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()).

Tests/LibCrypto/TestBigInteger.cpp

@@ -467,6 +467,12 @@ TEST_CASE(test_bigint_bitwise_and_different_lengths)
     EXPECT_EQ(num1.bitwise_and(num2), "1180290"_bigint);
 }
 
+TEST_CASE(test_signed_bigint_bitwise_not)
+{
+    EXPECT_EQ("3"_sbigint.bitwise_not(), "-4"_sbigint);
+    EXPECT_EQ("-1"_sbigint.bitwise_not(), "0"_sbigint);
+}
+
 TEST_CASE(test_signed_bigint_bitwise_and)
 {
     auto num1 = "-1234567"_sbigint;

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

Userland/Libraries/LibJS/Runtime/Value.cpp

@@ -881,10 +881,7 @@ ThrowCompletionOr<Value> bitwise_not(GlobalObject& global_object, Value lhs)
     auto lhs_numeric = TRY(lhs.to_numeric(global_object));
     if (lhs_numeric.is_number())
         return Value(~TRY(lhs_numeric.to_i32(global_object)));
-    auto big_integer_bitwise_not = lhs_numeric.as_bigint().big_integer();
-    big_integer_bitwise_not = big_integer_bitwise_not.plus(Crypto::SignedBigInteger { 1 });
-    big_integer_bitwise_not.negate();
-    return Value(js_bigint(vm, big_integer_bitwise_not));
+    return Value(js_bigint(vm, lhs_numeric.as_bigint().big_integer().bitwise_not()));
 }
 
 // 13.5.4 Unary + Operator, https://tc39.es/ecma262/#sec-unary-plus-operator

Userland/Libraries/LibJS/Tests/builtins/BigInt/bigint-basic.js

@@ -40,6 +40,8 @@ describe("correct behavior", () => {
         expect(1n | 2n).toBe(3n);
         expect(5n ^ 3n).toBe(6n);
         expect(~1n).toBe(-2n);
+        expect(~-1n).toBe(0n);
+
         expect(5n << 2n).toBe(20n);
         expect(7n >> 1n).toBe(3n);
     });