#include <stdlib.h>
#include <stddef.h>
#include <stdbool.h> // pre bool

typedef int list_element_t;

typedef struct {
    size_t length;
    list_element_t elements[];
} list_t;

// ==== Deklarácie funkcií (tzv. prototypy) ====

list_t *new_list(size_t length, list_element_t default_value);
list_t *map_list(list_t *list, list_element_t (*map_fn)(list_element_t));
list_t *filter_list(list_t *list, bool (*filter_fn)(list_element_t));
list_element_t foldl_list(list_t *list, list_element_t initial, list_element_t (*fold_fn)(list_element_t, list_element_t));
list_element_t foldr_list(list_t *list, list_element_t initial, list_element_t (*fold_fn)(list_element_t, list_element_t));
list_t *reverse_list(list_t *list);
void delete_list(list_t *list);

// ==== Implementácie ====

list_t *new_list(size_t length, list_element_t default_value) {
    list_t *list = malloc(sizeof(list_t) + length * sizeof(list_element_t));
    if (list == NULL) {
        return NULL;
    }

    list->length = length;
    for (size_t i = 0; i < length; i++) {
        list->elements[i] = default_value;
    }

    return list;
}

list_t *map_list(list_t *list, list_element_t (*map_fn)(list_element_t)) {
    list_t *result = malloc(sizeof(list_t) + sizeof(list_element_t) * list->length);
    if (result == NULL) {
        return NULL;
    }

    result->length = list->length;

    for (size_t i = 0; i < list->length; i++) {
        result->elements[i] = map_fn(list->elements[i]);
    }

    return result;
}

list_t *filter_list(list_t *list, bool (*filter_fn)(list_element_t)) {
    size_t count = 0;

    for (size_t i = 0; i < list->length; i++) {
        if (filter_fn(list->elements[i])) {
            count++;
        }
    }

    list_t *result = malloc(sizeof(list_t) + sizeof(list_element_t) * count);
    if (result == NULL) {
        return NULL;
    }

    result->length = count;
    size_t index = 0;

    for (size_t i = 0; i < list->length; i++) {
        if (filter_fn(list->elements[i])) {
            result->elements[index] = list->elements[i];
            index++;
        }
    }

    return result;
}

list_element_t foldl_list(list_t *list, list_element_t initial, list_element_t (*fold_fn)(list_element_t, list_element_t)) {
    list_element_t acc = initial;

    for (size_t i = 0; i < list->length; i++) {
        acc = fold_fn(acc, list->elements[i]);
    }

    return acc;
}

list_element_t foldr_list(list_t *list, list_element_t initial, list_element_t (*fold_fn)(list_element_t, list_element_t)) {
    list_element_t acc = initial;

    for (size_t i = list->length; i > 0; i--) {
        acc = fold_fn(list->elements[i - 1], acc);
    }

    return acc;
}

list_t *reverse_list(list_t *list) {
    list_t *result = malloc(sizeof(list_t) + sizeof(list_element_t) * list->length);
    if (result == NULL) {
        return NULL;
    }

    result->length = list->length;

    for (size_t i = 0; i < list->length; i++) {
        result->elements[i] = list->elements[list->length - 1 - i];
    }

    return result;
}

void delete_list(list_t *list) {
    free(list);
}