Incorrect results from `BigInteger.shl` for negative shift values

[hakanai]

Describe the bug
Various calls to BigInteger.shl(n) with n < 0 produced results differing from that of java.math.BigInteger.

To Reproduce

Try calling shl with any of the following samples:

• Results differ (-4194304 != 0) for operation: -8388608.shl(-1)
• Results differ (8 != 0) for operation: 8388608.shl(-20)
• Results differ (8 != 0) for operation: 512.shl(-6)
• Results differ (28 != 6353246884247830528) for operation: 2078458800722539264.shl(-56)

Expected behavior
Since the method doesn't document the behaviour, I assumed behaviour consistent with java.math.BigInteger - shl(-n) should behave the same as shr(n), and vice versa. Although, I have only detected this inconsistency with shl. I don't know whether shr has a similar issue, as my own test suite is yet to complain about it.

Platform

• JVM

Additional context
I have been hunting for a drop-in replacement for BigInteger. After reading through some of the API docs for this one, it seems it might not have been a candidate anyway, as not() is explicitly documented as not being consistent with Java's.

[hakanai]

Since I was curious, I wrote an explicit test for shr as well, and it looks like it behaves similarly.

The test:

            42.toBigInt().shr(-2) shouldBe 168.toBigInt()

Result:

Expected :168
Actual   :0

And just as for shl, non-negative values passed.

[ionspin]

Thanks for reporting, we'll look into it.

I have been hunting for a drop-in replacement for BigInteger. After reading through some of the API docs for this one, it seems it might not have been a candidate anyway, as not() is explicitly documented as not being consistent with Java's.

Yeah, while Java BigInteger was used for inspiration and to test against to validate results, and there are a lot of similarities the APIs are different, so it's not a drop in replacement.

[ionspin]

Btw, I still haven't forgot about your series based functions pull request. It's just that this decade has been kinda busy so far :)