On 02/24/2018 11:26 AM, TheFlyingFiddle wrote:
On Saturday, 24 February 2018 at 09:48:13 UTC, Joel wrote:
[...]
void popFront() { head = head.next; if (head !is null) head.prev =
null; }
void popBack() { tail = tail.prev; if (tail !is null) tail.next = null; }
Head and tail will point to the same nodes and you are setting the
head.prev to null. Which would make the tail.prev null when iterating it
backwards. Similarly if you started iterating it backwards and then
iterating it forward it would not work.
To expand on this:
The goal of this was apparently to have `popBack` stop before reaching
nodes that have already been `popFront`ed. I.e., `retro` should respect
the earlier `dropOne`.
To do that properly, you can look for `head is tail`. If `head is tail`,
the range is at its last element. Doesn't matter if there is another
`next` or `prev`. `popFront` and `popBack` can just declare the range as
empty then, e.g. by setting `head = null; tail = null;`.
So:
----
void popFront()
{
if (head is tail) { head = null; tail = null; }
else head = head.next;
}
void popBack()
{
if (head is tail) { head = null; tail = null; }
else tail = tail.prev;
}
----
Generally, don't change the data's structure in a range. In a range over
a linked list, don't touch the links.
Also, the `List` here functions both as a container (adding/removing
links) and as a range (traversing the links). Separating the two into
different types might make it easier to see what a method should be
allowed to do.
