Update int object to cache values between -1 and 255 by default.

This adds the `Int::new_obj_uncached()` function, which will create a new Int
object without caching it. The `Int::new_obj()` will cache the value if
it inclusively lies between -1 and 255. This is a minor optimization
that should hopefully go a long way with speed and memory footprint.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/obj/int.rs

@@ -4,11 +4,21 @@ use crate::{
 };
 use once_cell::sync::Lazy;
 use shredder::Scan;
-use std::fmt::{self, Debug, Formatter};
+use std::{collections::HashMap, fmt::{self, Debug, Formatter}, sync::Mutex};
 
+/// Integer object ref type alias.
 pub type IntRef = ObjRef<Int>;
+
+/// The native integer value type alias.
 pub type IntValue = i128;
 
+const INT_MIN_CACHE: IntValue = -1;
+const INT_MAX_CACHE: IntValue = 255;
+static INT_VALUE_CACHE: Lazy<Mutex<HashMap<IntValue, IntRef>>> = Lazy::new(||
+    Default::default()
+);
+
+/// Integer object.
 #[derive(Scan)]
 pub struct Int {
     value: IntValue,
@@ -17,8 +27,25 @@ pub struct Int {
 }
 
 impl Int {
-    pub fn new_obj(value: IntValue) -> ObjRef<Self> {
+    /// Create a newly allocated integer object on the heap.
-        // TODO(cache) : cache int values
+    ///
+    /// This will look up the integer in the local cache of integers and return the cached value.
+    pub fn new_obj(value: IntValue) -> IntRef {
+        // before making the int value, check if it exists in the cache
+        if value >= INT_MIN_CACHE && value <= INT_MAX_CACHE {
+            let mut cache = INT_VALUE_CACHE.lock().unwrap();
+            cache.entry(value)
+                .or_insert_with(|| Int::new_obj_uncached(value))
+                .clone()
+        } else {
+            Int::new_obj_uncached(value)
+        }
+    }
+
+    /// Create a newly allocated integer object on the heap.
+    ///
+    /// This will look up the integer in the local cache of integers.
+    pub fn new_obj_uncached(value: IntValue) -> IntRef {
         self_referring_obj! {
             Self {
                 value,