Getting the same result from Ruby as Javascript for bitwise XOR

Question

In Ruby:

-1104507 ^ 3965973030 => -3966969949

In Javascript:

-1104507 ^ 3965973030 => 327997347

Someone asked a similar question here but the answer just pointed to a wrapper for Closure. I need a way to get the same answers from Ruby as I get for JavaScript so I can port this code over.

I need a way of being able to get the JavaScript result from any A ^ B in Ruby for any integers A and B.

You'll need to use a bignum library which supports bitwise XOR, or write your own. — Matt Ball
– Matt Ball, Commented Jun 12, 2013 at 1:23

Mark Adler · Accepted Answer · 2013-06-12 20:53:00Z

1

Those two are the same result, modulo 2³². In Ruby you could & 4294967295 to make the result the same as in Javascript.

To cover all the cases, you need to take into account that Javascript considers binary values to be signed 32-bit integers. Ruby on the other hand will produce unsigned 32-bit integers from the & 4294967295 operation.

So, in Javascript simply:

c = a ^ b

To get the same thing in Ruby:

c = (a ^ b) & 4294967295
c -= 4294967296 if c > 2147483647

edited Jun 12, 2013 at 20:53

answered Jun 12, 2013 at 5:01

Mark Adler

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2 Comments

Ben G Over a year ago

what about 332763 ^ 2714866558. This one gives 2715058341 in ruby and -1579908955 in JS. And if you do 2715058341 & 4294967295 it's still 4294967295

Mark Adler Over a year ago

You mean it's still 2715058341. See my answer.

Ben G · Accepted Answer · 2013-06-12 19:22:00Z

0

Thanks to Mark Adler for the initial tip, I think this is the way to do it algorithmically:

max_32_int = (2**32)

c = a ^ b

if c > (max_32_int/2)
  c = c - max_32_int
elsif c < -(max_32_int/2)
  c = c + max_32_int
end

answered Jun 12, 2013 at 19:22

Ben G

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4 Comments

Mark Adler Over a year ago

You only need one if. See my answer.

Ben G Over a year ago

well, yours has & and -. Mine uses + and -. Same number of operators, no?

Mark Adler Over a year ago

if statements slow things down since they mess with the look-forward execution units. Even if you had more operators, but fewer if's, that would be a win. So one less if and the same number of operators is definitely a win. I should see if I can get rid of the remaining if with more operators.

Mark Adler Over a year ago

Not that that really matters much, since ruby is an interpreter.

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