Re: [PATCH 1/1] scripts: Changed potential O(n) file size calculation to O(1)

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, Nov 21, 2025 at 11:16:11AM +0100, Jan Kiszka wrote:
> You want to add
> 
> From: Konrad Schwarz <konrad.schwarz@siemens.com>
> 
> to the top of the commit message when submitting via a different account
> so that author and signer are aligned. Or use git sendemail which will
> do that when author and submitter address differ.
> 
> On 19.11.25 18:28, konrad.schwarz@gmail.com wrote:
> > The mkemmc.sh script calculates file sizes via `wc -c'.  `wc'
> > normally works by reading the entire file, resulting in O(n) performance.
> > 
> > Unix file systems obviously know a file's size and POSIX `ls' reports this
> > information unambiguously, so replacing `wc' with `ls' ensures O(1)
> > performance.  The files in question tend to be large making this change
> > worthwhile.
> 
> Valid point.
> 
> > 
> > Signed-off-by: Konrad Schwarz <konrad.schwarz@siemens.com>
> > ---
> >  scripts/mkemmc.sh | 10 ++++++++--
> >  1 file changed, 8 insertions(+), 2 deletions(-)
> > 
> > diff --git a/scripts/mkemmc.sh b/scripts/mkemmc.sh
> > index 45dc3f08fa..d2c4e84b16 100755
> > --- a/scripts/mkemmc.sh
> > +++ b/scripts/mkemmc.sh
> > @@ -37,13 +37,19 @@ usage() {
> >      exit "$1"
> >  }
> > 
> > +file_size() {
> > +	ls_line=$(ls -Hdog "$1") || return
> 
> This will not suppress the error message when a file does not exist or
> is not accessible, so:
> 
> ls_line=$(ls -Hdog "$1" 2>/dev/null) || return
> 
> > +	printf %s\\n "$ls_line" | cut -d\  -f3
> > +	unset ls_line

This parsing of 'ls' output could be simplified by
using the 'stat' command with a format string to
request only the file size.

    stat --format=%s "$1"


With regards,
Daniel
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