diff --git a/src/lib.rs b/src/lib.rs
index fc97503d..a500ad4c 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -8,17 +8,17 @@ use std::thread;
 use fancy_regex::Regex;
 use pyo3::exceptions;
 use pyo3::prelude::*;
+use pyo3::pyclass;
 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],
+fn _byte_pair_merge(
     ranks: &HashMap<Vec<u8>, Rank>,
-    f: impl Fn(std::ops::Range<usize>) -> T,
-) -> Vec<T> {
+    piece: &[u8],
+) -> Vec<(usize, Rank)> {
     // 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.
@@ -93,25 +93,24 @@ fn _byte_pair_merge<T>(
             break;
         }
     }
-    let mut out: Vec<T> = Vec::with_capacity(parts.len() - 1);
-    for i in 0..parts.len() - 1 {
-        out.push(f(parts[i].0..parts[i + 1].0));
-    }
-    out
+
+    parts
 }
 
 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]])
+    assert!(piece.len() > 1);
+    _byte_pair_merge(&ranks, &piece)
+        .windows(2)
+        .map(|part| ranks[&piece[part[0].0..part[1].0]])
+        .collect()
 }
 
 pub fn byte_pair_split<'a>(piece: &'a [u8], ranks: &HashMap<Vec<u8>, Rank>) -> Vec<&'a [u8]> {
-    if piece.len() == 1 {
-        return vec![piece];
-    }
-    _byte_pair_merge(piece, ranks, |p| &piece[p.start..p.end])
+    assert!(piece.len() > 1);
+    _byte_pair_merge(&ranks, &piece)
+        .windows(2)
+        .map(|part| &piece[part[0].0..part[1].0])
+        .collect()
 }
 
 // Various performance notes: