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ryu
This is a Go implementation of Ryu, a fast algorithm for converting floating-point numbers to strings.
The API is:
func AppendFloat32(b []byte, f float32) []byte
func AppendFloat64(b []byte, f float64) []byte
func FormatFloat32(f float32) string
func FormatFloat64(f float64) string
These functions are the equivalents of calling strconv.FormatFloat or
strconv.AppendFloat using the formatter 'e' and precision -1:
// These are the same:
const f float32 = 1.234
s := ryu.FormatFloat32(f)
s := strconv.FormatFloat(float64(f), 'e', -1, 32)
Benchmarks
These benchmarks were taken with Go 1.12beta1 on Linux/amd64 using an Intel i7-8700K.
name old time/op new time/op delta
FormatFloat32-12 128ns ± 1% 50ns ± 2% -60.82% (p=0.000 n=7+8)
FormatFloat64-12 129ns ± 4% 65ns ± 5% -49.54% (p=0.000 n=7+8)
AppendFloat32/0e+00-12 24.4ns ± 1% 3.0ns ± 1% -87.88% (p=0.000 n=8+8)
AppendFloat32/1e+00-12 26.5ns ± 1% 13.2ns ± 3% -49.98% (p=0.000 n=8+8)
AppendFloat32/3e-01-12 52.2ns ± 1% 32.5ns ± 2% -37.73% (p=0.000 n=8+8)
AppendFloat32/1e+06-12 41.2ns ± 1% 17.9ns ± 1% -56.45% (p=0.000 n=8+7)
AppendFloat32/-1.2345e+02-12 83.3ns ± 2% 34.2ns ± 1% -58.90% (p=0.000 n=8+8)
AppendFloat64/0e+00-12 24.5ns ± 2% 3.3ns ± 2% -86.50% (p=0.000 n=8+8)
AppendFloat64/1e+00-12 26.9ns ± 1% 14.5ns ± 1% -46.06% (p=0.001 n=8+6)
AppendFloat64/3e-01-12 53.0ns ± 1% 42.5ns ± 0% -19.75% (p=0.001 n=8+6)
AppendFloat64/1e+06-12 41.4ns ± 1% 21.1ns ± 1% -49.05% (p=0.000 n=8+8)
AppendFloat64/-1.2345e+02-12 83.8ns ± 1% 43.3ns ± 1% -48.32% (p=0.000 n=8+8)
AppendFloat64/6.226662346353213e-309-12 25.5µs ± 1% 0.0µs ± 1% -99.84% (p=0.000 n=8+8)
The test TestRandomBenchmark gathers statistics about the distribution of call
latencies for random float64 values. Here is the summary for one sample of 10,000
random floats:
ryu_test.go:279: after sampling 50000 float64s:
ryu: min = 2ns max = 90ns median = 41ns mean = 41ns
strconv (stdlib): min = 8ns max = 25845ns median = 106ns mean = 154ns
The strconv.FormatFloat latency is bimodal because of an infrequently-taken
slow path that is orders of magnitude more expensive
(https://golang.org/issue/15672).
Size optimization
The Ryu algorithm requires several lookup tables. Ulf Adams's C library
implements a size optimization (RYU_OPTIMIZE_SIZE) which greatly reduces the
size of the float64 tables in exchange for a little more CPU cost.
I have a WIP implementation of this optimization on the size branch. A binary
built using that version is 7.96 kB smaller. The benchmark results take a hit as
compared with the non-size-optimized build:
name old time/op new time/op delta
FormatFloat32-12 50.0ns ± 2% 49.4ns ± 1% ~ (p=0.183 n=8+8)
FormatFloat64-12 65.0ns ± 5% 72.1ns ± 5% +10.96% (p=0.000 n=8+8)
AppendFloat32/0e+00-12 2.95ns ± 1% 2.98ns ± 1% ~ (p=0.072 n=8+8)
AppendFloat32/1e+00-12 13.2ns ± 3% 13.1ns ± 1% ~ (p=0.275 n=8+8)
AppendFloat32/3e-01-12 32.5ns ± 2% 32.4ns ± 1% ~ (p=0.742 n=8+8)
AppendFloat32/1e+06-12 17.9ns ± 1% 17.6ns ± 1% -2.12% (p=0.001 n=7+8)
AppendFloat32/-1.2345e+02-12 34.2ns ± 1% 34.4ns ± 1% ~ (p=0.426 n=8+8)
AppendFloat64/0e+00-12 3.31ns ± 2% 3.29ns ± 1% ~ (p=0.394 n=8+8)
AppendFloat64/1e+00-12 14.5ns ± 1% 14.6ns ± 4% ~ (p=0.641 n=6+8)
AppendFloat64/3e-01-12 42.5ns ± 0% 50.0ns ± 1% +17.44% (p=0.001 n=6+8)
AppendFloat64/1e+06-12 21.1ns ± 1% 21.1ns ± 2% ~ (p=0.452 n=8+8)
AppendFloat64/-1.2345e+02-12 43.3ns ± 1% 50.9ns ± 1% +17.57% (p=0.000 n=8+8)
AppendFloat64/6.226662346353213e-309-12 40.6ns ± 1% 47.7ns ± 1% +17.38% (p=0.000 n=8+8)
However, it's still generally faster than strconv:
name old time/op new time/op delta
FormatFloat32-12 129ns ± 2% 49ns ± 1% -61.72% (p=0.000 n=8+8)
FormatFloat64-12 130ns ± 3% 72ns ± 5% -44.32% (p=0.000 n=7+8)
AppendFloat32/0e+00-12 24.5ns ± 2% 3.0ns ± 1% -87.83% (p=0.000 n=8+8)
AppendFloat32/1e+00-12 26.4ns ± 1% 13.1ns ± 1% -50.26% (p=0.000 n=7+8)
AppendFloat32/3e-01-12 52.6ns ± 2% 32.4ns ± 1% -38.43% (p=0.000 n=8+8)
AppendFloat32/1e+06-12 41.3ns ± 2% 17.6ns ± 1% -57.51% (p=0.000 n=8+8)
AppendFloat32/-1.2345e+02-12 83.5ns ± 1% 34.4ns ± 1% -58.82% (p=0.000 n=8+8)
AppendFloat64/0e+00-12 24.6ns ± 2% 3.3ns ± 1% -86.63% (p=0.000 n=8+8)
AppendFloat64/1e+00-12 26.7ns ± 1% 14.6ns ± 4% -45.51% (p=0.000 n=8+8)
AppendFloat64/3e-01-12 52.7ns ± 1% 50.0ns ± 1% -5.17% (p=0.000 n=8+8)
AppendFloat64/1e+06-12 41.2ns ± 1% 21.1ns ± 2% -48.61% (p=0.000 n=7+8)
AppendFloat64/-1.2345e+02-12 83.7ns ± 1% 50.9ns ± 1% -39.17% (p=0.000 n=8+8)
AppendFloat64/6.226662346353213e-309-12 25.8µs ± 2% 0.0µs ± 1% -99.81% (p=0.000 n=8+8)
Notes
This package is a fairly direct Go translation of Ulf Adams's C library at https://github.com/ulfjack/ryu. This code is also licensed with Apache 2.0 as a derived work of that code.
This package requires Go 1.12 (expected to be released February 2019).
For a small fraction of inputs, Ryu gives a different value than strconv does for the last digit. This is due to a bug in strconv: https://golang.org/issue/29491.
Future work
My plan is to incorporate this into strconv (see https://golang.org/issue/15672). Then everyone will benefit from the faster algorithm and there will be no need for this library.
If you would like to contribute, I'm interested in any bugfixes or clear-cut optimizations, but given the above I don't intend to add more features or APIs to this package.