for statements (ch23)

README.md

@@ -13,7 +13,7 @@ While reading [Crafting Interpreters](https://craftinginterpreters.com/), after
 - [x] 20 - Hash Tables
 - [x] 21 - Global Variables
 - [x] 22 - Local Variables
-- [ ] 23 - Jumping Back and Forth
+- [x] 23 - Jumping Back and Forth
 - [ ] 24 - Calls and Functions
 - [ ] 25 - Closures
 - [ ] 26 - Garbage Collection

samples/ch23_for.lox

@@ -0,0 +1,3 @@
+for(var i = 0; i < 10; i = i + 1) {
+    print i;
+}

src/compile.zig

@@ -360,6 +360,8 @@ const Parser = struct {
     fn statement(self: *Parser) ParsingError!void {
         if (self.match(TokenType.PRINT)) {
             try self.print_statement();
+        } else if (self.match(TokenType.FOR)) {
+            try self.for_statement();
         } else if (self.match(TokenType.IF)) {
             try self.if_statement();
         } else if (self.match(TokenType.WHILE)) {
@@ -644,6 +646,54 @@ const Parser = struct {
         try self.emit_byte(@intCast((offset >> 8) & 0xff));
         try self.emit_byte(@intCast(offset & 0xff));
     }
+
+    fn for_statement(self: *Parser) ParsingError!void {
+        self.begin_scope();
+
+        self.consume(TokenType.LEFT_PAREN, "Expect '(' after 'for'.");
+        if (self.match(TokenType.SEMICOLON)) {
+            // No initializer
+        } else if (self.match(TokenType.VAR)) {
+            try self.var_declaration();
+        } else {
+            try self.expression_statement();
+        }
+
+        var loop_start = self.chunk.count;
+
+        var exit_jump: ?usize = null;
+
+        if (!self.match(TokenType.SEMICOLON)) {
+            try self.expression();
+            self.consume(TokenType.SEMICOLON, "Expect ';' after loop condition.");
+
+            // Jump out of the loop if the condition is false.
+            exit_jump = try self.emit_jump(@intFromEnum(OpCode.OP_JUMP_IF_FALSE));
+            _ = try self.emit_byte(@intFromEnum(OpCode.OP_POP)); // Condition
+        }
+
+        if (!self.match(TokenType.RIGHT_PAREN)) {
+            const body_jump = try self.emit_jump(@intFromEnum(OpCode.OP_JUMP));
+            const increment_start = self.chunk.count;
+            try self.expression();
+            try self.emit_byte(@intFromEnum(OpCode.OP_POP));
+            self.consume(TokenType.RIGHT_PAREN, "Expect ')' after for clauses.");
+
+            try self.emit_loop(loop_start);
+            loop_start = increment_start;
+            self.patch_jump(body_jump);
+        }
+
+        try self.statement();
+        try self.emit_loop(loop_start);
+
+        if (exit_jump != null) {
+            self.patch_jump(exit_jump.?);
+            try self.emit_byte(@intFromEnum(OpCode.OP_POP));
+        }
+
+        try self.end_scope();
+    }
 };
 
 const Compiler = struct {