I'm trying to replicate a javascript check digit function in Ruby. The results are differing and it looks as though it is related to the size of the integer.
in Ruby:
puts "#{1421974191} | #{(1421974191 << 5)}"
produces 1421974191 | 45503174112
in javascript:
alert(1421974191 + ' | ' + (1421974191 << 5))
produces 1421974191 | -1741466144
I'd be grateful for any advice on why this is happening, and how I can replicate the javascript in Ruby.
Thanks in advance
Dan
JS' bitwise operators are limited to 32-bit values, so you're getting an overflow. From the MDN page:
The operands of all bitwise operators are converted to signed 32-bit integers in two's complement format.
Modern JS engines will actually prefer the two's complement format and bail out to a 64-bit floating point value if you push them, but the bitwise operators force a conversion back to 32-bit integers before doing anything.
The spec is very clear about this. From section 12.8.3.1:
- Return the result of left shifting lnum by shiftCount bits. The result is a signed 32-bit integer.
To mimic integer overflow in Ruby (or any language with large numbers), you can use the formula (taken from this Python answer):
((n + 2147483647) % 4294967294) - 2147483647
As the answer notes, this only works with left-shifting, not division or right-shifting.
