shad-go/speller/speller.go

//go:build !solution

package speller

import (
	"strings"
)

var numToDigits = []string{
	"", "one", "two", "three", "four", "five",
	"six", "seven", "eight", "nine",
}

var teens = []string{
	"ten", "eleven", "twelve", "thirteen", "fourteen",
	"fifteen", "sixteen", "seventeen", "eighteen", "nineteen",
}

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

var 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, digits := strings.Builder{}, digitsReversedSlice(n)
	// 35 is the optimal number which gives approx needed
	// space for the number in 1 allocation
	num.Grow(len(digits) * 35)
	if sign {
		num.WriteString("minus ")
	}
	largeIndex, iters := 0, 0
	for i := len(digits) - 1; i >= 0; i, largeIndex = i-3, largeIndex+1 {
		first, second, third := digits[i-2], digits[i-1], digits[i]
		if first == 0 && second == 0 && third == 0 {
			continue
		}
		if iters != 0 {
			num.WriteRune(' ')
		}
		hundreds := numToDigits[third]
		if hundreds != "" {
			num.WriteString(hundreds)
			num.WriteString(" hundred")
			if second != 0 || first != 0 {
				num.WriteRune(' ')
			}
		}
		if second > 1 {
			num.WriteString(decs[second])
			if first > 0 {
				num.WriteRune('-')
				num.WriteString(numToDigits[first])
			}
		} else if second == 1 {
			num.WriteString(teens[first])
		} else {
			if first > 0 {
				num.WriteString(numToDigits[first])
			}
		}
		if largeIndex < len(large) {
			num.WriteRune(' ')
			num.WriteString(large[largeIndex])
		}
		iters++
	}
	return num.String()
}