@node FAQ
@cindex FAQ
@unnumbered FAQ

@anchor{Stamping}
@cindex hashing
@cindex stamping
@section Hashing and stamping

All targets are checksummed if target's size, @code{ctime}/@code{mtime}
differs from the previous one (depending on @ref{OOD, @env{$REDO_INODE_TRUST}}
value). @command{apenwarr/redo} gives
@url{https://redo.readthedocs.io/en/latest/FAQImpl/#why-not-always-use-checksum-based-dependencies-instead-of-timestamps, many reasons}
why every time checksumming is bad, but in my opinion in practice all of
them do not apply.

@itemize
@item Aggregate targets and willing to be out-of-date ones just must not
  produce empty output files. @command{apenwarr/*}, @command{redo-c} and
  @command{goredo} implementations treat non existing file as an
  out-of-date target
@item If you really wish to produce an empty target file, just touch @file{$3}
@end itemize

Those who create an empty file if no @file{stdout} was written -- are
failed implementations.

redo is a tool to help people. Literally all targets can be safely
@code{redo-stamp < $3}-ed, reducing false positive out-of-dates. Of
course, with the correct @file{stdout}/@file{$3} working and placing
necessary results in @file{$3}, instead of just silently feeding them in
@command{redo-stamp}.

redo implementations already automatically record -ifchange on
@file{.do} files and -ifcreate on non-existing @file{.do} files. So why
they can not record @command{redo-stamp} the same way implicitly? No,
Zen of Python is not applicable there, because -ifchange/-ifcreate
contradicts it already.

Modern cryptographic hash algorithms and CPUs are so fast, that even all
read and writes to or from hard drive arrays can be easily checksummed
and transparently compressed, as ZFS with LZ4/Zstandard and
Skein/BLAKE[23] algorithms demonstrate us.

@command{goredo} includes @command{redo-stamp}, that really records the
stamp in the @file{.rec} file, but it does not play any role later. It
is stayed just for compatibility.

@section Can removed .do lead to permanent errors of its non existence?

Yes, because dependency on it was recorded previously. Is it safe to
assume that @file{.do}-less target now is an ordinary source-file? I
have no confidence in such behaviour. So it is user's decision how to
deal with it, probably it was just his inaccuracy mistake. If you really
want to get rid of that dependency knowledge for @file{foo/bar} target,
then just remove @file{foo/.redo/bar.rec}.

@section Does redo-always always rebuilds target?

By definition, it should be built always, as
@url{http://news.dieweltistgarnichtso.net/bin/redo-sh.html#why-built-twice,
redo-sh} and @command{redo-c} implementations do. But that ruins the
whole redo usability, potentially rebuilding everything again and again.
@command{apenwarr/redo} and @command{goredo} tries to build
always-targets only once per @strong{run}, as a some kind of optimization.

For example if you need to rebuild TeX documents (case mentioned in
redo-sh's FAQ) until all references and numbers are ready, then you must
naturally expectedly explicitly use while cycle in your @file{.do}, as
@command{apenwarr/redo} already suggests.

@section What to do with OOD targets, that has not changed their output?

How to prevent building of targets, who depend on the OOD target, that
produced the same output? If the target is already decided to be OOD,
then the whole tree becomes OOD too. It is clear, simple, reliable and
honest way of do-ing things.

Building of the lower level OOD target first is unfair and dishonest
thing to do, because probably top level target, being the OOD, won't be
dependant on lower level target anymore at all. Actually @command{goredo}
does this as a shameful hack and only for always-targets, still existing
in the wild. The whole @command{redo-always} idea is considered harmful.

If you wish to run something and decide if it must OOD the toplevel
targets, then just run @command{redo} to forcefully rebuild it and then
run the ordinary top level targets.