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/// Parse Roman (classic or Excel variants) → number
pub fn from_roman(s: &str) -> Result<u32, String> {
if s.is_empty() {
return Err("empty numeral".into());
}
fn val(c: char) -> Option<u32> {
Some(match c {
'I' => 1,
'V' => 5,
'X' => 10,
'L' => 50,
'C' => 100,
'D' => 500,
'M' => 1000,
_ => return None,
})
}
// Accept the union of subtractive pairs used by the tables above (Excel-compatible).
fn allowed_subtractive(a: char, b: char) -> bool {
matches!(
(a, b),
// classic:
('I','V')|('I','X')|('X','L')|('X','C')|('C','D')|('C','M')
// Excel forms:
|('V','L')|('L','D')|('L','M') // VL, LD, LM
|('X','D')|('X','M') // XD, XM
|('V','M') // VM
|('I','L')|('I','C')|('I','D')|('I','M') // IL, IC, ID, IM
|('V','D')|('V','C') // VD, VC
)
}
let chars: Vec<char> = s.chars().map(|c| c.to_ascii_uppercase()).collect();
let mut total = 0u32;
let mut i = 0usize;
// Repetition rules similar to classic Romans:
// V, L, D cannot repeat; I, X, C, M max 3 in a row.
let mut last_char: Option<char> = None;
let mut run_len = 0usize;
while i < chars.len() {
let c = chars[i];
let v = val(c).ok_or_else(|| format!("invalid character '{c}'"))?;
if Some(c) == last_char {
run_len += 1;
match c {
'V' | 'L' | 'D' => return Err(format!("invalid repetition of '{c}'")),
_ if run_len >= 3 => return Err(format!("invalid repetition of '{c}'")),
_ => {}
}
} else {
last_char = Some(c);
run_len = 0;
}
if i + 1 < chars.len() {
let c2 = chars[i + 1];
let v2 = val(c2).ok_or_else(|| format!("invalid character '{c2}'"))?;
if v < v2 {
if !allowed_subtractive(c, c2) {
return Err(format!("invalid subtractive pair '{c}{c2}'"));
}
// Disallow stacked subtractives like IIV, XXL:
if run_len > 0 {
return Err(format!("malformed numeral near position {i}"));
}
total += v2 - v;
i += 2;
last_char = None;
run_len = 0;
continue;
}
}
total += v;
i += 1;
}
Ok(total)
}
/// Classic Roman (strict) encoder used as a base for all forms.
fn to_roman(mut n: u32) -> Result<String, String> {
if !(1..=3999).contains(&n) {
return Err("value out of range (must be 1..=3999)".into());
}
const MAP: &[(u32, &str)] = &[
(1000, "M"),
(900, "CM"),
(500, "D"),
(400, "CD"),
(100, "C"),
(90, "XC"),
(50, "L"),
(40, "XL"),
(10, "X"),
(9, "IX"),
(5, "V"),
(4, "IV"),
(1, "I"),
];
let mut out = String::with_capacity(15);
for &(val, sym) in MAP {
while n >= val {
out.push_str(sym);
n -= val;
}
if n == 0 {
break;
}
}
Ok(out)
}
/// Excel/Google Sheets compatible ROMAN(number, [form]) encoder.
/// `form`: 0..=4 (0=Classic, 4=Simplified).
pub fn to_roman_with_form(n: u32, form: i32) -> Result<String, String> {
let mut s = to_roman(n)?;
if form == 0 {
return Ok(s);
}
if !(0..=4).contains(&form) {
return Err("form must be between 0 and 4".into());
}
// Base rules (apply for all f >= 1)
let base_rules: &[(&str, &str)] = &[
// C(D|M)XC -> L$1XL
("CDXC", "LDXL"),
("CMXC", "LMXL"),
// C(D|M)L -> L$1
("CDL", "LD"),
("CML", "LM"),
// X(L|C)IX -> V$1IV
("XLIX", "VLIV"),
("XCIX", "VCIV"),
// X(L|C)V -> V$1
("XLV", "VL"),
("XCV", "VC"),
];
// Level 2 extra rules
let lvl2_rules: &[(&str, &str)] = &[
// V(L|C)IV -> I$1
("VLIV", "IL"),
("VCIV", "IC"),
// L(D|M)XL -> X$1
("LDXL", "XD"),
("LMXL", "XM"),
// L(D|M)VL -> X$1V
("LDVL", "XDV"),
("LMVL", "XMV"),
// L(D|M)IL -> X$1IX
("LDIL", "XDIX"),
("LMIL", "XMIX"),
];
// Level 3 extra rules
let lvl3_rules: &[(&str, &str)] = &[
// X(D|M)V -> V$1
("XDV", "VD"),
("XMV", "VM"),
// X(D|M)IX -> V$1IV
("XDIX", "VDIV"),
("XMIX", "VMIV"),
];
// Level 4 extra rules
let lvl4_rules: &[(&str, &str)] = &[
// V(D|M)IV -> I$1
("VDIV", "ID"),
("VMIV", "IM"),
];
// Helper to apply a batch of (from -> to) globally, in order.
fn apply_rules(mut t: String, rules: &[(&str, &str)]) -> String {
for (from, to) in rules {
if t.contains(from) {
t = t.replace(from, to);
}
}
t
}
s = apply_rules(s, base_rules);
if form >= 2 {
s = apply_rules(s, lvl2_rules);
}
if form >= 3 {
s = apply_rules(s, lvl3_rules);
}
if form >= 4 {
s = apply_rules(s, lvl4_rules);
}
Ok(s)
}