mirror of
https://github.com/shopspring/decimal.git
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130 lines
4.6 KiB
Markdown
130 lines
4.6 KiB
Markdown
# decimal
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[](https://travis-ci.org/shopspring/decimal) [](https://godoc.org/github.com/shopspring/decimal) [](https://goreportcard.com/report/github.com/shopspring/decimal)
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Arbitrary-precision fixed-point decimal numbers in go.
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NOTE: can "only" represent numbers with a maximum of 2^31 digits after the decimal point.
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## Features
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* the zero-value is 0, and is safe to use without initialization
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* addition, subtraction, multiplication with no loss of precision
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* division with specified precision
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* database/sql serialization/deserialization
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* json and xml serialization/deserialization
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## Install
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Run `go get github.com/shopspring/decimal`
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## Requirements
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Decimal library requires Go version `>=1.7`
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## Usage
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```go
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package main
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import (
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"fmt"
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"github.com/shopspring/decimal"
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)
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func main() {
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price, err := decimal.NewFromString("136.02")
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if err != nil {
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panic(err)
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}
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quantity := decimal.NewFromInt(3)
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fee, _ := decimal.NewFromString(".035")
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taxRate, _ := decimal.NewFromString(".08875")
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subtotal := price.Mul(quantity)
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preTax := subtotal.Mul(fee.Add(decimal.NewFromFloat(1)))
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total := preTax.Mul(taxRate.Add(decimal.NewFromFloat(1)))
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fmt.Println("Subtotal:", subtotal) // Subtotal: 408.06
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fmt.Println("Pre-tax:", preTax) // Pre-tax: 422.3421
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fmt.Println("Taxes:", total.Sub(preTax)) // Taxes: 37.482861375
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fmt.Println("Total:", total) // Total: 459.824961375
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fmt.Println("Tax rate:", total.Sub(preTax).Div(preTax)) // Tax rate: 0.08875
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}
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```
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## Documentation
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http://godoc.org/github.com/shopspring/decimal
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## Production Usage
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* [Spring](https://shopspring.com/), since August 14, 2014.
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* If you are using this in production, please let us know!
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## FAQ
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#### Why don't you just use float64?
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Because float64s (or any binary floating point type, actually) can't represent
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numbers such as 0.1 exactly.
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Consider this code: http://play.golang.org/p/TQBd4yJe6B You might expect that
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it prints out `10`, but it actually prints `9.999999999999831`. Over time,
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these small errors can really add up!
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#### Why don't you just use big.Rat?
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big.Rat is fine for representing rational numbers, but Decimal is better for
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representing money. Why? Here's a (contrived) example:
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Let's say you use big.Rat, and you have two numbers, x and y, both
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representing 1/3, and you have `z = 1 - x - y = 1/3`. If you print each one
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out, the string output has to stop somewhere (let's say it stops at 3 decimal
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digits, for simplicity), so you'll get 0.333, 0.333, and 0.333. But where did
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the other 0.001 go?
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Here's the above example as code: http://play.golang.org/p/lCZZs0w9KE
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With Decimal, the strings being printed out represent the number exactly. So,
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if you have `x = y = 1/3` (with precision 3), they will actually be equal to
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0.333, and when you do `z = 1 - x - y`, `z` will be equal to .334. No money is
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unaccounted for!
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You still have to be careful. If you want to split a number `N` 3 ways, you
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can't just send `N/3` to three different people. You have to pick one to send
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`N - (2/3*N)` to. That person will receive the fraction of a penny remainder.
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But, it is much easier to be careful with Decimal than with big.Rat.
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#### Why isn't the API similar to big.Int's?
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big.Int's API is built to reduce the number of memory allocations for maximal
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performance. This makes sense for its use-case, but the trade-off is that the
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API is awkward and easy to misuse.
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For example, to add two big.Ints, you do: `z := new(big.Int).Add(x, y)`. A
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developer unfamiliar with this API might try to do `z := a.Add(a, b)`. This
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modifies `a` and sets `z` as an alias for `a`, which they might not expect. It
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also modifies any other aliases to `a`.
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Here's an example of the subtle bugs you can introduce with big.Int's API:
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https://play.golang.org/p/x2R_78pa8r
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In contrast, it's difficult to make such mistakes with decimal. Decimals
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behave like other go numbers types: even though `a = b` will not deep copy
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`b` into `a`, it is impossible to modify a Decimal, since all Decimal methods
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return new Decimals and do not modify the originals. The downside is that
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this causes extra allocations, so Decimal is less performant. My assumption
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is that if you're using Decimals, you probably care more about correctness
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than performance.
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## License
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The MIT License (MIT)
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This is a heavily modified fork of [fpd.Decimal](https://github.com/oguzbilgic/fpd), which was also released under the MIT License.
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