>> error fixed + test with a python impl

src/fib.rs

@@ -5,20 +5,19 @@
 pub fn fast2(n: u64) -> u64 {
     let (mut a, mut b) = (0, 1);
     let m = n;
-    let mut mask = (u64::MAX >> (m.leading_zeros() + 1)) + 1;
+    let bit_len = u64::BITS - m.leading_zeros(); // copilot 
+    let mut mask = if bit_len == 0 { 0 } else { 1 << (bit_len - 1) };
     while mask > 0 {
         let c = a * (2 * b - a);
         let d = a * a + b * b;
         if m & mask == 0 {
             (a, b) = (c, d);
-        }
-        else {
+        } else {
             (a, b) = (d, c + d);
         }
         mask >>= 1;
     }
     a
-
 }
 // 0, 1, 3, 7
 // -> -1 /2 -1 /2 -1 /2

src/lib.rs

@@ -5,6 +5,7 @@ pub mod pyexport {
 
     /// this shit is the thing now in python
     #[pyfunction]
+    #[pyo3(signature = (n), text_signature = "(n: int) -> int")]
     fn fib(n: u64) -> u64 {
         crate::fib::fast2(n)
         // currently use u64 as a large number tupe, can use num_bigint