//go:build !solution

package speller

import (
	"strings"
)

var (
	digits = []string{
		"", "one", "two", "three", "four", "five",
		"six", "seven", "eight", "nine",
	}
	teens = []string{
		"ten", "eleven", "twelve", "thirteen", "fourteen",
		"fifteen", "sixteen", "seventeen", "eighteen", "nineteen",
	}
	decs = []string{
		"", "", "twenty", "thirty", "forty", "fifty",
		"sixty", "seventy", "eighty", "ninety",
	}
	large = []string{
		"billion", "million", "thousand",
	}
)

func digitsReversedSlice(n int64) []uint8 {
	digits := make([]uint8, 12)
	for i := 0; i < len(digits); i, n = i+1, n/10 {
		digits[i] = uint8(n % 10)
	}
	return digits
}

func Spell(n int64) string {
	if n == 0 {
		return "zero"
	}
	sign := n < 0
	if sign {
		n = -n
	}
	num, numDigits := strings.Builder{}, digitsReversedSlice(n)
	// 35 is the optimal number which gives approx needed
	// space for the number in 1 allocation
	num.Grow(len(numDigits) * 35)
	if sign {
		num.WriteString("minus ")
	}
	for i, largeIndex := len(numDigits)-1, 0; i >= 0; i, largeIndex = i-3, largeIndex+1 {
		hundreds, tens, ones := numDigits[i], numDigits[i-1], numDigits[i-2]
		hundredsPresent, tensPesent, onesPresent := hundreds != 0, tens != 0, ones != 0
		if !hundredsPresent && !tensPesent && !onesPresent {
			continue
		}
		if hundredsPresent {
			num.WriteString(digits[hundreds])
			num.WriteString(" hundred")
			if tensPesent || onesPresent {
				num.WriteRune(' ')
			}
		}
		if tensPesent {
			if tens == 1 {
				num.WriteString(teens[ones])
			} else {
				num.WriteString(decs[tens])
				if onesPresent {
					num.WriteRune('-')
				}
			}
		}
		if onesPresent && tens != 1 {
			num.WriteString(digits[ones])
		}
		if largeIndex < len(large) {
			num.WriteRune(' ')
			num.WriteString(large[largeIndex])
		}
		num.WriteRune(' ')
	}
	output := num.String()
	output = output[:len(output)-1]
	return output
}