diff --git a/int2intmaplike.py b/int2intmaplike.py
index 593176d..35b40e2 100644
--- a/int2intmaplike.py
+++ b/int2intmaplike.py
@@ -16,6 +16,7 @@ class Int2IntMapLike():
     def __init__(self, file):
         self._file = file
         self._handle = open(file, "w+t")
+        self._buffer = ""
 
     def close(self):
         if not self._handle.closed:
@@ -27,13 +28,22 @@ class Int2IntMapLike():
     def add(self, start: int, length: int, val: int):
         line = "%d,%d,%d\n" % (start, length, val)
         length = len(line)
-        offset = self._handle.tell()
+        offset = self._handle.tell() + len(self._buffer)
         if offset % self.blocksize + length > self.blocksize:
+            # end of block: fill block
             fill_bytes = self.blocksize - offset % self.blocksize
-            self._handle.write("\n" * fill_bytes)
-        self._handle.write(line)
+            self._buffer = self._buffer + ("\n" * fill_bytes)
+        else:
+            self._buffer = self._buffer + line
+
+    def _flush_buffer(self):
+        self._handle.write(self._buffer)
+        self._buffer = ""
+        self._handle.flush()
 
     def find(self, key: int) -> Optional[int]:
+        if (len(self._buffer)) > 0:
+            self._flush_buffer()
         size = os.stat(self._file).st_size
         if size == 0:
             return None
diff --git a/testint2intmaplike.py b/testint2intmaplike.py
index 3be2038..b5fcec7 100644
--- a/testint2intmaplike.py
+++ b/testint2intmaplike.py
@@ -16,6 +16,35 @@ class Int2IntMapLikeTest(unittest.TestCase):
         self.map.close()
         self.test_dir.cleanup()
 
+    def test_empty_map(self):
+        map = self.map
+        self.assertEqual(None, map.find(0))
+
+    def test_one_line_one_byte(self):
+        map = self.map
+        map.add(10, 1, 1)  # add only the key 10
+        self.assertEqual(None, map.find(9))  # directly before
+        self.assertEqual(1, map.find(10))
+        self.assertEqual(None, map.find(11))  # directly after
+
+    def test_one_line_two_bytes(self):
+        map = self.map
+        map.add(10, 2, 1)  # added keys 10 and 11
+        self.assertEqual(None, map.find(9))  # directly before
+        self.assertEqual(1, map.find(10))
+        self.assertEqual(1, map.find(11))
+        self.assertEqual(None, map.find(12))  # directly after
+
+    def test_two_lines(self):
+        map = self.map
+        map.add(10, 1, 1)  # added key 10
+        map.add(12, 1, 2)  # added key 12
+        self.assertEqual(None, map.find(9))  # directly before
+        self.assertEqual(1, map.find(10))
+        self.assertEqual(None, map.find(11))  # between
+        self.assertEqual(2, map.find(12))
+        self.assertEqual(None, map.find(13))  # directly after
+
     def test_fill_map(self):
         map = self.map
         map.blocksize = 64