diff --git a/assignment1-18/chessboard.c b/assignment1-18/chessboard.c
index dfac173..ae8e258 100644
--- a/assignment1-18/chessboard.c
+++ b/assignment1-18/chessboard.c
@@ -13,8 +13,10 @@
 #define DEBUG(...) (void) 0;
 #endif
 
-static inline void cb_move(struct chessboard* cb,
+static inline enum pieces cb_move(struct chessboard* cb,
         int from_row, int from_col, int to_row, int to_col);
+static inline void cb_move_restore(struct chessboard* cb, enum pieces piece,
+        int ex_from_row, int ex_from_col, int ex_to_row, int ex_to_col);
 static inline int sign(int n);
 static inline enum player player(struct chessboard* cb, int row, int col);
 static inline bool is_empty(struct chessboard* cb, int row, int col);
@@ -93,10 +95,19 @@ void print_chessboard(struct chessboard * cb) {
 
 }
 
-inline void cb_move(struct chessboard* cb,
+inline enum pieces cb_move(struct chessboard* cb,
         int from_row, int from_col, int to_row, int to_col) {
+    enum pieces tmp = cb->position[to_row][to_col];
     cb->position[to_row][to_col] = cb->position[from_row][from_col];
     cb->position[from_row][from_col] = EMPTY;
+    return tmp;
+}
+
+inline void cb_move_restore(struct chessboard* cb, enum pieces piece,
+        int ex_from_row, int ex_from_col, int ex_to_row, int ex_to_col) {
+    cb->position[ex_from_row][ex_from_col] =
+        cb->position[ex_to_row][ex_to_col];
+    cb->position[ex_to_row][ex_to_col] = piece;
 }
 
 inline bool is_empty(struct chessboard* cb, int row, int col) {
@@ -285,6 +296,44 @@ bool player_checks(struct chessboard *cb, enum player p) {
     return false;
 }
 
+bool player_checkmates(struct chessboard *cb, enum player p) {
+    int i, j;
+    for (i = 0; i < 8; i++) {
+        for (j = 0; j < 8; j++) {
+            if (is_empty(cb, i, j) || player(cb, i, j) == p) {
+                continue;
+            }
+
+            int k, l;
+            for (k = 0; k < 8; k++) {
+                for (l = 0; l < 8; l++) {
+                    if ((k == i && l == j) ||
+                        is_empty(cb, k, l) ||
+                        player(cb, k, l) != p) {
+                        continue;
+                    }
+
+                    bool can = can_move(cb, p == BLACK ? WHITE : BLACK,
+                            i, j, k, l);
+                    if (!can) {
+                        continue;
+                    }
+
+                    enum pieces tmp = cb_move(cb, i, j, k, l);
+                    bool check = player_checks(cb, p);
+                    cb_move_restore(cb, tmp, i, j, k, l);
+
+                    if (!check) {
+                        return false;
+                    }
+                }
+            }
+        }
+    }
+
+    return true;
+}
+
 enum mstatus move(
         struct chessboard * cb, enum player p,
         const char * from, const char * to) {
@@ -331,21 +380,24 @@ enum mstatus move(
         return INVALID;
     }
 
-    bool check_before_move = player_checks(cb, p == WHITE ? BLACK : WHITE);
+    bool check_before = player_checks(cb, p == WHITE ? BLACK : WHITE);
     bool can = can_move(cb, p, from_row, from_col, to_row, to_col);
 
     if (!can) {
         return INVALID;
     } else {
         // FIXME: check if move is allowed in check state
-        cb_move(cb, from_row, from_col, to_row, to_col);
-        if (player_checks(cb, p)) {
-            if (check_before_move) {
-                DEBUG("move: move does not end check state, invalid");
-                return INVALID;
+        enum pieces tmp = cb_move(cb, from_row, from_col, to_row, to_col);
+        if (player_checks(cb, p == WHITE ? BLACK : WHITE) && check_before) {
+            DEBUG("move: move does not end check state, invalid");
+            cb_move_restore(cb, tmp, from_row, from_col, to_row, to_col);
+            return INVALID;
+        } else if (player_checks(cb, p)) {
+            if (player_checkmates(cb, p)) {
+                return CHECK_MATE;
+            } else {
+                return CHECK;
             }
-            // TODO: check checkmate
-            return CHECK;
         } else {
             return VALID;
         }
@@ -357,18 +409,19 @@ int main() {
     init_chessboard(&cb);
     enum player p = WHITE;
     char from[5], to[5];
-    from[0] = '\0';
-    while(1) {
+    while (true) {
         print_chessboard(&cb);
         printf("%s moves.\n", p == WHITE ? "White" : "Black");
         printf("From: ");
         fgets(from, 5, stdin);
         // remove trailing \n: https://stackoverflow.com/q/2693776
         strtok(from, "\n");
+
         if (strcmp(from, "exit") == 0) {
             printf("Exiting...\n");
             return 0;
         }
+
         printf("To: ");
         fgets(to, 5, stdin);
         strtok(to, "\n");
@@ -386,6 +439,7 @@ int main() {
             case CHECK_MATE:
                 printf("Checkmate, %s wins.\n", p == WHITE ? "white" : "black");
         }
+
         if (s != INVALID) {
             p = !p; // switch player: this works since WHITE = 0 and BLACK = 1
         }