From a661a2febb6ecabd6b6f2b8ea131abab70498726 Mon Sep 17 00:00:00 2001
From: Noah Hellman <noah@hllmn.net>
Date: Mon, 23 Jan 2023 21:41:04 +0100
Subject: [PATCH] parser: parse AlphaLower/Upper numbers

---
 src/lib.rs | 29 ++++++++++++++++++++++++++++-
 1 file changed, 28 insertions(+), 1 deletion(-)

diff --git a/src/lib.rs b/src/lib.rs
index 187e211..3494cd9 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -267,7 +267,26 @@ impl OrderedListNumbering {
     fn parse_number(self, n: &str) -> u32 {
         match self {
             Self::Decimal => n.parse().unwrap(),
-            Self::AlphaLower | Self::AlphaUpper => 1,
+            Self::AlphaLower | Self::AlphaUpper => {
+                let d0 = u32::from(if matches!(self, Self::AlphaLower) {
+                    b'a'
+                } else {
+                    b'A'
+                });
+                let weights = (1..=n.len()).scan(1, |a, _| {
+                    let prev = *a;
+                    *a *= 26;
+                    Some(prev)
+                });
+                n.as_bytes()
+                    .iter()
+                    .rev()
+                    .copied()
+                    .map(u32::from)
+                    .zip(weights)
+                    .map(|(d, w)| w * (d - d0 + 1))
+                    .sum()
+            }
             Self::RomanLower => 1,
             Self::RomanUpper => 1,
         }
@@ -1102,4 +1121,12 @@ mod test {
             }),
         );
     }
+
+    #[test]
+    fn numbering_alpha() {
+        assert_eq!(AlphaLower.parse_number("a"), 1);
+        assert_eq!(AlphaUpper.parse_number("B"), 2);
+        assert_eq!(AlphaUpper.parse_number("Z"), 26);
+        assert_eq!(AlphaLower.parse_number("aa"), 27);
+    }
 }