/*
Convert numbers to English words in Kotlin
------------------------------------------
Supports numbers from 0 to 999,999,999,999,999.
Strategy:
- Break the number into named groups: trillions, billions, millions,
thousands, hundreds.
- Convert each 0–999 group using a helper function.
- Append group names ("trillion", "billion", etc.) when needed.
- Build the final string using a MutableList<String> and joinToString().
*/
private val ones: List<String> = listOf(
"zero", "one", "two", "three", "four",
"five", "six", "seven", "eight", "nine",
"ten", "eleven", "twelve", "thirteen", "fourteen",
"fifteen", "sixteen", "seventeen", "eighteen", "nineteen"
)
private val tensWords: List<String> = listOf(
"", "", "twenty", "thirty", "forty",
"fifty", "sixty", "seventy", "eighty", "ninety"
)
/*
appendWord:
Adds a word to the output list.
*/
private fun appendWord(out: MutableList<String>, word: String) {
if (word.isNotEmpty()) out += word
}
/*
convertHundreds:
Converts numbers from 0 to 999 into words.
*/
private fun convertHundreds(n0: Long, out: MutableList<String>) {
var n: Long = n0
if (n == 0L) return
if (n >= 100) {
appendWord(out, ones[(n / 100).toInt()])
appendWord(out, "hundred")
n %= 100
if (n > 0) appendWord(out, "and")
}
if (n >= 20) {
appendWord(out, tensWords[(n / 10).toInt()])
if (n % 10 != 0L) appendWord(out, ones[(n % 10).toInt()])
} else if (n > 0) {
appendWord(out, ones[n.toInt()])
}
}
/*
numberToWords:
Converts any number from 0 to 999,999,999,999,999 into English words.
*/
fun numberToWords(num: Long): String {
if (num == 0L) return "zero"
val out: MutableList<String> = mutableListOf()
val trillions: Long = num / 1_000_000_000_000
val billions: Long = (num / 1_000_000_000) % 1000
val millions: Long = (num / 1_000_000) % 1000
val thousands: Long = (num / 1_000) % 1000
val hundreds: Long = num % 1000
if (trillions > 0) {
convertHundreds(trillions, out)
appendWord(out, "trillion")
}
if (billions > 0) {
convertHundreds(billions, out)
appendWord(out, "billion")
}
if (millions > 0) {
convertHundreds(millions, out)
appendWord(out, "million")
}
if (thousands > 0) {
convertHundreds(thousands, out)
appendWord(out, "thousand")
}
if (hundreds > 0) {
convertHundreds(hundreds, out)
}
return out.joinToString(" ")
}
/*
main:
Demonstrates usage with multiple test numbers.
*/
fun main() {
val tests: List<Long> = listOf(
1_234,
12_345,
123_456,
1_234_567,
123_456_789,
12_345_678_901,
1_234_567_890_123,
999_999_999_999_999
)
for (n in tests) {
println("$n -> ${numberToWords(n)}")
}
}
/*
run:
1234 -> one thousand two hundred and thirty four
12345 -> twelve thousand three hundred and forty five
123456 -> one hundred and twenty three thousand four hundred and fifty six
1234567 -> one million two hundred and thirty four thousand five hundred and sixty seven
123456789 -> one hundred and twenty three million four hundred and fifty six thousand seven hundred and eighty nine
12345678901 -> twelve billion three hundred and forty five million six hundred and seventy eight thousand nine hundred and one
1234567890123 -> one trillion two hundred and thirty four billion five hundred and sixty seven million eight hundred and ninety thousand one hundred and twenty three
999999999999999 -> nine hundred and ninety nine trillion nine hundred and ninety nine billion nine hundred and ninety nine million nine hundred and ninety nine thousand nine hundred and ninety nine
*/
