Refactor: Use root_purity instead of global_purity

This commit refactors the purity analysis from using a
`HashMap<GlobalIdx, Purity>` to a `Vec<Purity>` indexed by
`GlobalIdx.0`.

This change simplifies the data structure and makes purity lookups more
efficient. The `Binder`'s `globals` field has also been removed as it
was not being used.

src/ast/compiler/analyzer.rs

@@ -6,7 +6,7 @@ use std::collections::{HashMap, HashSet};
 use std::rc::Rc;
 
 pub struct Analyzer<'a> {
-    global_purity: &'a HashMap<GlobalIdx, Purity>,
+    root_purity: &'a [Purity],
     /// Stack of currently visiting lambdas to detect direct recursion.
     lambda_stack: Vec<crate::ast::types::Identity>,
     /// Map of global index to its Lambda identity if known.
@@ -18,10 +18,10 @@ pub struct Analyzer<'a> {
 impl<'a> Analyzer<'a> {
     pub fn analyze(
         node: &TypedNode,
-        global_purity: &'a HashMap<GlobalIdx, Purity>,
+        root_purity: &'a [Purity],
     ) -> AnalyzedNode {
         let mut analyzer = Self {
-            global_purity,
+            root_purity,
             lambda_stack: Vec::new(),
             globals_to_lambdas: HashMap::new(),
             recursive_identities: HashSet::new(),
@@ -70,7 +70,7 @@ impl<'a> Analyzer<'a> {
             BoundKind::Get { addr, name } => {
                 let p = match addr {
                     Address::Global(idx) => {
-                        self.global_purity.get(idx).cloned().unwrap_or(Purity::Pure)
+                        self.root_purity.get(idx.0 as usize).cloned().unwrap_or(Purity::Pure)
                     }
                     _ => Purity::Pure,
                 };
@@ -206,8 +206,8 @@ impl<'a> Analyzer<'a> {
                     ..
                 } = &callee.kind
                 {
-                    self.global_purity
-                        .get(idx)
+                    self.root_purity
+                        .get(idx.0 as usize)
                         .cloned()
                         .unwrap_or(Purity::Impure)
                 } else {