ex.11.6.c

#include <stdio.h>

struct entry
{
	int				value;
	struct entry	*previous;
	struct entry	*next;
};

void insertBefore (struct entry *insertion, struct entry *next)
{
	struct entry *previous = next->previous;

	if ( previous != (struct entry *) 0 ) {
		previous->next = insertion;
	}

	next->previous = insertion;

	insertion->previous = previous;
	insertion->next     = next;
}

void insertAfter (struct entry *insertion, struct entry *previous)
{
	struct entry *next = previous->next;

	if ( next != (struct entry *) 0 ) {
		next->previous  = insertion;
	}

	previous->next = insertion;

	insertion->previous = previous;
	insertion->next     = next;
}

void roundTrip (const struct entry *ptr)
{
	while ( ptr->next != (struct entry *) 0 ) {
		printf ("%i\n", ptr->value);
		ptr = ptr->next;
	}
	while ( ptr->previous != (struct entry *) 0 ) {
		printf ("%i\n", ptr->value);
		ptr = ptr->previous;
	}
	printf ("%i\n\n", ptr->value);
}

int main (void)
{
	struct entry *nNull = (struct entry *) 0;

	struct entry n1 = { .value = 1, .previous = nNull };
	struct entry n2 = { .value = 2, .previous = &n1 };
	struct entry n3 = { .value = 3, .previous = &n2 };

	n1.next = &n2;
	n2.next = &n3;
	n3.next = nNull;

	roundTrip (&n1);

	struct entry n9 = { .value = 9 };
	insertBefore (&n9, &n1);
	roundTrip (&n9);

	struct entry n8 = { .value = 8 };
	insertAfter (&n8, &n3);
	roundTrip (&n9);

	return 0;
}