Add board to fen function

engine/include/fen.h

@@ -1 +1,2 @@
 int load_fen();
+void board_to_fen(struct Board *board, char *out, size_t out_sz);

engine/src/fen.c

@@ -10,6 +10,11 @@ int square_index(int file, int rank) {
     return rank*8 + file;
 }
 
+static const char PIECE_CH[12] = {
+    'P','N','B','R','Q','K',
+    'p','n','b','r','q','k'
+};
+
 int char_to_piece_index(char c) {
     switch (c) {
         case 'P': return P; case 'N': return N; case 'B': return B;
@@ -117,4 +122,73 @@ int load_fen(struct Board *board, const char *fen) {
     board->occ[BOTH] = board->occ[WHITE] | board->occ[BLACK];
 
     return 0;
+}
+
+void board_to_fen(const struct Board *board, char *out, size_t out_sz) {
+    char fen[256];
+    char *p = fen;
+
+    // 1. Piece placement
+    for (int rank = 7; rank >= 0; rank--) {
+        int empty = 0;
+        for (int file = 0; file < 8; file++) {
+            int sq = rank * 8 + file;
+            char piece = 0;
+
+            for (int i = 0; i < 12; i++) {
+                if (board->pieces[i] & (1ULL << sq)) {
+                    piece = PIECE_CH[i];
+                    break;
+                }
+            }
+
+            if (piece) {
+                if (empty > 0) {
+                    *p++ = '0' + empty;
+                    empty = 0;
+                }
+                *p++ = piece;
+            } else {
+                empty++;
+            }
+        }
+        if (empty > 0) *p++ = '0' + empty;
+        if (rank > 0) *p++ = '/';
+    }
+
+    // 2. Side to move
+    *p++ = ' ';
+    *p++ = (board->side_to_move == WHITE) ? 'w' : 'b';
+
+    // 3. Castling rights
+    *p++ = ' ';
+    bool any = false;
+    if (board->castling_rights & CASTLE_WK) { *p++ = 'K'; any = true; }
+    if (board->castling_rights & CASTLE_WQ) { *p++ = 'Q'; any = true; }
+    if (board->castling_rights & CASTLE_BK) { *p++ = 'k'; any = true; }
+    if (board->castling_rights & CASTLE_BQ) { *p++ = 'q'; any = true; }
+    if (!any) *p++ = '-';
+
+    // 4. En passant
+    *p++ = ' ';
+    if (board->ep_square >= 0) {
+        int file = board->ep_square % 8;
+        int rank = board->ep_square / 8;
+        *p++ = 'a' + file;
+        *p++ = '1' + rank;
+    } else {
+        *p++ = '-';
+    }
+
+    // 5. Halfmove clock
+    p += sprintf(p, " %d", board->halfmove_clock);
+
+    // 6. Fullmove number
+    p += sprintf(p, " %d", board->fullmove_number);
+
+    *p = '\0';
+
+    // Copy safely into output buffer
+    strncpy(out, fen, out_sz);
+    out[out_sz - 1] = '\0';
 }