conversions: Add roman numberals

conversions/src/lib.rs

@@ -20,6 +20,7 @@ mod excel_title_to_column;
 pub use excel_title_to_column::excel_title_to_column;
 pub mod energy;
 pub mod molecular_chemistry;
+pub mod roman_numerals;
 pub mod speed;
 pub mod volume;
 

conversions/src/roman_numerals.rs

@@ -0,0 +1,111 @@
+// Copyright (c) 2023 Xu Shaohua <shaohua@biofan.org>. All rights reserved.
+// Use of this source is governed by General Public License that can be found
+// in the LICENSE file.
+
+use std::ops::{Div, Rem};
+
+const ROMAN: &[(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"),
+];
+
+const ROMAN_INVERSE: &[(u8, u32)] = &[
+    (b'I', 1),
+    (b'V', 5),
+    (b'X', 10),
+    (b'L', 50),
+    (b'C', 100),
+    (b'D', 500),
+    (b'M', 1000),
+];
+
+#[allow(clippy::cast_possible_truncation)]
+fn to_int(byte: u8) -> u32 {
+    for (k, v) in ROMAN_INVERSE {
+        if k == &byte {
+            return *v;
+        }
+    }
+    0
+}
+
+/// Given a roman numeral, convert it to an integer.
+///
+/// [Roman Numerals](https://en.wikipedia.org/wiki/Roman_numerals)
+#[must_use]
+pub fn roman_to_int(roman: &str) -> u32 {
+    let mut total = 0;
+    let mut place = 0;
+    let len = roman.len();
+    while place < len {
+        let p0 = to_int(roman.as_bytes()[place]);
+        if place + 1 < len {
+            let p1 = to_int(roman.as_bytes()[place + 1]);
+            if p0 < p1 {
+                total += p1 - p0;
+                place += 2;
+                continue;
+            }
+        }
+        total += p0;
+        place += 1;
+    }
+    total
+}
+
+/// Given a integer, convert it to an roman numeral.
+///
+/// [Roman Numerals](https://en.wikipedia.org/wiki/Roman_numerals)
+#[must_use]
+pub fn int_to_roman(mut number: u32) -> String {
+    let mut result = String::new();
+    for (arabic, roman) in ROMAN {
+        let factor = number.div(arabic);
+        number = number.rem(arabic);
+        for _i in 0..factor {
+            result.push_str(roman);
+        }
+        if number == 0 {
+            break;
+        }
+    }
+    result
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{int_to_roman, roman_to_int};
+
+    const PAIRS: &[(&str, u32)] = &[
+        ("III", 3),
+        ("CLIV", 154),
+        ("MIX", 1009),
+        ("MMD", 2500),
+        ("MMMCMXCIX", 3999),
+    ];
+
+    #[test]
+    fn test_roman_to_int() {
+        for (s, val) in PAIRS {
+            assert_eq!(roman_to_int(s), *val);
+        }
+    }
+
+    #[test]
+    fn test_int_to_roman() {
+        for (s, val) in PAIRS {
+            assert_eq!(&int_to_roman(*val), s);
+        }
+    }
+}