diff --git a/a3/snake.c b/a3/snake.c
index e69de29..c959a3f 100644
--- a/a3/snake.c
+++ b/a3/snake.c
@@ -0,0 +1,86 @@
+#include "snake.h"
+#include <stdlib.h>
+
+struct snake* add_snake(struct snake* snake, int x, int y) {
+    struct snake* head = malloc(sizeof(struct snake));
+    if (head == NULL) {
+        return NULL;
+    }
+    head->x=x;
+    head->y=y;
+    head->next=snake;
+    return head;
+}
+
+struct snake* remove_snake(struct snake* snake) {
+    if (snake == NULL || snake->next == NULL) {
+        free(snake);
+        return NULL;
+    }
+    struct snake* lastbo = snake;
+    while (lastbo->next->next != NULL) {
+        lastbo=lastbo->next;
+    }
+    free(lastbo->next);
+    lastbo->next=NULL;
+    return snake;
+}
+
+int is_snake(struct snake* snake, int x, int y) {
+    while (snake != NULL) {
+        if (snake->x == x && snake->y == y) {
+            return 1;
+        }
+        snake=snake->next;
+    }
+    return 0;
+}
+
+void free_snake(struct snake* sn) {
+    while (sn != NULL) {
+        struct snake* a =sn;
+        sn=sn->next;
+        free(a);
+    }
+}
+
+int step_state(struct state* st) {
+    int nx = (st->snake->x + st->sx);
+    int ny = (st->snake->y + st->sy);
+
+    if (nx<0 || ny<0 || nx>=st->width || ny>=st->height) {
+        return END_WALL;
+    }
+
+    if (is_snake(st->snake, nx, ny)) {
+        return END_SNAKE;
+    }
+
+    int ate = 0;
+    for (int i = 0; i < FOOD_COUNT; i++) {
+        if (st->foodx[i]==nx && st->foody[i] == ny) {
+            st->foodx[i]=-1;
+            st->foody[i]=-1;
+            ate=1;
+            break;
+        }
+    }
+
+    st->snake = add_snake(st->snake, nx, ny);
+    if (ate == 0) {
+        st->snake=remove_snake(st->snake);
+    }
+
+    int foodleft = 0;
+    for (int i = 0; i < FOOD_COUNT; i++) {
+        if (st->foodx[i] >= 0 && st->foody[i] >= 0) {
+            foodleft = 1;
+            break;
+        }
+    }
+
+    if (!foodleft) {
+        return END_FOOD;
+    }
+    return END_CONTINUE;
+}