Hashes module produces different results in C and JS.

[treeform]

Running this code produces different results in C and JS:

echo hash(123)
echo hash(123.457)
echo hash("hi there")

in C:

123
4638458299730989416
-8297432703265153370

in JS:

123
124.457
1638780415

Should hashing be consistent across compiler backends?

[dom96]

yeah, it probably should be. I ended up using this in my program:

{.push overflowChecks: off.}
proc customHash*(x: string): int32 =
  ## JS/C backend-compatible hashing
  ##
  ## Initially based on https://gist.github.com/hyamamoto/fd435505d29ebfa3d9716fd2be8d42f0
  ## but JS strings are UTF16...

  for i in 0 ..< x.len:
    result = (31*result + ord(x[i]).int32) or 0

    # echo(result, " ", x[i], " ", ord(x[i]))

{.pop.}

[treeform]

How did you hash floats?

[dom96]

Haven't needed that so far, but if you search around for a JS implementation you should be able to find something :)

[andreaferretti]

It's especially bizarre that hashing a float does not return an int! Moreover, it is not that good of a hash if hash(123.457) == 124.457 :-/

[Araq]

@andreaferretti This + 1.0 in the hashing logic was stolen from Lua's implementation where it was used this way in production for over a decade. I'm sure we can do better now but it was not done carelessly.

[andreaferretti]

I wonder what is the rationale for this

[mratsim]

What are your use-cases to have the same hashes in C and JS (and I guess the VM)?

[Araq]

@andreaferretti Normalizing -0.0 to +0.0 (which should compare equal and hence hash equally).

[treeform]

@mratsim I want to make sure that the data structure I am synchronizing is the same on the server and browser. I wanted to hash both sides and if hash is different there is a desync bug some place.

[krux02]

The problem for the different Hash results is Here:
https://github.com/nim-lang/Nim/blob/2a3b0563141f2cfdee4cbeef85167c25610ff71f/lib/pure/hashes.nim#L146
This is technically only correct on 64 bit native target.

[Araq]

@treeform That is a bad idea because then the details are tied to a very specific unspecificied hash algorithm that the stdlib happens to use.