From d0d81e470ec07745efffbb81becfb4bfca89bb5b Mon Sep 17 00:00:00 2001
From: Tim Ledbetter <timledbetter@gmail.com>
Date: Mon, 13 May 2024 18:57:38 +0100
Subject: [PATCH] AK: Fix off by one error in integral `ceil_log2()`

Previously, certain values of `ceil_log2(x)` would be 1 smaller than
`ceil(log2(x))`.
---
 AK/IntegralMath.h            |  6 ++----
 Tests/AK/TestIntegerMath.cpp | 34 ++++++++++++++++++++++++++++++++++
 2 files changed, 36 insertions(+), 4 deletions(-)

diff --git a/AK/IntegralMath.h b/AK/IntegralMath.h
index c1051fed843..ebd279768fa 100644
--- a/AK/IntegralMath.h
+++ b/AK/IntegralMath.h
@@ -27,12 +27,10 @@ constexpr T log2(T x)
 template<Integral T>
 constexpr T ceil_log2(T x)
 {
-    if (!x)
+    if (x <= 1)
         return 0;
 
-    T log = AK::log2(x);
-    log += (x & ((((T)1) << (log - 1)) - 1)) != 0;
-    return log;
+    return AK::log2(x - 1) + 1;
 }
 
 template<Integral I>
diff --git a/Tests/AK/TestIntegerMath.cpp b/Tests/AK/TestIntegerMath.cpp
index 3e1d742ce5e..192543a494a 100644
--- a/Tests/AK/TestIntegerMath.cpp
+++ b/Tests/AK/TestIntegerMath.cpp
@@ -64,3 +64,37 @@ TEST_CASE(exp2)
     EXPECT_EQ(AK::exp2<i64>(62), 4611686018427387904);
     EXPECT_EQ(AK::exp2<u64>(63), 9223372036854775808ull);
 }
+
+TEST_CASE(ceil_log2)
+{
+    EXPECT_EQ(AK::ceil_log2<u64>(0), 0ull);
+    EXPECT_EQ(AK::ceil_log2<u64>(1), 0ull);
+    EXPECT_EQ(AK::ceil_log2<u8>(2), 1);
+    EXPECT_EQ(AK::ceil_log2<u8>(3), 2);
+    EXPECT_EQ(AK::ceil_log2<u8>(6), 3);
+    EXPECT_EQ(AK::ceil_log2<i8>(96), 7);
+    EXPECT_EQ(AK::ceil_log2<i8>(127), 7);
+    EXPECT_EQ(AK::ceil_log2<u8>(128), 7);
+    EXPECT_EQ(AK::ceil_log2<u8>(255), 8);
+    EXPECT_EQ(AK::ceil_log2<i16>(256), 8);
+    EXPECT_EQ(AK::ceil_log2<i16>(257), 9);
+    EXPECT_EQ(AK::ceil_log2<i16>(384), 9);
+    EXPECT_EQ(AK::ceil_log2<i16>(24576), 15);
+    EXPECT_EQ(AK::ceil_log2<i16>(32767), 15);
+    EXPECT_EQ(AK::ceil_log2<i32>(32768), 15);
+    EXPECT_EQ(AK::ceil_log2<i32>(32769), 16);
+    EXPECT_EQ(AK::ceil_log2<i32>(98304), 17);
+    EXPECT_EQ(AK::ceil_log2<i32>(1610612736), 31);
+    EXPECT_EQ(AK::ceil_log2<i32>(2147483647), 31);
+    EXPECT_EQ(AK::ceil_log2<u32>(2147483648), 31u);
+    EXPECT_EQ(AK::ceil_log2<u32>(2147483649), 32u);
+    EXPECT_EQ(AK::ceil_log2<u32>(3221225472), 32u);
+    EXPECT_EQ(AK::ceil_log2<u32>(4294967295), 32u);
+    EXPECT_EQ(AK::ceil_log2<i64>(4294967296), 32);
+    EXPECT_EQ(AK::ceil_log2<i64>(4294967297), 33);
+    EXPECT_EQ(AK::ceil_log2<i64>(9223372036854775807), 63ll);
+    EXPECT_EQ(AK::ceil_log2<u64>(9223372036854775808ull), 63ull);
+    EXPECT_EQ(AK::ceil_log2<u64>(9223372036854775809ull), 64ull);
+    EXPECT_EQ(AK::ceil_log2<u64>(13835058055282163712ull), 64ull);
+    EXPECT_EQ(AK::ceil_log2<u64>(18446744073709551615ull), 64ull);
+}