"Dr.Ruud" <rvtol+use...@isolution.nl> writes:
> On 12/06/2013 10:27, lee wrote:
>
>> File sizes do not reliably indicate whether a file has been modified or
>> not.
>
> If the file size has changed, then your file has changed. That is 100%
> reliable, and is a quick and cheap check.
It works only one way: different size --> file has changed. What
doesn't work is: file has changed --> different size.
> But if the size hasn't changed, then you still need to check something
> else. You can do another light check, or decide to do the heavy one.
>
> This is also important because a hash-value is only a fingerprint, so
> different files have (a small chance on having) the same hash value.
>
> The file size check makes the chance even smaller that you don't
> detect the change.
Hm ok, this kinda sucks ... Imagine I check size and mtime and I have
a SHA-256 hash. Now there are the following cases:
+ mtime AND size changed: file has changed, put into report, update index
+ EITHER mtime changed, size is same, OR size changed, mtime is same:
* compute hash
* hash is different: file has changed, put into report, update index
* hash is the same: manual intervention is required to decide
whether the file should be in the report or not
+ NEITHER mtime, NOR size changed: do nothing
How likely is it that the hash is the same though the file did change?
And when you think of it, it's even possible (yet unlikely) that the
file changed _and_ has still the same size and same mtime.
So how do you reliably detect whether a file has changed or not? Create
a copy of the file as it went into the report and when generating
another report, compare the current file with the backup? Maybe not a
bad idea, it would be pretty reliable without any complicated ado.
--
"Object-oriented programming languages aren't completely convinced that
you should be allowed to do anything with functions."
http://www.joelonsoftware.com/items/2006/08/01.html
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