var x = (4 * y) + "px";

Thomas Depierre

haruspex.dev

@Di4naO

floating-point constant

float-to-string conversion

floating-point multiplication

string concatenation

https://slides.com/di4nao/string-that-float-codemesh

0.3

https://slides.com/di4nao/string-that-float-codemesh

sign

exponent

mantissa

(-1)^s * (1.mantissa)2 * 2^(exponent-bias)

0011001101

0.300048828125

exactly

https://slides.com/di4nao/string-that-float-codemesh

sign

exponent

mantissa

0011001101

0.300048828125

exactly

0.2999267578125

0.3001708984375

https://slides.com/di4nao/string-that-float-codemesh

0.300048828125

0.3?

https://slides.com/di4nao/string-that-float-codemesh

Dragon4 (Steele and White, 1990)
- Slow (uses bignums), complete
- Improved by Gay (1990), Burger and Dybvig(1996)
Grisu2 (Loitsch, 2010 –intermediate result)
- Incorrect output –don't use!
Grisu3 (Loitsch, 2010)Fast, incomplete (needs fallback algorithm)
Errol3 (Andrysco, Jhala, and Lerner, 2016)
- Moderate speed, complete
Ryu (Ulf Adams, 2018)
- Fastest, complete

Solutions

Most libc and stdlib

SUPER FAST

https://slides.com/di4nao/string-that-float-codemesh

 Name           ips        average  deviation         median         99th %
Ryu          80.26       12.46 ms     ±7.14%       12.19 ms       14.98 ms
OTP          25.36       39.43 ms    ±14.13%       38.02 ms       62.33 ms
Comparison: 
Ryu          80.26
OTP          25.36 - 3.16x slower +26.97 ms

SUPER FAST

https://slides.com/di4nao/string-that-float-codemesh

Sources

Stephan T. Lavavej, “Floating-Point ＜charconv＞: Making Your Code 10x Faster With C++17's Final Boss”
- https://www.youtube.com/watch?v=4P_kbF0EbZM
Ulf Adams, "Ryū: Fast Float-to-String Conversion"
- https://www.youtube.com/watch?v=kw-U6smcLzk
https://github.com/ulfjack/ryu

https://slides.com/di4nao/string-that-float-codemesh