crafting-interpreters/rust/rox/src/interpreter.rs

use thiserror::Error;

use crate::{
    expression::Expression,
    token::{Literal, Token, TokenType},
    value::Value,
};

#[derive(Error, Debug)]
pub enum InterpreterError {
    #[error("line {0}: MINUS unary expression expects a number on the right")]
    UnaryExpressionNotANumber(usize),
    #[error("line {0}: unknown unary operator: {1}")]
    UnaryOperatorUnknown(usize, String),
    #[error("line {0}: unknown binary operator: {1}")]
    BinaryOperatorUnknown(usize, String),
}

/// Try to evaluate an expression and return its result.
pub fn evaluate(expression: Expression) -> Result<Value, InterpreterError> {
    match expression {
        Expression::Literal { value } => literal(value),
        Expression::Grouping { expression } => grouping(*expression),
        Expression::Unary {
            operator: op,
            right,
        } => unary(op, *right),
        Expression::Binary {
            left,
            operator,
            right,
        } => binary(*left, operator, *right),
    }
}

/// Convert the literal value into a Value.
fn literal(literal: Literal) -> Result<Value, InterpreterError> {
    Ok(literal.into())
}

/// Evaluate the inner expression.
fn grouping(inner: Expression) -> Result<Value, InterpreterError> {
    evaluate(inner)
}

/// Evaluate the expression on the right and use its result when evaluating the unary operator.
fn unary(op: Token, right: Expression) -> Result<Value, InterpreterError> {
    let right = evaluate(right)?;

    match op.token_type {
        TokenType::Minus => {
            if let Value::Number(val) = right {
                Ok(Value::Number(-val))
            } else {
                Err(InterpreterError::UnaryExpressionNotANumber(op.line))
            }
        }
        TokenType::Bang => Ok(Value::Boolean(!right.is_truthy())),
        _ => Err(InterpreterError::UnaryOperatorUnknown(op.line, op.lexeme)),
    }
}

/// Calculate number operations.
fn number_op(left: f64, op: TokenType, right: f64) -> f64 {
    match op {
        TokenType::Minus => left - right,
        TokenType::Plus => left + right,
        TokenType::Slash => left / right,
        TokenType::Star => left * right,
        _ => unreachable!(),
    }
}

/// Calculate boolean operations.
fn boolean_op(left: f64, op: TokenType, right: f64) -> bool {
    match op {
        TokenType::Greater => left > right,
        TokenType::GreaterEqual => left >= right,
        TokenType::Less => left < right,
        TokenType::LessEqual => left <= right,
        _ => unreachable!(),
    }
}

/// Evaluate the left and right expressions (in that order) and then combine them with the
/// specified operator.
fn binary(left: Expression, op: Token, right: Expression) -> Result<Value, InterpreterError> {
    let left = evaluate(left)?;
    let right = evaluate(right)?;

    match op.token_type {
        TokenType::Minus | TokenType::Slash | TokenType::Star | TokenType::Plus => {
            if let (Value::Number(left), Value::Number(right)) = (left, right) {
                Ok(Value::Number(number_op(left, op.token_type, right)))
            } else {
                todo!()
            }
        }
        TokenType::Greater | TokenType::GreaterEqual | TokenType::Less | TokenType::LessEqual => {
            if let (Value::Number(left), Value::Number(right)) = (left, right) {
                Ok(Value::Boolean(boolean_op(left, op.token_type, right)))
            } else {
                todo!()
            }
        }
        TokenType::BangEqual => Ok(Value::Boolean(left != right)),
        TokenType::EqualEqual => Ok(Value::Boolean(left == right)),
        _ => Err(InterpreterError::BinaryOperatorUnknown(op.line, op.lexeme)),
    }
}