--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/src/chess/fen.c Sun Jun 07 16:56:22 2026 +0200
@@ -0,0 +1,171 @@
+/*
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS HEADER.
+ *
+ * Copyright 2026 Mike Becker. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+#include "fen.h"
+
+#include <stdlib.h>
+#include <stdio.h>
+
+static size_t fen_pieces(char *str, GameState *gamestate) {
+ size_t i = 0;
+ for (int row = 7 ; row >= 0 ; row--) {
+ unsigned int skip = 0;
+ for (int file = 0 ; file < 8 ; file++) {
+ if (gamestate->board[row][file]) {
+ if (skip > 0) {
+ str[i++] = '0'+skip;
+ skip = 0;
+ }
+ switch (gamestate->board[row][file] & ~ENPASSANT_THREAT) {
+ case WHITE|KING: str[i++] = 'K'; break;
+ case WHITE|QUEEN: str[i++] = 'Q'; break;
+ case WHITE|BISHOP: str[i++] = 'B'; break;
+ case WHITE|KNIGHT: str[i++] = 'N'; break;
+ case WHITE|ROOK: str[i++] = 'R'; break;
+ case WHITE|PAWN: str[i++] = 'P'; break;
+ case BLACK|KING: str[i++] = 'k'; break;
+ case BLACK|QUEEN: str[i++] = 'q'; break;
+ case BLACK|BISHOP: str[i++] = 'b'; break;
+ case BLACK|KNIGHT: str[i++] = 'n'; break;
+ case BLACK|ROOK: str[i++] = 'r'; break;
+ case BLACK|PAWN: str[i++] = 'p'; break;
+ }
+ } else {
+ skip++;
+ }
+ }
+ if (skip > 0) {
+ str[i++] = '0'+skip;
+ }
+ if (row > 0) {
+ str[i++] = '/';
+ }
+ }
+
+ return i;
+}
+
+static size_t fen_color(char *str, GameState *gamestate) {
+ uint8_t color = opponent_color(gamestate->movecount > 0 ?
+ (last_move(gamestate).piece & COLOR_MASK) : BLACK);
+
+ str[0] = color == WHITE ? 'w' : 'b';
+
+ return 1;
+}
+
+static bool fen_castling_chkmoved(GameState *gamestate,
+ uint8_t row, uint8_t file) {
+
+ for (unsigned i = 0 ; i < gamestate->movecount ; i++) {
+ if (gamestate->moves[i].fromfile == file
+ && gamestate->moves[i].fromrow == row) {
+ return true;
+ }
+ }
+
+ return false;
+}
+
+static size_t fen_castling(char *str, GameState *gamestate) {
+ bool K, Q, k, q;
+
+ if (fen_castling_chkmoved(gamestate, rowidx('1'), fileidx('e'))) {
+ K = Q = false;
+ } else {
+ K = !fen_castling_chkmoved(gamestate, rowidx('1'), fileidx('h'));
+ Q = !fen_castling_chkmoved(gamestate, rowidx('1'), fileidx('a'));
+ }
+ if (fen_castling_chkmoved(gamestate, rowidx('8'), fileidx('e'))) {
+ k = q = false;
+ } else {
+ k = !fen_castling_chkmoved(gamestate, rowidx('8'), fileidx('h'));
+ q = !fen_castling_chkmoved(gamestate, rowidx('8'), fileidx('a'));
+ }
+
+ size_t i = 0;
+ if (K) str[i++] = 'K';
+ if (Q) str[i++] = 'Q';
+ if (k) str[i++] = 'k';
+ if (q) str[i++] = 'q';
+ if (!i) str[i++] = '-';
+
+ return i;
+}
+
+static size_t fen_enpassant(char *str, GameState *gamestate) {
+
+ str[0] = '-'; str[1] = '\0';
+
+ for (int file = 0 ; file < 8 ; file++) {
+ if (gamestate->board[3][file] & ENPASSANT_THREAT) {
+ str[0] = filechr(file);
+ str[1] = rowchr(2);
+ }
+ if (gamestate->board[4][file] & ENPASSANT_THREAT) {
+ str[0] = filechr(file);
+ str[1] = rowchr(5);
+ }
+ }
+
+ return str[0] == '-' ? 1 : 2;
+}
+
+static size_t fen_halfmove(char *str, GameState *gamestate) {
+ unsigned int hm = 0;
+ for (unsigned int i = 0; i < gamestate->movecount; i++) {
+ if (gamestate->moves[i].capture
+ || (gamestate->moves[i].piece & PIECE_MASK) == PAWN) {
+ hm = 0;
+ } else {
+ hm++;
+ }
+ }
+
+ return sprintf(str, "%u", hm);
+}
+
+static size_t fen_movenr(char *str, GameState *gamestate) {
+ return sprintf(str, "%u", gamestate->movecount);
+}
+
+void fen_compute(char *str, GameState *gamestate) {
+ str += fen_pieces(str, gamestate);
+ *str = ' '; str++;
+ str += fen_color(str, gamestate);
+ *str = ' '; str++;
+ str += fen_castling(str, gamestate);
+ *str = ' '; str++;
+ str += fen_enpassant(str, gamestate);
+ *str = ' '; str++;
+ str += fen_halfmove(str, gamestate);
+ *str = ' '; str++;
+ str += fen_movenr(str, gamestate);
+ str[0] = '\0';
+}