[jlox] Add Resolution and Binding

A very subtle bug: The resolution relies on the fact that Java has a
concept of object identity: Even when the "Contents" of two objects are
equal, those objects are not considered equal.

This does not hold for records:
```
record Test(int val) {}

var a = new Test(1);
var b = new Test(1);

var m = new HashMap<>();
m.put(a, "Hello");
m.get(b); // returns "Hello"
```

tl;dr - got bitten by trying to do things fancy

jlox/lox/src/main/java/xyz/ctsk/lox/Environment.java

@@ -20,6 +20,16 @@ public class Environment {
         values.put(name, value);
     }
 
+    private Environment ancestor(int distance) {
+        Environment environment = this;
+
+        for (int i = 0; i < distance; i++) {
+            environment = environment.enclosing;
+        }
+
+        return environment;
+    }
+
     void assign(Token name, Object value) {
         if (values.containsKey(name.lexeme())) {
             values.put(name.lexeme(), value);
@@ -30,6 +40,10 @@ public class Environment {
         }
     }
 
+    void assignAt(int distance, Token name, Object value) {
+        ancestor(distance).values.put(name.lexeme(), value);
+    }
+
     Object get(Token name) {
         if (values.containsKey(name.lexeme())) {
             return values.get(name.lexeme());
@@ -40,4 +54,9 @@ public class Environment {
             throw new RuntimeError(name, message);
         }
     }
+
+    Object getAt(int distance, String name) {
+        return ancestor(distance).values.get(name);
+    }
+
 }

jlox/lox/src/main/java/xyz/ctsk/lox/FunctionType.java

@@ -0,0 +1,7 @@
+package xyz.ctsk.lox;
+
+public enum FunctionType {
+    NONE,
+    FUNCTION,
+    METHOD
+}

jlox/lox/src/main/java/xyz/ctsk/lox/Interpreter.java

@@ -1,10 +1,13 @@
 package xyz.ctsk.lox;
 
+import java.util.HashMap;
 import java.util.List;
+import java.util.Map;
 
 public class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
     final Environment globals = new Environment();
     private Environment environment = globals;
+    private final Map<Expr, Integer> locals = new HashMap<>();
 
 
     Interpreter() {
@@ -54,6 +57,10 @@ public class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
         return expr.accept(this);
     }
 
+    void resolve(Expr expr, int depth) {
+        locals.put(expr, depth);
+    }
+
     static String stringify(Object object) {
         if (object == null) return "nil";
 
@@ -79,7 +86,7 @@ public class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
 
     @Override
     public Void visitFunctionStmt(Stmt.Function stmt) {
-        environment.define(stmt.name().lexeme(), new LoxFunction(stmt));
+        environment.define(stmt.name().lexeme(), new LoxFunction(stmt, environment));
         return null;
     }
 
@@ -124,7 +131,14 @@ public class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
     @Override
     public Object visitAssignExpr(Expr.Assign expr) {
         var value = evaluate(expr.value());
-        environment.assign(expr.name(), value);
+
+        Integer distance = locals.get(expr);
+        if (distance != null) {
+            environment.assignAt(distance, expr.name(), value);
+        } else {
+            globals.assign(expr.name(), value);
+        }
+
         return value;
     }
 
@@ -218,7 +232,16 @@ public class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
 
     @Override
     public Object visitVariableExpr(Expr.Variable expr) {
-        return environment.get(expr.name());
+        return lookupVariable(expr.name(), expr);
+    }
+
+    private Object lookupVariable(Token name, Expr.Variable expr) {
+        var distance = locals.get(expr);
+        if (distance != null) {
+            return environment.getAt(distance, name.lexeme());
+        } else {
+            return globals.get(name);
+        }
     }
 
 

jlox/lox/src/main/java/xyz/ctsk/lox/Lox.java

@@ -45,6 +45,11 @@ public class Lox {
 
         if (hadError) return;
 
+        var resolver = new Resolver(interpreter);
+        resolver.resolve(statements);
+
+        if (hadError) return;
+
         interpreter.interpret(statements);
     }
 
@@ -61,7 +66,7 @@ public class Lox {
         }
     }
 
-    private static void runFile(String path) throws IOException {
+    protected static void runFile(String path) throws IOException {
         byte[] bytes = Files.readAllBytes(Paths.get(path));
         run(new String(bytes, Charset.defaultCharset()));
 

jlox/lox/src/main/java/xyz/ctsk/lox/LoxFunction.java

@@ -4,9 +4,11 @@ import java.util.List;
 
 public class LoxFunction implements LoxCallable {
     private final Stmt.Function declaration;
+    private final Environment closure;
 
-    public LoxFunction(Stmt.Function declaration) {
+    public LoxFunction(Stmt.Function declaration, Environment closure) {
         this.declaration = declaration;
+        this.closure = closure;
     }
 
     @Override
@@ -16,7 +18,7 @@ public class LoxFunction implements LoxCallable {
 
     @Override
     public Object call(Interpreter interpreter, List<Object> arguments) {
-        Environment environment = new Environment(interpreter.globals);
+        Environment environment = new Environment(closure);
         for (int i = 0; i < declaration.params().size(); i++) {
             environment.define(declaration.params().get(i).lexeme(),
                     arguments.get(i));

jlox/lox/src/main/java/xyz/ctsk/lox/Parser.java

@@ -430,5 +430,4 @@ public class Parser {
     }
 
     private static class ParseError extends RuntimeException {}
-    private enum FunctionType { FUNCTION }
 }

jlox/lox/src/main/java/xyz/ctsk/lox/Resolver.java

@@ -0,0 +1,197 @@
+package xyz.ctsk.lox;
+
+
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Stack;
+
+public class Resolver implements Expr.Visitor<Void>, Stmt.Visitor<Void> {
+    private final Interpreter interpreter;
+    private final Stack<Map<String, Boolean>> scopes = new Stack<>();
+    private FunctionType currentFuntion = FunctionType.NONE;
+
+
+    Resolver(Interpreter interpreter) {
+        this.interpreter = interpreter;
+    }
+
+    private void resolve(Stmt stmt) {
+        stmt.accept(this);
+    }
+
+    void resolve(List<Stmt> statements) {
+        statements.forEach(this::resolve);
+    }
+
+    private void resolve(Expr expr) {
+        expr.accept(this);
+    }
+
+
+    private void beginScope() {
+        scopes.push(new HashMap<>());
+    }
+
+    private void endScope() {
+        scopes.pop();
+    }
+
+    private void declare(Token name) {
+        if (scopes.isEmpty()) return;
+        var scope = scopes.peek();
+        if (scope.containsKey(name.lexeme())) {
+            Lox.error(name, "Already a variable with this name in this scope.");
+        }
+        scope.put(name.lexeme(), false);
+    }
+
+    private void define(Token name) {
+        if (scopes.isEmpty()) return;
+        scopes.peek().put(name.lexeme(), true);
+    }
+
+    private void resolveFunction(Stmt.Function function, FunctionType functionType) {
+        var enclosingFunction = currentFuntion;
+        currentFuntion = functionType;
+        beginScope();
+        for (var param : function.params()) {
+            declare(param);
+            define(param);
+        }
+        resolve(function.body());
+        endScope();
+        currentFuntion = enclosingFunction;
+    }
+
+
+    private void resolveLocal(Expr expr, Token name) {
+        for (int i = scopes.size() - 1; i >= 0; i--) {
+            if (scopes.get(i).containsKey(name.lexeme())) {
+                interpreter.resolve(expr, scopes.size() - 1 - i);
+                return;
+            }
+        }
+    }
+
+    @Override
+    public Void visitBlockStmt(Stmt.Block stmt) {
+        beginScope();
+        resolve(stmt.statements());
+        endScope();
+        return null;
+    }
+
+
+    @Override
+    public Void visitExpressionStmt(Stmt.Expression stmt) {
+        resolve(stmt.expression());
+        return null;
+    }
+
+    @Override
+    public Void visitFunctionStmt(Stmt.Function stmt) {
+        declare(stmt.name());
+        define(stmt.name());
+        resolveFunction(stmt, FunctionType.FUNCTION);
+        return null;
+    }
+
+    @Override
+    public Void visitIfStmt(Stmt.If stmt) {
+        resolve(stmt.condition());
+        resolve(stmt.thenBranch());
+        if (stmt.elseBranch() != null) resolve(stmt.elseBranch());
+        return null;
+    }
+
+    @Override
+    public Void visitPrintStmt(Stmt.Print stmt) {
+        resolve(stmt.expression());
+        return null;
+    }
+
+    @Override
+    public Void visitReturnStmt(Stmt.Return stmt) {
+        if (currentFuntion == FunctionType.NONE) {
+            Lox.error(stmt.keyword(), "Can't return fom top-level code.");
+        }
+
+        if (stmt.value() != null) {
+            resolve(stmt.value());
+        }
+        return null;
+    }
+
+    @Override
+    public Void visitVarStmt(Stmt.Var stmt) {
+        declare(stmt.name());
+        if (stmt.initializer() != null) {
+            resolve(stmt.initializer());
+        }
+        define(stmt.name());
+        return null;
+    }
+
+    @Override
+    public Void visitWhileStmt(Stmt.While stmt) {
+        resolve(stmt.condition());
+        resolve(stmt.body());
+        return null;
+    }
+
+
+    @Override
+    public Void visitAssignExpr(Expr.Assign expr) {
+        resolve(expr.value());
+        resolveLocal(expr, expr.name());
+        return null;
+    }
+
+    @Override
+    public Void visitBinaryExpr(Expr.Binary expr) {
+        resolve(expr.left());
+        resolve(expr.right());
+        return null;
+    }
+
+    @Override
+    public Void visitCallExpr(Expr.Call expr) {
+        resolve(expr.callee());
+        expr.arguments().forEach(this::resolve);
+        return null;
+    }
+
+    @Override
+    public Void visitGroupingExpr(Expr.Grouping expr) {
+        resolve(expr.expression());
+        return null;
+    }
+
+    @Override
+    public Void visitLiteralExpr(Expr.Literal expr) {
+        return null;
+    }
+
+    @Override
+    public Void visitLogicalExpr(Expr.Logical expr) {
+        resolve(expr.left());
+        resolve(expr.right());
+        return null;
+    }
+
+    @Override
+    public Void visitUnaryExpr(Expr.Unary expr) {
+        resolve(expr.right());
+        return null;
+    }
+
+    @Override
+    public Void visitVariableExpr(Expr.Variable expr) {
+        if (!scopes.isEmpty() && scopes.peek().get(expr.name().lexeme()) == Boolean.FALSE) {
+            Lox.error(expr.name(), "Can't read local variable in its own initializer.");
+        }
+        resolveLocal(expr, expr.name());
+        return null;
+    }
+}

jlox/lox/src/main/java/xyz/ctsk/lox/Scanner.java

@@ -44,7 +44,7 @@ public class Scanner {
     }
 
     private void addToken(TokenType type, Object literal) {
-        tokens.add(new Token(type, currentMatch(), literal, line));
+        tokens.add(new Token(type, currentMatch(), literal, line, current));
     }
 
     private void addToken(TokenType type) {
@@ -191,7 +191,7 @@ public class Scanner {
             scanToken();
         }
 
-        tokens.add(new Token(EOF, "", line));
+        tokens.add(new Token(EOF, "", line, current));
         return tokens;
     }
 }

jlox/lox/src/main/java/xyz/ctsk/lox/Token.java

@@ -1,8 +1,8 @@
 package xyz.ctsk.lox;
 
-public record Token(TokenType type, String lexeme, Object literal, int line) {
-    public Token(TokenType type, String lexeme, int line) {
-        this(type, lexeme, null, line);
+public record Token(TokenType type, String lexeme, Object literal, int line, int position) {
+    public Token(TokenType type, String lexeme, int line, int position) {
+        this(type, lexeme, null, line, position);
     }
 
     @Override

jlox/lox/src/test/java/xyz/ctsk/lox/ResolverTest.java

@@ -0,0 +1,13 @@
+package xyz.ctsk.lox;
+
+import org.junit.jupiter.api.Test;
+
+import java.io.IOException;
+
+public class ResolverTest {
+    @Test
+    void forLoopBug() throws IOException {
+        var canary = this.getClass().getResource("ResolverBug.lox").getPath();
+        Lox.runFile(canary);
+    }
+}

jlox/lox/src/test/resources/xyz/ctsk/lox/ResolverBug.lox

@@ -0,0 +1 @@
+for (var i = 0; i < 2; i = i + 1) print i;