zig

commit 925f82da098300ac7015d7897fc5202a7ea275a0 (tree)
parent 99229ceb55be717aa1995f1a0105bc6b96a910f6
Author: hemisputnik <hemisputnik@proton.me>
Date:   Sat, 28 Feb 2026 00:46:05 +0200

std.math.big.int: add log2

Diffstat:
M
lib/std/math/big/int.zig
 | 
5
+++++
M
lib/std/math/big/int_test.zig
 | 
17
+++++++++++++++++

2 files changed, 22 insertions(+), 0 deletions(-)
diff --git a/lib/std/math/big/int.zig b/lib/std/math/big/int.zig
@@ -2670,6 +2670,11 @@ pub const Const = struct {
         }
         return @min(result, bits);
     }
+
+    /// Calculate the base 2 logarithm, rounded down.
+    pub fn log2(a: Const) Limb {
+        return a.limbs.len * @bitSizeOf(Limb) - 1 - @clz(a.limbs[a.limbs.len - 1]);
+    }
 };
 
 /// An arbitrary-precision big integer along with an allocator which manages the memory.
diff --git a/lib/std/math/big/int_test.zig b/lib/std/math/big/int_test.zig
@@ -4072,3 +4072,20 @@ test "ctz" {
     try testing.expectEqual(0, limb_max_squared.ctz(@bitSizeOf(Limb) * 2));
     try testing.expectEqual(0, limb_max_squared.ctz(@bitSizeOf(Limb) * 2 + 1));
 }
+
+test "log2" {
+    var a = try Managed.init(testing.allocator);
+    defer a.deinit();
+
+    try a.setString(2, "1");
+    try testing.expectEqual(0, a.toConst().log2());
+
+    try a.setString(2, "1111011");
+    try testing.expectEqual(6, a.toConst().log2());
+
+    try a.setString(2, "10100111011101010");
+    try testing.expectEqual(16, a.toConst().log2());
+
+    try a.setString(16, "a22d71c87a9ce406da4f5895f9f3cc3d603192baf6c8a2b5c32649d0465bf188fe799b3618085e49d71bdaec01");
+    try testing.expectEqual(359, a.toConst().log2());
+}