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LibCrypto: Fix subtracting two negative SignedBigIntegers

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Currently, we get the following results

    -1 - -2 = -1
    -2 - -1 =  1

Correct would be:

    -1 - -2 =  1
    -2 - -1 = -1

This was already attempted to be fixed in 7ed8970, but that change was
incorrect. This directly translates to LibJS BigInts having the same
incorrect behavior - it even was tested.
This commit is contained in:
Linus Groh 2021-11-16 09:17:55 +00:00
parent 014840eeca
commit 58c6a156bf
Notes: sideshowbarker 2024-07-18 01:04:35 +09:00 
Author: https://github.com/linusg
Commit: https://github.com/SerenityOS/serenity/commit/58c6a156bf3
Pull-request: https://github.com/SerenityOS/serenity/pull/10936
Reviewed-by: https://github.com/alimpfard ✅
3 changed files with 17 additions and 4 deletions
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9
Tests/LibCrypto/TestBigInteger.cpp
View File

@ -543,6 +543,15 @@ TEST_CASE(test_signed_subtraction_simple_subtraction_negative_result)
EXPECT_EQ(num1.minus(num2), Crypto::SignedBigInteger { -20 });
}
TEST_CASE(test_signed_subtraction_both_negative)
{
Crypto::SignedBigInteger num1(-50);
Crypto::SignedBigInteger num2(-70);
EXPECT_EQ(num1.minus(num2), Crypto::SignedBigInteger { 20 });
EXPECT_EQ(num2.minus(num1), Crypto::SignedBigInteger { -20 });
}
TEST_CASE(test_signed_subtraction_simple_subtraction_with_borrow)
{
Crypto::SignedBigInteger num1(Crypto::UnsignedBigInteger { UINT32_MAX });

10
Userland/Libraries/LibCrypto/BigInt/SignedBigInteger.cpp
View File

@ -109,11 +109,15 @@ FLATTEN SignedBigInteger SignedBigInteger::minus(const SignedBigInteger& other)
// -x - -y = y - x
if (m_unsigned_data < other.m_unsigned_data) {
// The result will be positive.
return SignedBigInteger { other.m_unsigned_data.minus(m_unsigned_data), true };
return SignedBigInteger { other.m_unsigned_data.minus(m_unsigned_data) };
}
// The result will be either zero, or negative.
// y - x = - (x - y)
return SignedBigInteger { m_unsigned_data.minus(other.m_unsigned_data) };
if (m_unsigned_data > other.m_unsigned_data) {
// The result will be negative.
return SignedBigInteger { m_unsigned_data.minus(other.m_unsigned_data), true };
}
// Both operands have the same magnitude, the result is positive zero.
return SignedBigInteger { 0 };
}
FLATTEN SignedBigInteger SignedBigInteger::plus(const UnsignedBigInteger& other) const

2
Userland/Libraries/LibJS/Tests/builtins/BigInt/bigint-minus.js
View File

@ -2,7 +2,7 @@ describe("minus behavior", () => {
test("the basics", () => {
expect(3n - 4n).toBe(-1n);
expect(3n - -4n).toBe(7n);
expect(-3n - -4n).toBe(-1n);
expect(-3n - -4n).toBe(1n);
expect(-3n - 4n).toBe(-7n);
});
});