Graph viewer on ch4 and25

19 files changed, 540 insertions, 404 deletions

diff --git a/chapter04/Makefile b/chapter04/Makefile
index e6c8f275..931ad8ef 100644
--- a/chapter04/Makefile
+++ b/chapter04/Makefile
@@ -81,6 +81,10 @@ fuzzer: $(default_targets)
 	@echo "fuzzing " $(test_cp)
 	@$(JVM) org.junit.runner.JUnitCore com.seaofnodes.simple.FuzzerWrap
 
+.PHONY: view
+view: build
+	java -ea -cp "$(CLZDIR)/main" com.seaofnodes.simple.print.JSViewer
+
 
 .PHONY: clean
 clean:
diff --git a/chapter04/src/main/java/com/seaofnodes/simple/Parser.java b/chapter04/src/main/java/com/seaofnodes/simple/Parser.java
index ff2c0504..360a7be9 100644
--- a/chapter04/src/main/java/com/seaofnodes/simple/Parser.java
+++ b/chapter04/src/main/java/com/seaofnodes/simple/Parser.java
@@ -1,5 +1,6 @@
 package com.seaofnodes.simple;
 
+import com.seaofnodes.graph.GraphObserver;
 import com.seaofnodes.simple.node.*;
 import com.seaofnodes.simple.type.*;
 
@@ -23,6 +24,11 @@ public class Parser {
      * To make the compiler multithreaded, this field will have to move into a TLS.
      */
     public static StartNode START;
+    // Current compilation context, like START; the observer belongs to the parser.
+    public static Parser PARSER;
+    public GraphObserver<Node> _obs;
+    public ReturnNode _ret;
+    public int pos() { return _lexer._position; }
 
     // The Lexer.  Thin wrapper over a byte[] buffer with a cursor.
     private final Lexer _lexer;
@@ -43,6 +49,7 @@ public class Parser {
 
 
     public Parser(String source, TypeInteger arg) {
+        PARSER = this;
         Node.reset();
         _lexer = new Lexer(source);
         _scope = new ScopeNode();
@@ -76,6 +83,8 @@ public class Parser {
         _scope.pop();
         if (!_lexer.isEOF()) throw error("Syntax error, unexpected " + _lexer.getAnyNextToken());
         if( show ) showGraph();
+        _ret = ret;
+        if( _obs != null ) _obs.phase("Parse");
         return ret;
     }
 
diff --git a/chapter04/src/main/java/com/seaofnodes/simple/node/Node.java b/chapter04/src/main/java/com/seaofnodes/simple/node/Node.java
index 2c0e3832..58516151 100644
--- a/chapter04/src/main/java/com/seaofnodes/simple/node/Node.java
+++ b/chapter04/src/main/java/com/seaofnodes/simple/node/Node.java
@@ -1,6 +1,7 @@
 package com.seaofnodes.simple.node;
 
 import com.seaofnodes.simple.Utils;
+import com.seaofnodes.simple.Parser;
 import com.seaofnodes.simple.type.Type;
 
 import java.util.*;
@@ -202,7 +203,7 @@ public abstract class Node {
     }
 
     // Mostly used for asserts and printing.
-    boolean isDead() { return isUnused() && nIns()==0 && _type==null; }
+    public boolean isDead() { return isUnused() && nIns()==0 && _type==null; }
 
     // Shortcuts to stop DCE mid-parse
     // Add bogus null use to keep node alive
@@ -234,11 +235,21 @@ public abstract class Node {
      * </ul>
      */
     public final Node peephole( ) {
+        var obs = Parser.PARSER == null ? null : Parser.PARSER._obs;
+        Type old = _type;
+        if( obs != null ) obs.before(this);
+        Node n = peepholeOpt();
+        Node rez = n == null ? this : n;
+        if( obs != null ) obs.after(this, n != null || old != _type ? rez : null, false);
+        return rez;
+    }
+
+    private Node peepholeOpt() {
         // Compute initial or improved Type
         Type type = _type = compute();
 
         if (_disablePeephole)
-            return this;        // Peephole optimizations turned off
+            return null;        // Peephole optimizations turned off
 
         // Replace constant computations from non-constants with a constant node
         if (!(this instanceof ConstantNode) && type.isConstant())
@@ -252,7 +263,7 @@ public abstract class Node {
             // Recursively optimize
             return deadCodeElim(n.peephole());
 
-        return this;            // No progress
+        return null;            // No progress
     }
 
     // m is the new Node, self is the old.
diff --git a/chapter04/src/main/java/com/seaofnodes/simple/print/JSViewer.java b/chapter04/src/main/java/com/seaofnodes/simple/print/JSViewer.java
new file mode 100644
index 00000000..43b0be2f
--- /dev/null
+++ b/chapter04/src/main/java/com/seaofnodes/simple/print/JSViewer.java
@@ -0,0 +1,26 @@
+package com.seaofnodes.simple.print;
+
+import com.seaofnodes.graph.*;
+import com.seaofnodes.simple.Parser;
+import java.io.IOException;
+
+public class JSViewer extends GraphViewer {
+    public static void main(String[] args) throws IOException {
+        try( var view = new JSViewer() ) { view.run(); }
+    }
+    JSViewer() throws IOException {}
+
+    @Override protected void compile(String src) {
+        Parser parser = new Parser(src);
+        parser._obs = new SimpleGraphObserver(parser) {
+            @Override protected void frame(GraphSnapshot snap, int pos, GraphEvent evt) throws IOException {
+                JSViewer.this.frame(snap, pos, evt);
+            }
+        };
+        try {
+            parser.parse();
+        } finally {
+            parser._obs = null;
+        }
+    }
+}
diff --git a/chapter04/src/main/java/com/seaofnodes/simple/print/SimpleGraphAdapter.java b/chapter04/src/main/java/com/seaofnodes/simple/print/SimpleGraphAdapter.java
index 0755613b..72a11a87 100644
--- a/chapter04/src/main/java/com/seaofnodes/simple/print/SimpleGraphAdapter.java
+++ b/chapter04/src/main/java/com/seaofnodes/simple/print/SimpleGraphAdapter.java
@@ -9,6 +9,7 @@ import java.util.ArrayList;
 
 /** Chapter 4's view of the IR; no knowledge of browser, transport or layout. */
 public class SimpleGraphAdapter extends GraphAdapter<Node> {
+    @Override protected boolean dead(Node n) { return n.isDead(); }
     @Override protected int id(Node n) { return n._nid; }
     @Override protected int nIns(Node n) { return n.nIns(); }
     @Override protected Node in(Node n, int idx) { return n.in(idx); }
diff --git a/chapter04/src/main/java/com/seaofnodes/simple/print/SimpleGraphObserver.java b/chapter04/src/main/java/com/seaofnodes/simple/print/SimpleGraphObserver.java
new file mode 100644
index 00000000..865d1e20
--- /dev/null
+++ b/chapter04/src/main/java/com/seaofnodes/simple/print/SimpleGraphObserver.java
@@ -0,0 +1,20 @@
+package com.seaofnodes.simple.print;
+
+import com.seaofnodes.graph.GraphCapture;
+import com.seaofnodes.simple.Parser;
+import com.seaofnodes.simple.node.Node;
+import java.util.ArrayList;
+
+/** Chapter 4 needs only a parser; it has no worklist or distant dependencies. */
+public abstract class SimpleGraphObserver extends GraphCapture<Node> {
+    private final Parser _parser;
+    protected SimpleGraphObserver(Parser parser) { super(new SimpleGraphAdapter()); _parser = parser; }
+    @Override protected String phase() { return "Parse"; }
+    @Override protected int pos() { return _parser.pos(); }
+    @Override protected void detach() { _parser._obs = null; }
+    @Override protected void roots(ArrayList<Node> roots) {
+        roots.add(Parser.START);
+        roots.add(_parser._scope);
+        roots.add(_parser._ret);
+    }
+}
diff --git a/chapter25/src/main/java/com/seaofnodes/simple/print/JSViewer.java b/chapter25/src/main/java/com/seaofnodes/simple/print/JSViewer.java
index 10c07730..03842119 100644
--- a/chapter25/src/main/java/com/seaofnodes/simple/print/JSViewer.java
+++ b/chapter25/src/main/java/com/seaofnodes/simple/print/JSViewer.java
@@ -1,72 +1,20 @@
 package com.seaofnodes.simple.print;
 
-import com.seaofnodes.graph.GraphJson;
-import com.seaofnodes.graph.GraphEvent;
-import com.seaofnodes.graph.GraphSnapshot;
+import com.seaofnodes.graph.*;
 import com.seaofnodes.simple.codegen.CodeGen;
 import java.io.IOException;
-import java.nio.file.Paths;
-import java.nio.file.Files;
-import java.nio.file.Path;
-import java.net.URI;
 
-public class JSViewer implements AutoCloseable {
-    // Display programs in an endless loop
-    public static void main( String[] args ) throws Exception {
-        try(var js = new JSViewer()) {
-            js.run();
-        }
-    }
-
-    // WebSocket to the browser for display
-    private final SimpleWebSocket _server;
-
-    JSViewer() throws Exception {
-        // Launch server; handshake
-        _server = new SimpleWebSocket(viewerURI(),12345);
-    }
-
-    // The browser application is shared by all chapters. Search upwards so
-    // chapter, repository-root, and linearized checkouts use the same assets.
-    static URI viewerURI() throws IOException {
-        String url = System.getProperty("simple.graph.url");
-        if( url != null ) return URI.create(url);
-        for( Path dir = Paths.get("").toAbsolutePath(); dir != null; dir = dir.getParent() ) {
-            Path page = dir.resolve("graph/web/index.html");
-            if( Files.isRegularFile(page) ) return page.toUri();
-        }
-        throw new IOException("Cannot find graph/web/index.html; run inside the Simple checkout "
-                              + "or set -Dsimple.graph.url=<viewer URL>");
-    }
-
-    void run( ) throws Exception {
-        _server.put("!");
-        while( true ) {
-            String src = _server.get();
-            switch( src ) {
-            case null: return;
-            case "null": return;
-            case "+": break;    // Client requests more frames, but we send them all anyway
-            default:
-                System.out.println(src);
-                try {
-                    compile(src);
-                    // Catch and ignore Parser errors
-                } catch(RuntimeException re) {
-                    System.err.println(re);
-                } finally {
-                    _server.put("#"); // Final frame
-                }
-                break;
-            }
-        }
+public class JSViewer extends GraphViewer {
+    public static void main(String[] args) throws IOException {
+        try( var view = new JSViewer() ) { view.run(); }
     }
+    JSViewer() throws IOException {}
 
-    private void compile(String src) {
+    @Override protected void compile(String src) {
         CodeGen code = new CodeGen(src);
         code._obs = new SimpleGraphObserver(code) {
             @Override protected void frame(GraphSnapshot snap, int pos, GraphEvent evt) throws IOException {
-                _server.put(GraphJson.frame(snap, pos, evt));
+                JSViewer.this.frame(snap, pos, evt);
             }
         };
         try {
@@ -75,6 +23,4 @@ public class JSViewer implements AutoCloseable {
             code._obs = null;
         }
     }
-
-    @Override public void close() throws IOException { _server.close(); }
 }
diff --git a/chapter25/src/main/java/com/seaofnodes/simple/print/SimpleGraphAdapter.java b/chapter25/src/main/java/com/seaofnodes/simple/print/SimpleGraphAdapter.java
index 592abb5c..e9b1dc6d 100644
--- a/chapter25/src/main/java/com/seaofnodes/simple/print/SimpleGraphAdapter.java
+++ b/chapter25/src/main/java/com/seaofnodes/simple/print/SimpleGraphAdapter.java
@@ -10,6 +10,9 @@ import java.util.ArrayList;
 
 /** Chapter 25's view of the IR, including partially constructed parse-time graphs. */
 public class SimpleGraphAdapter extends GraphAdapter<Node> {
+    @Override protected boolean dead(Node n) { return n.isDead(); }
+    @Override protected int nDeps(Node n) { return n.nDeps(); }
+    @Override protected Node dep(Node n, int idx) { return n.dep(idx); }
     @Override protected int id(Node n) { return n._nid; }
     @Override protected int nIns(Node n) { return n.nIns(); }
     @Override protected Node in(Node n, int idx) { return n.in(idx); }
diff --git a/chapter25/src/main/java/com/seaofnodes/simple/print/SimpleGraphObserver.java b/chapter25/src/main/java/com/seaofnodes/simple/print/SimpleGraphObserver.java
index 5d511287..0318b793 100644
--- a/chapter25/src/main/java/com/seaofnodes/simple/print/SimpleGraphObserver.java
+++ b/chapter25/src/main/java/com/seaofnodes/simple/print/SimpleGraphObserver.java
@@ -1,130 +1,22 @@
 package com.seaofnodes.simple.print;
 
-import com.seaofnodes.graph.GraphEvent;
-import com.seaofnodes.graph.GraphObserver;
-import com.seaofnodes.graph.GraphSnapshot;
+import com.seaofnodes.graph.GraphCapture;
 import com.seaofnodes.simple.codegen.CodeGen;
 import com.seaofnodes.simple.node.Node;
-import java.io.IOException;
 import java.util.ArrayList;
-import java.util.BitSet;
-import java.util.UUID;
 
-/** Capture events and neighborhoods on the compiler thread. Transport is supplied by the viewer. */
-public abstract class SimpleGraphObserver extends GraphObserver<Node> {
+/** Chapter 25's compilation context; capture and event bookkeeping are shared. */
+public abstract class SimpleGraphObserver extends GraphCapture<Node> {
     private final CodeGen _code;
-    private final SimpleGraphAdapter _graph = new SimpleGraphAdapter();
-    private final String _comp = UUID.randomUUID().toString();
-    private final ArrayList<Peep> _peeps = new ArrayList<>();
-    private long _next, _step;
-    private boolean _off;
-
-    protected SimpleGraphObserver(CodeGen code) { _code = code; }
-
-    protected abstract void frame(GraphSnapshot snap, int pos, GraphEvent evt) throws IOException;
-
-    private static class Peep {
-        final Node node;
-        final long id, up;
-        final String phase;
-        final int pos;
-        final BitSet near = new BitSet();
-        GraphSnapshot pre;
-        boolean sent;
-
-        Peep(Node n, long id, long up, String phase, int pos) {
-            node = n; this.id = id; this.up = up; this.phase = phase; this.pos = pos;
-        }
-    }
-
-    @Override public void before(Node n) {
-        if( _off ) return;
-        var p = new Peep(n, ++_next, _peeps.isEmpty() ? 0 : _peeps.getLast().id,
-                         _code._phase.name(), pos());
-        _peeps.add(p);
-        near(p, n);
-        // Buffer until we know that this attempt (or a nested one) made progress.
-        // Include n explicitly: parse-time nodes need not have any uses yet.
-        p.pre = snap(n);
-    }
-
-    @Override public void after(Node n, Node repl, boolean applied) {
-        if( _off ) return;
-        Peep p = _peeps.getLast();
-        assert p.node == n;
-        near(p, n);
-        near(p, repl);
-        for( Peep up : _peeps ) up.near.or(p.near);
-        if( repl != null || p.sent ) {
-            // Emit ancestors first so a nested rewrite always has its context.
-            for( Peep up : _peeps ) {
-                if( up.sent ) continue;
-                emit(up.pre, up.pos, evt(up, GraphEvent.Kind.BEFORE, 0));
-                if( _off ) return;
-                up.pre = null;
-                up.sent = true;
-            }
-            Node rez = repl == null ? n : repl;
-            emit(snap(rez.isDead() ? null : rez), pos(),
-                 evt(p, applied ? GraphEvent.Kind.APPLY : GraphEvent.Kind.RETURN,
-                     rez.isDead() ? 0 : rez._nid));
-        }
-        _peeps.removeLast();
-    }
-
-    @Override public void dep(Node n, Node def) {
-        if( _off || _peeps.isEmpty() ) return;
-        Peep p = _peeps.getLast();
-        add(p, n);
-        add(p, def);
-    }
-
-    @Override public void phase(String phase) {
-        if( _off ) return;
-        emit(snap(null), pos(), new GraphEvent(GraphEvent.Kind.PHASE, 0, 0, phase, 0, 0, new int[0]));
-    }
-
-    private GraphEvent evt(Peep p, GraphEvent.Kind kind, int repl) {
-        return new GraphEvent(kind, p.id, p.up, p.phase, p.node._nid, repl, p.near.stream().toArray());
-    }
-
-    private void add(Peep p, Node n) { if( n != null ) p.near.set(n._nid); }
-
-    private void near(Peep p, Node n) {
-        if( n == null ) return;
-        add(p, n);
-        for( Node def : n._inputs  ) add(p, def);
-        for( Node use : n._outputs ) add(p, use);
-        // These are wake-up dependents, not all nodes inspected by a rule.
-        // Read before the compiler drains the list; never move/clear it here.
-        for( int i = 0; i < n.nDeps(); i++ ) add(p, n.dep(i));
-    }
-
-    private int pos() { return _code._phase == CodeGen.Phase.Parse ? _code.P.pos() : -1; }
-
-    private GraphSnapshot snap(Node extra) {
-        var roots = new ArrayList<Node>();
+    protected SimpleGraphObserver(CodeGen code) { super(new SimpleGraphAdapter()); _code = code; }
+    @Override protected String phase() { return _code._phase.name(); }
+    @Override protected int pos() { return _code._phase == CodeGen.Phase.Parse ? _code.P.pos() : -1; }
+    @Override protected void detach() { _code._obs = null; }
+    @Override protected void roots(ArrayList<Node> roots) {
         roots.add(_code._stop);
         if( _code._phase == CodeGen.Phase.Parse ) {
             roots.add(_code.P._scope);
             roots.addAll(_code.P._xScopes);
         }
-        for( Peep p : _peeps ) if( !p.node.isDead() ) roots.add(p.node);
-        roots.add(extra);
-        return _graph.snap(_comp, 0, roots.toArray(Node[]::new));
-    }
-
-    private void emit(GraphSnapshot snap, int pos, GraphEvent evt) {
-        if( _off ) return;
-        // Assign steps on emission; attempts without progress do not consume frames.
-        snap = new GraphSnapshot(snap.ver(), _comp, _step++, snap.roots(), snap.nodes());
-        try {
-            frame(snap, pos, evt);
-        } catch( IOException e ) {
-            // A disconnected display must not interrupt optimization.
-            _off = true;
-            _code._obs = null;
-            System.err.println("Graph viewer disconnected: " + e.getMessage());
-        }
     }
 }
diff --git a/chapter25/src/main/java/com/seaofnodes/simple/print/SimpleWebSocket.java b/chapter25/src/main/java/com/seaofnodes/simple/print/SimpleWebSocket.java
deleted file mode 100644
index ae500879..00000000
--- a/chapter25/src/main/java/com/seaofnodes/simple/print/SimpleWebSocket.java
+++ /dev/null
@@ -1,193 +0,0 @@
-package com.seaofnodes.simple.print;
-
-import com.seaofnodes.simple.util.Utils;
-import java.io.*;
-import java.net.ServerSocket;
-import java.net.Socket;
-import java.net.URI;
-import java.nio.charset.StandardCharsets;
-import java.security.MessageDigest;
-import java.security.NoSuchAlgorithmException;
-import java.util.Base64;
-import java.net.InetSocketAddress;
-import java.nio.file.Path;
-import com.sun.net.httpserver.HttpServer;
-import com.sun.net.httpserver.SimpleFileServer;
-
-// Why-Oh-Why is this needed?
-
-// Pass in a URI and a port.
-// Opens the client; client has to open a websocket at the given port.
-// Connects, handshakes.
-// Thereafter you can call get/put to swap frames with client.
-
-class SimpleWebSocket extends ServerSocket {
-    final  InputStream _in;
-    final OutputStream _out;
-    private HttpServer _http;
-    private Socket _client;
-
-    SimpleWebSocket(URI uri, int port) throws IOException, NoSuchAlgorithmException {
-        super(port);
-        try {
-            // Use an ordinary web URL; desktop dispatch of file: URLs can silently
-            // raise the browser without opening the page.
-            if( "file".equals(uri.getScheme()) ) {
-                Path page = Path.of(uri);
-                _http = SimpleFileServer.createFileServer(
-                    new InetSocketAddress("127.0.0.1",0), page.getParent(),
-                    SimpleFileServer.OutputLevel.NONE);
-                _http.start();
-                uri = URI.create("http://127.0.0.1:" + _http.getAddress().getPort()
-                                 + "/" + page.getFileName());
-            }
-            // Print the address even if the desktop silently fails to open a tab.
-            System.out.println("Graph viewer: " + uri);
-            if( Boolean.parseBoolean(System.getProperty("simple.graph.open", "true")) )
-                java.awt.Desktop.getDesktop().browse(uri);
-            // Look for client; get in/out streams
-            Socket sock = accept();
-            _client = sock;
-            _in  = sock. getInputStream();
-            _out = sock.getOutputStream();
-
-
-            // Establish web socket with handshake; easier with a buffered reader
-            BufferedReader br = new BufferedReader(new InputStreamReader(_in, StandardCharsets.US_ASCII));
-            String key = null;
-            // TCP can deliver the request in several chunks. ready() is not an
-            // end-of-headers test; wait for the terminating empty line.
-            String line;
-            while( (line = br.readLine()) != null && !line.isEmpty() ) {
-                if( line.startsWith("Sec-WebSocket-Key: ") )
-                    key = line.substring(19);
-            }
-            if( key==null || line==null ) {
-                sock.close();
-                super.close();
-                throw new IOException("Incomplete WebSocket upgrade request");
-            }
-            // Magic handshake
-            byte[] response = ("HTTP/1.1 101 Switching Protocols\r\n"
-                               + "Connection: Upgrade\r\n"
-                               + "Upgrade: websocket\r\n"
-                               + "Sec-WebSocket-Accept: "
-                               + Base64.getEncoder().encodeToString(MessageDigest.getInstance("SHA-1").digest((key + "258EAFA5-E914-47DA-95CA-C5AB0DC85B11").getBytes( StandardCharsets.UTF_8 )))
-                               + "\r\n\r\n").getBytes( StandardCharsets.UTF_8 );
-            _out.write(response,0,response.length);
-            _out.flush();
-        } catch( IOException | NoSuchAlgorithmException | RuntimeException | Error failure ) {
-            if( _http != null ) _http.stop(0);
-            if( _client != null ) {
-                try { _client.close(); } catch( IOException ignored ) {}
-            }
-            try { super.close(); } catch( IOException ignored ) {}
-            throw failure;
-        }
-    }
-
-    final byte[] _ins = new byte[1024];
-    final byte[] _str = new byte[1024];
-    final byte[] _key = new byte[4];
-    int _x, _len;
-    private int rawget() { assert _x<_len; return _ins[_x++]&0xFF; }
-
-    // TODO: Flip to Direct Byte Buffer, this is just a poor-mans version
-    private void flipRead() throws IOException {
-        // Move _x to _len to offset 0
-        System.arraycopy(_ins,_x,_ins,0,(_len-_x));
-        _len = _len-_x;
-        _x=0;
-        // Read more
-        int len = _in.read(_ins,_len,_ins.length-_len);
-        if( len == -1 )
-            throw Utils.TODO(); // EOF
-        _len += len;
-    }
-    // Block until you read at least len
-    private void atLeast(int len) throws IOException {
-        if( _x+len <= _len ) return;
-        flipRead();
-    }
-
-
-    // Classic read from browser client.  Encoded, so decoded here
-    public String get() throws IOException {
-        atLeast(1);             // Need 1 byte for opcode
-        int op = rawget();
-        if( (op&0x80)!=0x80 )   // FIN bit not set?
-            throw Utils.TODO( "Multi-part from client: " + op ); // Multipart message
-        op &= 0x7F;             // Strip FIN bit
-        switch( op&15) {
-        case 1: {
-            atLeast(5);         // Min message length
-            // Message length
-            int len = rawget()-128;
-            if( len > 125 ) {
-                if( len > 126 )
-                    throw Utils.TODO(); // Multi-byte length
-                len = (rawget()<<8) | rawget();
-            }
-            atLeast(len+4);     // Rest of message
-            // Encoding key
-            _key[0] = (byte)rawget();
-            _key[1] = (byte)rawget();
-            _key[2] = (byte)rawget();
-            _key[3] = (byte)rawget();
-
-            // Decode bytes
-            for( int i=0; i<len; i++ )
-                _str[i] = (byte)(rawget() ^ _key[i & 3]);
-
-            return new String(_str,0,len,StandardCharsets.UTF_8);
-        }
-            // Client closes
-        case 8: return null;
-            // Ping
-        case 9:
-            // Send a PONG back.  TODO: echo PING data in the PONG
-            _out.write(0x8A); // FIN + PONG
-            _out.write(0x00); // LENGTH==0
-            return get();
-            // Not (yet) implemented: continuation frames, ping/pong
-        default:
-            throw Utils.TODO("Unexpected op from client: "+op); // Multi-part message
-        }
-    }
-
-    // Server text frames are UTF-8 and unmasked.
-    public void put(String msg) throws IOException {
-        byte[] bytes = msg.getBytes(StandardCharsets.UTF_8);
-        int len = bytes.length;
-        _out.write(129); // FIN, opcode 1 - whole text message
-        if( len <= 125 ) {
-            _out.write(len);
-        } else if( len <= 65535 ) {
-            _out.write(126);
-            _out.write(len>>>8);
-            _out.write(len&255);
-        } else {
-            _out.write(127);
-            for( int shift = 56; shift >= 0; shift -= 8 )
-                _out.write((int)((long)len >>> shift) & 255);
-        }
-        _out.write(bytes);
-        _out.flush();
-    }
-
-    @Override public void close() throws IOException {
-        try {
-            if( _out != null ) {
-                _out.write(new byte[]{(byte)0x88, 0}); // Complete, empty close frame
-                _out.flush();
-            }
-        } finally {
-            if( _http != null ) _http.stop(0);
-            try {
-                if( _client != null ) _client.close();
-            } finally {
-                super.close();
-            }
-        }
-    }
-}
diff --git a/graph/README.md b/graph/README.md
index 27a96c01..e9cef3aa 100644
--- a/graph/README.md
+++ b/graph/README.md
@@ -1,21 +1,22 @@
 # Shared graph viewer
 
-The browser application lives in `web/` and is shared by chapters 18–25.
+The browser application lives in `web/` and is shared by chapters 4 and 18–25.
 `web/index.html` contains the page and `web/viewer.js` manages playback.
 `web/layout.js` runs ELK layout in a browser worker; `web/render.js` draws SVG
 and handles selection, pan and zoom. `web/legacy.js` loads Graphviz on demand
 for earlier chapters. Libraries live in `web/vendor/`, with one checked-in
 copy of each; chapters do not copy them during builds.
 
-From a chapter directory, run `make view`. The chapter's `JSViewer` finds
+From a chapter directory, run `make view`. For chapters 4 and 25, the shared
+`GraphViewer` finds
 `graph/web/index.html` by walking up from the working directory, serves its
 directory through the JDK's HTTP server on a local ephemeral port, and opens
 that web address in the browser. This works from a chapter directory, the
 repository root, or the root of a linearized checkout. Java launches from elsewhere can specify
 `-Dsimple.graph.url=<viewer URL>` before the main class.
 
-The viewer uses the existing WebSocket connection. Chapter 25 sends structured
-JSON snapshots in each frame; earlier chapters send bare DOT.
+The viewer uses a WebSocket connection. Chapters 4 and 25 send structured
+JSON snapshots in each frame; chapters 18–24 still send bare DOT.
 No JavaScript package installation, frontend build, or separate server process
 is required.
 For development over HTTP, run this from the repository root:
@@ -31,7 +32,7 @@ The initial program compiles on connection. For another program, click **Compile
 or press **Ctrl+Enter** (**Cmd+Enter** on macOS), then use the arrows to step
 through the captured frames. Keep `make view` running while using the tab.
 
-In chapter 25, scroll to zoom, drag to pan, and use **Fit** to show the whole
+In chapters 4 and 25, scroll to zoom, drag to pan, and use **Fit** to show the whole
 graph again. Clicking a node shows its full label, type and input slots, and
 highlights its edges. Selection follows that node's ID across frames; Escape
 clears it. **Associations** shows scope bindings and lifetime edges, which are
@@ -69,10 +70,24 @@ for Firefox testing. Test logs/screenshots are written under `build/graph-browse
 
 ## Boundary between chapters and the viewer
 
-Each chapter currently retains its Java graph exporter, compiler hooks, and
-WebSocket transport. The browser code has no dependency on a chapter's Java
-classes. The adapters below share snapshot capture; moving the transport into
-the shared Java code is a separate step.
+Chapters 4 and 25 share the Java viewer plumbing as well as the browser:
+
+- `GraphViewer` owns asset discovery, the source/compile loop, frame serialization,
+  and session shutdown. A chapter's `JSViewer` subclass only invokes its compiler
+  and attaches/detaches its observer.
+- `GraphSocket` serves local assets, opens the browser, and handles WebSocket
+  messages. One buffered stream handles both the upgrade and frames. Payloads
+  use exact reads and their advertised length, including 64-bit lengths;
+  continuation frames assemble UTF-8 before decoding, and PING data is echoed.
+- `GraphCapture<N>` owns nested attempts, buffered snapshots, event numbering,
+  neighborhood accumulation and disconnect handling. It extends `GraphObserver<N>`.
+  Chapter subclasses supply roots, phase, parser position and observer detachment.
+- `GraphAdapter<N>` supplies node facts and indexed graph/dependency access.
+  Dependency access defaults to an empty set in chapters without those lists.
+
+Chapter 25's private WebSocket implementation is removed. Chapters 18–24 retain
+their old transport until migrated. Shared Java has no dependency on chapter classes.
+The WebSocket listener and built-in asset server bind to 127.0.0.1.
 
 The connection messages are:
 
@@ -81,12 +96,13 @@ The connection messages are:
 | Compiler to browser | `!` | Request the source program |
 | Browser to compiler | Source text | Compile and capture a new sequence |
 | Compiler to browser | `digraph ...` | One complete DOT snapshot |
-| Compiler to browser | JSON `{snap, pos, evt}` | Chapter 25's complete graph, parser position and event |
+| Compiler to browser | JSON `{snap, pos, evt}` | Chapters 4/25's complete graph, parser position and event |
+| Compiler to browser | JSON `{error}` | Compilation failed; the session accepts another program |
 | Browser to compiler | `+` | Acknowledge/request another frame |
 | Compiler to browser | `#` | End of sequence |
 | Browser to compiler | `null` | End the session |
 
-Chapter 25's JSON frame has this shape:
+The JSON frame has this shape:
 
 ```json
 {
@@ -115,10 +131,10 @@ The browser caches the parsed objects in `frames`. It computes geometry on the
 first visit to a frame and keeps it in `frames[i].layout`; revisiting a frame
 uses exactly that geometry. Viewport changes do not rerun layout. Nodes and
 edges have stable SVG IDs within a compilation. There is no animation yet.
-One `+` acknowledges the whole frame. The chapter 25 writer sends UTF-8 bytes
+One `+` acknowledges the whole frame. The shared writer sends UTF-8 bytes
 and supports WebSocket's 64-bit length header for frames larger than 65535 bytes.
 
-Earlier chapters' DOT frames may contain a `// POS:` comment identifying the
+Legacy chapters' DOT frames may contain a `// POS:` comment identifying the
 parser position. The browser owns frame history and backward/forward playback.
 
 Chapter 25 no longer generates DOT for the interactive viewer. The browser
@@ -207,8 +223,8 @@ Functions and compilation-unit boundaries. These are existing IR facts, not
 computed region membership or a SESE hierarchy. Scope bindings and constant
 lifetime edges are associations, so they need not constrain CFG layout.
 Types may be absent on newly constructed nodes. Source locations, grouping,
-and delta encoding remain follow-up work. Chapter 25 adds observer events as
-described below; the chapter 4 adapter is still independent of an observer.
+and delta encoding remain follow-up work. Both chapters use the observer events
+described below.
 
 Make compiles these shared sources directly into each participating chapter's
 classes using `javac`. From the repository root:
@@ -223,22 +239,26 @@ browser. Shared source changes trigger recompilation. No Maven executable,
 Maven-built artifacts, generated source copies or separately installed graph
 JARs are needed. Existing `make tests`, `make release` and chapter 25's
 `make view` also include the shared sources through their normal prerequisites.
+Chapter 4's `make view` depends on `build` and runs the same shared viewer.
 
 For the existing Maven/CI build path, `build-helper:add-source` describes the
 same source directory. The two chapter POMs select `../graph`, while the root
 POM defaults to `graph` for linearized checkouts. These entries are independent
 of the Make build.
 
-## Peephole observer (chapter 25)
+## Peephole observers
 
 `GraphObserver<N>` is a shared abstract base for `before`, `after`, `phase`, and
-`dep` callbacks. `CodeGen._obs` owns the optional observer. Normal compilation
+`dep` callbacks. Chapter 25's `CodeGen._obs` and chapter 4's `Parser._obs` own the
+optional observer. Chapter 4 uses `Parser.PARSER` as its current compilation
+context, alongside its existing global `START`. Normal compilation
 leaves it null, so it does no snapshot capture or serialization. Assertions
 that probe peepholes with `_midAssert` set do not generate observer calls.
 Neither `Node` nor `IterPeeps` refers to `JSViewer`.
 
-`SimpleGraphObserver` supplies chapter 25's capture and neighborhood logic.
-`JSViewer` installs one for each compilation and supplies only the frame sink.
+`GraphCapture` supplies capture and neighborhood logic. Each chapter's small
+`SimpleGraphObserver` supplies its compilation context. `JSViewer` installs one
+for each compilation and forwards frames to `GraphViewer`.
 An I/O failure disables the observer and allows optimization to continue.
 The observer is detached in a `finally` block when compilation finishes or fails.
 
@@ -249,7 +269,7 @@ The event kinds distinguish the actual capture boundaries:
 | `BEFORE` | Graph before an attempt that made progress, or contained a nested rewrite |
 | `RETURN` | Recursive `Node.peephole()` finished, including its local DCE; the caller has not yet attached the returned node |
 | `APPLY` | A worklist attempt finished after substitution, dependency draining, and unused-node cleanup |
-| `PHASE` | Parse, Iter, or Opto completed, including work outside individual peepholes |
+| `PHASE` | Parse completed in chapter 4; Parse, Iter, or Opto completed in chapter 25, including work outside individual peepholes |
 
 `peep` identifies an attempt; `up` identifies its enclosing attempt, or zero.
 `node` and `repl` identify the original and result nodes. A zero `repl` on
@@ -288,8 +308,9 @@ Actual graph diffs and explicit rule annotations can refine this later.
 
 The browser keeps the event alongside its snapshot and layout. It labels and
 highlights the current step, but does not yet collapse nested rewrites or move
-the camera to their neighborhoods. The chapter 4 observer hookup is follow-up
-work; its adapter already targets the same graph model.
+the camera to their neighborhoods. Chapter 4 uses the same event model, with
+defs/uses for its neighborhoods and no worklist or dependency lists. Its parser
+retains the final Return as a root after scope cleanup.
 
 Chapter 25's parser no longer accepts `#showGraph;`; use the browser to inspect
 the graph and **Save SVG** for a static figure. Earlier chapters retain their
diff --git a/graph/src/main/java/com/seaofnodes/graph/GraphAdapter.java b/graph/src/main/java/com/seaofnodes/graph/GraphAdapter.java
index 77672c45..3e840b54 100644
--- a/graph/src/main/java/com/seaofnodes/graph/GraphAdapter.java
+++ b/graph/src/main/java/com/seaofnodes/graph/GraphAdapter.java
@@ -17,6 +17,9 @@ public abstract class GraphAdapter<N> {
     protected abstract int nOuts(N node);
     protected abstract N out(N node, int idx);
     protected abstract GraphSnapshot.Node desc(N node);
+    protected abstract boolean dead(N node);
+    protected int nDeps(N node) { return 0; }
+    protected N dep(N node, int idx) { throw new IndexOutOfBoundsException(idx); }
 
     protected final int ref(N node) { return node == null ? 0 : id(node); }
 
@@ -28,10 +31,16 @@ public abstract class GraphAdapter<N> {
      */
     @SafeVarargs
     public final GraphSnapshot snap(String comp, long step, N... roots) {
+        var list = new ArrayList<N>();
+        java.util.Collections.addAll(list, roots);
+        return snap(comp, step, list);
+    }
+
+    public final GraphSnapshot snap(String comp, long step, ArrayList<N> roots) {
         // Node IDs are dense; index directly without boxed keys or map entries.
         var seen = new ArrayList<N>();
         var todo = new ArrayDeque<N>();
-        var rids = new int[roots.length];
+        var rids = new int[roots.size()];
         int len = 0;
         for( N root : roots )
             if( enq(root, seen, todo) ) rids[len++] = id(root);
diff --git a/graph/src/main/java/com/seaofnodes/graph/GraphCapture.java b/graph/src/main/java/com/seaofnodes/graph/GraphCapture.java
new file mode 100644
index 00000000..0e2f538d
--- /dev/null
+++ b/graph/src/main/java/com/seaofnodes/graph/GraphCapture.java
@@ -0,0 +1,125 @@
+package com.seaofnodes.graph;
+
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.BitSet;
+import java.util.UUID;
+
+/** Shared capture, event nesting and neighborhoods. Chapter hooks only read the IR. */
+public abstract class GraphCapture<N> extends GraphObserver<N> {
+
+    private final GraphAdapter<N> _graph;
+    private final String _comp = UUID.randomUUID().toString();
+    private final ArrayList<Peep> _peeps = new ArrayList<>();
+    private long _next, _step;
+    private boolean _off;
+
+    protected GraphCapture(GraphAdapter<N> graph) { _graph = graph; }
+
+    protected abstract String phase();
+    protected abstract int pos();
+    protected abstract void roots(ArrayList<N> roots);
+    protected abstract void detach();
+
+    protected abstract void frame(GraphSnapshot snap, int pos, GraphEvent evt) throws IOException;
+
+    private class Peep {
+        final N node;
+        final long id, up;
+        final String phase;
+        final int pos;
+        final BitSet near = new BitSet();
+        GraphSnapshot pre;
+        boolean sent;
+
+        Peep(N n, long id, long up, String phase, int pos) {
+            node = n; this.id = id; this.up = up; this.phase = phase; this.pos = pos;
+        }
+    }
+
+    @Override public void before(N n) {
+        if( _off ) return;
+        var p = new Peep(n, ++_next, _peeps.isEmpty() ? 0 : _peeps.getLast().id,
+                         phase(), pos());
+        _peeps.add(p);
+        near(p, n);
+        // Buffer until we know that this attempt (or a nested one) made progress.
+        // Include n explicitly: parse-time nodes need not have any uses yet.
+        p.pre = snap(n);
+    }
+
+    @Override public void after(N n, N repl, boolean applied) {
+        if( _off ) return;
+        Peep p = _peeps.getLast();
+        assert p.node == n;
+        near(p, n);
+        near(p, repl);
+        for( Peep up : _peeps ) up.near.or(p.near);
+        if( repl != null || p.sent ) {
+            // Emit ancestors first so a nested rewrite always has its context.
+            for( Peep up : _peeps ) {
+                if( up.sent ) continue;
+                emit(up.pre, up.pos, evt(up, GraphEvent.Kind.BEFORE, 0));
+                if( _off ) return;
+                up.pre = null;
+                up.sent = true;
+            }
+            N rez = repl == null ? n : repl;
+            emit(snap(_graph.dead(rez) ? null : rez), pos(),
+                 evt(p, applied ? GraphEvent.Kind.APPLY : GraphEvent.Kind.RETURN,
+                     _graph.dead(rez) ? 0 : _graph.id(rez)));
+        }
+        _peeps.removeLast();
+    }
+
+    @Override public void dep(N n, N def) {
+        if( _off || _peeps.isEmpty() ) return;
+        Peep p = _peeps.getLast();
+        add(p, n);
+        add(p, def);
+    }
+
+    @Override public void phase(String phase) {
+        if( _off ) return;
+        emit(snap(null), pos(), new GraphEvent(GraphEvent.Kind.PHASE, 0, 0, phase, 0, 0, new int[0]));
+    }
+
+    private GraphEvent evt(Peep p, GraphEvent.Kind kind, int repl) {
+        return new GraphEvent(kind, p.id, p.up, p.phase, _graph.id(p.node), repl, p.near.stream().toArray());
+    }
+
+    private void add(Peep p, N n) { if( n != null ) p.near.set(_graph.id(n)); }
+
+    private void near(Peep p, N n) {
+        if( n == null ) return;
+        add(p, n);
+        for( int i = 0; i < _graph.nIns(n); i++ ) add(p, _graph.in(n, i));
+        for( int i = 0; i < _graph.nOuts(n); i++ ) add(p, _graph.out(n, i));
+        // These are wake-up dependents, not all nodes inspected by a rule.
+        // Read before the compiler drains the list; never move/clear it here.
+        for( int i = 0; i < _graph.nDeps(n); i++ ) add(p, _graph.dep(n, i));
+    }
+
+
+    private GraphSnapshot snap(N extra) {
+        var roots = new ArrayList<N>();
+        roots(roots);
+        for( Peep p : _peeps ) if( !_graph.dead(p.node) ) roots.add(p.node);
+        roots.add(extra);
+        return _graph.snap(_comp, 0, roots);
+    }
+
+    private void emit(GraphSnapshot snap, int pos, GraphEvent evt) {
+        if( _off ) return;
+        // Assign steps on emission; attempts without progress do not consume frames.
+        snap = new GraphSnapshot(snap.ver(), _comp, _step++, snap.roots(), snap.nodes());
+        try {
+            frame(snap, pos, evt);
+        } catch( IOException e ) {
+            // A disconnected display must not interrupt optimization.
+            _off = true;
+            detach();
+            System.err.println("Graph viewer disconnected: " + e.getMessage());
+        }
+    }
+}
diff --git a/graph/src/main/java/com/seaofnodes/graph/GraphJson.java b/graph/src/main/java/com/seaofnodes/graph/GraphJson.java
index c27076d3..8726d81b 100644
--- a/graph/src/main/java/com/seaofnodes/graph/GraphJson.java
+++ b/graph/src/main/java/com/seaofnodes/graph/GraphJson.java
@@ -2,6 +2,12 @@ package com.seaofnodes.graph;
 
 /** JSON for the shared graph model; no chapter or layout dependencies. */
 public class GraphJson {
+    public static String error(String msg) {
+        var sb = new StringBuilder("{\"error\":");
+        str(sb, msg);
+        return sb.append('}').toString();
+    }
+
     public static String write(GraphSnapshot snap) {
         var sb = new StringBuilder();
         snap(sb, snap);
diff --git a/graph/src/main/java/com/seaofnodes/graph/GraphSocket.java b/graph/src/main/java/com/seaofnodes/graph/GraphSocket.java
new file mode 100644
index 00000000..b9859b28
--- /dev/null
+++ b/graph/src/main/java/com/seaofnodes/graph/GraphSocket.java
@@ -0,0 +1,148 @@
+package com.seaofnodes.graph;
+
+import java.io.*;
+import java.net.*;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Path;
+import java.security.MessageDigest;
+import java.security.NoSuchAlgorithmException;
+import java.util.Base64;
+import com.sun.net.httpserver.HttpServer;
+import com.sun.net.httpserver.SimpleFileServer;
+
+/** One local viewer session. HTTP assets and compiler messages share its lifetime. */
+class GraphSocket extends ServerSocket {
+    private DataInputStream _in;
+    private OutputStream _out;
+    private HttpServer _http;
+    private Socket _client;
+
+    GraphSocket(URI uri, int port) throws IOException {
+        super(port, 1, InetAddress.getByName("127.0.0.1"));
+        try {
+            if( "file".equals(uri.getScheme()) ) {
+                Path page = Path.of(uri);
+                _http = SimpleFileServer.createFileServer(
+                    new InetSocketAddress("127.0.0.1", 0), page.getParent(),
+                    SimpleFileServer.OutputLevel.NONE);
+                _http.start();
+                uri = URI.create("http://127.0.0.1:" + _http.getAddress().getPort() + "/" + page.getFileName());
+            }
+            System.out.println("Graph viewer: " + uri);
+            if( Boolean.parseBoolean(System.getProperty("simple.graph.open", "true")) )
+                java.awt.Desktop.getDesktop().browse(uri);
+            _client = accept();
+            // Keep one buffer across the upgrade and all subsequent frames.
+            _in = new DataInputStream(new BufferedInputStream(_client.getInputStream()));
+            _out = new BufferedOutputStream(_client.getOutputStream());
+            upgrade();
+        } catch( IOException | RuntimeException | Error e ) {
+            try { close(); } catch( IOException ignored ) {}
+            throw e;
+        }
+    }
+
+    private String line() throws IOException {
+        var sb = new StringBuilder();
+        for( int b; (b = _in.read()) != '\n'; ) {
+            if( b == -1 ) throw new EOFException("Incomplete WebSocket upgrade");
+            if( b != '\r' ) sb.append((char)b);
+        }
+        return sb.toString();
+    }
+
+    private void upgrade() throws IOException {
+        String key = null;
+        for( String line; !(line = line()).isEmpty(); ) {
+            int colon = line.indexOf(':');
+            if( colon > 0 && line.substring(0, colon).equalsIgnoreCase("Sec-WebSocket-Key") )
+                key = line.substring(colon + 1).trim();
+        }
+        if( key == null ) throw new IOException("Missing WebSocket key");
+        String accept;
+        try {
+            byte[] hash = MessageDigest.getInstance("SHA-1").digest(
+                (key + "258EAFA5-E914-47DA-95CA-C5AB0DC85B11").getBytes(StandardCharsets.US_ASCII));
+            accept = Base64.getEncoder().encodeToString(hash);
+        } catch( NoSuchAlgorithmException e ) {
+            throw new IllegalStateException(e); // SHA-1 is required by the JDK.
+        }
+        _out.write(("HTTP/1.1 101 Switching Protocols\r\nConnection: Upgrade\r\nUpgrade: websocket\r\n"
+                    + "Sec-WebSocket-Accept: " + accept + "\r\n\r\n").getBytes(StandardCharsets.US_ASCII));
+        _out.flush();
+    }
+
+    public String get() throws IOException {
+        ByteArrayOutputStream msg = null;
+        while( true ) {
+            int head = _in.read();
+            if( head == -1 ) {
+                if( msg != null ) throw new EOFException("Incomplete WebSocket message");
+                return null;
+            }
+            boolean fin = (head & 128) != 0;
+            int op = head & 15;
+            int size = _in.readUnsignedByte();
+            if( (head & 112) != 0 || (size & 128) == 0 )
+                throw new IOException("Expected a masked WebSocket frame without extensions");
+            long len = size & 127;
+            if( len == 126 ) len = _in.readUnsignedShort();
+            else if( len == 127 ) len = _in.readLong();
+            if( len < 0 || len > Integer.MAX_VALUE ) throw new IOException("WebSocket frame too large");
+            if( op >= 8 && (!fin || len > 125) ) throw new IOException("Invalid WebSocket control frame");
+            byte[] mask = new byte[4];
+            _in.readFully(mask);
+            byte[] data = new byte[(int)len];
+            _in.readFully(data); // TCP may deliver any header or payload in pieces.
+            for( int i = 0; i < data.length; i++ ) data[i] ^= mask[i & 3];
+            switch( op ) {
+            case 8: return null;
+            case 9: put(10, data); continue; // PONG echoes PING, even between fragments.
+            case 10: continue;
+            case 1:
+                if( msg != null ) throw new IOException("Text frame inside an unfinished message");
+                if( fin ) return new String(data, StandardCharsets.UTF_8);
+                msg = new ByteArrayOutputStream();
+                break;
+            case 0:
+                if( msg == null ) throw new IOException("Continuation without a text frame");
+                break;
+            default: throw new IOException("Unsupported WebSocket opcode: " + op);
+            }
+            msg.write(data);
+            if( fin ) return msg.toString(StandardCharsets.UTF_8);
+        }
+    }
+
+    public void put(String msg) throws IOException { put(1, msg.getBytes(StandardCharsets.UTF_8)); }
+
+    private void put(int op, byte[] data) throws IOException {
+        int len = data.length;
+        _out.write(128 | op);
+        if( len <= 125 ) _out.write(len);
+        else if( len <= 65535 ) {
+            _out.write(126);
+            _out.write(len >>> 8);
+            _out.write(len & 255);
+        } else {
+            _out.write(127);
+            for( int shift = 56; shift >= 0; shift -= 8 )
+                _out.write((int)((long)len >>> shift) & 255);
+        }
+        _out.write(data);
+        _out.flush();
+    }
+
+    @Override public void close() throws IOException {
+        try {
+            if( _out != null && !isClosed() ) put(8, new byte[0]);
+        } finally {
+            if( _http != null ) _http.stop(0);
+            try {
+                if( _client != null ) _client.close();
+            } finally {
+                super.close();
+            }
+        }
+    }
+}
diff --git a/graph/src/main/java/com/seaofnodes/graph/GraphViewer.java b/graph/src/main/java/com/seaofnodes/graph/GraphViewer.java
new file mode 100644
index 00000000..dcb2e337
--- /dev/null
+++ b/graph/src/main/java/com/seaofnodes/graph/GraphViewer.java
@@ -0,0 +1,49 @@
+package com.seaofnodes.graph;
+
+import java.io.IOException;
+import java.net.URI;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+/** Shared launcher and source/compile loop. Subclasses supply their compiler. */
+public abstract class GraphViewer implements AutoCloseable {
+    private final GraphSocket _server;
+
+    protected GraphViewer() throws IOException { _server = new GraphSocket(viewerURI(), 12345); }
+    protected abstract void compile(String src);
+
+    protected final void frame(GraphSnapshot snap, int pos, GraphEvent evt) throws IOException {
+        _server.put(GraphJson.frame(snap, pos, evt));
+    }
+
+    private static URI viewerURI() throws IOException {
+        String url = System.getProperty("simple.graph.url");
+        if( url != null ) return URI.create(url);
+        for( Path dir = Path.of("").toAbsolutePath(); dir != null; dir = dir.getParent() ) {
+            Path page = dir.resolve("graph/web/index.html");
+            if( Files.isRegularFile(page) ) return page.toUri();
+        }
+        throw new IOException("Cannot find graph/web/index.html; run inside the Simple checkout "
+                              + "or set -Dsimple.graph.url=<viewer URL>");
+    }
+
+    public final void run() throws IOException {
+        _server.put("!");
+        while( true ) {
+            String src = _server.get();
+            if( src == null || src.equals("null") ) return;
+            if( src.equals("+") ) continue; // Frames are currently sent eagerly.
+            System.out.println(src);
+            try {
+                compile(src);
+            } catch( RuntimeException e ) {
+                System.err.println(e);
+                _server.put(GraphJson.error(e.toString()));
+            } finally {
+                _server.put("#");
+            }
+        }
+    }
+
+    @Override public final void close() throws IOException { _server.close(); }
+}
diff --git a/graph/test_browser.py b/graph/test_browser.py
index dbbfdd59..90223119 100644
--- a/graph/test_browser.py
+++ b/graph/test_browser.py
@@ -23,7 +23,7 @@ from playwright.sync_api import sync_playwright
 
 def check(chapter, browser_name):
     number = int(chapter.removeprefix("chapter"))
-    package = "com.seaofnodes.simple." + ("print." if number >= 20 else "")
+    package = "com.seaofnodes.simple." + ("print." if number == 4 or number >= 20 else "")
     artifacts = ROOT / "build/graph-browser" / (chapter + "-" + browser_name)
     artifacts.mkdir(parents=True, exist_ok=True)
     log = artifacts / "compiler.log"
@@ -53,8 +53,9 @@ def check(chapter, browser_name):
                 page.wait_for_function("current === 1 && !rendering")
                 page.locator("#doPrev").click()
                 page.wait_for_function("current === 0 && !rendering")
-                for source in ("return 1 + 2;",
-                               "int x=0; while(x<arg) { x=x+1; } return x;"):
+                sources = ("return 1 + 2;", "int x=arg+1; return x+x+2;" if number == 4 else
+                           "int x=0; while(x<arg) { x=x+1; } return x;")
+                for source in sources:
                     previous = page.evaluate("generation")
                     page.locator("#program").fill(source)
                     page.locator("#compile").click()
@@ -68,7 +69,7 @@ def check(chapter, browser_name):
                         page.wait_for_function("index => current === index && !rendering", arg=index)
                     assert page.locator("#doNext").is_disabled()
                     assert "error" not in page.locator("#status").inner_text().lower()
-                    if number == 25:
+                    if number in (4, 25):
                         assert page.locator("#elk g.node").count() == page.evaluate("frames[current].snap.nodes.length")
                         # Revisit cached geometry and keep the selected node across steps.
                         page.evaluate("""() => {
diff --git a/graph/test_transport.py b/graph/test_transport.py
index b33b4657..d0026b97 100644
--- a/graph/test_transport.py
+++ b/graph/test_transport.py
@@ -10,14 +10,45 @@ import subprocess
 import sys
 import time
 import urllib.request
+import json
+import struct
+
+
+def packet(data, op=1, fin=True):
+    n = len(data)
+    head = bytes([op | (128 if fin else 0)])
+    head += bytes([128 | n]) if n < 126 else (b'\xfe' + struct.pack('>H', n) if n <= 65535
+                                           else b'\xff' + struct.pack('>Q', n))
+    mask = b'abcd'
+    return head + mask + bytes(b ^ mask[i % 4] for i, b in enumerate(data))
+
+
+def frame(stream):
+    head = stream.read(2)
+    assert len(head) == 2, 'Incomplete server frame'
+    n = head[1]
+    if n == 126: n = struct.unpack('>H', stream.read(2))[0]
+    elif n == 127: n = struct.unpack('>Q', stream.read(8))[0]
+    data = stream.read(n)
+    assert len(data) == n
+    return head[0] & 15, data
+
+
+def compile_frames(stream):
+    frames = []
+    while True:
+        op, data = frame(stream)
+        assert op == 1
+        if data == b'#': return frames
+        frames.append(json.loads(data))
 
 
 def check(chapter):
     root = Path(__file__).resolve().parent.parent
     number = int(chapter.removeprefix("chapter"))
-    assert 18 <= number <= 25
+    assert number == 4 or 18 <= number <= 25
     chapter_dir = root / chapter
-    package = "com.seaofnodes.simple." + ("print." if number >= 20 else "")
+    package = "com.seaofnodes.simple." + ("print." if number == 4 or number >= 20 else "")
     log = root / "build" / "graph-transport" / (chapter + ".log")
     log.parent.mkdir(parents=True, exist_ok=True)
     with log.open("w") as output:
@@ -62,7 +93,9 @@ def check(chapter):
                 else:
                     raise AssertionError("Responded before headers ended: " + repr(early))
                 client.settimeout(3)
-                client.sendall(b"\r\n")
+                shared = number in (4, 25)
+                # A frame in the same write as the upgrade must not get lost in a reader buffer.
+                client.sendall(b"\r\n" + (packet(b'hello', op=9) if shared else b''))
                 with client.makefile("rb") as stream:
                     assert stream.readline() == b"HTTP/1.1 101 Switching Protocols\r\n"
                     headers = []
@@ -74,6 +107,25 @@ def check(chapter):
                         headers.append(line)
                     assert b"Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=\r\n" in headers
                     assert stream.read(3) == b"\x81\x01!", "Missing source request"
+                    if shared:
+                        assert frame(stream) == (10, b'hello')
+                        # Larger than both the old 1 KB buffer and the 16-bit length field.
+                        data = packet(b' ' * 70000 + b'return 3;')
+                        client.sendall(data[:3])
+                        time.sleep(.05)
+                        for start in range(3, len(data), 509):
+                            client.sendall(data[start:start + 509])
+                        frames = compile_frames(stream)
+                        assert frames and all('snap' in f for f in frames)
+                        # Split a UTF-8 code point across fragments, with an interleaved PING.
+                        client.sendall(packet(b'\xc3', fin=False) + packet(b'ping', op=9) + packet(b'\xa9', op=0))
+                        assert frame(stream) == (10, b'ping')
+                        errors = compile_frames(stream)
+                        assert any('error' in f for f in errors)
+                        assert '\u00e9' in log.read_text()
+                        client.sendall(packet(b'return 4;'))
+                        frames = compile_frames(stream)
+                        assert frames and all('snap' in f for f in frames)
                     # Masked 'null' asks the compiler to exit normally.
                     mask = b"abcd"
                     payload = bytes(byte ^ mask[i % 4] for i, byte in enumerate(b"null"))
@@ -91,7 +143,8 @@ def check(chapter):
             if process.poll() is None:
                 process.terminate()
                 process.wait(timeout=5)
-    print(chapter + ": HTTP assets, fragmented upgrade, greeting, and shutdown passed")
+    print(chapter + ": HTTP assets, fragmented upgrade, greeting, and shutdown passed" +
+          ("; large frames, UTF-8 fragments, ping, and compile-error recovery passed" if number in (4, 25) else ""))
 
 
 if __name__ == "__main__":
diff --git a/graph/web/viewer.js b/graph/web/viewer.js
index 9aaaa154..ed14651f 100644
--- a/graph/web/viewer.js
+++ b/graph/web/viewer.js
@@ -148,6 +148,11 @@ try {
       try {
         if (message.startsWith("{")) {
           frame = JSON.parse(message);
+          if (frame.error) {
+            failure = "Compile error: " + frame.error;
+            updateUI();
+            return;
+          }
           if (frame.snap.ver !== 1) throw new Error("Unsupported graph version: " + frame.snap.ver);
         } else {
           // Earlier chapters still send bare DOT.