Holy cow, I wrote a book!
Some people have noticed that NTFS automatically sorts filenames,
but does so in a manner different from Explorer.
Why is that?
For illustration purposes, I created files with the following names:
And here's the sort order for various scenarios,
at least on my machine.
(You'll later see why it's important whose machine you test on.)
First, notice that Explorer and "dir /on" agree on the alphabetic
(Once you throw digits into the mix, things diverge.)
This is not a coincidence.
Both are using the default locale's word sort algorithm.
Why does the raw NTFS sort order differ?
Because NTFS's raw sort order has different goals.
The "dir /on" and Explorer output are sorting the items for humans.
When sorting for humans, you need to respect their locale.
If my computer were in Sweden, Explorer and "dir /on" would have
sorted the items in a different order:
You can ask a Swede why this is the correct sort order
if you're that curious. My point is that different locales have
different sorting rules.
NTFS's raw sort order, on the other hand, is not for humans.
As we saw above, sorting for humans can result in different results
depending on which human you ask.
But there is only one order for files on the disk,
and NTFS needs to apply a consistent rule so that it can find a
file when asked for it later.
In order to maintain this consistency, the NTFS raw sort order
cannot be dependent upon such fickle properties as the current
user's locale. It needs to lock in a sort algorithm
and stick to it.
As Michael Kaplan pointed out earlier,
NTFS captures the case mapping table at the time the drive is formatted
and continues to use that table, even if the OS's
case mapping tables change subsequently.
Once the string has been converted to uppercase, it then needs
to be sorted.
Since this is not for humans, there's no need to implement the
complex rules regarding secondary and tertiary keys,
the interaction between alphanumerics and punctuation, and all
the other things that make sorting hard.
It just compares the code points as binary values,
also known as
an ordinal sort.
In summary, therefore,
Explorer sorts the items so you (a human) can find them.
NTFS sorts the items so it (the computer) can find them.
If you're writing a program and you
want the results of a directory listing to be sorted,
then sort it yourself according to the criteria of your choice.