fen: simplify fen_check() + fix e.p. when no possible capture

src/fen.c

@@ -68,7 +68,7 @@ static const char *castle_str = "KQkq";
  * - inconsistent castle flags (if K & R are not in correct position)
  * - inconsistent en-passant square (turn, bad pawn position)
  *
- * pos_check() is also called, leading to fatal errors if something is wrong.
+ * pos_ok() is also called, leading to fatal errors if something is wrong.
  *
  * @return: 0 if OK, 1 if OK after fix, -1 if fatal issue.
  */
@@ -76,19 +76,22 @@ static int fen_check(pos_t *pos)
 {
     char *colstr[2] = { "white", "black"};
     int warning = 0;
+    color_t us = pos->turn, them = OPPONENT(us);
+    int up = sq_up(us);                           /* pawn up */
+    square_t ep = pos->en_passant;
 
     /* en passant, depends on who plays next */
-    if (pos->en_passant != SQUARE_NONE) {
-        rank_t eprank = sq_rank(pos->en_passant);
-        file_t epfile = sq_file(pos->en_passant);
-        rank_t rank5 = sq_rel_rank(RANK_5, pos->turn);
-        rank_t rank6 = sq_rel_rank(RANK_6, pos->turn);
-        rank_t rank7 = sq_rel_rank(RANK_7, pos->turn);
-        piece_t pawn = pos->turn == WHITE? B_PAWN: W_PAWN;
+    if (ep != SQUARE_NONE) {
+        rank_t eprank = sq_rank(ep);
+        rank_t rank6 = sq_rel_rank(RANK_6, us);
+        piece_t pawn = MAKE_PIECE(PAWN, them);
+        bitboard_t att = bb_pawn_attacks[them][ep] & pos->bb[us][PAWN];
+
         if (warn(eprank != rank6 ||
-                 pos->board[sq_make(epfile, rank5)] != pawn ||
-                 pos->board[sq_make(epfile, rank6)] != EMPTY ||
-                 pos->board[sq_make(epfile, rank7)] != EMPTY,
+                 pos->board[ep - up] != pawn ||
+                 pos->board[ep] != EMPTY ||
+                 pos->board[ep + up] != EMPTY ||
+                 att == 0ull,
                  "fen warn: wrong en-passant settings. (fixed)\n")) {
 #           ifdef DEBUG_FEN
             printf("ep5=%o ep6=%o ep7=%o\n", sq_make(epfile, rank5),
@@ -100,7 +103,7 @@ static int fen_check(pos_t *pos)
     }
 
     for (int color = WHITE; color <= BLACK; ++color) {
-        rank_t rank1 = color == WHITE? RANK_1: RANK_8;
+        rank_t rank1 = sq_rel_rank(RANK_1, color);
 
         /* castling */
         /* where K and R should be for valid castle flag */
@@ -338,8 +341,8 @@ char *pos2fen(const pos_t *pos, char *fen)
     if (pos->en_passant == SQUARE_NONE) {
         fen[cur++] = '-';
     } else {
-        fen[cur++] = FILE2C(sq_file(pos->en_passant));
-        fen[cur++] = RANK2C(sq_rank(pos->en_passant));
+        strcpy(fen+cur, sq_to_string(pos->en_passant));
+        cur += 2;
     }
     fen[cur++] = ' ';
 

src/position.c

@@ -336,7 +336,7 @@ bitboard_t pos_king_blockers(const pos_t *pos, const color_t color, const bitboa
  * - side-to-move already checking opponent king
  * - side-to-move in check more than twice
  * - kings distance is 1
- * - TODO: discrepancy between piece bitboards and ALL_PIECES bitboards (per color)
+ * - TODO: - castling / e.p. flags
  *
  * In case of errors, and @strict is true, @bug_on() is called, and program will
  * be terminated.
@@ -349,6 +349,7 @@ bitboard_t pos_king_blockers(const pos_t *pos, const color_t color, const bitboa
 bool pos_ok(const pos_t *pos, const bool strict)
 {
     int n, count = 0, bbcount = 0, error = 0;
+    color_t us = pos->turn, them = OPPONENT(us);
 
     /* force BUG_ON and WARN_ON */
 #   pragma push_macro("BUG_ON")
@@ -391,12 +392,11 @@ bool pos_ok(const pos_t *pos, const bool strict)
     /* occupied board is different from bitboards */
     error += warn_on(count != bbcount);
     /* is opponent already in check ? */
-    error += warn_on(pos_checkers(pos, OPPONENT(pos->turn)));
+    error += warn_on(pos_checkers(pos, them));
     /* is color to play in check more than twice ? */
-    error += warn_on(popcount64(pos_checkers(pos, pos->turn)) > 2);
+    error += warn_on(popcount64(pos_checkers(pos, us)) > 2);
     /* kings distance is less than 2 */
     error += warn_on(sq_dist(pos->king[WHITE], pos->king[BLACK]) < 2);
-
     if (strict) {
         bug_on(error);
         /* not reached */

test/common-test.h

@@ -393,6 +393,14 @@ struct fentest {
       "simple movedo/undo: only 2 W knights",
       "5n2/1k6/8/8/5K2/8/P7/1N6 w - - 0 1"
     },
+    { __LINE__, FEN,
+      "legal EP",
+      "4k3/8/8/3pP3/8/8/8/4K3 w - d6 0 1"
+    },
+    { __LINE__, FEN,
+      "illegal EP, fix-able by fen parser",
+      "4k3/8/8/3p1P2/8/8/8/4K3 w - d6 0 1"
+    },
     { __LINE__, FEN,
       "illegal EP and castle flags, fix-able by fen parser, SF crash",
       "4k3/8/8/8/7B/8/8/4K3 w KQkq e6 0 1"

test/fen-test.c

@@ -31,8 +31,7 @@ int main(__unused int ac, __unused char**av)
     bitboard_init();
 
     while ((fen = next_fen(FEN))) {
-        printf("***** [%s] ", fen);
-        fflush(stdout);
+        printf("line %3d: [%s] ", cur_line(), fen);
         if (!(pos = fen2pos(NULL, fen))) {
             printf("**INVALID\n");
         } else {

test/perft-test.c

@@ -287,7 +287,7 @@ int main(int __unused ac, __unused char**av)
     while ((fen = next_fen(PERFT | MOVEDO))) {
         test_line = cur_line();
         if (!(fenpos = fen2pos(pos, fen))) {
-            printf("wrong fen %d: [%s]\n", i, fen);
+            printf("wrong fen line = %d: [%s]\n", test_line, fen);
             continue;
         }
         pos = fenpos;