#include <stdlib.h>
#include <stdio.h>
#include <string.h>

int ID = 1;
struct List list;

struct Person
{
    // Unique identifier for the person
    int id;
    // Information about person
    char name[20];
    int age;
    // Pointer to next person in list
    struct Person *next;
};

struct List
{
    // First person in the list. A value equal to NULL indicates that the
    // list is empty.
    struct Person *head;
    // Current person in the list. A value equal to NULL indicates a
    // past-the-end position.
    struct Person *current;
    // Pointer to the element appearing before 'current'. It can be NULL if
    // 'current' is NULL, or if 'current' is the first element in the list.
    struct Person *previous;
    // Number of persons in the list
    int count;
};

// Give an initial value to all the fields in the list.
void ListInitialize(struct List *list)
{
    list->head = NULL;
    list->current = NULL;
    list->previous = NULL;
    list->count = 0;
}


// Move the current position in the list one element forward. If last element
// is exceeded, the current position is set to a special past-the-end value.
void ListNext(struct List *list)
{
    if (list->current)
    {
        list->previous = list->current;
        list->current = list->current->next;
    }

}

// Move the current position to the first element in the list.
void ListHead(struct List *list)
{
    list->previous = NULL;
    list->current = list->head;
}

// Get the element at the current position, or NULL if the current position is
// past-the-end.
struct Person *ListGet(struct List *list)
{
    return list->current;
}

// Set the current position to the person with the given id. If no person
// exists with that id, the current position is set to past-the-end.
void ListFind(struct List *list, int id)
{
    ListHead(list);
    while (list->current && list->current->id != id)
    ListNext(list);
}


// Insert a person before the element at the current position in the list. If
// the current position is past-the-end, the person is inserted at the end of
// the list. The new person is made the new current element in the list.
void ListInsert(struct List *list, struct Person *person)
{
    // Set 'next' pointer of current element
    person->next = list->current;
    // Set 'next' pointer of previous element. Treat the special case where
    // the current element was the head of the list.
    if (list->current == list->head)
        list->head = person;
    else
        list->previous->next = person;
    // Set the current element to the new person
    list->current = person;
}


// Remove the current element in the list. The new current element will be the
// element that appeared right after the removed element.
void ListRemove(struct List *list)
{
    // Ignore if current element is past-the-end
    if (!list->current)
        return;
    // Remove element. Consider special case where the current element is
    // in the head of the list.
    if (list->current == list->head)
        list->head = list->current->next;
    else
        list->previous->next = list->current->next;
    // Free element, but save pointer to next element first.
    struct Person *next = list->current->next;
    free(list->current);
    // Set new current element
    list->current = next;
}


void PrintPerson(struct Person *person)
{
    printf("Person with ID %d:\n", person->id);
    printf("\tName: %s\n", person->name);
    printf("\tAge: %d\n\n", person->age);
}


struct Person * makePerson()
{
    struct Person *p = malloc(sizeof(struct Person));
    printf("\nYou selected \"Add a Person\".");
        printf("\nEnter name: ");
        char name[20];
        scanf("%s", name);
        printf("\nEnter an age: ");
        int age;
        scanf("%d", &age);
        p->age = age;
        strcpy(p->name, name);
        p->id = ID;
        ID++;
    return p;

}


void findPerson()

{
    printf("\nYou selected \"Find a person\".");
    printf("\nEnter an Id:  ");
    int userId;
    scanf("%d", &userId);
    printf("\n");
    ListFind(&list, userId);
    struct Person *p = ListGet(&list);
    if(p)
    {
        PrintPerson(p);
    }
    else
    {
        printf("\nCannot find specified user");
    }
}

void removePerson()
{
        printf("\nYou selected \"Remove a person\".");
        printf("\nEnter an Id:  ");
        int userId;
        scanf("%d", &userId);
        printf("\n");
        ListFind(&list, userId);
        struct Person *p = ListGet(&list);
        if(p)
        {
        ListRemove(&list);
                printf("Removed a person with Id: %d\n", userId);
        }
        else
        {
                printf("\nCannot find specified user");
        }

}

void printList()
{
    ListHead(&list);
        while (list.current)
    {
        struct Person *p = ListGet(&list);
        PrintPerson(p);
            ListNext(&list);
    }

}

int main()
{
    int cond = 1;
    ListInitialize(&list);
        while(cond) { // Loop the menu
        int choice;
        printf("\n------------------------------------\nMain menu:\n\n1. Add a Person\n2. Find a person\n3. Remove a person\n4. Print the List\n5. Exit\n\nSelect an option:");
        scanf("%d", &choice); // Read in user choice
        switch(choice){
            case 1:
            {
                struct Person *p = makePerson();
                    ListInsert(&list, p);
                break;
            }
            case 2:
            {
                findPerson();
                break;
            }
            case 3:
                removePerson();
                break;
            case 4:
                printList();
                break;
            case 5:
                printf("\nYou selected \"Exit\".\n");
                    cond = 0; // Break out of while loop
                break;
            default:
                break;
         }
        printf("\n\n\n");
    }
}