Use a linked list for the Env

crates/uplc/Cargo.toml

@@ -26,7 +26,7 @@ secp256k1 = "0.30.0"
 thiserror = "1.0.63"
 
 [dev-dependencies]
-criterion = { version = "0.5.1", features = ["html_reports"] }
+criterion = { version = "0.7.0", features = ["html_reports"] }
 insta = "1.40.0"
 itertools = "0.13.0"
 ouroboros = "0.18.4"

crates/uplc/src/machine/env.rs

@@ -1,31 +1,52 @@
-use bumpalo::{collections::Vec as BumpVec, Bump};
+use bumpalo::Bump;
 
 use crate::binder::Eval;
 
 use super::value::Value;
 
 #[derive(Debug)]
-pub struct Env<'a, V>(BumpVec<'a, &'a Value<'a, V>>)
+pub enum Env<'a, V>
 where
-    V: Eval<'a>;
+    V: Eval<'a>,
+{
+    Empty,
+    Cons {
+        data: &'a Value<'a, V>,
+        next: &'a Env<'a, V>,
+    },
+}
 
 impl<'a, V> Env<'a, V>
 where
     V: Eval<'a>,
 {
     pub fn new_in(arena: &'a Bump) -> &'a Self {
-        arena.alloc(Self(BumpVec::new_in(arena)))
+        arena.alloc(Self::Empty)
     }
 
-    pub fn push(&'a self, arena: &'a Bump, argument: &'a Value<'a, V>) -> &'a Self {
-        let mut new_env = self.0.clone();
-
-        new_env.push(argument);
-
-        arena.alloc(Self(new_env))
+    pub fn push(&'a self, arena: &'a Bump, arg: &'a Value<'a, V>) -> &'a Self {
+        arena.alloc(Self::Cons {
+            data: arg,
+            next: self,
+        })
     }
 
-    pub fn lookup(&'a self, name: usize) -> Option<&'a Value<'a, V>> {
-        self.0.get(self.0.len() - name).copied()
+    // De Bruijn indices are 1-based
+    // So the data at the env[i] is at De Bruijn index i-1
+    pub fn lookup(&self, index: usize) -> Option<&'a Value<'a, V>> {
+        if index == 0 {
+            return None;
+        }
+
+        match self {
+            Env::Empty => None,
+            Env::Cons { data, next: parent } => {
+                if index == 1 {
+                    return Some(data);
+                }
+
+                parent.lookup(index - 1)
+            }
+        }
     }
 }