bitwise: add ctz, aka count trailing zeroes

bitwise.hpp

@@ -107,6 +107,31 @@ namespace cruft {
     }
 
 
+    ///////////////////////////////////////////////////////////////////////////
+    /// Count trailing zeroes
+    constexpr unsigned int
+    ctz (unsigned int x) noexcept
+    {
+        return __builtin_ctz (x);
+    }
+
+
+    //-------------------------------------------------------------------------
+    constexpr unsigned long
+    ctz (unsigned long x) noexcept
+    {
+        return __builtin_ctzl (x);
+    }
+
+
+    //-------------------------------------------------------------------------
+    constexpr unsigned long long
+    ctz (unsigned long long x) noexcept
+    {
+        return __builtin_ctzll (x);
+    }
+
+
     ///////////////////////////////////////////////////////////////////////////
     /// returns the integral value composed of the bits from `val' in the
     /// inclusive range [lo, hi].

test/bitwise.cpp

@@ -87,5 +87,7 @@ main (int, char**)
     test_reverse (tap);
     test_bitfield (tap);
 
+    tap.expect_eq (cruft::ctz (0b11110000u), 4u, "count-trailing-zero trivial test");
+
     return tap.status ();
 }