From dbb35e40ecb1ee6d34df419531d8ee4a083bf30a Mon Sep 17 00:00:00 2001
From: Jake Lishman <jake.lishman@ibm.com>
Date: Wed, 13 Dec 2023 18:05:48 +0000
Subject: [PATCH] Use custom ilog2

---
 crates/accelerate/src/sparse_pauli_op.rs | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/crates/accelerate/src/sparse_pauli_op.rs b/crates/accelerate/src/sparse_pauli_op.rs
index 5b0e98021f9f..abfe8536f893 100644
--- a/crates/accelerate/src/sparse_pauli_op.rs
+++ b/crates/accelerate/src/sparse_pauli_op.rs
@@ -17,8 +17,8 @@ use pyo3::Python;
 
 use hashbrown::HashMap;
 use ndarray::{s, Array1, Array2, ArrayView1, ArrayView2, Axis};
-use num_traits::Zero;
 use num_complex::Complex64;
+use num_traits::Zero;
 use numpy::{IntoPyArray, PyArray1, PyArray2, PyReadonlyArray2};
 
 /// Find the unique elements of an array.
@@ -97,7 +97,9 @@ pub fn decompose_dense(
     operator: PyReadonlyArray2<Complex64>,
     tolerance: f64,
 ) -> PyResult<ZXPaulis> {
-    let num_qubits = operator.shape()[0].ilog2() as usize;
+    // Can't use `{integer}::ilog2` until Rust 1.67+.
+    let ilog2 = |val: usize| std::mem::size_of::<usize>() * 8 - val.leading_zeros() as usize - 1;
+    let num_qubits = ilog2(operator.shape()[0]);
     let size = 1 << num_qubits;
     if operator.shape() != [size, size] {
         return Err(PyValueError::new_err(format!(