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float to string is incredibly fast now - look at Ulf Adams’ Ryu and Ryu Printf algorithms, which I’ve used to implement C++17 <charconv> in Visual Studio 2019 (16.2 has everything implemented except general precision; the upcoming 16.4 release adds that).

I don’t know of truly fast algorithms for string to float, although I improved upon our CRT’s performance by 40%.

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Formatting is much faster than before, but still terribly slow compared to simply rounding numbers using math and floor and ceiling appropriately.

Ryu is more than 100x slower than something like

rval = floor(100*val+0.5)/100.0

(which is not quite right due to numerical issues, but close, and illustrates the idea).

Formatting, to get a rounded float, is terribly slow.

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