diff --git a/src/lib.rs b/src/lib.rs
index 9caee98..9aa1c0a 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -94,7 +94,6 @@ pub struct ByteNgramReader<R: Read> {
     inner: Bytes<R>,
     token: u64,
     count: u64,
-    index: u64,
     mask: u64,
 }
 
@@ -104,7 +103,6 @@ impl<R: Read> ByteNgramReader<R> {
             inner: inner.bytes(),
             token: 0,
             count: 0,
-            index: 0,
             mask: 0,
         }
     }
@@ -114,7 +112,7 @@ impl<R: Read> Iterator for ByteNgramReader<R> {
     type Item = ByteNgram;
 
     fn next(&mut self) -> Option<Self::Item> {
-        if self.index == 0 {
+        if self.mask == 0 {
             if let Some(Ok(byte)) = self.inner.next() {
                 self.token += u64::from(byte);
                 self.token <<= 8;
@@ -122,21 +120,18 @@ impl<R: Read> Iterator for ByteNgramReader<R> {
                 if self.count < 6 {
                     self.count += 1;
 
-                    self.index = self.count;
-                    self.mask = (256u64.pow(self.count as u32) - 1) << 8;
+                    self.mask = ((256u64.pow(self.count as u32) - 1) << 8) + self.count;
                 } else {
-                    self.index = 7;
-                    self.mask = 0xFFFF_FFFF_FFFF_FF00;
+                    self.mask = 0xFFFF_FFFF_FFFF_FF07;
                 }
             } else {
                 return None;
             }
         }
 
-        let token = ByteNgram((self.token & self.mask & !0xFF) + self.index);
+        let token = ByteNgram((self.token & self.mask & !0xFF) + (self.mask & 0xFF));
 
-        self.index -= 1;
-        self.mask >>= 8;
+        self.mask = ((self.mask >> 8) & !0xFF) + (self.mask & 0xFF) - 1;
 
         Some(token)
     }