add support for `smallvec`

Cargo.lock

···
 version = 3
 
 [[package]]
+name = "smallvec"
+version = "1.13.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3c5e1a9a646d36c3599cd173a41282daf47c44583ad367b8e6837255952e5c67"
+
+[[package]]
 name = "um"
 version = "0.1.0"
+dependencies = [
+ "smallvec",
+]

Cargo.toml

···
 version = "0.1.0"
 edition = "2021"
 
+[dependencies]
+smallvec = { version = "1.13.2", optional = true }
+
 [features]
 default = ["reclaim-memory"]
 reclaim-memory = []
+smallvec = ["dep:smallvec"]

src/main.rs

···
+#[cfg(feature = "smallvec")]
+use smallvec::SmallVec;
+use std::time::Instant;
+
+#[cfg(feature = "smallvec")]
+const SMALLVEC_SIZE: usize = 24;
+
 fn main() {
     let mut program = Vec::new();
     for arg in std::env::args().skip(1) {
···
 pub struct Um {
     program_counter: Platter,
     registers: [Platter; 8],
+    #[cfg(feature = "smallvec")]
+    memory: Vec<SmallVec<[Platter; SMALLVEC_SIZE]>>,
+    #[cfg(not(feature = "smallvec"))]
     memory: Vec<Vec<Platter>>,
     #[cfg(feature = "reclaim-memory")]
     free_blocks: Vec<Platter>,
···
     pub fn new(program: Vec<Platter>) -> Self {
         let ops = decode_ops(&program);
         Self {
-            memory: vec![program],
+            memory: vec![program.into()],
             ops,
             ..Default::default()
         }
···
 
     #[cfg(not(feature = "reclaim-memory"))]
     fn allocate_memory(&mut self, length: Platter) -> Platter {
-        self.memory.push(vec![0; length as usize]);
+        self.memory.push(Self::new_block(length as usize));
         (self.memory.len() - 1) as Platter
     }
 
     #[cfg(feature = "reclaim-memory")]
     fn allocate_memory(&mut self, length: Platter) -> Platter {
         if let Some(index) = self.free_blocks.pop() {
-            self.memory[index as usize] = vec![0; length as usize];
+            self.memory[index as usize] = Self::new_block(length as usize);
             index as Platter
         } else {
-            self.memory.push(vec![0; length as usize]);
+            self.memory.push(Self::new_block(length as usize));
             (self.memory.len() - 1) as Platter
         }
     }
···
         #[cfg(feature = "reclaim-memory")]
         {
             self.free_blocks.push(block);
-            self.memory[block as usize] = vec![];
+            self.memory[block as usize] = Self::new_block(0);
         }
     }
 
···
             "universal machine failure: instruction: {:08x}, program_counter: {:08x}, registers: {:08x?}",
             self.memory[0][self.program_counter as usize], self.program_counter, self.registers
         )
+    }
+
+    #[cfg(feature = "smallvec")]
+    fn new_block(len: usize) -> SmallVec<[Platter; SMALLVEC_SIZE]> {
+        smallvec::smallvec![0; len]
+    }
+
+    #[cfg(not(feature = "smallvec"))]
+    fn new_block(len: usize) -> Vec<Platter> {
+        vec![0; len]
     }
 }