I saw Dave Cheney tweet this the other day.
My understanding is this:
First one - Dave's working one
The <number_here>:
is an index in the array. This "sets" the current index.. which is why further into the declaration the index must be "reset" back in the array. So, the first number is 5
(index 0), the second "entry" has an index of 4:
.. so the value 1
will be at index 4:
5 _ _ _ 1 _
^ index is currently here
..the next one has no index but it will continue after the last index given. which is 4+1
.. so index 5
gets the value 0
:
5 _ _ _ 1 0
^ index is here.. it needs to be reset
Now the index will overrun .. so its gets set further back. The next one is 2:
.. so that puts the value below with the value 3
:
5 _ 3 _ 1 0
^ index is here
Next one again, continues on, since it has no index:
5 _ 3 2 1 0
^ index is here
Then the last one has index 1:
.. with the value 4:
5 4 3 2 1 0
Second one - your broken one.
The second one is the same thing - but you haven't protected the over-writing of a currently placed index. Lets step through it:
Value 5
at index 0:
5 _ _ _ _ _ _ _ _
^ index is here
Value 1
at index 4:
5 _ _ _ 1 _ _ _ _
^ index is here
Value 0
at index 5 (remember, it continues on):
5 _ _ _ 1 0 _ _ _
^ index is here
Value 3
at index 2:
5 _ 3 _ 1 0 _ _ _
^ index is here
Value 2
at index 3 (again, it continues on:
5 _ 3 2 1 0 _ _ _
^ index is here
Value 4
at index 1:
5 4 3 2 1 0 _ _ _
^ index is here ... you're awfully close to overwriting the next value
Value 12
at index 2:
5 4 12 2 1 0 _ _ _
^^^^^ BOOOM
Boom..
..you've overwritten the value 3 and will continue to do so given where the index is for the remaining values. This is the problem..