From bc4541a50e9e3bc413357982f23dd221321940b7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Emmanuel=20Beno=C3=AEt?= <tseeker@nocternity.net>
Date: Sat, 31 Dec 2022 13:41:44 +0100
Subject: [PATCH] Interpreter - Initial implementation of the expression
 evaluator

---
 src/interpreter.rs | 149 ++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 148 insertions(+), 1 deletion(-)

diff --git a/src/interpreter.rs b/src/interpreter.rs
index 4f531c7..d4476cd 100644
--- a/src/interpreter.rs
+++ b/src/interpreter.rs
@@ -1,8 +1,155 @@
+use crate::{
+    ast,
+    errors::InterpreterError,
+    tokens::{Token, TokenType},
+};
+
 /// A value being handled by the interpreter.
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, PartialEq)]
 pub enum Value {
     Nil,
     Boolean(bool),
     String(String),
     Number(f64),
 }
+
+impl Value {
+    /// Check whether a value is truthy or not.
+    pub fn is_truthy(&self) -> bool {
+        if self == &Value::Nil {
+            false
+        } else if let Value::Boolean(b) = self {
+            *b
+        } else {
+            true
+        }
+    }
+}
+
+/// An Interpretable can be evaluated and will return a value.
+pub trait Interpretable {
+    fn interprete(&self) -> Result<Value, InterpreterError>;
+}
+
+
+/* -------------------------------- *
+ * INTERPRETER FOR EXPRESSION NODES *
+ * -------------------------------- */
+
+impl Interpretable for ast::ExprNode {
+    fn interprete(&self) -> Result<Value, InterpreterError> {
+        match self {
+            ast::ExprNode::Binary {
+                left,
+                operator,
+                right,
+            } => self.on_binary(left, operator, right),
+            ast::ExprNode::Unary { operator, right } => self.on_unary(operator, right),
+            ast::ExprNode::Grouping { expression } => expression.interprete(),
+            ast::ExprNode::Litteral { value } => self.on_litteral(value),
+        }
+    }
+}
+
+impl ast::ExprNode {
+    /// Evaluate a binary operator.
+    fn on_binary(
+        &self,
+        left: &ast::ExprNode,
+        operator: &Token,
+        right: &ast::ExprNode,
+    ) -> Result<Value, InterpreterError> {
+        let left_value = left.interprete()?;
+        let right_value = right.interprete()?;
+        match operator.token_type {
+            TokenType::Plus => match (left_value, right_value) {
+                (Value::Number(a), Value::Number(b)) => Ok(Value::Number(a + b)),
+                (Value::String(a), Value::String(b)) => Ok(Value::String(a + &b)),
+                _ => Err(InterpreterError::new(operator, "type error")),
+            },
+
+            TokenType::Minus => match (left_value, right_value) {
+                (Value::Number(a), Value::Number(b)) => Ok(Value::Number(a - b)),
+                _ => Err(InterpreterError::new(operator, "type error")),
+            },
+
+            TokenType::Star => match (left_value, right_value) {
+                (Value::Number(a), Value::Number(b)) => Ok(Value::Number(a * b)),
+                (Value::String(a), Value::Number(b)) => Ok(Value::String(a.repeat(b as usize))),
+                _ => Err(InterpreterError::new(operator, "type error")),
+            },
+
+            TokenType::Slash => match (left_value, right_value) {
+                (Value::Number(a), Value::Number(b)) => {
+                    if b == 0. {
+                        Err(InterpreterError::new(operator, "division by zero"))
+                    } else {
+                        Ok(Value::Number(a / b))
+                    }
+                }
+                _ => Err(InterpreterError::new(operator, "type error")),
+            },
+
+            TokenType::Greater => match (left_value, right_value) {
+                (Value::Number(a), Value::Number(b)) => Ok(Value::Boolean(a > b)),
+                _ => Err(InterpreterError::new(operator, "type error")),
+            },
+
+            TokenType::GreaterEqual => match (left_value, right_value) {
+                (Value::Number(a), Value::Number(b)) => Ok(Value::Boolean(a >= b)),
+                _ => Err(InterpreterError::new(operator, "type error")),
+            },
+
+            TokenType::Less => match (left_value, right_value) {
+                (Value::Number(a), Value::Number(b)) => Ok(Value::Boolean(a < b)),
+                _ => Err(InterpreterError::new(operator, "type error")),
+            },
+
+            TokenType::LessEqual => match (left_value, right_value) {
+                (Value::Number(a), Value::Number(b)) => Ok(Value::Boolean(a <= b)),
+                _ => Err(InterpreterError::new(operator, "type error")),
+            },
+
+            TokenType::Equal => Ok(Value::Boolean(left_value == right_value)),
+            TokenType::BangEqual => Ok(Value::Boolean(left_value != right_value)),
+
+            _ => panic!(
+                "Unsupported token type for binary operator (token {:?})",
+                operator
+            ),
+        }
+    }
+
+    /// Evaluate an unary operator.
+    fn on_unary(&self, operator: &Token, right: &ast::ExprNode) -> Result<Value, InterpreterError> {
+        let right_value = right.interprete()?;
+        match operator.token_type {
+            TokenType::Minus => {
+                if let Value::Number(n) = right_value {
+                    Ok(Value::Number(-n))
+                } else {
+                    Err(InterpreterError::new(operator, "number expected"))
+                }
+            }
+
+            TokenType::Bang => Ok(Value::Boolean(!right_value.is_truthy())),
+
+            _ => panic!(
+                "Unsupported token type for unary operator (token {:?})",
+                operator
+            ),
+        }
+    }
+
+    /// Evaluate a litteral.
+    fn on_litteral(&self, value: &Token) -> Result<Value, InterpreterError> {
+        match &value.token_type {
+            TokenType::Nil => Ok(Value::Nil),
+            TokenType::True => Ok(Value::Boolean(true)),
+            TokenType::False => Ok(Value::Boolean(false)),
+            TokenType::Number(n) => Ok(Value::Number(*n)),
+            TokenType::String(s) => Ok(Value::String(s.clone())),
+            _ => panic!("Unsupported token type for litteral (token {:?})", value),
+        }
+    }
+}