Make en_passant_square an Option<u8>

src/fen.rs

@@ -101,7 +101,7 @@ fn castling_ability(castling: &str) -> Result<u8, FenError> {
 
 use std::collections::HashMap;
 
-fn en_passant_target_square(square: &str) -> Result<u8, FenError> {
+fn en_passant_target_square(square: &str) -> Result<Option<u8>, FenError> {
     let mut sqr = square.chars();
 
     let mut files: HashMap<char, u8> = HashMap::new();
@@ -120,14 +120,15 @@ fn en_passant_target_square(square: &str) -> Result<u8, FenError> {
 
     match sqr.next() {
         Some(file) if files.contains_key(&file) => match sqr.next() {
-            Some(rank) if ranks.contains_key(&rank) => Ok(ranks.get(&rank).expect("Invalid rank")
-                * 8
-                + files.get(&file).expect("Invalid file")),
+            Some(rank) if ranks.contains_key(&rank) => Ok(Some(
+                ranks.get(&rank).expect("Invalid rank") * 8
+                    + files.get(&file).expect("Invalid file"),
+            )),
             Some(_) | None => Err(FenError::from(
                 "Not a valid rank (3 or 6) for an en passant target square",
             )),
         },
-        Some('-') => Ok(0),
+        Some('-') => Ok(None),
         Some(_) | None => Err(FenError::from("Not a file (a..h) or dash (-) character")),
     }
 }