gno.land/p/moul/x/daily/romannum/v0⧉

1// Package romannum is a pure port of the classic Roman-numeral converter kata:2// ToRoman / FromRoman, valid for 1..3999. Deterministic — no time, randomness3// or I/O — and free of any realm coupling, so it is reusable as a library.4//5// A live demo of this package (an interactive integer↔Roman converter) is at6// [r/moul/x/daily/romannumdemo](/r/moul/x/daily/romannumdemo/v0).7package romannum89import (10	"strconv"11	"strings"12)1314// romanUnits is the greedy subtractive-notation table, largest value first.15var romanUnits = []struct {16	val int17	sym string18}{19	{1000, "M"}, {900, "CM"}, {500, "D"}, {400, "CD"},20	{100, "C"}, {90, "XC"}, {50, "L"}, {40, "XL"},21	{10, "X"}, {9, "IX"}, {5, "V"}, {4, "IV"}, {1, "I"},22}2324// ToRoman renders an integer in 1..3999 as a Roman numeral.25// It panics if n is out of range.26func ToRoman(n int) string {27	if n < 1 || n > 3999 {28		panic("romannum: out of range (want 1..3999): " + strconv.Itoa(n))29	}30	var b strings.Builder31	for _, u := range romanUnits {32		for n >= u.val {33			b.WriteString(u.sym)34			n -= u.val35		}36	}37	return b.String()38}3940// FromRoman parses a Roman numeral back to an integer.41// It panics on any malformed input (e.g. "IIII", "IC", "VV").42func FromRoman(s string) int {43	n, ok := parseRoman(s)44	if !ok {45		panic("romannum: invalid roman numeral: " + s)46	}47	return n48}4950// charVal maps a single Roman digit to its value, or 0 if unknown.51func charVal(c byte) int {52	switch c {53	case 'I':54		return 155	case 'V':56		return 557	case 'X':58		return 1059	case 'L':60		return 5061	case 'C':62		return 10063	case 'D':64		return 50065	case 'M':66		return 100067	}68	return 069}7071// parseRoman is the pure, panic-free core used by FromRoman.72// It returns (value, true) only for a canonical numeral: it accepts input73// iff ToRoman(value) reproduces it exactly, which rejects malformed forms.74func parseRoman(s string) (int, bool) {75	s = strings.ToUpper(strings.TrimSpace(s))76	if s == "" {77		return 0, false78	}79	total, prev := 0, 080	for i := len(s) - 1; i >= 0; i-- {81		v := charVal(s[i])82		if v == 0 {83			return 0, false84		}85		if v < prev {86			total -= v87		} else {88			total += v89			prev = v90		}91	}92	if total < 1 || total > 3999 || ToRoman(total) != s {93		return 0, false94	}95	return total, true96}97