("\\(([^()]+)\\)"), "$1"),
+            new Replace(Pattern.compile("\\b\\w+\\b"), "1"),
+            new Replace(Pattern.compile("\\b\\w+\\b"), "0"),
+            new Replace(Pattern.compile("\\d+\\+"), ""),
+            new Replace(Pattern.compile("\\+\\d+"), ""),
+            new Replace(Pattern.compile("(?<=\\n|^)[^\\n]*(?=\\n|$)"), "\n"),
+            new Replace(Pattern.compile("\\n+"), "\n"),
+            new Replace(Pattern.compile("^\\n+|\\n+$"), ""),
+            new Replace(Pattern.compile("else *"), ""),
+            new Replace(Pattern.compile("\\bif\\([^()]+\\) *"), ""),
+            new Replace(Pattern.compile("\\bwhile\\([^()]+\\) *"), ""),
+    };
+
+    /**
+     * 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, String with) {
+        var matcher = pattern.matcher(script);
+        var sb = new StringBuilder();
+        int last = 0;
+        while (matcher.find()) {
+            var pos = sb.length();
+            matcher.appendReplacement(sb, with);
+            var tail = sb.length();
+            matcher.appendTail(sb);
+            if (tester.test(sb.toString())) {
+                sb.setLength(tail);
+            } else {
+                sb.setLength(pos);
+                sb.append(script, last, matcher.end());
+            }
+            last = matcher.end();
+        }
+        matcher.appendTail(sb);
+        return sb.toString();
+    }
+
+    /**
+     * Clean up variable names from the random ones generated by the fuzzer.
+     */
+    private String cleanVariables(String script) {
+        var matcher = Pattern.compile("\\bint ([0-9a-zA-Z_]+)").matcher(script);
+        int num = 0;
+        while (matcher.find()) {
+            var var = matcher.group(1);
+            if (var.matches("v\\d+")) continue;
+            while (script.contains("v"+num)) num++;
+            var n = script.replaceAll("\\b"+var+"\\b", "v" + num);
+            if (tester.test(n)) {
+                script = n;
+            }
+        }
+        return script;
+    }
+
+    /**
+     * Run all rewrite rules once.
+     */
+    private String doAllReplacements(String script) {
+        for (var replace: REPLACEMENTS) {
+            script = doReplacement(script, replace.p, replace.r);
+        }
+        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);
+        });
+    }
+
+}
diff --git a/src/test/java/com/seaofnodes/simple/fuzzer/ScriptGenerator.java b/src/test/java/com/seaofnodes/simple/fuzzer/ScriptGenerator.java
new file mode 100644
index 00000000..148c2319
--- /dev/null
+++ b/src/test/java/com/seaofnodes/simple/fuzzer/ScriptGenerator.java
@@ -0,0 +1,462 @@
+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;
+
+/**
+ * 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 {
+
+    /**
+     * Get the list of keywords from the parser.
+     */
+    @SuppressWarnings("unchecked")
+    private static HashSet<String> getKeywords() {
+        try {
+            var field = Parser.class.getDeclaredField("KEYWORDS");
+            field.setAccessible(true);
+            return (HashSet<String>) field.get(new Parser(""));
+        } catch (Throwable e) {
+            throw new ExceptionInInitializerError(e);
+        }
+    }
+
+    /**
+     * 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";
+    /**
+     * List of keywords not valid for identifiers.
+     */
+    private static final HashSet<String> KEYWORDS = getKeywords();
+
+    /**
+     * 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;
+    /**
+     * Binary operators.
+     */
+    private static final String[] BINARY_OP = {"==", "!=", "<", "<=", ">", ">=", "+", "-", "*", "/"};
+    /**
+     * Unary operators.
+     */
+    private static final String[] UNARY_OP = {"-"};
+
+
+    /**
+     * 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 = 7;
+    /**
+     * The depth of expressions allowed.
+     */
+    private int exprDepth = 3;
+    /**
+     * Current variables in scope.
+     */
+    private final ArrayList<String> variables = new ArrayList<>();
+    /**
+     * If this needs to generate valid scripts or is also allowed to generate invalid ones.
+     */
+    private final 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(10) + 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;
+    }
+
+    /**
+     * 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 (variables.lastIndexOf(v) < currScopeStart) return v;
+            if (!generateValid) {
+                maybeInvalid = true;
+                return v;
+            }
+        }
+        // Generate a new random variable name
+        StringBuilder sb = getRandomName();
+        var v = sb.toString();
+        if (!generateValid && (KEYWORDS.contains(v) || variables.lastIndexOf(v) >= currScopeStart)) {
+            maybeInvalid = true;
+            return v;
+        }
+        while (KEYWORDS.contains(v) || variables.lastIndexOf(v) >= currScopeStart) {
+            sb.append(VAR_CHARS.charAt(random.nextInt(VAR_CHARS.length())));
+            v = sb.toString();
+        }
+        return v;
+    }
+
+
+
+    /**
+     * 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() {
+        this.maybeInvalid = false;
+        variables.add(ScopeNode.ARG0);
+        currScopeStart = variables.size();
+        if ((genStatements() & FLAG_STOP) == 0) {
+            if (generateValid || random.nextInt(10)<7) {
+                genReturn();
+            } else {
+                this.maybeInvalid = true;
+            }
+        }
+        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(10);
+        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(10)) {
+            case 1 -> genBlock();
+            case 2 -> genIf();
+            case 3 -> genWhile();
+            case 4, 5 -> genDecl();
+            case 6, 7, 8 -> genAssignment();
+            default -> genExit();
+        };
+    }
+
+    /**
+     * 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() {
+        indentation += INDENTATION;
+        int res;
+        if (depth == 0) {
+            res = random.nextBoolean() ? genAssignment() : genExit();
+        } else {
+            depth--;
+            res = switch (random.nextInt(10)) {
+                case 1, 2, 3, 4, 5 -> {
+                    depth++;
+                    indentation -= INDENTATION;
+                    var ret = genBlock();
+                    indentation += INDENTATION;
+                    depth--;
+                    yield ret;
+                }
+                case 6 -> genIf();
+                case 7 -> genWhile();
+                case 8 -> genAssignment();
+                default -> genExit();
+            };
+            depth++;
+        }
+        indentation -= INDENTATION;
+        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;
+        }
+        depth--;
+        int oldCSS = currScopeStart;
+        currScopeStart = variables.size();
+        sb.append("{\n");
+        indentation += INDENTATION;
+        var stop = genStatements();
+        indentation -= INDENTATION;
+        printIndentation().append("}");
+        while (variables.size() > currScopeStart)
+            variables.removeLast();
+        currScopeStart = oldCSS;
+        depth++;
+        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();
+        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 while loop.
+     * @return 0
+     */
+    public int genWhile() {
+        sb.append("while(");
+        genExpression();
+        sb.append(") ");
+        loopDepth++;
+        genStatementBlock();
+        loopDepth--;
+        return 0;
+    }
+
+    /**
+     * Generate a declaration statement.
+     * @return 0
+     */
+    public int genDecl() {
+        String name = getVarName();
+        sb.append("int ").append(name).append("=");
+        genExpression();
+        sb.append(";");
+        variables.add(name);
+        return 0;
+    }
+
+    /**
+     * Generate an assignment statement.
+     * @return 0
+     */
+    public int genAssignment() {
+        if (variables.isEmpty()) return genDecl();
+        genVariable();
+        sb.append("=");
+        genExpression();
+        sb.append(";");
+        return 0;
+    }
+
+    /**
+     * Generate a return statement.
+     * @return FLAG_STOP
+     */
+    public int genReturn() {
+        sb.append("return ");
+        genExpression();
+        sb.append(";");
+        return FLAG_STOP;
+    }
+
+
+    /**
+     * Generate a binary expression.
+     * This method does not care about operator precedence.
+     */
+    public void genExpression() {
+        var num = randLog(10);
+        while(num-->0) {
+            genUnary();
+            sb.append(BINARY_OP[random.nextInt(BINARY_OP.length)]);
+        }
+        genUnary();
+    }
+
+    /**
+     * Generate a unary expression.
+     */
+    public void genUnary() {
+        var num = randLog(6);
+        while(num-->0) {
+            sb.append(UNARY_OP[random.nextInt(UNARY_OP.length)]);
+        }
+        genPrimary();
+    }
+
+    /**
+     * Generate a primary expression.
+     */
+    public void genPrimary() {
+        switch (random.nextInt(10)) {
+            case 0:
+                sb.append("true");
+                break;
+            case 1:
+                sb.append("false");
+                break;
+            case 2:
+                if (exprDepth == 0) {
+                    // Ensure termination of the generator.
+                    if (random.nextBoolean()) {
+                        genNumber();
+                    } else {
+                        genVariable();
+                    }
+                } else {
+                    sb.append("(");
+                    exprDepth--;
+                    genExpression();
+                    exprDepth++;
+                    sb.append(")");
+                }
+                break;
+            case 3, 4, 5, 6:
+                genNumber();
+                break;
+            default:
+                genVariable();
+        }
+    }
+
+    /**
+     * Generate a number.
+     */
+    public void genNumber() {
+        sb.append(random.nextInt(100));
+    }
+
+    /**
+     * 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() {
+        if (!generateValid && random.nextInt(100)>97) {
+            sb.append(getRandomName());
+            maybeInvalid = true;
+            return;
+        }
+        if (variables.isEmpty()) {
+            genNumber();
+        } else {
+            sb.append(variables.get(random.nextInt(variables.size())));
+        }
+    }
+
+}