Add bit_len() function

[Timmmm]

It would be useful to have a function that gets the bit length of unsigned integers. I.e.

0b0 -> 0
0b101 -> 3
0b1110 -> 4

Python has this via 7.bit_length(). Rust almost has it via 7.ilog2(), but it's actually not the same because it panics for 0.

You can do either of these instead:

n.checked_ilog2().map(|n| n + 1).unwrap_or(0)

T::BITS - n.leading_zeros()

However both are worse than n.bit_len() (or .bit_length() if you prefer).

They both produce fairly simple assembly on x86:

first_version:
    bsr     eax, edi
    inc     eax
    test    edi, edi
    cmove   eax, edi
    ret

second_version:
    mov     eax, 63
    bsr     eax, edi
    xor     eax, -32
    add     eax, 33
    ret

I think the second version is probably the better option.

[JohnScience]

I'd love that in the standard library too!

[Jules-Bertholet]

The best way to get a simple addition like this into the standard library is to file a T-libs-api ACP.

[jrose]

Previously:

Add methods that return the number of bits necessary to represent an integer in binary to the standard library

Hi internals, Currently, the number of bits necessary to represent an integer in binary to can be obtained by: For unsigned integers: let answer: u16 = 42; // 0b10_1010 assert_eq!(answer.checked_ilog2().map_or(0, |n| n + 1), 6); For signed integers (excluding the sign bit and leading zeros): let n: i16 = -37; // -0b10_0101 assert_eq!(n.unsigned_abs().checked_ilog2().map_or(0, |n| n + 1), 6); Methods for this purpose appear to exist in the standard library of several languages: int.bit_le…