I'm planning to use this function to combine hashCodes. Does it make sense? ```fun bitLength(n: Int)...

# announcements

[Daniel]

I'm planning to use this function to combine hashCodes. Does it make sense?

fun bitLength(n: Int) = floor(log2(n.toFloat())).toInt() + 1

fun combine(vararg numbers: Int): Int {
    var total = 0
    var prevBitLength = 0
    
    for (num in numbers) {
        total += num shl prevBitLength
        prevBitLength += bitLength(num)
    }
    
    return total
}

[diesieben07]

You might want to check out the source code for java.util.Arrays.hashCode

[Daniel]

I realized that two numbers could overlap, so I changed it to

private fun bitCombine(vararg numbers: Int): Int {
        var total = 0
        var prevBitLength = 0

        for (num in numbers) {
            total += num shl prevBitLength
            prevBitLength += Int.SIZE_BITS
        }

        return total
    }

[nanodeath]

I think you're going to want to xor things rather than + them.

also the for loop does an allocation.
and the vararg.

[Daniel]

Why xor?

[nanodeath]

I guess I don't understand how that method doesn't int overflow immediately, since the second element is shifting left by the number of bits int supports

[Daniel]

Thank you!

[nanodeath]

xor because there aren't a lot of ways to combine ints of size 0-2^32 without overflowing, and if you're just going for unique, it's fine

[nanodeath]

I think IntelliJ does something like "thing1.hashcode + thing2.hashcode * 31 + thing3.hashcode * 31", where 31 is some prime

[nanodeath]

generated by IntelliJ

Hashcode.kt

[nanodeath]

okay so that's not xor obviously, but I guess it works

[Daniel]

That makes sense, I guess

[Nir]

xor is not good for combining hashes either

[Daniel]

Why not?

[Nir]

because if the two hashes are equal you get zero

[Daniel]

good point

[Nir]

so it has a huge blind spot

[Nir]

with stuff like hashes, you basically never want to improvise tbh

[Nir]

look up proper algorithms

[Nir]

https://stackoverflow.com/questions/2590677/how-do-i-combine-hash-values-in-c0x

[Nir]

here's an example I'm familiar with from my work (it's in C++ but trivial to adapt obviously)

[nanodeath]

I think if you multiplied by a prime first the xor might be okay. but yeah, it's one reason why I like data classes -- I don't have to think about it, and it doesn't get out of sync with new fields

[Nir]

yeah a dataclass is a pretty simple solution for this

[Daniel]

Yeah, I just get a warning in the IDE that I shouldn't be allocating in onDraw when I use a dataclass