convertroman/romannumeral.go

package convertroman

// Currently only supports Roman Numerals without viniculum (1-3999) and will throw an error for
// numbers outside of that range. See here for details on viniculum:
// https://en.wikipedia.org/wiki/Roman_numerals#Large_numbers

import (
	"errors"
)

var (
	// IntegerOutOfBounds - error for when the integer provided is invalid and unable to be converted to a roman numeral
	IntegerOutOfBounds = errors.New("integer must be between 1 and 3999")

	// lookup arrays used for converting from an int to a roman numeral extremely quickly.
	// method from here: https://rosettacode.org/wiki/Roman_numerals/Encode#Go
	r0 = [10]string{"", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX"}
	r1 = [10]string{"", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC"}
	r2 = [10]string{"", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM"}
	r3 = [4]string{"", "M", "MM", "MMM"}
)

// FromInt converts an integer value to a roman numeral string.
// An error is returned if the integer is not between 1 and 3999.
func FromInt(input int) (string, error) {
	// ensure provided integer is within the valid range
	if input < 1 || input > 3999 {
		return "OOB", IntegerOutOfBounds
	}

	// convert the integer to a roman numeral string and return it
	return r3[input%1e4/1e3] + r2[input%1e3/1e2] + r1[input%100/10] + r0[input%10], nil
}