Replacement for snprintf

[berni44]

On Wednesday, 30 October 2019 at 20:46:07 UTC, Stefan Koch wrote:
> If you could post that so I can have a look over the WIP that'd 
> be nice.

See https://github.com/berni44/phobos/tree/printf

The function can be found at the end of std/format.d. I had to 
comment out some unittests, because e and g qualifiers are not 
yet supported. I put several comments in the code, so I hope it's 
clear, what always happens. If not, feel free to ask. (I'll be 
offline during the weekend.)

I also added a diagram for speed comparison. See 
https://github.com/berni44/phobos/blob/printf/diagram.png

Blue and green use "%.10f" while black and red use "%.100f". Blue 
and red is my function, while green and black is snprintf. The 
X-axis gives the value in the exponent from 0 to 255, the y-axis 
gives the average time in nanoseconds. The green bottom line at 
the left is approx at 600ns. For each exponent there have been 
approx 217886 numbers checked (the same set for both functions).

As you can see, at the left side, snprintf is faster, having an 
almost constant time, while the time of mine is slightly 
increasing when exponents get smaller. I scanned the snprintf 
implementation to find out, what they do - see my comment in the 
implementation for details.