How to determine the bit-width of the Number type in Vimscript?

Question

I want to figure out the bit-width of the built-in Number (integer actually) of Vimscript . In particular, I want to tell whether a 32-bit or 64-bit integer is in use. Currently I test whether the maximum constants of both widths can be represented:

let s:INT32_MAX = 2147483647 " 2^31-1
let s:INT64_MAX = 9223372036854775807 " 2^63-1

if s:INT64_MAX > 0
  let s:INT_WIDTH = 64
  let s:INT_MAX = s:INT64_MAX
else
  let s:INT_WIDTH = 32
  let s:INT_MAX = s:INT32_MAX
endif
let s:INT_MIN = -s:INT_MAX - 1

This approach has the annoying magic numbers in code. Although it can be replaced by float2nr(pow(2, 31)), which is exactly s:INT_MAX above, this requires has('float'). I wonder if there is any feature that can be tested like has('num64'). I searched through the :h +feature-list but found nothing more relevant than OS information like +win32. Any insight on this question is appreciated.

Answer 1

You can directly test for the 64-bit number support feature using:

has('num64')

The minimum and maximum numbers can be obtained using 0/0 and 1/0 respectively:

let VARNUM_MIN = 0/0
let VARNUM_MAX = 1/0

To get the bit-depth of these numbers, you can use printf with a binary format.

let bits = len(printf('%b', 1/0))         " returns 63
let bits_nan = len(printf('%b', 0/0))     " returns 64

You can just check if bits is greater than 32, or less than or equal to 32.