rlox/src/frontend/parser.rs

use crate::{
    common::{
        ast::{AstNode, Expr, Stmt},
        base_value::{BaseValue, LoxFunction},
        lox_result::{parse_error, runtime_error, LoxError, LoxResult},
    },
    frontend::{
        source_registry::SourceSlice,
        tokens::{Token, TokenType},
    },
};

pub struct Parser {
    tokens: Vec<Token>,
    current: usize,
}

impl Parser {
    pub fn new(tokens: Vec<Token>) -> Self {
        Self { tokens, current: 0 }
    }

    pub fn parse(&mut self) -> LoxResult<Vec<AstNode<Stmt>>> {
        let mut statements = Vec::new();
        let mut errors = Vec::new();
        while !self.is_at_end() {
            match self.statement(None) {
                Ok(stmt) => {
                    statements.push(stmt.clone());
                }
                Err(err) => {
                    errors.push(err);
                    self.synchronize();
                }
            }
        }

        if !errors.is_empty() {
            return Err(errors.into_iter().next().unwrap());
        }

        Ok(statements)
    }

    fn statement(&mut self, label: Option<String>) -> LoxResult<AstNode<Stmt>> {
        match (&self.peek().token_type, &self.peek_next().token_type) {
            (TokenType::Print, _) => self.print_statement(),
            (TokenType::Return, _) => self.return_statement(label),
            (TokenType::Break, _) => self.return_statement(Some("loop".to_string())),
            (TokenType::StartBlock, _) => self.block_statement(label),
            (TokenType::Var, TokenType::Identifier) => {
                self.advance();
                self.var_statement()
            }
            (TokenType::Identifier, TokenType::Colon) => self.var_statement(),
            (TokenType::Identifier, TokenType::Equal) => self.assignment_statement(),
            (TokenType::If, _) => self.if_statement(),
            (TokenType::While, _) => self.while_statement(),
            (TokenType::For, _) => self.for_statement(),
            _ => self.expression_statement(),
        }
    }

    fn for_statement(&mut self) -> LoxResult<AstNode<Stmt>> {
        let start_slice = self.peek().source_slice.clone();
        self.advance(); // consume 'for'

        let variable = self.var_statement()?;
        let condition = self.expression()?;
        self.consume(TokenType::Semicolon, "Expected ';' after for condition")?;
        let increment = self.assignment_statement()?;
        let body = self.statement(Some("loop".to_string()))?;

        let end_slice = self.previous().source_slice.clone();
        let combined_slice = SourceSlice::from_positions(
            start_slice.source_id,
            start_slice.start_position,
            end_slice.end_position,
        );

        Ok(AstNode::new(
            Stmt::For {
                variable: Box::new(variable),
                condition: Box::new(condition),
                increment: Box::new(increment),
                body: Box::new(body),
                return_value: Box::new(BaseValue::Nil),
            },
            combined_slice,
        ))
    }

    // fn for_statement(&mut self) -> LoxResult<AstNode<Stmt>> {
    //     let start_slice = self.peek().source_slice.clone();
    //     self.advance();
    //     let variable = if let Token {
    //         token_type: TokenType::Identifier,
    //         literal: Some(variable_name),
    //         source_slice,
    //         ..
    //     } = self.peek()
    //     {
    //         AstNode {
    //             node: Expr::Literal {
    //                 value: variable_name.clone(),
    //             },
    //             source_slice: source_slice.clone(),
    //         }
    //     } else {
    //         return parse_error(
    //             self.peek().source_slice.clone(),
    //             "Expect variable name after 'for' keyword.",
    //         );
    //     };
    //     self.advance();
    //     self.consume(TokenType::In, "Expect 'in' after for variable.")?;
    //     let iterable = self.expression()?;
    //     let body = self.statement(None)?;
    //     let end_slice = body.source_slice.clone();
    //     let combined_slice = SourceSlice::from_positions(
    //         start_slice.source_id,
    //         start_slice.start_position,
    //         end_slice.end_position,
    //     );
    //     Ok(AstNode::new(
    //         Stmt::For {
    //             variable: Box::new(variable),
    //             iterable: Box::new(iterable),
    //             body: Box::new(body),
    //         },
    //         combined_slice,
    //     ))
    // }

    fn while_statement(&mut self) -> LoxResult<AstNode<Stmt>> {
        let start_slice = self.peek().source_slice.clone();
        self.advance();
        let condition = self.expression()?;
        let body = self.statement(Some("loop".to_string()))?;
        let end_slice = body.source_slice.clone();
        let combined_slice = SourceSlice::from_positions(
            start_slice.source_id,
            start_slice.start_position,
            end_slice.end_position,
        );
        Ok(AstNode::new(
            Stmt::While {
                condition: Box::new(condition),
                body: Box::new(body),
                return_value: Box::new(BaseValue::Nil),
            },
            combined_slice,
        ))
    }

    fn if_statement(&mut self) -> LoxResult<AstNode<Stmt>> {
        let start_slice = self.peek().source_slice.clone();
        self.advance(); // consume 'if'
        let condition = self.expression()?;
        self.consume(TokenType::Then, "Expect 'then' after if condition.")?;
        let then_branch = self.statement(None)?;
        let mut elif_branches = Vec::new();
        let mut last_slice = then_branch.source_slice.clone();

        while self.peek().token_type == TokenType::Elif {
            self.advance(); // consume 'elif'
            let elif_condition = self.expression()?;
            self.consume(TokenType::Then, "Expect 'then' after elif condition.")?;
            let elif_branch = self.statement(None)?;
            last_slice = elif_branch.source_slice.clone();
            elif_branches.push((Box::new(elif_condition), Box::new(elif_branch)));
        }

        let else_branch = if self.peek().token_type == TokenType::Else {
            self.advance(); // consume 'else'
            let else_stmt = self.statement(None)?;
            last_slice = else_stmt.source_slice.clone();
            Some(Box::new(else_stmt))
        } else {
            None
        };

        let combined_slice = SourceSlice::from_positions(
            start_slice.source_id,
            start_slice.start_position,
            last_slice.end_position,
        );

        Ok(AstNode::new(
            Stmt::If {
                condition: Box::new(condition),
                then_branch: Box::new(then_branch),
                elif_branch: elif_branches,
                else_branch: else_branch,
                return_value: Box::new(BaseValue::Nil),
            },
            combined_slice,
        ))
    }

    fn var_statement(&mut self) -> LoxResult<AstNode<Stmt>> {
        let start_slice = self.peek().source_slice.clone();
        let name = self.consume(TokenType::Identifier, "Expect variable name.")?;
        let name_lexeme = name.lexeme.clone();

        let mut _type_annotation = BaseValue::Nil;
        self.consume(TokenType::Colon, "Expect column")?;
        if self.peek().token_type == TokenType::Identifier {
            // TODO manage type annotation
            self.consume(TokenType::Identifier, "Expect type name.")?;
        }

        match self.peek().token_type {
            TokenType::Equal | TokenType::Identifier => {
                self.variable_declaration(start_slice, name_lexeme)
            }
            TokenType::Colon => self.function_declaration(start_slice, name_lexeme),
            _ => runtime_error(start_slice, "this is not supposed to be here"),
        }
    }

    fn function_declaration(
        &mut self,
        start_slice: SourceSlice,
        name_lexeme: String,
    ) -> LoxResult<AstNode<Stmt>> {
        self.advance();
        self.consume(TokenType::Fn, "Expect 'fn' after '::'")?;
        self.consume(TokenType::LeftParen, "Expect '(' after 'fn' ")?;
        let mut parameters: Vec<(String, String)> = vec![];
        while self.peek().token_type != TokenType::RightParen {
            let expr = self
                .consume(TokenType::Identifier, "Expect identifier after '('")?
                .clone();
            if self.peek().token_type == TokenType::Colon {
                self.advance();
                let type_ =
                    self.consume(TokenType::Identifier, "Expected identifier after ':' ")?;
                parameters.push((expr.lexeme, type_.lexeme.clone()));
            } else {
                parameters.push((expr.lexeme, "Any".to_string()));
            }
            if self.peek().token_type == TokenType::Comma {
                self.advance();
            }
        }
        self.consume(TokenType::RightParen, "Expected ')' after parameters")?;
        let end_position = self.peek().source_slice.clone();
        let combine_position = SourceSlice {
            source_id: start_slice.source_id,
            start_position: start_slice.start_position.clone(),
            end_position: end_position.end_position,
        };

        let mut guard: Option<Box<AstNode<Expr>>> = None;
        if self.peek().token_type == TokenType::LeftBrace {
            self.consume(TokenType::LeftBrace, "Expected '{' after guard expression")?;
            guard = Some(Box::new(self.expression()?));
            self.consume(TokenType::RightBrace, "Expected '}' after guard expression")?;
        }

        let body = self.statement(None)?;
        let node = AstNode {
            node: Expr::Literal {
                value: BaseValue::Function(LoxFunction {
                    parameters,
                    return_type: None,
                    body,
                    closure: None,
                    guard,
                }),
            },
            source_slice: combine_position.clone(),
        };
        Ok(AstNode::new(
            Stmt::VarDeclaration {
                name: name_lexeme,
                initializer: Some(Box::new(node)),
                return_value: Box::new(BaseValue::Nil),
            },
            combine_position.clone(),
        ))
    }

    fn variable_declaration(
        &mut self,
        start_slice: SourceSlice,
        name_lexeme: String,
    ) -> LoxResult<AstNode<Stmt>> {
        if self.peek().token_type == TokenType::Identifier {
            self.advance();
            if self.peek().token_type == TokenType::Identifier {
                self.advance();
            }
            // todo: make type annotation
        }
        let mut value = AstNode {
            node: Expr::Literal {
                value: BaseValue::Nil,
            },
            source_slice: start_slice.clone(),
        };
        if self.peek().token_type == TokenType::Equal {
            self.advance();
            value = self.expression()?;
        }
        let semicolon = self.consume(
            TokenType::Semicolon,
            "Expect ';' after variable declaration.",
        )?;
        let end_slice = semicolon.source_slice.clone();
        let combined_slice = SourceSlice::from_positions(
            start_slice.source_id,
            start_slice.start_position,
            end_slice.end_position,
        );
        Ok(AstNode::new(
            Stmt::VarDeclaration {
                name: name_lexeme,
                initializer: Some(Box::new(value)),
                return_value: Box::new(BaseValue::Nil),
            },
            combined_slice,
        ))
    }
    fn assignment_statement(&mut self) -> LoxResult<AstNode<Stmt>> {
        let start_slice = self.peek().source_slice.clone();
        let name = self.consume(TokenType::Identifier, "Expect variable name.")?;
        let name_lexeme = name.lexeme.clone();
        self.consume(TokenType::Equal, "Expect '=' after variable name.")?;
        let value = self.expression()?;
        let semicolon = self.consume(
            TokenType::Semicolon,
            "Expect ';' after variable declaration.",
        )?;
        let end_slice = semicolon.source_slice.clone();
        let combined_slice = SourceSlice::from_positions(
            start_slice.source_id,
            start_slice.start_position,
            end_slice.end_position,
        );
        Ok(AstNode::new(
            Stmt::VarAssigment {
                name: name_lexeme,
                value: Box::new(value),
                return_value: Box::new(BaseValue::Nil),
            },
            combined_slice,
        ))
    }

    fn print_statement(&mut self) -> LoxResult<AstNode<Stmt>> {
        // consume the print keyword
        let start_slice = self.peek().source_slice.clone();
        self.advance();
        let expr = self.expression()?;
        let semicolon = self.consume(TokenType::Semicolon, "Expect ';' after value.")?;
        let end_slice = semicolon.source_slice.clone();
        let combined_slice = SourceSlice::from_positions(
            start_slice.source_id,
            start_slice.start_position,
            end_slice.end_position,
        );
        Ok(AstNode::new(
            Stmt::Print {
                expression: Box::new(expr),
                return_value: Box::new(BaseValue::Nil),
            },
            combined_slice,
        ))
    }

    fn block_statement(&mut self, label: Option<String>) -> LoxResult<AstNode<Stmt>> {
        let start_slice = self.peek().source_slice.clone();
        self.advance();
        let mut statements = Vec::new();

        while self.peek().token_type != TokenType::EndBlock && !self.is_at_end() {
            let stmt = self.statement(label.clone())?;
            statements.push(stmt);
        }
        let end_token = self.consume(TokenType::EndBlock, "Expect 'end' after block.")?;
        let end_slice = end_token.source_slice.clone();
        let combined_slice = SourceSlice::from_positions(
            start_slice.source_id,
            start_slice.start_position,
            end_slice.end_position,
        );
        let label_str = if let Some(label) = label {
            label
        } else {
            String::default()
        };
        Ok(AstNode::new(
            Stmt::Block {
                statements: Box::new(statements),
                label: label_str,
                return_value: Box::new(BaseValue::Nil),
            },
            combined_slice,
        ))
    }

    fn expression_statement(&mut self) -> LoxResult<AstNode<Stmt>> {
        let start_slice = self.peek().source_slice.clone();
        let expr = self.expression()?;
        if self.peek().token_type == TokenType::Semicolon {
            let semicolon = self.advance();
            let end_slice = semicolon.source_slice.clone();
            let combined_slice = SourceSlice::from_positions(
                start_slice.source_id,
                start_slice.start_position,
                end_slice.end_position,
            );
            return Ok(AstNode::new(
                Stmt::Expression {
                    expression: Box::new(expr),
                    return_value: Box::new(BaseValue::Nil),
                },
                combined_slice,
            ));
        }
        // Use the expression's source slice for expression statements without semicolon
        let expr_slice = expr.source_slice.clone();
        Ok(AstNode::new(
            Stmt::Expression {
                expression: Box::new(expr),
                return_value: Box::new(BaseValue::Nil),
            },
            expr_slice,
        ))
    }

    fn return_statement(&mut self, label: Option<String>) -> LoxResult<AstNode<Stmt>> {
        let start_slice = self.peek().source_slice.clone();
        self.advance();
        let expr = match self.expression() {
            Ok(expr) => expr,
            Err(LoxError::ParseError {
                message,
                source_slice,
            }) => {
                if message == "Expect expression." {
                    AstNode {
                        node: Expr::Literal {
                            value: BaseValue::Nil,
                        },
                        source_slice: start_slice.clone(),
                    }
                } else {
                    return Err(LoxError::ParseError {
                        message,
                        source_slice,
                    });
                }
            }
            Err(err) => return Err(err),
        };
        let semicolon = self.consume(TokenType::Semicolon, "Expect ';' after return value.")?;
        let end_slice = semicolon.source_slice.clone();
        let combined_slice = SourceSlice::from_positions(
            start_slice.source_id,
            start_slice.start_position,
            end_slice.end_position,
        );
        let label_str = if let Some(label) = label {
            label
        } else {
            String::default()
        };
        Ok(AstNode::new(
            Stmt::Return {
                expression: Box::new(expr),
                return_value: Box::new(BaseValue::Nil),
                label: label_str,
            },
            combined_slice,
        ))
    }

    fn expression(&mut self) -> LoxResult<AstNode<Expr>> {
        self.or_and()
    }

    fn or_and(&mut self) -> LoxResult<AstNode<Expr>> {
        let mut expr = self.logical_is()?;

        while [TokenType::Or, TokenType::And].contains(&self.peek().token_type) {
            let start_slice = expr.source_slice.clone();
            let operator = self.peek().token_type.clone();
            self.advance();
            let right = self.logical_is()?;
            let end_slice = right.source_slice.clone();
            let combined_slice = SourceSlice::from_positions(
                start_slice.source_id,
                start_slice.start_position,
                end_slice.end_position,
            );
            expr = AstNode::new(
                Expr::Binary {
                    left: Box::new(expr),
                    operator,
                    right: Box::new(right),
                },
                combined_slice,
            );
        }

        Ok(expr)
    }

    fn logical_is(&mut self) -> LoxResult<AstNode<Expr>> {
        let mut expr = self.equality()?;

        while self.peek().token_type == TokenType::Is {
            let start_slice = expr.source_slice.clone();
            let operator = self.peek().token_type.clone();
            self.advance();
            let right = self.equality()?;
            let end_slice = right.source_slice.clone();
            let combined_slice = SourceSlice::from_positions(
                start_slice.source_id,
                start_slice.start_position,
                end_slice.end_position,
            );
            expr = AstNode::new(
                Expr::Binary {
                    left: Box::new(expr),
                    operator,
                    right: Box::new(right),
                },
                combined_slice,
            );
        }

        Ok(expr)
    }

    fn equality(&mut self) -> LoxResult<AstNode<Expr>> {
        let mut expr = self.comparison()?;

        while [TokenType::BangEqual, TokenType::EqualEqual].contains(&self.peek().token_type) {
            let start_slice = expr.source_slice.clone();
            let operator = self.peek().token_type.clone();
            self.advance();
            let right = self.comparison()?;
            let end_slice = right.source_slice.clone();
            let combined_slice = SourceSlice::from_positions(
                start_slice.source_id,
                start_slice.start_position,
                end_slice.end_position,
            );
            expr = AstNode::new(
                Expr::Binary {
                    left: Box::new(expr),
                    operator,
                    right: Box::new(right),
                },
                combined_slice,
            );
        }

        Ok(expr)
    }

    fn comparison(&mut self) -> LoxResult<AstNode<Expr>> {
        let mut expr = self.term()?;

        while [
            TokenType::Greater,
            TokenType::GreaterEqual,
            TokenType::Less,
            TokenType::LessEqual,
        ]
        .contains(&self.peek().token_type)
        {
            let start_slice = expr.source_slice.clone();
            let operator = self.peek().token_type.clone();
            self.advance();
            let right = self.term()?;
            let end_slice = right.source_slice.clone();
            let combined_slice = SourceSlice::from_positions(
                start_slice.source_id,
                start_slice.start_position,
                end_slice.end_position,
            );
            expr = AstNode::new(
                Expr::Binary {
                    left: Box::new(expr),
                    operator,
                    right: Box::new(right),
                },
                combined_slice,
            );
        }

        Ok(expr)
    }

    fn term(&mut self) -> LoxResult<AstNode<Expr>> {
        let mut expr = self.factor()?;

        while [TokenType::Plus, TokenType::Minus].contains(&self.peek().token_type) {
            let start_slice = expr.source_slice.clone();
            let operator = self.peek().token_type.clone();
            self.advance();
            let right = self.factor()?;
            let end_slice = right.source_slice.clone();
            let combined_slice = SourceSlice::from_positions(
                start_slice.source_id,
                start_slice.start_position,
                end_slice.end_position,
            );
            expr = AstNode::new(
                Expr::Binary {
                    left: Box::new(expr),
                    operator,
                    right: Box::new(right),
                },
                combined_slice,
            );
        }

        Ok(expr)
    }

    fn factor(&mut self) -> LoxResult<AstNode<Expr>> {
        let mut expr = self.unary()?;

        while [TokenType::Slash, TokenType::Star].contains(&self.peek().token_type) {
            let start_slice = expr.source_slice.clone();
            let operator = self.peek().token_type.clone();
            self.advance();
            let right = self.unary()?;
            let end_slice = right.source_slice.clone();
            let combined_slice = SourceSlice::from_positions(
                start_slice.source_id,
                start_slice.start_position,
                end_slice.end_position,
            );
            expr = AstNode::new(
                Expr::Binary {
                    left: Box::new(expr),
                    operator,
                    right: Box::new(right),
                },
                combined_slice,
            );
        }

        Ok(expr)
    }

    fn unary(&mut self) -> LoxResult<AstNode<Expr>> {
        if [TokenType::Bang, TokenType::Minus].contains(&self.peek().token_type) {
            let operator = self.peek().token_type.clone();
            let source_slice = self.peek().source_slice.clone();
            self.advance();
            let right = self.unary()?;
            return Ok(AstNode::new(
                Expr::Unary {
                    operator,
                    operand: Box::new(right),
                },
                source_slice,
            ));
        }

        self.call()
    }

    fn call(&mut self) -> LoxResult<AstNode<Expr>> {
        let mut expr = self.primary()?;

        loop {
            if self.peek().token_type == TokenType::LeftParen {
                expr = self.finish_call(expr)?;
            } else {
                break;
            }
        }

        Ok(expr)
    }
    fn finish_call(&mut self, callee: AstNode<Expr>) -> LoxResult<AstNode<Expr>> {
        let start_slice = callee.source_slice.start_position.clone();
        self.advance(); // Consume '('

        let mut arguments = Vec::new();

        if self.peek().token_type != TokenType::RightParen {
            loop {
                arguments.push(self.expression()?);
                if self.peek().token_type == TokenType::Comma {
                    self.advance();
                } else {
                    break;
                }
            }
        }

        self.consume(TokenType::RightParen, "Expect ')' after arguments.")?;

        // Estendi il source_slice per includere le parentesi di chiusura
        let end_slice = self.previous().source_slice.end_position.clone();
        let full_slice = SourceSlice {
            source_id: callee.source_slice.source_id.clone(),
            start_position: start_slice.clone(),
            end_position: end_slice.clone(),
        };

        Ok(AstNode::new(
            Expr::Call {
                callee: Box::new(callee),
                arguments,
            },
            full_slice,
        ))
    }

    fn primary(&mut self) -> LoxResult<AstNode<Expr>> {
        match self.peek().token_type {
            TokenType::False => {
                let source_slice = self.peek().source_slice.clone();
                self.advance();
                Ok(AstNode::new(
                    Expr::Literal {
                        value: BaseValue::Boolean(false),
                    },
                    source_slice,
                ))
            }
            TokenType::True => {
                let source_slice = self.peek().source_slice.clone();
                self.advance();
                Ok(AstNode::new(
                    Expr::Literal {
                        value: BaseValue::Boolean(true),
                    },
                    source_slice,
                ))
            }
            TokenType::Nil => {
                let source_slice = self.peek().source_slice.clone();
                self.advance();
                Ok(AstNode::new(
                    Expr::Literal {
                        value: BaseValue::Nil,
                    },
                    source_slice,
                ))
            }
            TokenType::Number => {
                let source_slice = self.peek().source_slice.clone();
                let literal = self.peek().literal.clone();
                self.advance();
                if let Some(literal) = literal {
                    Ok(AstNode::new(Expr::Literal { value: literal }, source_slice))
                } else {
                    parse_error(self.peek().source_slice.clone(), "Expected number literal")
                }
            }
            TokenType::String => {
                let source_slice = self.peek().source_slice.clone();
                let literal = self.peek().literal.clone();
                self.advance();
                if let Some(literal) = literal {
                    Ok(AstNode::new(Expr::Literal { value: literal }, source_slice))
                } else {
                    parse_error(self.peek().source_slice.clone(), "Expected string literal")
                }
            }
            TokenType::LeftParen => {
                let source_slice = self.peek().source_slice.clone();
                self.advance();
                let expr = self.expression()?;
                self.consume(TokenType::RightParen, "Expect ')' after expression.")?;
                Ok(AstNode::new(
                    Expr::Grouping {
                        expression: Box::new(expr),
                    },
                    source_slice,
                ))
            }
            TokenType::Identifier => {
                let name = self.peek().lexeme.clone();
                let source_slice = self.peek().source_slice.clone();
                self.advance();
                Ok(AstNode::new(Expr::Identifier { name }, source_slice))
            }
            _ => parse_error(self.peek().source_slice.clone(), "Expect expression."),
        }
    }

    fn is_at_end(&self) -> bool {
        if self.current >= self.tokens.len() {
            true
        } else {
            self.tokens[self.current].token_type == TokenType::Eof
        }
    }

    fn advance(&mut self) -> &Token {
        if !self.is_at_end() {
            self.current += 1;
        }
        self.previous()
    }

    fn peek(&self) -> &Token {
        if self.is_at_end() {
            &self.tokens.last().unwrap()
        } else {
            &self.tokens[self.current]
        }
    }

    fn peek_next(&self) -> &Token {
        if self.is_at_end() {
            &self.peek()
        } else {
            &self.tokens[self.current + 1]
        }
    }

    fn previous(&self) -> &Token {
        if self.current == 0 {
            &self.tokens[0]
        } else {
            &self.tokens[self.current - 1]
        }
    }

    fn consume(&mut self, token_type: TokenType, message: &str) -> LoxResult<&Token> {
        if self.peek().token_type == token_type {
            self.advance();
            Ok(self.previous())
        } else {
            return parse_error(self.peek().source_slice.clone(), message);
        }
    }

    fn synchronize(&mut self) {
        self.advance();

        while !self.is_at_end() {
            if self.previous().token_type == TokenType::Semicolon {
                return;
            }

            match self.peek().token_type {
                TokenType::Class
                | TokenType::Fun
                | TokenType::For
                | TokenType::If
                | TokenType::While
                | TokenType::Print
                | TokenType::Return => return,
                _ => {}
            }

            self.advance();
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::common::base_value::Number;
    use crate::frontend::lexer::Lexer;

    /// Lex and parse `src`, returning the parser's result.
    fn parse_source(src: &str) -> LoxResult<Vec<AstNode<Stmt>>> {
        let tokens = Lexer::new(src.to_string(), 0)
            .scans_tokens()
            .expect("source should lex without errors");
        Parser::new(tokens).parse()
    }

    /// Parse `src`, panicking if parsing fails.
    fn parse_ok(src: &str) -> Vec<AstNode<Stmt>> {
        parse_source(src).expect("expected source to parse")
    }

    /// Extract the inner expression of an expression statement.
    fn expression_of(stmt: &AstNode<Stmt>) -> &AstNode<Expr> {
        match &stmt.node {
            Stmt::Expression { expression, .. } => expression,
            other => panic!("expected expression statement, got {:?}", other),
        }
    }

    #[test]
    fn parses_number_literal_expression() {
        let stmts = parse_ok("42;");
        assert_eq!(stmts.len(), 1);
        match &expression_of(&stmts[0]).node {
            Expr::Literal { value } => {
                assert_eq!(*value, BaseValue::Number(Number::I32(42)));
            }
            other => panic!("expected literal, got {:?}", other),
        }
    }

    #[test]
    fn respects_multiplication_precedence_over_addition() {
        // 1 + 2 * 3 should parse as 1 + (2 * 3)
        let stmts = parse_ok("1 + 2 * 3;");
        match &expression_of(&stmts[0]).node {
            Expr::Binary {
                operator, right, ..
            } => {
                assert_eq!(*operator, TokenType::Plus);
                match &right.node {
                    Expr::Binary { operator, .. } => {
                        assert_eq!(*operator, TokenType::Star)
                    }
                    other => panic!("expected nested binary, got {:?}", other),
                }
            }
            other => panic!("expected binary expression, got {:?}", other),
        }
    }

    #[test]
    fn parses_unary_negation() {
        let stmts = parse_ok("-5;");
        match &expression_of(&stmts[0]).node {
            Expr::Unary { operator, .. } => assert_eq!(*operator, TokenType::Minus),
            other => panic!("expected unary expression, got {:?}", other),
        }
    }

    #[test]
    fn parses_grouping_to_change_precedence() {
        // (1 + 2) * 3 should have a grouping on the left of the multiply.
        let stmts = parse_ok("(1 + 2) * 3;");
        match &expression_of(&stmts[0]).node {
            Expr::Binary { operator, left, .. } => {
                assert_eq!(*operator, TokenType::Star);
                assert!(matches!(left.node, Expr::Grouping { .. }));
            }
            other => panic!("expected binary expression, got {:?}", other),
        }
    }

    #[test]
    fn parses_comparison_expression() {
        let stmts = parse_ok("1 < 2;");
        match &expression_of(&stmts[0]).node {
            Expr::Binary { operator, .. } => assert_eq!(*operator, TokenType::Less),
            other => panic!("expected binary expression, got {:?}", other),
        }
    }

    #[test]
    fn parses_print_statement() {
        let stmts = parse_ok("print 1;");
        assert!(matches!(stmts[0].node, Stmt::Print { .. }));
    }

    #[test]
    fn parses_var_declaration_with_initializer() {
        let stmts = parse_ok("var x: Int = 5;");
        match &stmts[0].node {
            Stmt::VarDeclaration {
                name, initializer, ..
            } => {
                assert_eq!(name, "x");
                assert!(initializer.is_some());
            }
            other => panic!("expected var declaration, got {:?}", other),
        }
    }

    #[test]
    fn parses_assignment_statement() {
        let stmts = parse_ok("x = 5;");
        match &stmts[0].node {
            Stmt::VarAssigment { name, .. } => assert_eq!(name, "x"),
            other => panic!("expected assignment statement, got {:?}", other),
        }
    }

    #[test]
    fn parses_block_statement() {
        let stmts = parse_ok("do print 1; print 2; end");
        match &stmts[0].node {
            Stmt::Block { statements, .. } => assert_eq!(statements.len(), 2),
            other => panic!("expected block statement, got {:?}", other),
        }
    }

    #[test]
    fn parses_if_statement() {
        let stmts = parse_ok("if true then print 1;");
        assert!(matches!(stmts[0].node, Stmt::If { .. }));
    }

    #[test]
    fn parses_while_statement() {
        let stmts = parse_ok("while true do print 1; end");
        assert!(matches!(stmts[0].node, Stmt::While { .. }));
    }

    #[test]
    fn errors_on_missing_semicolon_after_print() {
        assert!(parse_source("print 1").is_err());
    }

    #[test]
    fn errors_on_unclosed_grouping() {
        assert!(parse_source("(1 + 2;").is_err());
    }
}