diff -r c33567d61ba7 -r 5762f2b5f87a src/chess/fen.c
--- /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';
+}