Fibonacci with big numbers (and progress)

fib.go

package main

import (
	"bufio"
	"fmt"
	"math/big"
	"os"
	"strconv"
	"strings"
	"time"

	"gopkg.in/cheggaaa/pb.v1"
)

// getNumberInput reads input from the stdin and returns it parsed as a number.
func getNumberInput(prompt string) (int, error) {
	reader := bufio.NewReader(os.Stdin)

	for {
		fmt.Printf("%v", prompt)
		if text, err := reader.ReadString('\n'); err != nil {
			return -1, err
		} else {
			text = strings.Replace(text, "\n", "", -1)
			if value, err := strconv.Atoi(text); err == nil {
				return value, nil
			}
		}
	}

	return -1, fmt.Errorf("Unexpected end of infinite loop.")
}

// fib calculates the n'th fibonacci number.
// Progress is reported via the progress channel while the final result is transmitted via the result channel.
func fib(n int64, result chan big.Int, progress chan float64) {
	if n < 1 {
		result <- big.Int{}
		return
	}

	// initial values
	a, b := big.NewInt(0), big.NewInt(1)

	// constants
	one := big.NewInt(1)
	nBig := big.NewInt(n)

	// report start
	progress <- 0.0
	for i := big.NewInt(0); i.Cmp(nBig) == -1; i.Add(i, one) {
		//a, b = a+b, a
		tmp := a
		a.Add(a, b)
		b.Set(tmp)

		// progress reporting
		iBig := big.NewInt(0).Set(i)
		iBig.Mul(iBig, big.NewInt(10000))
		p := big.NewInt(0).Div(iBig, nBig)
		progress <- (float64(p.Int64()) / 10000.0)
	}

	// report end
	progress <- 1.0

	result <- *a
}

func main() {
	n, err := getNumberInput("fib(n) = x; n = ")
	if err != nil {
		panic(err)
	}
	result := make(chan big.Int)
	progress := make(chan float64)

	go fib(int64(n), result, progress)

	// wait for it to start
	<-progress
	bar := pb.New(10000)
	bar.ShowCounters = false
	bar.ShowTimeLeft = false
	bar.ShowBar = false
	bar.ShowPercent = false
	bar.Prefix(fmt.Sprintf("Calculating fib(%v)... ", n))
	bar.Start()

	running := true
	startTimeout := time.After(3 * time.Second)
	for running {
		select {
		case <-startTimeout:
			// show bar only after timeout
			bar.ShowPercent = true
			bar.ShowBar = true
			bar.ShowSpeed = true
		case p := <-progress:
			if bar.ShowBar {
				barValue := int(p * 10000)
				bar.Set(barValue)
			}

			if p >= 1.0 {
				running = false
			}
		}
	}

	bar.FinishPrint("Done!")
	r := <-result
	fmt.Printf("fib(%v) = %v\n", n, &r)
}