Conversion to roman numerals

core/src/ast.rs

@@ -9,7 +9,7 @@ use crate::serialize::{Deserialize, Serialize};
 use crate::value::{built_in_function::BuiltInFunction, ApplyMulHandling, Value};
 use crate::Attrs;
 use std::sync::Arc;
-use std::{cmp, fmt, io};
+use std::{borrow, cmp, fmt, io};
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub(crate) enum BitwiseBop {
@@ -621,6 +621,15 @@ fn evaluate_as<I: Interrupt>(
 				}
 				return Err(FendError::ExpectedANumber);
 			}
+			"roman" | "roman_numeral" => {
+				let a = evaluate(a, scope, attrs, context, int)?
+					.expect_num()?
+					.try_as_usize(int)?;
+				if a == 0 {
+					return Err(FendError::RomanNumeralZero);
+				}
+				return Ok(Value::String(borrow::Cow::Owned(to_roman(a))));
+			}
 			_ => (),
 		}
 	}
@@ -754,3 +763,29 @@ pub(crate) fn resolve_identifier<I: Interrupt>(
 		_ => return crate::units::query_unit(ident.as_str(), attrs, context, int),
 	})
 }
+
+fn to_roman(mut num: usize) -> String {
+	let mut result = String::new();
+	for (r, n) in [
+		("M", 1000),
+		("CM", 900),
+		("D", 500),
+		("CD", 400),
+		("C", 100),
+		("XC", 90),
+		("L", 50),
+		("XL", 40),
+		("X", 10),
+		("IX", 9),
+		("V", 5),
+		("IV", 4),
+		("I", 1),
+	] {
+		let q = num / n;
+		num -= q * n;
+		for _ in 0..q {
+			result.push_str(r);
+		}
+	}
+	result
+}

core/src/error.rs

@@ -94,6 +94,7 @@ pub(crate) enum FendError {
 		before: date::Date,
 		after: date::Date,
 	},
+	RomanNumeralZero,
 }
 
 impl fmt::Display for FendError {
@@ -248,6 +249,7 @@ impl fmt::Display for FendError {
 					"{month} {expected_day}, {year} does not exist, did you mean {before} or {after}?",
 				)
 			}
+			Self::RomanNumeralZero => write!(f, "zero cannot be represented as a roman numeral"),
 		}
 	}
 }

core/tests/integration_tests.rs

@@ -5899,3 +5899,40 @@ fn test_equality() {
 	test_eval("2.010m == 200cm", "false");
 	test_eval("2.000m == approx. 200cm", "true");
 }
+
+#[test]
+fn test_roman() {
+	expect_error(
+		"0 to roman",
+		Some("zero cannot be represented as a roman numeral"),
+	);
+	test_eval_simple("1 to roman", "I");
+	test_eval_simple("2 to roman", "II");
+	test_eval_simple("3 to roman", "III");
+	test_eval_simple("4 to roman", "IV");
+	test_eval_simple("5 to roman", "V");
+	test_eval_simple("6 to roman", "VI");
+	test_eval_simple("7 to roman", "VII");
+	test_eval_simple("8 to roman", "VIII");
+	test_eval_simple("9 to roman", "IX");
+	test_eval_simple("10 to roman", "X");
+	test_eval_simple("11 to roman", "XI");
+	test_eval_simple("12 to roman", "XII");
+	test_eval_simple("13 to roman", "XIII");
+	test_eval_simple("14 to roman", "XIV");
+	test_eval_simple("15 to roman", "XV");
+	test_eval_simple("16 to roman", "XVI");
+	test_eval_simple("17 to roman", "XVII");
+	test_eval_simple("18 to roman", "XVIII");
+	test_eval_simple("19 to roman", "XIX");
+	test_eval_simple("20 to roman", "XX");
+	test_eval_simple("21 to roman", "XXI");
+	test_eval_simple("22 to roman", "XXII");
+	test_eval_simple("45 to roman", "XLV");
+	test_eval_simple("134 to roman", "CXXXIV");
+	test_eval_simple("1965 to roman", "MCMLXV");
+	test_eval_simple("2020 to roman", "MMXX");
+	test_eval_simple("3456 to roman", "MMMCDLVI");
+	test_eval_simple("1452 to roman", "MCDLII");
+	test_eval_simple("20002 to roman", "MMMMMMMMMMMMMMMMMMMMII");
+}