Javascript bitwise to c#

Question

I have some issue to convert a javascript code into c# the issue is with bitwise operator:

Javascript function: return (s - (s | 0x0)) * 0x100000000 | 0x0;

C# function; return (long)((s - ((long)s)) * 0x100000000);

If s = 1.7320508075688772 on Javascript report -1150833019 on c# report 3144134277

other example can be Javascript: (1779033703 << 0x1e) = -1073741824 c# (1779033703 << 0x1e) = 1910222893216694272

What i need is translate Javascript function into c# with same number result.

Thanks for help.

I don’t know C# syntax that well, but (int)3144134277U or (int)3144134277UL, maybe? >> produces a signed 32-bit integer in JavaScript when used on a number. — Ry-
– Ry- ♦, Commented Sep 23, 2019 at 20:24
you cant use int cause is over range (32-bit signed integer type -2,147,483,648 to 2,147,483,647) — Roberto Rossi
– Roberto Rossi, Commented Sep 23, 2019 at 20:34
Do you know what the purpose of this operation is? It looks like you're trying to reinterpret an int as a uint and sign-extend it to a long. Whatever it is, though, there's probably a more natural, idiomatic way of doing in C# whatever it's intended to accomplish but we need to examine its purpose first. — madreflection
– madreflection, Commented Sep 23, 2019 at 20:43

GokuMizuno · Accepted Answer · 2019-09-23 20:34:25Z

3

So, there are a few things going on here.

You have a type mismatch in your JavaScript. In Hex, 3144134277 is BB67AE85, and -1150833019 is FFFFFFFFBB67AE85. So, we can see that the JavaScript int32 is being implicitly converted to an unsigned int64.
You can't bitshift by 0. Bitshifting is dividing by 2^n, where n is, in this case, 0. That returns the same number, as 2^0 = 1.
(long)((ulong)(…) That's a double cast, and is considered bad form. Your number literal will be cast to an unsigned long, then cast again to a long. This just wastes cycles.
Your cast is a C style cast, in C# casting is more often done as object.ToInt()

So, in review, you have a bug in your JavaScript.

answered Sep 23, 2019 at 20:34

GokuMizuno

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Guven Sezgin Kurt · Accepted Answer · 2019-09-23 21:29:36Z

1

You can't expect the same behavior on C# by default. Because:

In JavaScript, a number is stored as a 64-bit floating point number but the bit-wise operation is performed on a 32-bit binary number
So to perform a bit-operation JavaScript converts the number into a 32-bit binary number, perform the operation and convert back the result to a 64-bit number.

So in your case you might be trying to cast a 64-bit number to 32-bit one and get a faulty result from there. Which in C# it wouldn't be a good thing to have in my opinion.

answered Sep 23, 2019 at 21:29

Guven Sezgin Kurt

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madreflection Over a year ago

This underscores the need to understand the function's purpose and try to match that instead of doing a direct translation.

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