Re: [PATCH 4/8] git-remote-mediawiki: get rid of O(N^2) loop

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Matthieu Moy <Matthieu.Moy@xxxxxxx> writes:

> The algorithm to find a path from the local revision to the remote one
> was calling "git rev-list" and parsing its output N times. Run rev-list
> only once, and fill a hashtable with the result to optimize the body of
> the loop.

Good thinking.  I wonder if it would further reduce the overhead if
you stop using --children and do this using --parents instead, as
you will be reading the parsed_sha1..local range either way yourself
anyway.

> Signed-off-by: Matthieu Moy <Matthieu.Moy@xxxxxxx>
> ---
>  contrib/mw-to-git/git-remote-mediawiki | 25 ++++++++++++++++++-------
>  1 file changed, 18 insertions(+), 7 deletions(-)
>
> diff --git a/contrib/mw-to-git/git-remote-mediawiki b/contrib/mw-to-git/git-remote-mediawiki
> index 8e46e4e..f9c0cc6 100755
> --- a/contrib/mw-to-git/git-remote-mediawiki
> +++ b/contrib/mw-to-git/git-remote-mediawiki
> @@ -1196,16 +1196,27 @@ sub mw_push_revision {
>  	if ($last_local_revid > 0) {
>  		my $parsed_sha1 = $remoteorigin_sha1;
>  		# Find a path from last MediaWiki commit to pushed commit
> +		print STDERR "Computing path from local to remote ...\n";
> +		my @local_ancestry = split(/\n/, run_git("rev-list --boundary --children $local ^$parsed_sha1"));
> +		my %local_ancestry;
> +		foreach my $line (@local_ancestry) {
> +			if (my ($parent, $child) = $line =~ m/^-?([a-f0-9]+) ([a-f0-9]+)/) {
> +				$local_ancestry{$parent} = $child;
> +				if ($parent eq $parsed_sha1 || $child eq $parsed_sha1) {
> +					print STDERR "$parent -> $child\n";
> +				}
> +			} elsif (!$line =~ m/^([a-f0-9]+)/) {
> +				die "Unexpected output from git rev-list: $line";
> +			}
> +		}
>  		while ($parsed_sha1 ne $HEAD_sha1) {
> -			my @commit_info =  grep(/^$parsed_sha1/, split(/\n/, run_git("rev-list --children $local")));
> -			if (!@commit_info) {
> +			my $child = $local_ancestry{$parsed_sha1};
> +			if (!$child) {
> +				printf STDERR "Cannot find a path in history from remote commit to last commit\n";
>  				return error_non_fast_forward($remote);
>  			}
> -			my @commit_info_split = split(/ |\n/, $commit_info[0]);
> -			# $commit_info_split[1] is the sha1 of the commit to export
> -			# $commit_info_split[0] is the sha1 of its direct child
> -			push(@commit_pairs, \@commit_info_split);
> -			$parsed_sha1 = $commit_info_split[1];
> +			push(@commit_pairs, [$parsed_sha1, $child]);
> +			$parsed_sha1 = $child;
>  		}
>  	} else {
>  		# No remote mediawiki revision. Export the whole
--
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