To digit simplification

[gilescope]

I found out the other day that all the ascii digits have the first four bits as one would hope them to. (Eg. char 2 ends 0b0010). There are two bits to indicate it's in the digit range ( 0b0011_0000). If it is a true digit then all the higher bits aside from these two will be 0 (as ascii is the lowest part of the unicode u32 spectrum). So XORing with 0b11_0000 should mean we either get the number 0-9 or alternativly we get a larger number in the u32 space. If we get something that's not 0-9 then it will be discarded as it will be greater than the radix.

The code seems so fast though that there's quite a lot of noise in the benchmarks so it's not that easy to prove conclusively that it's faster as well as less instructions.

The non-fast path I was toying with as well wondering if we could do this as then we'd only have one return and less instructions still:

           match self {
                'a'..='z' => self as u32 - 'a' as u32 + 10,
                'A'..='Z' => self as u32 - 'A' as u32 + 10,
                _ => { radix = 10; self as u32 ^ ASCII_DIGIT_MASK},
            }

Here's the godbolt.

( H/T to @byteshadow for pointing out xor was what I needed)

[rust-highfive]

r? @dtolnay

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[m-ou-se]

@bors r+

[bors]

📌 Commit d2ba68b has been approved by m-ou-se