Golang : How to convert a number to words

I need to translate a given number into words such as 100 to one hundred. The code below is a translation of Christian d'Heureuse's work at http://www.source-code.biz/snippets/java/13.htm. It shows how to convert a number up to billion into words.

Here you go!

 package main

 import (
 "fmt"
 )

 var lowNames = []string{"zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen"}

 var tensNames = []string{"twenty", "thirty", "forty", "fifty", "sixty", "seventy", "eighty", "ninety"}

 var bigNames = []string{"thousand", "million", "billion"}

 func convert999(num int) string {
 s1 := lowNames[num/100] + " hundred"
 s2 := convert99(num % 100)
 if num <= 99 {
 return s2
 }
 if num0 == 0 {
 return s1
 } else {
 return s1 + " " + s2
 }
 }

 func convert99(num int) string {
 if num < 20 {
 return lowNames[num]
 }
 s := tensNames[num/10-2]
 if num == 0 {
 return s
 }
 return s + "-" + lowNames[num]
 }

 func convertNum2Words(num int) string {
 if num < 0 {
 return "negative " + convertNum2Words(-num)
 }

 if num <= 999 {
 return convert999(num)
 }

 s := ""
 t := 0
 for num > 0 {
 if num00 != 0 {
 s2 := convert999(num % 1000)
 if t > 0 {
 s2 = s2 + " " + bigNames[t-1]
 }
 if s == "" {
 s = s2
 } else {
 s = s2 + ", " + s
 }
 }
 num /= 1000
 t++
 }
 return s
 }

 func main() {
 fmt.Println("100 in words : ", convertNum2Words(100))
 fmt.Println("0123 in words : ", convertNum2Words(0123))
 fmt.Println("-99 in words : ", convertNum2Words(-99))
 fmt.Println("54312337 in words : ", convertNum2Words(54312337))
 fmt.Println("932476182910 in words : ", convertNum2Words(932476182910))
 fmt.Println("-932476182910 in words : ", convertNum2Words(-932476182910))

 // because we only convert up to billion, any number exceeding 9 zeros will cause
 // runtime error: index out of range
 //fmt.Println("9324761829101 in words : " , convertNum2Words(9324761829101))
 }

Output:

100 in words : one hundred

0123 in words : eighty-three <------ wrong!

-99 in words : negative ninety-nine

54312337 in words : fifty-four million, three hundred twelve thousand, three hundred thirty-seven

932476182910 in words : nine hundred thirty-two billion, four hundred seventy-six million, one hundred eighty-two thousand, nine hundred ten

-932476182910 in words : negative nine hundred thirty-two billion, four hundred seventy-six million, one hundred eighty-two thousand, nine hundred ten

NOTES:

You need to put in a sanity checker to test if the input number is a sane number. As demonstrated, having a given number that starts with zero will cause the translation to go crazy. For example, the translation of 0123 to eighty-three just doesn't make sense.

How to write the sanity checker? That's the exercise for this tutorial. ;-)

Reference:

http://www.source-code.biz/snippets/java/13.htm