diff --git a/src/lib.rs b/src/lib.rs
index ab89657..d1d0cf1 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,7 +1,6 @@
-use pyo3::prelude::*;
 use external_sort::{ExternalSorter, ExternallySortable};
-use itertools::Itertools;
-use std::io::{BufRead, BufReader, Read, Write};
+use pyo3::prelude::*;
+use std::io::{BufRead, BufReader, BufWriter, Read, SeekFrom, Write};
 
 // Define a string structure that can be sorted externally
 #[derive(Serialize, Deserialize, Clone, PartialEq, Eq, PartialOrd, Ord)]
@@ -38,6 +37,34 @@ fn sort_lines(lines: Vec<String>) -> Vec<String> {
     lines
 }
 
+fn streaming_sort_until<'a, IN, OUT, STR>(
+    input: IN,
+    mut output: OUT,
+    end: &str,
+) -> std::io::Result<()>
+where
+    // can take &str or &String
+    STR: AsRef<str>,
+    IN: Iterator<Item = STR>,
+    OUT: Write,
+{
+    let input = input
+        .take_while(|l| l.as_ref() != end)
+        .map(|l| TsvLine::new(l.as_ref()));
+    // Do the external sort
+    let sorted = ExternalSorter::new(1000000, None)
+        .sort_by(input, |a, b| {
+            tsv_cmp(a.the_line.as_str(), b.the_line.as_str())
+        })
+        .unwrap();
+    // Write the sorted lines to the output file
+    for line in sorted {
+        writeln!(&mut output, "{}", line.unwrap().the_line)?;
+    }
+    writeln!(&mut output, "{end}")?;
+    Ok(())
+}
+
 /// Merge sort a range of lines from an input file and write the result to another file.
 ///
 /// The function `sort_file_lines` seeks to the given start position in the input file, reads
@@ -76,31 +103,22 @@ fn sort_lines(lines: Vec<String>) -> Vec<String> {
 ///
 #[pyfunction]
 fn sort_file_lines(input: &str, output: &str, start: u64, end: &str) -> PyResult<u64> {
-    // Open the input file and seek to the start position
-    let mut input_file = std::fs::File::open(input)?;
-    input_file.seek(std::io::SeekFrom::Start(start))?;
-    // Wrap the input file in a buffered reader
-    let mut input = BufReader::new(&mut input_file);
-    // Create an iterator which reads lines until the end marker and doesn't consume the end marker
-    let mut binding = input.by_ref().lines().peekable();
-    let lines = binding
-        .peeking_take_while(|line| line.as_ref().map(|l| l != end).unwrap_or(false))
-        .map(|line| TsvLine::new(&line.unwrap()));
-    // Do the external sort
-    let iter = ExternalSorter::new(1000000, None).sort_by(
-        lines,
-        |a, b| tsv_cmp(a.the_line.as_str(), b.the_line.as_str()),
-    ).unwrap();
-    // Write the sorted lines to the output file
-    let output_file = std::fs::File::create(output)?;
-    let mut output = std::io::BufWriter::new(output_file);
-    for line in iter {
-        writeln!(output, "{}", line.unwrap().the_line)?;
-    }
-    // Write the end marker (which was not consumed by peeking_take_while)
-    writeln!(output, "{}", binding.next().unwrap().unwrap())?;
+    // Open the input file with a buffer
+    let mut input = BufReader::new(File::open(input)?);
+
+    // Seek to the start position
+    input.seek(SeekFrom::Start(start))?;
+
+    // Open the output file
+    let mut output = BufWriter::new(File::create(output)?);
+
+    // Create the lines iterator
+    let lines = input.by_ref().lines().map(|l| l.unwrap());
+    streaming_sort_until(lines, output, end)?;
+    // sort_until(input.as_ref().lines(), &mut output)?;
+
     // return the stream position from the counting reader object
-    Ok(input.stream_position().unwrap())
+    Ok(input.stream_position().unwrap() - (end.bytes().len() as u64))
 }
 
 /// A Python module implemented in Rust.
@@ -413,4 +431,10 @@ mod tests {
             expected as i8,
         );
     }
+    #[test]
+    fn streaming_sort_smoke() {
+        let mut res = Vec::new();
+        streaming_sort_until("1\n3\n2\nEND".lines(), &mut res, "END").unwrap();
+        assert_eq!(std::str::from_utf8(&res).unwrap(), "1\n2\n3\nEND\n");
+    }
 }