diff --git a/du5/snake.c b/du5/snake.c
new file mode 100644
index 0000000..82ec194
--- /dev/null
+++ b/du5/snake.c
@@ -0,0 +1,87 @@
+#include "snake.h"
+#include <stdlib.h>
+
+struct snake* add_snake(struct snake* head, int x, int y) {
+    struct snake* node = (struct snake*)malloc(sizeof(struct snake));
+    if (!node) return NULL;
+
+    node->x = x;
+    node->y = y;
+    node->next = head;
+    return node;
+}
+
+struct snake* remove_snake(struct snake* head) {
+    if (!head) return NULL;
+    if (!head->next) {
+        free(head);
+        return NULL;
+    }
+
+    struct snake* current = head;
+    while (current->next && current->next->next) {
+        current = current->next;
+    }
+
+    free(current->next);
+    current->next = NULL;
+    return head;
+}
+
+void free_snake(struct snake* head) {
+    while (head) {
+        struct snake* temp = head;
+        head = head->next;
+        free(temp);
+    }
+}
+
+int is_snake(struct snake* head, int x, int y) {
+    struct snake* temp = head;
+    while (temp) {
+        if (temp->x == x && temp->y == y) {
+            return 1;
+        }
+        temp = temp->next;
+    }
+    return 0;
+}
+
+int step_state(struct state* st) {
+    int new_x = st->snake->x + st->sx;
+    int new_y = st->snake->y + st->sy;
+
+    // Collision with walls
+    if (new_x < 0 || new_y < 0 || new_x >= st->width || new_y >= st->height) {
+        return END_WALL;
+    }
+
+    // Collision with itself
+    if (is_snake(st->snake, new_x, new_y)) {
+        return END_SNAKE;
+    }
+
+    // Check food collision
+    for (int i = 0; i < FOOD_COUNT; ++i) {
+        if (st->foodx[i] == new_x && st->foody[i] == new_y) {
+            st->foodx[i] = -1;
+            st->foody[i] = -1;
+            st->snake = add_snake(st->snake, new_x, new_y);
+
+            int food_remaining = 0;
+            for (int j = 0; j < FOOD_COUNT; ++j) {
+                if (st->foodx[j] >= 0 && st->foody[j] >= 0) {
+                    food_remaining = 1;
+                    break;
+                }
+            }
+
+            return food_remaining ? END_CONTINUE : END_FOOD;
+        }
+    }
+
+    // Normal move: add head, remove tail
+    st->snake = add_snake(st->snake, new_x, new_y);
+    st->snake = remove_snake(st->snake);
+    return END_CONTINUE;
+}