Move castling move generation logic to movegen.rs and refactor related repeated code

src/fen.rs

@@ -1,5 +1,5 @@
 use crate::board::{Board, Color, Kind, Piece};
-use crate::game::{Game, State};
+use crate::game::{Castle, Game, State};
 use String as FenError;
 
 pub fn from_fen(fen: &str) -> Result<Game, FenError> {
@@ -78,17 +78,15 @@ fn side_to_move(side: &str) -> Result<Color, FenError> {
     }
 }
 
-/// Return castling ability as a bitflag. LSb K, 2nd LSb Q, 3rd LSb k, 4th LSb q. Ignore 4 MSb's
-fn castling_ability(castling: &str) -> Result<u8, FenError> {
-    let mut rights = 0b0;
-
+fn castling_ability(castling: &str) -> Result<[Castle; 2], FenError> {
+    let mut bitflag = 0b0;
     for c in castling.chars() {
         match c {
-            'K' => rights |= 0b1,
-            'Q' => rights |= 0b10,
-            'k' => rights |= 0b100,
-            'q' => rights |= 0b1000,
-            '-' => rights |= 0b0,
+            'K' => bitflag |= 0b1000,
+            'Q' => bitflag |= 0b100,
+            'k' => bitflag |= 0b10,
+            'q' => bitflag |= 0b1,
+            '-' => break,
             _ => {
                 return Err(FenError::from(
                     "Found invalid character while parsing in castling_ability",
@@ -96,7 +94,30 @@ fn castling_ability(castling: &str) -> Result<u8, FenError> {
             }
         };
     }
-    Ok(rights)
+
+    let mut castling_ability: [Castle; 2] = [Castle::None, Castle::None];
+
+    let white_king_and_queen = bitflag & 0b11 == 0b11;
+    let white_king = (bitflag >> 1) & 1 == 1;
+    let white_queen = bitflag & 1 == 1;
+    castling_ability[0] = match (white_king_and_queen, white_king, white_queen) {
+        (true, _, _) => Castle::Both,
+        (_, true, _) => Castle::Short,
+        (_, _, true) => Castle::Long,
+        _ => Castle::None,
+    };
+
+    let black_king_and_queen = (bitflag >> 2) & 0b11 == 0b11;
+    let black_king = (bitflag >> 3) & 1 == 1;
+    let black_queen = (bitflag >> 2) & 1 == 1;
+    castling_ability[1] = match (black_king_and_queen, black_king, black_queen) {
+        (true, _, _) => Castle::Both,
+        (_, true, _) => Castle::Short,
+        (_, _, true) => Castle::Long,
+        _ => Castle::None,
+    };
+
+    Ok(castling_ability)
 }
 
 use std::collections::HashMap;