Is ampersand followed by int.MaxValue rounding down?

Question

I have a piece of C# code that another developer has copied from a blog post which is used to encode/obfuscate an integer. this code contains some syntax that i am unfamiliar with. It looks like it might be rounding down the result of the calculation to prevent it from exceeding the maximum size of an integer, if that is the case i am worried that two input values could potentially result in the same output. The obfuscated values need to be unique so i'm worried about using this code without understanding how it works

this is a simplified version of the code:

public static int DecodeNumber(int input)
{
    return (input * PrimeInverse) & int.MaxValue;
}

so my question is what is the meaning of the ampersand in this context and will this code produce an output that is unique to the input?

Answer 1

No, there is no "rounding" going on here. This is a sneaky way of truncating the most significant bit when multiplication results in overflow.

According to the documentation, int.MaxValue is 2,147,483,647, which is 0x7FFFFFFF in hex. Performing a bitwise AND with this value simply clears out the most significant bit.

Since the intention of the code is to use int.MaxValue for its binary pattern, rather than for its numeric value of the highest int that could be represented by Int32, I would recommend either using 0x7FFFFFFF constant explicitly, or computing it with ~ expressionL

return (input * PrimeInverse) & ~(1 << 31);