I've been working (for far too long) on a mail-script to deliver to 50,000 users (more added every week). It's got to the point where it's grinding the mail-server, particularly if there are invalid addresses in the list.

It occurred to me that I could optimise this by delivering to the most frequent addresses first: should save on nslookups, and frequently-appearing hostnames are more likely to be valid than the "go@away.no.spam" variety. About 80% of the list should be processed before I run into the single-user hostnames. The most frequently occurring hostname is on the same host as the mailing program, so this 40% of the list should be delivered almost immediately. (BTW, the host doesn't use sendmail - I don't get to choose the MTA.)

Here's the outline of the code I produced, and I'd be grateful for any comments about how I could do this more efficiently. It's not bad atm, sorting the 50,000 addresses in ~ 3 secs, but I'm sure there's another and probably more efficient way to do it. 

# unsorted addresses start off in the @addresses array.
# sorted ones end up in the @recipients array.

foreach my $address (@addresses) {
    my ($name, $host) = split(/\@/, $address);
    # various checks on the validity of name and host, snipped
    push @{$host}, "$name\@$host";
    $frequency{$host}++;
}

foreach my $host (reverse sort { $frequency{$a} <=> $frequency{$b} } k
+eys %frequency) {
    foreach my $address (@{$host}) {
        push @recipients, $address;
    }
    @{$host} = -1;
}

[download]

So I'm naturally concerned about the wild proliferation of (often one-element) arrays, even though these are exterminated later. TIA.

In reply to Sorting a list by frequency of items by yojimbo

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