diff -r e3a1a794351e -r 564af8a16828 src/chess/rules.c
--- a/src/chess/rules.c	Tue Aug 28 15:45:44 2018 +0200
+++ b/src/chess/rules.c	Tue Aug 28 15:56:33 2018 +0200
@@ -327,17 +327,22 @@
         return result;
     }
     
+    /* simulate move for check validation */
+    GameState simulation = gamestate_copy_sim(gamestate);
+    Move simmove = *move;
+    apply_move_impl(&simulation, &simmove, 1);
+    
     /* find kings for check validation */
     uint8_t piececolor = (move->piece & COLOR_MASK);
     
     uint8_t mykingfile = 0, mykingrow = 0, opkingfile = 0, opkingrow = 0;
     for (uint8_t row = 0 ; row < 8 ; row++) {
         for (uint8_t file = 0 ; file < 8 ; file++) {
-            if (gamestate->board[row][file] ==
+            if (simulation.board[row][file] ==
                     (piececolor == WHITE?WKING:BKING)) {
                 mykingfile = file;
                 mykingrow = row;
-            } else if (gamestate->board[row][file] ==
+            } else if (simulation.board[row][file] ==
                     (piececolor == WHITE?BKING:WKING)) {
                 opkingfile = file;
                 opkingrow = row;
@@ -345,11 +350,6 @@
         }
     }
     
-    /* simulate move for check validation */
-    GameState simulation = gamestate_copy_sim(gamestate);
-    Move simmove = *move;
-    apply_move_impl(&simulation, &simmove, 1);
-    
     /* don't move into or stay in check position */
     if (is_covered(&simulation, mykingrow, mykingfile,
         opponent_color(piececolor))) {
@@ -495,19 +495,20 @@
 _Bool is_pinned(GameState *gamestate, Move *move) {
     uint8_t color = move->piece & COLOR_MASK;
 
+    GameState simulation = gamestate_copy_sim(gamestate);
+    Move simmove = *move;
+    apply_move(&simulation, &simmove);
+    
     uint8_t kingfile = 0, kingrow = 0;
     for (uint8_t row = 0 ; row < 8 ; row++) {
         for (uint8_t file = 0 ; file < 8 ; file++) {
-            if (gamestate->board[row][file] == (color|KING)) {
+            if (simulation.board[row][file] == (color|KING)) {
                 kingfile = file;
                 kingrow = row;
             }
         }
     }
-
-    GameState simulation = gamestate_copy_sim(gamestate);
-    Move simmove = *move;
-    apply_move(&simulation, &simmove);
+    
     _Bool covered = is_covered(&simulation,
         kingrow, kingfile, opponent_color(color));
     gamestate_cleanup(&simulation);
@@ -570,7 +571,7 @@
         int reason = INVALID_POSITION;
         
         // find threats for the specified position
-        for (uint8_t i = 0 ; i < threatcount ; i++) {
+        for (uint8_t i = 0 ; i < threatcount ; i++) {            
             if ((threats[i].piece & (PIECE_MASK | COLOR_MASK))
                     == move->piece &&
                     (move->fromrow == POS_UNSPECIFIED ||