diff --git a/du4/program.c b/du4/program.c
new file mode 100644
index 0000000..d120ffa
--- /dev/null
+++ b/du4/program.c
@@ -0,0 +1,81 @@
+#include "snake.h"
+#include <stdlib.h>
+
+static struct snake* get_tail(struct snake* snake) {
+    struct snake* tail = snake;
+    while (tail != NULL && tail->next != NULL) {
+        tail = tail->next;
+    }
+    return tail;
+}
+
+struct snake* add_snake(struct snake* snake, int x, int y) {
+    struct snake* head = calloc(1, sizeof(struct snake));
+    if (head == NULL) {
+        return snake;
+    }
+    head->x = x;
+    head->y = y;
+    head->next = snake;
+    return head;
+}
+
+struct snake* remove_snake(struct snake* snake) {
+    if (snake == NULL) {
+        return NULL;
+    }
+    if (snake->next == NULL) {
+        free(snake);
+        return NULL;
+    }
+    struct snake* prev = snake;
+    struct snake* last = snake->next;
+    while (last->next != NULL) {
+        prev = last;
+        last = last->next;
+    }
+    prev->next = NULL;
+    free(last);
+    return snake;
+}
+
+void free_snake(struct snake* sn) {
+    while (sn != NULL) {
+        struct snake* next = sn->next;
+        free(sn);
+        sn = next;
+    }
+}
+
+int is_snake(struct snake* snake, int x, int y) {
+    for (struct snake* node = snake; node != NULL; node = node->next) {
+        if (node->x == x && node->y == y) {
+            return 1;
+        }
+    }
+    return 0;
+}
+
+int step_state(struct state* st) {
+    int nx = st->snake->x + st->sx;
+    int ny = st->snake->y + st->sy;
+    int ate_food = nx == st->food_x && ny == st->food_y;
+    struct snake* tail = get_tail(st->snake);
+
+    if (nx < 0 || ny < 0 || nx >= st->width || ny >= st->height) {
+        return END_GAME;
+    }
+    if (is_snake(st->snake, nx, ny)) {
+        if (ate_food || tail == NULL || tail->x != nx || tail->y != ny) {
+            return END_GAME;
+        }
+    }
+    st->snake = add_snake(st->snake, nx, ny);
+    if (st->snake == NULL) {
+        return END_GAME;
+    }
+    if (!ate_food) {
+        st->snake = remove_snake(st->snake);
+    }
+    return END_CONTINUE;
+}
\ No newline at end of file