From 1ec86d4845ea4d613408f59debe34840a9f95df3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Vili=20Sinerv=C3=A4?= <vili.m.sinerva@gmail.com>
Date: Tue, 4 Feb 2025 00:34:56 +0200
Subject: [PATCH] Refactor symbol table to be non-horrible :D

---
 src/compiler/interpreter.rs | 22 +++++---------------
 src/compiler/symtab.rs      | 41 ++++++++++++++++++++++++++-----------
 2 files changed, 34 insertions(+), 29 deletions(-)

diff --git a/src/compiler/interpreter.rs b/src/compiler/interpreter.rs
index 888a648..bbc9a48 100644
--- a/src/compiler/interpreter.rs
+++ b/src/compiler/interpreter.rs
@@ -1,5 +1,3 @@
-use std::collections::HashMap;
-
 use crate::compiler::{
     ast::Expression::{self, *},
     symtab::SymTab,
@@ -13,7 +11,7 @@ pub struct Interpreter<'source> {
 impl<'source> Interpreter<'source> {
     pub fn new() -> Self {
         Interpreter {
-            symbols: SymTab::new_global(),
+            symbols: SymTab::new(),
         }
     }
 
@@ -89,10 +87,8 @@ impl<'source> Interpreter<'source> {
                 }
             },
             VarDeclaration(_, name, expr) => {
-                let value = self.interpret(expr);
-                if self.symbols.locals.insert(name, value).is_some() {
-                    panic!("Variable {} already defined in this scope!", name)
-                }
+                let val = self.interpret(expr);
+                self.symbols.insert(name, val);
                 Value::None()
             }
             Conditional(_, condition_expr, then_expr, else_expr) => {
@@ -137,22 +133,14 @@ impl<'source> Interpreter<'source> {
                 function(&arg_values)
             }
             Block(_, expressions) => {
-                self.symbols = SymTab {
-                    locals: HashMap::new(),
-                    parent: Some(Box::new(std::mem::take(&mut self.symbols))),
-                };
+                self.symbols.push_level();
 
                 let mut val = Value::None();
                 for expression in expressions {
                     val = self.interpret(expression);
                 }
 
-                if let Some(symbols) = &mut self.symbols.parent {
-                    self.symbols = std::mem::take(symbols);
-                } else {
-                    panic!("Non-global symbol table without parent!");
-                }
-
+                self.symbols.remove_level();
                 val
             }
         }
diff --git a/src/compiler/symtab.rs b/src/compiler/symtab.rs
index 1f90f20..0ccd020 100644
--- a/src/compiler/symtab.rs
+++ b/src/compiler/symtab.rs
@@ -3,23 +3,21 @@ use std::collections::HashMap;
 
 #[derive(Default)]
 pub struct SymTab<'source> {
-    pub locals: HashMap<&'source str, Value>,
-    pub parent: Option<Box<SymTab<'source>>>,
+    tables: Vec<HashMap<&'source str, Value>>,
 }
 
 impl<'source> SymTab<'source> {
     pub fn get(&mut self, symbol: &str) -> &mut Value {
-        if let Some(val) = self.locals.get_mut(symbol) {
-            val
-        } else if let Some(parent) = &mut self.parent {
-            parent.get(symbol)
-        } else {
-            panic!("No symbol {} found!", symbol);
+        for i in (0..self.tables.len()).rev() {
+            if self.tables[i].contains_key(symbol) {
+                return self.tables[i].get_mut(symbol).unwrap();
+            }
         }
+        panic!("No symbol {} found!", symbol);
     }
 
-    pub fn new_global() -> SymTab<'source> {
-        let locals = HashMap::from([
+    pub fn new() -> SymTab<'source> {
+        let globals = HashMap::from([
             ("+", Value::Func(Value::add)),
             ("*", Value::Func(Value::mul)),
             ("-", Value::Func(Value::sub)),
@@ -36,8 +34,27 @@ impl<'source> SymTab<'source> {
         ]);
 
         SymTab {
-            locals,
-            parent: None,
+            tables: vec![globals],
+        }
+    }
+
+    pub fn push_level(&mut self) {
+        self.tables.push(HashMap::new());
+    }
+
+    pub fn remove_level(&mut self) {
+        self.tables.pop();
+    }
+
+    pub fn insert(&mut self, name: &'source str, val: Value) {
+        if self
+            .tables
+            .last_mut()
+            .expect("Symbols table should never be empty!")
+            .insert(name, val)
+            .is_some()
+        {
+            panic!("Variable {} already defined in this scope!", name)
         }
     }
 }