#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;
}