Fibonacci closure in go

Question

I am following the go tour on their official website and I have been asked to write a Fibonacci generator. Here it is:

 package main

import "fmt"

// fibonacci is a function that returns
// a function that returns an int.
func fibonacci() func() int {
    first := 0
    second := 0
    return func() int{
        if(first == 0) {
         first = 1
         second = 1
         return 0
        }else {
            current := first   
            firstc := second
            second = first + second
            first = firstc
            return current
        }



    }
}

func main() {
    f := fibonacci()
    for i := 0; i < 10; i++ {
        fmt.Println(f())
    }
}

It works. However I consider it very ugly and I'm sure there has to be a better solution. I have been thinking about posting this on the code-review however since I'm asking for a better approach I thought this is the right place to post it.

Is there a better way to write this code?

Here is the task:

Implement a fibonacci function that returns a function (a closure) that returns successive fibonacci numbers.

Answer 1

My favorite clean way to implement iterating through the Fibonacci numbers is to use first as f_{i - 1}, and second as f_i. The Fibonacci equation states that:

f_{i + 1} = f_i + f_{i - 1}

Except when we write this in code, in the next round we're incrementing i. So we're effectively doing:

f_{next i} = f_{current i} + f_{current i - 1}

and

f_{next i - 1} = f_{current i}

The way I like to implement this in code is:

first, second = second, first + second

The first = second part corresponds to updating f_{next i - 1} = f_{current i}, and the second = first + second part corresponds to updating f_{next i} = f_{current i} + f_{current i - 1}.

Then all we have left to do is return the old value of first, so we'll store it in a temp variable out of the way before doing the update. In total, we get:

// fibonacci returns a function that returns
// successive fibonacci numbers from each
// successive call
func fibonacci() func() int {
    first, second := 0, 1
    return func() int {
        ret := first
        first, second = second, first+second
        return ret
    }
}

See it in action on the Go Playground.