Iterative BitSizeL implementation

Standard/src/Math/Functions.qs

@@ -551,12 +551,6 @@ namespace Microsoft.Quantum.Math {
         return AccumulatedBitsizeI(a, 0);
     }
 
-    /// # Summary
-    /// Helper function used to recursively calculate the bitsize of a value.
-    internal function AccumulatedBitsizeL(val : BigInt, bitsize : Int) : Int {
-        return val == 0L ? bitsize | AccumulatedBitsizeL(val / 2L, bitsize + 1);
-    }
-
 
     /// # Summary
     /// For a non-negative integer `a`, returns the number of bits required to represent `a`.
@@ -572,7 +566,13 @@ namespace Microsoft.Quantum.Math {
     /// The bit-size of `a`.
     function BitSizeL(a : BigInt) : Int {
         Fact(a >= 0L, $"`a` must be non-negative");
-        return AccumulatedBitsizeL(a, 0);
+        mutable bitsize = 0;
+        mutable val = a;
+        while (val != 0L) {
+            set bitsize += 1;
+            set val /= 2L;
+        } 
+        return bitsize;
     }
 
 

Standard/tests/Math/MathTests.qs

@@ -68,6 +68,14 @@ namespace Microsoft.Quantum.Canon {
         EqualityFactI(BitSizeI(3), 2, $"BitSizeI(3) must be 2");
         EqualityFactI(BitSizeI(7), 3, $"BitSizeI(7) must be 2");
     }
+
+    @Test("QuantumSimulator")
+    function CanComputeBitSizeFromLargeNumbers () : Unit {
+        for (k in 1 .. 100) {
+            let exp = 128 * k;
+            EqualityFactI(BitSizeL(1L <<< exp), exp + 1, $"unexpected bitsize for exponent {exp} (k = {k})");
+        }
+    }
 
     @Test("QuantumSimulator")
     function ExpModIsCorrect () : Unit {