More on atof()

Some more background on atof(). I analyzing sequencing data. That means, I am processing files with billions of lines. I haven't yet mastered properly how to do all I need in a parallel manner, so some of my operations are linear. That's where it all starts. At first, I had programs written in perl+python to analyze the data. The problem was that each file took many hours to precess. The rate of processing (as measured by pv) was round 300kb/s. Simply running 'cat' on the file works at a rate of 100Mb/s. Quite a lot faster. I was trying to do all my processing at the rate of 'cat'. Here is where the problems start - other than the fact that I switched to flex/C/C++.... The data to be analyzed has lots of numbers, and so I read and write a lot of numbers, and every read/write operation on numbers takes a long time, basically because computers store data in binary, and I'm storing text files with a decimal representation of numbers.
There are 3 solutions to this problem:

1. Store numbers in binary in the file. For that one needs to define a good format for storing binary data. That wuold be really the best approach.
2. Write or find a good BCD (Binary-coded decimal) library. I know... sounds old. A bit like perl, I guess. But, as long as one doesn't to too complicated math, this will be almost as fast as 1.
3. improve read/write of numbers.

How much was there to improve? With sprintf my programs were running at rates of 10-30 MB/s, or 3-10 times "too slow".
I went on a hunt for an efficient itoa, but didn't really find much.
In the end, I wrote the itoa mentioned in my last post. That itoa was fixed width, now I also added variable width.
Here is a table of the relative speeds of various implementations:

size of number	my itoa	my itoa_fill0	itoa1	ufast_itoa10	new_itoa	sstream	i32toa	sprintf
10^1	1.00	2.25	1.27	1.10	1.81	6.04	9.98	18.64
10^2	1.00	2.21	1.33	1.09	1.91	6.07	9.91	18.62
10^3	1.00	2.45	1.80	1.41	3.01	6.68	12.16	22.06
10^4	1.00	2.33	2.25	1.38	2.47	6.24	12.06	21.12
10^5	1.00	1.33	1.80	2.76	4.46	4.18	7.98	14.48
10^6	1.00	1.12	1.87	2.27	3.85	3.52	6.86	12.76
10^7	1.00	1.21	2.27	2.97	4.75	3.79	7.90	14.05
10^8	1.00	1.27	2.78	3.07	4.74	4.00	6.72	15.08
10^9	1.23	1.00	2.70	3.85	5.11	3.49	6.08	13.73

It is easy to see what the problem is. sprintf is really slow. sstream is actually surprisingly fast.... atoi next. Then, just reading and writing from files....