LibJS+LibCrypto: Fix SignedBitInteger::bitwise_not and use it in LibJS

Bitwise operators are defined on two's complement, but SignedBitInteger
uses sign-magnitude. Correctly convert between the two.

Let LibJS delegate to SignedBitInteger for bitwise_not, like it does
for all other bitwise_ operations on bigints.

No behavior change (LibJS is now the only client of
SignedBitInteger::bitwise_not()).

1 files changed, 6 insertions, 1 deletions

diff --git a/Userland/Libraries/LibCrypto/BigInt/SignedBigInteger.cpp b/Userland/Libraries/LibCrypto/BigInt/SignedBigInteger.cpp
index 0787f90eb1..9d860f4c43 100644
--- a/Userland/Libraries/LibCrypto/BigInt/SignedBigInteger.cpp
+++ b/Userland/Libraries/LibCrypto/BigInt/SignedBigInteger.cpp
@@ -160,7 +160,12 @@ FLATTEN SignedBigInteger SignedBigInteger::bitwise_xor(const UnsignedBigInteger&
 
 FLATTEN SignedBigInteger SignedBigInteger::bitwise_not() const
 {
-    return { unsigned_value().bitwise_not(), !m_sign };
+    // Bitwise operators assume two's complement, while SignedBigInteger uses sign-magnitude.
+    // In two's complement, -x := ~x + 1.
+    // Hence, ~x == -x -1 == -(x + 1).
+    SignedBigInteger result = plus(SignedBigInteger { 1 });
+    result.negate();
+    return result;
 }
 
 FLATTEN SignedBigInteger SignedBigInteger::multiplied_by(UnsignedBigInteger const& other) const