book list

book.c

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

typedef struct _book {
	char author[50];
	int page;
	
	int is_sentinel;
	struct _book *next;
	struct _book *prev;
} book;

int count_pages(book *head, char *author) {
	head = head->next;
	int sum = 0;
	
	while (head->is_sentinel == 0) {
		if ( strcmp(head->author, author) == 0) {
			sum += head->page;
		}
		
		head = head->next;
	}
	
	return sum;
} 

/*TEST ONLY*/
book *create_book(char author[50], int page) {
	book *item;
	
	item = (book *)malloc(sizeof(book));
	if (item == NULL) { exit(1); }
	
	strcpy(item->author, author);
	item->page = page;
	item->is_sentinel = 0;
	
	return item;
}

/*TEST ONLY*/
book *create_book_list() {
	book *head, *tail;

	head = create_book("", 0);
	tail = create_book("", 0);
	
	head->is_sentinel = 1;
	tail->is_sentinel = 1;
	
	head->prev = NULL;
	head->next = tail;
	tail->prev = head;
	tail->next = NULL;
	
	return head;
}

/*TEST ONLY*/
void append_book(book *head, book *item) {
	while(head->next->is_sentinel == 0) {
		head = head->next;
	}
	
	item->next = head->next;
	head->next->prev = item;
	
	head->next = item;
	item->prev = head;
}

/*TEST ONLY*/
void delete_book_list(book *head) {
	book *next;
	
	while(head) {
		next = head->next;
		free(head);
		head = next;
	}
}

/*TEST ONLY*/
int main() {
	book *head = create_book_list();
	
	append_book(head, create_book("foo", 5));
	append_book(head, create_book("bar", 10));
	append_book(head, create_book("baz", 8));
	append_book(head, create_book("foo", 24));
	append_book(head, create_book("foo", 1));
	
	printf("%d\n", count_pages(head, "foo"));
	
	delete_book_list(head);

    return 0;
}