From 0a892df176374687da46b442bc2bcbe45c8809a9 Mon Sep 17 00:00:00 2001
From: Mariano Riefolo <rfmariano@protonmail.com>
Date: Fri, 28 Jun 2024 11:12:26 +0200
Subject: [PATCH] Parsing expressions chapter

---
 src/lox/Lox.java    |  17 ++++-
 src/lox/Parser.java | 170 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 184 insertions(+), 3 deletions(-)
 create mode 100644 src/lox/Parser.java

diff --git a/src/lox/Lox.java b/src/lox/Lox.java
index 8262a47..03f3b87 100644
--- a/src/lox/Lox.java
+++ b/src/lox/Lox.java
@@ -46,9 +46,12 @@ public class Lox {
         Scanner scanner = new Scanner(source);
         List<Token> tokens = scanner.scanTokens();
 
-        for (Token token : tokens) {
-            System.out.println(token);
-        }
+        Parser parser = new Parser(tokens);
+        Expr expression = parser.parse();
+
+        if (hadError) return;
+
+        System.out.println(new AstPrinter().print(expression));
     }
 
     static void error(int line, String message) {
@@ -59,4 +62,12 @@ public class Lox {
         System.err.println("[line " + line + "] Error" + where + ": " + message);
         hadError = true;
     }
+
+    static void error(Token token, String message) {
+        if (token.type == TokenType.EOF) {
+            report(token.line, " at end", message);
+        } else {
+            report(token.line, " at '" + token.lexeme + "'", message);
+        }
+    }
 }
diff --git a/src/lox/Parser.java b/src/lox/Parser.java
new file mode 100644
index 0000000..9ebd54a
--- /dev/null
+++ b/src/lox/Parser.java
@@ -0,0 +1,170 @@
+package lox;
+
+import java.util.List;
+
+import static lox.TokenType.*;
+
+public class Parser {
+    private static class ParseError extends RuntimeException {}
+
+    private final List<Token> tokens;
+    private int current;
+
+    Parser(List<Token> tokens) {
+        this.tokens = tokens;
+    }
+
+    Expr parse() {
+        try {
+            return expression();
+        } catch (ParseError error) {
+            return null;
+        }
+    }
+
+    private Expr expression() {
+        return equality();
+    }
+
+    private Expr equality() {
+        Expr expr = comparison();
+
+        while (match(BANG_EQUAL, EQUAL_EQUAL)) {
+            Token operator = previous();
+            Expr right = comparison();
+            expr = new Expr.Binary(expr, operator, right);
+        }
+
+        return expr;
+    }
+
+    private Expr comparison() {
+        Expr expr = term();
+
+        while (match(GREATER, GREATER_EQUAL, LESS, LESS_EQUAL)) {
+            Token operator = previous();
+            Expr right = term();
+            expr = new Expr.Binary(expr, operator, right);
+        }
+
+        return expr;
+    }
+
+    private Expr term() {
+        Expr expr = factor();
+
+        while (match(MINUS, PLUS)) {
+            Token operator = previous();
+            Expr right = factor();
+            expr = new Expr.Binary(expr, operator, right);
+        }
+
+        return expr;
+    }
+
+    private Expr factor() {
+        Expr expr = unary();
+
+        while (match(SLASH, STAR)) {
+            Token operator = previous();
+            Expr right = unary();
+            expr = new Expr.Binary(expr, operator, right);
+        }
+
+        return expr;
+    }
+
+    private Expr unary() {
+        if (match(BANG, MINUS)) {
+            Token operator = previous();
+            Expr right = unary();
+            return new Expr.Unary(operator, right);
+        }
+
+        return primary();
+    }
+
+    private Expr primary() {
+        if (match(FALSE)) return new Expr.Literal(false);
+        if (match(TRUE)) return new Expr.Literal(true);
+        if (match(NIL)) return new Expr.Literal(null);
+
+        if (match(NUMBER, STRING)) {
+            return new Expr.Literal(previous().literal);
+        }
+
+        if (match(LEFT_PAREN)) {
+            Expr expr = expression();
+            consume(RIGHT_PAREN, "Expect ')' after expression.");
+            return new Expr.Grouping(expr);
+        }
+
+        throw error(peek(), "Expect expression");
+    }
+
+    private boolean match(TokenType... types) {
+        for (TokenType type : types) {
+            if (check(type)) {
+                advance();
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    private Token consume(TokenType type, String message) {
+        if (check(type)) return advance();
+
+        throw error(peek(), message);
+    }
+
+    private void synchronize() {
+        advance();
+
+        while (!isAtEnd()) {
+            if (previous().type == SEMICOLON) return;
+
+            switch (peek().type) {
+                case CLASS:
+                case FUN:
+                case VAR:
+                case FOR:
+                case IF:
+                case WHILE:
+                case PRINT:
+                case RETURN:
+                    return;
+            }
+
+            advance();
+        }
+    }
+
+    private boolean check(TokenType type) {
+        if (isAtEnd()) return false;
+        return peek().type == type;
+    }
+
+    private Token advance() {
+        if (!isAtEnd()) current++;
+        return previous();
+    }
+
+    private boolean isAtEnd() {
+        return peek().type == EOF;
+    }
+
+    private Token peek() {
+        return tokens.get(current);
+    }
+
+    private Token previous() {
+        return tokens.get(current - 1);
+    }
+
+    private ParseError error(Token token, String message) {
+        Lox.error(token, message);
+        return new ParseError();
+    }
+}