Error-free calculation with big.Float of golang

Introduction

When performing the four arithmetic operations of ETH with golang, a floating point calculation with 18 digits after the decimal point is required. If you are not aware of overflow, there will be an error in the calculation, so use the double precision floating point type in the correct usage.

let's try it

Things necessary

only golang

What kind of error will occur?

ETH is subtracted from the budget of 1000 ETH and then re-added. If there is no error, it should return to 1000 ETH.

error.go

package main

import (
	"fmt"
	"math/big"
	"strings"
)

func main() {
	budget, _ := new(big.Float).SetString("1000")
	amount, _ := new(big.Float).SetString("0.123456789012345678")
	balance := new(big.Float).Sub(budget, amount)
	summary := new(big.Float).SetPrec(128).Add(balance, amount)

	fmt.Printf("budget  = %v\n", formatFloatPrice(budget, 18))
	fmt.Printf("amount  = %v\n\n", formatFloatPrice(amount, 18))

	fmt.Printf("balance = budget - amount = %v\n", formatFloatPrice(balance, 18))
	fmt.Printf("budget = balance + amount = %v\n", formatFloatPrice(summary, 18))
}

func formatFloatPrice(amount *big.Float, decimals int) string {
	str := amount.Text('f', decimals)
	if strings.Contains(str, ".") {
		str = strings.TrimRight(str, "0")
		if strings.HasSuffix(str, ".") {
			str = str + "0"
		}
	}
	return str
}

When I run it, it should be 1000 ETH, but it doesn't work due to the error.

Execution result

budget  = 1000.0
amount  = 0.123456789012345678

balance = budget - amount = 999.876543210987654309
budget = balance + amount = 999.999999999999999987

How to eliminate the error?

Use the magic method SetPrec () to improve the accuracy of floating point numbers.

• In the case of ETC20 token, the issuer can specify the number of digits arbitrarily, so there is a possibility of overflow even with the following precision.

ok.go

package main

import (
	"fmt"
	"math/big"
	"strings"
)

func main() {
	budget, _ := new(big.Float).SetPrec(128).SetString("1000")
	amount, _ := new(big.Float).SetPrec(128).SetString("0.123456789012345678")
	balance := new(big.Float).SetPrec(128).Sub(budget, amount)

	summary := new(big.Float).SetPrec(128).Add(balance, amount)

	fmt.Printf("budget  = %v\n", formatFloatPrice(budget, 18))
	fmt.Printf("amount  = %v\n\n", formatFloatPrice(amount, 18))

	fmt.Printf("balance = budget - amount = %v\n", formatFloatPrice(balance, 18))
	fmt.Printf("budget = balance + amount = %v\n", formatFloatPrice(summary, 18))
}

func formatFloatPrice(amount *big.Float, decimals int) string {
	str := amount.Text('f', decimals)
	if strings.Contains(str, ".") {
		str = strings.TrimRight(str, "0")
		if strings.HasSuffix(str, ".") {
			str = str + "0"
		}
	}
	return str
}

I have returned to 1000 ETH safely!

Execution result

budget  = 1000.0
amount  = 0.123456789012345678

balance = budget - amount = 999.876543210987654322
budget = balance + amount = 1000.0