implement chapter 11 in rust

rust/rox/src/environment.rs

@@ -8,6 +8,8 @@ use crate::{token::Token, value::Value};
 pub enum EnvironmentError {
     #[error("line {0}: undefined variable: {1}")]
     UndefinedVariable(usize, String),
+    #[error("invalid environment distance")]
+    InvalidDistance,
 }
 
 /// Environment mapping variable names to their respective values. Can have an optional enclosing
@@ -50,6 +52,18 @@ impl Environment {
         }
     }
 
+    pub fn assign_at(
+        &mut self,
+        distance: usize,
+        token: &Token,
+        value: Value,
+    ) -> Result<(), EnvironmentError> {
+        self.ancestor(distance)?
+            .values
+            .insert(token.lexeme.clone(), value);
+        Ok(())
+    }
+
     /// Get the value of an existing (defined) variable. Error if there is no such variable in
     /// this environment or any of the enclosing ones.
     pub fn get(&self, token: &Token) -> Result<Value, EnvironmentError> {
@@ -64,4 +78,29 @@ impl Environment {
             ))
         }
     }
+
+    pub fn get_at(&self, distance: usize, token: &Token) -> Result<Value, EnvironmentError> {
+        if let Some(v) = self.ancestor(distance)?.values.get(token.lexeme.as_str()) {
+            Ok(v.clone())
+        } else {
+            Err(EnvironmentError::UndefinedVariable(
+                token.line,
+                token.lexeme.clone(),
+            ))
+        }
+    }
+
+    fn ancestor(&self, distance: usize) -> Result<Environment, EnvironmentError> {
+        let mut environment = self.clone();
+
+        for _ in 0..distance {
+            let enclosing = match &environment.enclosing {
+                Some(enc) => enc.borrow().clone(),
+                None => return Err(EnvironmentError::InvalidDistance),
+            };
+            environment = enclosing;
+        }
+
+        Ok(environment)
+    }
 }