diff --git a/src/lib.rs b/src/lib.rs
index 3d930655..fc97503d 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -2,28 +2,31 @@
 #![allow(clippy::borrow_deref_ref)]
 
 use std::collections::HashSet;
+use std::num::NonZeroU64;
 use std::thread;
 
 use fancy_regex::Regex;
 use pyo3::exceptions;
 use pyo3::prelude::*;
-use pyo3::types::{PyBytes, PyList, PyTuple};
 use pyo3::PyResult;
+use pyo3::types::{PyBytes, PyList, PyTuple};
 use rustc_hash::FxHashMap as HashMap;
 
+type Rank = u32;
+
 fn _byte_pair_merge<T>(
     piece: &[u8],
-    ranks: &HashMap<Vec<u8>, usize>,
+    ranks: &HashMap<Vec<u8>, Rank>,
     f: impl Fn(std::ops::Range<usize>) -> T,
 ) -> Vec<T> {
     // This is a vector of (start, rank).
     // The rank is of the byte pair starting at position start.
     // The rank of the last item in the vector is not a valid value.
-    let mut parts: Vec<(usize, usize)> = (0..piece.len() + 1).map(|i| (i, usize::MAX)).collect();
+    let mut parts: Vec<(usize, Rank)> = (0..piece.len() + 1).map(|i| (i, Rank::MAX)).collect();
 
     let get_rank = {
         #[inline(always)]
-        |parts: &Vec<(usize, usize)>, start_idx: usize, skip: usize| {
+        |parts: &Vec<(usize, Rank)>, start_idx: usize, skip: usize| {
             if (start_idx + skip + 2) < parts.len() {
                 ranks
                     .get(&piece[parts[start_idx].0..parts[start_idx + skip + 2].0])
@@ -39,8 +42,8 @@ fn _byte_pair_merge<T>(
     for i in 0..parts.len() - 2 {
         match get_rank(&parts, i, 0) {
             Some(rank) => {
-                // usize::MAX is a sentinel value and cannot be a valid rank
-                debug_assert!(rank != usize::MAX);
+                // Rank::MAX is a sentinel value and cannot be a valid rank
+                debug_assert!(rank != Rank::MAX);
                 parts[i].1 = rank;
             }
             None => {
@@ -63,16 +66,16 @@ fn _byte_pair_merge<T>(
             break;
         }
 
-        // usize::MAX is a sentinel rank value allowing us to
+        // Rank::MAX is a sentinel rank value allowing us to
         // take the min more quickly
-        let mut min_rank: (usize, usize) = (usize::MAX, 0);
+        let mut min_rank: (Rank, usize) = (Rank::MAX, 0);
         for (i, &(_, rank)) in parts[..parts.len() - 1].iter().enumerate() {
             if rank < min_rank.0 {
                 min_rank = (rank, i);
             }
         }
 
-        if min_rank.0 != usize::MAX {
+        if min_rank.0 != Rank::MAX {
             let i = min_rank.1;
 
             // NOTE: We are about to remove parts[i + 1]. We do not do it
@@ -80,9 +83,9 @@ fn _byte_pair_merge<T>(
             // parts[i] and parts[i-1] before removing, which could thrash
             // the cache. Thus, we update the rank calculation by skipping over
             // parts[i + 1], by invoking `get_rank!` with `skip = 1`.
-            parts[i].1 = get_rank(&parts, i, 1).unwrap_or(usize::MAX);
+            parts[i].1 = get_rank(&parts, i, 1).unwrap_or(Rank::MAX);
             if i > 0 {
-                parts[i - 1].1 = get_rank(&parts, i - 1, 1).unwrap_or(usize::MAX);
+                parts[i - 1].1 = get_rank(&parts, i - 1, 1).unwrap_or(Rank::MAX);
             }
 
             parts.remove(i + 1);
@@ -97,14 +100,14 @@ fn _byte_pair_merge<T>(
     out
 }
 
-pub fn byte_pair_encode(piece: &[u8], ranks: &HashMap<Vec<u8>, usize>) -> Vec<usize> {
+pub fn byte_pair_encode(piece: &[u8], ranks: &HashMap<Vec<u8>, Rank>) -> Vec<Rank> {
     if piece.len() == 1 {
         return vec![ranks[piece]];
     }
     _byte_pair_merge(piece, ranks, |p| ranks[&piece[p.start..p.end]])
 }
 
-pub fn byte_pair_split<'a>(piece: &'a [u8], ranks: &HashMap<Vec<u8>, usize>) -> Vec<&'a [u8]> {
+pub fn byte_pair_split<'a>(piece: &'a [u8], ranks: &HashMap<Vec<u8>, Rank>) -> Vec<&'a [u8]> {
     if piece.len() == 1 {
         return vec![piece];
     }
@@ -152,7 +155,6 @@ pub fn byte_pair_split<'a>(piece: &'a [u8], ranks: &HashMap<Vec<u8>, usize>) ->
 // The current implementation ends up doing a lot of hashing of bytes. In theory, this could be made
 // to be hashing of two-tuples of ints, which looks like it may also be a couple percent faster.
 
-use std::num::NonZeroU64;
 pub struct FakeThreadId(NonZeroU64);
 
 fn hash_current_thread() -> usize {
@@ -169,12 +171,13 @@ fn hash_current_thread() -> usize {
 }
 
 const MAX_NUM_THREADS: usize = 128;
+
 #[pyclass]
 struct CoreBPE {
-    encoder: HashMap<Vec<u8>, usize>,
-    special_tokens_encoder: HashMap<String, usize>,
-    decoder: HashMap<usize, Vec<u8>>,
-    special_tokens_decoder: HashMap<usize, Vec<u8>>,
+    encoder: HashMap<Vec<u8>, Rank>,
+    special_tokens_encoder: HashMap<String, Rank>,
+    decoder: HashMap<Rank, Vec<u8>>,
+    special_tokens_decoder: HashMap<Rank, Vec<u8>>,
     regex_tls: Vec<Regex>,
     special_regex_tls: Vec<Regex>,
     sorted_token_bytes: Vec<Vec<u8>>,
@@ -192,7 +195,7 @@ impl CoreBPE {
         &self.special_regex_tls[hash_current_thread() % MAX_NUM_THREADS]
     }
 
-    fn _decode_native(&self, tokens: &[usize]) -> Vec<u8> {
+    fn _decode_native(&self, tokens: &[Rank]) -> Vec<u8> {
         let mut ret = Vec::with_capacity(tokens.len() * 2);
         for token in tokens {
             let token_bytes = self
@@ -204,7 +207,7 @@ impl CoreBPE {
         ret
     }
 
-    fn _encode_ordinary_native(&self, text: &str) -> Vec<usize> {
+    fn _encode_ordinary_native(&self, text: &str) -> Vec<Rank> {
         // This is the core of the encoding logic; the other functions in here
         // just make things complicated :-)
         let regex = self._get_tl_regex();
@@ -220,7 +223,7 @@ impl CoreBPE {
         ret
     }
 
-    fn _encode_native(&self, text: &str, allowed_special: &HashSet<&str>) -> (Vec<usize>, usize) {
+    fn _encode_native(&self, text: &str, allowed_special: &HashSet<&str>) -> (Vec<Rank>, usize) {
         let special_regex = self._get_tl_special_regex();
         let regex = self._get_tl_regex();
         let mut ret = vec![];
@@ -278,9 +281,9 @@ impl CoreBPE {
 
     fn _increase_last_piece_token_len(
         &self,
-        tokens: Vec<usize>,
+        tokens: Vec<Rank>,
         mut last_piece_token_len: usize,
-    ) -> (Vec<usize>, usize) {
+    ) -> (Vec<Rank>, usize) {
         // Unfortunately, the locations where our regex splits can be unstable.
         // For the purposes of determining unstable tokens, unstable regex splitting
         // is only a problem if a split that was present disappears, since this can
@@ -319,7 +322,7 @@ impl CoreBPE {
         &self,
         text: &str,
         allowed_special: &HashSet<&str>,
-    ) -> (Vec<usize>, HashSet<Vec<usize>>) {
+    ) -> (Vec<Rank>, HashSet<Vec<Rank>>) {
         let (tokens, last_piece_token_len) = self._encode_native(text, allowed_special);
         if last_piece_token_len == 0 {
             // If last_piece_token_len is zero, the last token was a special token and we have
@@ -436,8 +439,8 @@ impl CoreBPE {
 impl CoreBPE {
     #[new]
     fn new(
-        encoder: HashMap<Vec<u8>, usize>,
-        special_tokens_encoder: HashMap<String, usize>,
+        encoder: HashMap<Vec<u8>, Rank>,
+        special_tokens_encoder: HashMap<String, Rank>,
         pattern: &str,
     ) -> PyResult<Self> {
         let regex = Regex::new(pattern)
@@ -452,7 +455,7 @@ impl CoreBPE {
                 .map_err(|e| PyErr::new::<exceptions::PyValueError, _>(e.to_string()))?
         };
 
-        let decoder: HashMap<usize, Vec<u8>> =
+        let decoder: HashMap<Rank, Vec<u8>> =
             encoder.iter().map(|(k, v)| (*v, k.clone())).collect();
 
         assert!(
@@ -460,7 +463,7 @@ impl CoreBPE {
             "Encoder and decoder must be of equal length; maybe you had duplicate token indices in your encoder?"
         );
 
-        let special_tokens_decoder: HashMap<usize, Vec<u8>> = special_tokens_encoder
+        let special_tokens_decoder: HashMap<Rank, Vec<u8>> = special_tokens_encoder
             .iter()
             .map(|(k, v)| (*v, k.as_bytes().to_vec()))
             .collect();
@@ -486,15 +489,15 @@ impl CoreBPE {
     // Encoding
     // ====================
 
-    fn encode_ordinary(&self, py: Python, text: &str) -> Vec<usize> {
+    fn encode_ordinary(&self, py: Python, text: &str) -> Vec<Rank> {
         py.allow_threads(|| self._encode_ordinary_native(text))
     }
 
-    fn encode(&self, py: Python, text: &str, allowed_special: HashSet<&str>) -> Vec<usize> {
+    fn encode(&self, py: Python, text: &str, allowed_special: HashSet<&str>) -> Vec<Rank> {
         py.allow_threads(|| self._encode_native(text, &allowed_special).0)
     }
 
-    fn _encode_bytes(&self, py: Python, bytes: &[u8]) -> Vec<usize> {
+    fn _encode_bytes(&self, py: Python, bytes: &[u8]) -> Vec<Rank> {
         py.allow_threads(|| {
             match std::str::from_utf8(bytes) {
                 Ok(text) => self._encode_ordinary_native(text),
@@ -534,7 +537,7 @@ impl CoreBPE {
         (tokens, py_completions).into_py(py)
     }
 
-    fn encode_single_token(&self, piece: &[u8]) -> PyResult<usize> {
+    fn encode_single_token(&self, piece: &[u8]) -> PyResult<Rank> {
         if let Some(token) = self.encoder.get(piece).copied() {
             return Ok(token);
         }
@@ -546,7 +549,7 @@ impl CoreBPE {
         Err(PyErr::new::<exceptions::PyKeyError, _>(piece.to_owned()))
     }
 
-    fn encode_single_piece(&self, piece: &[u8]) -> Vec<usize> {
+    fn encode_single_piece(&self, piece: &[u8]) -> Vec<Rank> {
         if let Some(token) = self.encoder.get(piece) {
             return vec![*token];
         }
@@ -557,12 +560,12 @@ impl CoreBPE {
     // Decoding
     // ====================
 
-    fn decode_bytes(&self, py: Python, tokens: Vec<usize>) -> Py<PyBytes> {
+    fn decode_bytes(&self, py: Python, tokens: Vec<Rank>) -> Py<PyBytes> {
         let bytes = py.allow_threads(|| self._decode_native(&tokens));
         PyBytes::new(py, &bytes).into()
     }
 
-    fn decode_single_token_bytes(&self, py: Python, token: usize) -> PyResult<Py<PyBytes>> {
+    fn decode_single_token_bytes(&self, py: Python, token: Rank) -> PyResult<Py<PyBytes>> {
         if let Some(bytes) = self.decoder.get(&token) {
             return Ok(PyBytes::new(py, bytes).into());
         }