Add @intlog2 for Floored CT log-base2 (#2355)

* Add `@intlog2` for Floored CT log-base2
---------

Co-authored-by: Christoffer Lerno <christoffer@aegik.com>

lib/std/math/math.c3

@@ -344,6 +344,23 @@ macro log(x, base)
 *>
 macro log2(x) => $$log2(values::promote_int(x));
 
+<*
+ @require values::@is_int($x) : `The input value must be an integer.`
+ @require $x >= 0 : `The input value must be a positive integer.`
+ @return `A floored base-2 log of an input integer value.`
+*>
+macro @intlog2($x)
+{
+	$if $x <= 1:
+		return 0;
+	$endif
+
+	$typeof($x) $z = 0;
+	$for var $y = $x; $y > 0; $y >>= 1, ++$z: $endfor
+
+	return $z - 1;
+}
+
 <*
  @require values::@is_promotable_to_floatlike(x) : `The input must be a number or a float vector`
 *>

releasenotes.md

@@ -4,6 +4,7 @@
 
 ### Changes / improvements
 - Support `alias foo = module std::io` module aliasing.
+- Add compile-time `@intlog2` macro to math.
 
 ### Fixes
 - List.remove_at would incorrectly trigger ASAN.

test/unit/stdlib/math/math.c3

@@ -457,6 +457,24 @@ fn void test_log() @test
 	}
 }
 
+fn void test_ct_intlog2() @test @if($feature(SLOW_TESTS))
+{
+	uint128 actual, expected;
+	$for var $x = 0; $x <= 128; ++$x :
+		expected = (uint128)math::floor(math::log2($x));
+		actual = (uint128)math::@intlog2($x);
+		assert(expected == actual, "input %d: floor(log2($x)) -> %d is not equal to @intlog2($x) -> %d", $x, expected, actual);
+	$endfor
+
+	var $logme = (uint128)1;
+	$for var $x = 0; $x < 8192; ++$x :
+		$logme *= 13;
+		expected = (uint128)math::floor(math::log2((uint128)$logme));
+		actual = (uint128)math::@intlog2((uint128)$logme);
+		assert(expected == actual, "input %d (idx %d): floor(log2(|$logme|)) -> %d is not equal to @intlog2(|$logme|) -> %d", $logme, $x, expected, actual);
+	$endfor
+}
+
 fn void test_pow() @test
 {
 	int[<10>] e =  { 2, 1, 0, -1, -2, 2, 1, 0, -1, -2 };