--- a/src/chess/fen.c Thu Sep 03 18:32:47 2026 +0200
+++ b/src/chess/fen.c Fri Sep 04 13:49:48 2026 +0200
@@ -31,8 +31,9 @@
#include <stdlib.h>
#include <stdio.h>
+#include <string.h>
-static size_t fen_pieces(char *str, GameState *gamestate) {
+static size_t fen_pieces(char *str, const GameState *gamestate) {
size_t i = 0;
Rank rank = 7;
do {
@@ -72,12 +73,12 @@
return i;
}
-static size_t fen_color(char *str, GameState *gamestate) {
+static size_t fen_color(char *str, const GameState *gamestate) {
str[0] = current_color(gamestate) == WHITE ? 'w' : 'b';
return 1;
}
-static size_t fen_castling(char *str, GameState *gamestate) {
+static size_t fen_castling(char *str, const GameState *gamestate) {
size_t i = 0;
if (!gamestate->castling.K) str[i++] = 'K';
if (!gamestate->castling.Q) str[i++] = 'Q';
@@ -88,7 +89,7 @@
return i;
}
-static size_t fen_enpassant(char *str, GameState *gamestate) {
+static size_t fen_enpassant(char *str, const GameState *gamestate) {
str[0] = '-';
@@ -106,7 +107,8 @@
return str[0] == '-' ? 1 : 2;
}
-static size_t fen_halfmove(char *str, GameState *gamestate) {
+static size_t fen_halfmove(char *str, const GameState *gamestate) {
+ // TODO: respect a possible fifty_ctr_start
unsigned int hm = 0;
for (unsigned int i = 0; i < gamestate->movecount; i++) {
if (gamestate->moves[i].capture
@@ -120,8 +122,9 @@
return sprintf(str, "%u", hm);
}
-static size_t fen_movenr(char *str, GameState *gamestate) {
- return sprintf(str, "%u", 1 + gamestate->movecount / 2);
+static size_t fen_movenr(char *str, const GameState *gamestate) {
+ unsigned mc = gamestate->movecount + gamestate->move_start;
+ return sprintf(str, "%u", 1 + mc / 2);
}
static size_t fen_space(char *str) {
@@ -129,7 +132,7 @@
return 1;
}
-void fen_compute(char *str, GameState *gamestate) {
+void fen_compute(char *str, const GameState *gamestate) {
str += fen_pieces(str, gamestate);
str += fen_space(str);
str += fen_color(str, gamestate);
@@ -143,3 +146,153 @@
str += fen_movenr(str, gamestate);
*str = '\0';
}
+
+static unsigned fen_parse_number(const char *str, unsigned *target) {
+ unsigned l = 0;
+ *target = 0;
+ while (str[l] >= '0' && str[l] <= '9') {
+ unsigned n = str[l] - '0';
+ *target *= 10;
+ *target += n;
+ l++;
+ }
+ /* safety precaution - reject unreasonable high numbers */
+ if (l > 5) return 0;
+ return l;
+}
+
+int fen_parse(const char *str, GameState *gamestate) {
+ const char * const fen_start = str;
+ // TODO: think about error reporting that is as good as for PGNs
+
+ if (str == NULL) return 1;
+
+ /* zero-initialize the game state */
+ memset(gamestate, 0, sizeof(GameState));
+
+ /* parse the board (FEN starts top-left at "a8") */
+ Rank r = 7;
+ File f = 0;
+ while (true) {
+ switch (*str) {
+ case 'K': gamestate->board[r][f] = WKING; break;
+ case 'Q': gamestate->board[r][f] = WQUEEN; break;
+ case 'B': gamestate->board[r][f] = WBISHOP; break;
+ case 'N': gamestate->board[r][f] = WKNIGHT; break;
+ case 'R': gamestate->board[r][f] = WROOK; break;
+ case 'P': gamestate->board[r][f] = WPAWN; break;
+ case 'k': gamestate->board[r][f] = BKING; break;
+ case 'q': gamestate->board[r][f] = BQUEEN; break;
+ case 'b': gamestate->board[r][f] = BBISHOP; break;
+ case 'n': gamestate->board[r][f] = BKNIGHT; break;
+ case 'r': gamestate->board[r][f] = BROOK; break;
+ case 'p': gamestate->board[r][f] = BPAWN; break;
+ case '1': break;
+ case '2': f += 1; break;
+ case '3': f += 2; break;
+ case '4': f += 3; break;
+ case '5': f += 4; break;
+ case '6': f += 5; break;
+ case '7': f += 6; break;
+ case '8': f += 7; break;
+ default: return 1;
+ }
+ f++;
+ str++;
+ if (f == 8) {
+ /* rank complete - test for separator or ending space */
+ if (r > 0) {
+ if (*str != '/') return 1;
+ str++;
+ f = 0;
+ r--;
+ } else {
+ if (*str != ' ') return 1;
+ str++;
+ break;
+ }
+ }
+ }
+
+ /* whose turn is it? */
+ bool white_to_move;
+ if (str[0] == 'w') {
+ white_to_move = true;
+ } else if (str[0] == 'b') {
+ white_to_move = false;
+ } else {
+ return 1;
+ }
+ if (str[1] != ' ') return 1;
+ str += 2;
+
+ /* castling rights */
+ gamestate->castling.K = gamestate->castling.Q = true;
+ gamestate->castling.k = gamestate->castling.q = true;
+ if (*str == '-') {
+ str++;
+ } else {
+ char cstl[5] = "KQkq";
+ bool found = false;
+ for (unsigned i = 0 ; i < 4 ; i++) {
+ if (*str == cstl[i]) {
+ found = true;
+ switch (i) {
+ case 0: gamestate->castling.K = false; break;
+ case 1: gamestate->castling.Q = false; break;
+ case 2: gamestate->castling.k = false; break;
+ case 3: gamestate->castling.q = false; break;
+ }
+ str++;
+ }
+ }
+ if (!found) return 1; /* no castling info found */
+ }
+ if (*str != ' ') return 1;
+ str++;
+
+ /* is there an en-passant threat? */
+ if (*str == '-') {
+ str++;
+ } else {
+ if (isfile(str[0]) && isrank(str[1])) {
+ f = fileidx(str[0]);
+ r = rankidx(str[1]);
+ if (r == 2) {
+ r = 3;
+ } else if (r == 5) {
+ r = 4;
+ } else {
+ return 1;
+ }
+ /* the threat is applied to the pawn, not the field it passed */
+ enpassant_threat_add(gamestate, f, r);
+ } else {
+ return 1;
+ }
+ }
+ if (*str != ' ') return 1;
+ str++;
+
+ /* fifty-moves counter */
+ unsigned mnr;
+ unsigned mlen;
+ mlen = fen_parse_number(str, &mnr);
+ if (mlen == 0) return 1;
+ if (str[mlen] != ' ') return 1;
+ str += mlen+1;
+ gamestate->fifty_cntr_start = mnr;
+
+ /* move number */
+ mlen = fen_parse_number(str, &mnr);
+ if (mlen == 0) return 1;
+ if (mnr == 0) return 1;
+ if (str[mlen] != '\0') return 1;
+ str += mlen+1;
+ gamestate->move_start = 2*mnr - 1;
+ if (white_to_move) gamestate->move_start--;
+
+ /* only copy the fen string if everything is a success */
+ gamestate->fen_start = strdup(fen_start);
+ return 0;
+}
\ No newline at end of file