If you parse a string into a big.Float like f.SetString("0.001"), then multiply it, I'm seeing a loss of precision. If I use f.SetFloat64(0.001), I don't lose precision. Even doing a strconv.ParseFloat("0.001", 64), then calling f.SetFloat() works.
Full example of what I'm seeing here:
https://play.golang.org/p/_AyTHJJBUeL
Expanded from this question: https://stackoverflow.com/a/47546136/105562
The difference in output is due to imprecise representation of base 10 floating point numbers in float64 (IEEE-754 format) and the default precision and rounding of big.Float.
See this simple code to verify:
fmt.Printf("%.30f
", 0.001)
f, ok := new(big.Float).SetString("0.001")
fmt.Println(f.Prec(), ok)
Output of the above (try it on the Go Playground):
0.001000000000000000020816681712
64 true
So what we see is that the float64 value 0.001 is not exactly 0.001, and the default precision of big.Float is 64.
If you increase the precision of the number you set via a string value, you will see the same output:
s := "0.001"
f := new(big.Float)
f.SetPrec(100)
f.SetString(s)
fmt.Println(s)
fmt.Println(BigFloatToBigInt(f))
Now output will also be the same (try it on the Go Playground):
0.001
1000000000000000