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    # read input old and new stanzas
    my ($old, $new) = (shift, shift);
    my ($length_old, $length_new) = (length($old // ""), length($new // ""));

    # init output diff value
    my $diff = undef;

    # set diff undef if old and new are both undefined
    if (not defined $old and not defined $new) {
        $diff = undef;

    # set diff if old stanza is undefined
    } elsif (not defined $old) {
        $diff = "undef: old";

    # set diff if new stanza is undefined
    } elsif (not defined $new) {
        $diff = "undef: new";

    # set diff to null if old and new stanzas match
    } elsif ($old eq $new) {
        $diff = "";

    # set diff to first old or new line that doesn't match
    #   loop through old lines, looking for equivalant new lines
    #   look for additional new lines that are not present in old
    #   set diff to other if we don't know why old is not equal to new
    } else {
        my @new = split(/\n/, $new);
        my $num = 0;
        foreach my $line (split(/\n/, $old)) {
            $num++;
            if (defined $new[0] and $new[0] eq $line) {
                shift @new;
            } else {
                $diff = "line $num: $line";
                last;
            }
        }
        $num++;
        $diff = "line $num: $new[0]" if defined $new[0] and not defined $diff;
        $diff = "other" if not defined $diff;

    # finished setting output diff
    }

    # finished diff function, return diff text
    return $diff;
}



sub ios {

=head2 ios

    $output = Mnet::Stanza::ios($template, $config)

The Mnet::Stanza::ios fucntion uses a template to check a config for needed
config changes, outputting a generated list of overlay config commands that can
be applied to the device to bring it into compliance.

The script dies with an error if the template argument is missing. The second
config argument is optional and can be set to the current device config.

The output from this function will be commands that need to be added and/or
removed from the input config, generated by comparing the input template to the
input config. The output will be null if the input config does not need to be
updated.

This function is designed to work on templates and configs made up of indented
stanzas, as in the following ios config example, showing a global snmp command
and a named access-list stanza:

    ! global commands are not indented
    snmp-server community global

    ! a stanza includes lines indented underneath
    ip access-list stanza
     permit ip any any
     deny ip any any

Template lines should start with one of the following characters:

    !   comment

            comments are ignored, they are not propagated to output

    +   add line

            config line should be added if not already present
            to add lines under a stanza refer to '>' below

    >   find stanza

            use to find or create a stanza in the input config
            found stanzas can have '+', '=', and/or '-' lines underneath
            found stanzas are output if child lines generate output

    =   match stanza

            output stanza if not already present and an exact match
            indented lines undeneath to match must also start with '='
            possible to '>' find a stanza and '=' match child sub-stanza

    -   remove line or stanza if present

            remove global command, command under a stanza, or a stanza
            wildcard '*!*' at end of line matches one or more characters
            does not remove lines already checked in the current stanza
            lines to be removed are prefixed by 'no' in the output
            lines already starting with 'no' get the 'no' removed
            to remove commands under a stanza refer above to '>'

Following is an example of how this function can be used to remediate complex
ios feature configs:

    # use this module
    use Mnet::Stanza;

    # read current config from standard input
    my $sh_run = undef;
    $sh_run .= "$_\n" while <STDIN>;

    # define ios feature update template string
    #   can be programmatically generated from parsed config
    my $update_remplate = "

        ! check numbered acl, ensure no extra lines
        +access-list 1 permit ip any any
        -access-list 1 *!*

        ! check that this stanza matches exactly
        =ip access-list test
         =permit ip any any

        ! find vlan stanza and ensure acls are applied
        >interface Vlan1
         +ip access-group 1 in
         +ip access-group test out
    ";

    # define ios feature remove template string
    #   used to remove any old config before applying update
    my $remove_template = "

        ! acl automatically removed from interface
        -access-list 1
        -ip access-list test

    ";

    # output overlay config if update is needed
    #   overlay will remove old config before updating with new config
    if (Mnet::Stanza::ios($update_template, $sh_run)) {
        print Mnet::Stanza::ios($remove_template, $sh_run);
        print Mnet::Stanza::ios($update_template);
    }

Note that extra spaces are removed from the template and config inputs using
the Mnet::Stanza::trim function. Refer to that function for more info.

=cut

    # read input template and config args
    my $template = shift // croak "missing template arg";
    my $config = shift // "";

    # note: this function is called recursively for each find stanza '>'
    #   processing starts with input template and config supplied by caller
    #   template lines with the same indent level are processed, top to bottom
    #   find sub-stanzas causes recursive calls with stanza subcommands inputs

    # indent arg used by recursive ios find calls, abort if set by other caller
    #   setup some things when initially called by user script, indent is undef
    #   set indent null, trim input/template, everything starts on left margin
    #   set template errors sub now, so die always refers to original caller
    #   croak with an error if called by user script with indent arg set
    my $indent = shift;
    if (not defined $indent) {
        $indent = "";
        $config = Mnet::Stanza::trim($config);
        $template = Mnet::Stanza::trim($template);
        sub _ios_error_in_template {
            my $template = shift // croak "undefined arg";
            $template =~ /^(\s*\S.*)/m;
            die "ios error in template line '$1'".Carp::shortmess()."\n";
        }
    } elsif (caller ne "Mnet::Stanza::_ios_find") {
        croak("Mnet::Stanza::ios called with too many args")
    }

    # init output config overlay
    my $output = "";

    # track lines checked in current stanza
    #   key is set for each line checked with add/find/match/remove operation
    #   used by _ios_remove sub to ensure lines already checked are not removed
    #   example, adds test 1-2, removes all others: +test 1, +test 2, -test *!*
    my $checked = {};

    # clear lines starting with the ios comment character from template
    $template =~ s/^\s*!.*//mg;

    # parse list of lines/stanzas from input config at current indent level
    my @template_stanzas = Mnet::Stanza::parse($template, qr/^$indent\S/);
    croak("no text at indent ".length($indent)) if not $template_stanzas[0];

    # loop to process parsed template stanzas
    foreach my $template_stanza (@template_stanzas) {

        # add line '+'
        if ($template_stanza =~ /^\+/) {
            $output .= _ios_add($template_stanza, $config, $checked, $indent);

        # find stanza '>'
        #   which recursively calls this function to process subcommands
        } elsif ($template_stanza =~ /^\>/) {
            $output .= _ios_find($template_stanza, $config, $checked, $indent);

        # match stanza '='
        } elsif ($template_stanza =~ /^\=/) {
            $output .= _ios_match($template_stanza, $config, $checked, $indent);

        # remove line or stanza '-'
        } elsif ($template_stanza =~ /^\-/) {
            $output .= _ios_remove($template_stanza, $config, $checked,$indent);

        # otherwise abort with template error
        } else {
            _ios_error_in_template($template_stanza);
        }

    # continue processing parsed template stanzas
    }

    # finished ios function, return output config overlay
    return $output;
}



sub _ios_add {

# $output = _ios_add($template, $config, \%checked, $indent)
# purpose: ios template '+' add line if missing
# $output: missing config commands that need to be added
# $template: current template line to add, expected to be on left margin
# $config: ios config to work on, expected to be on left margin
# \%checked: keys for each add/find/match/remove in current stanza
# $indent: current template indent, zero or more spaces

    # read input args and initialize output config overlay
    my ($template, $config, $checked, $indent) = (shift, shift, shift, shift);
    my $output = "";

    # parse single line to add
    _ios_error_in_template($template) if $template !~ /^\+(\S.*)$/;
    my $add_line = $1;

    # append line to output if not already present with same indent
    $output .= $indent.$add_line."\n" if $config !~ /^$indent\Q$add_line\E$/m;

    # note that this line was checked
    $checked->{$indent.$add_line}++;

    # finished _ios_add, return output config overlay
    return $output;
}



sub _ios_find {

# $output = _ios_find($template, $config, \%checked, $indent)
# purpose: ios template '>' find stanza
# $output: missing config commands that need to be added
# $template: current template line to find, expected to be on left margin
# $config: ios config to work on, expected to be on left margin
# \%checked: keys for each add/find/match/remove in current stanza
# $indent: current template indent, zero or more spaces

    # read input args and initialize output config overlay
    my ($template, $config, $checked, $indent) = (shift, shift, shift, shift);
    my $output = "";

    # parse template first line to find
    _ios_error_in_template($template) if $template !~ /^\>(\S.*)$/m;
    my $find_line = $1;

    # parse template subcommands under first line to find
    my $find_subcommands = undef;
    foreach my $line (split(/\n/, $template)) {
        $find_subcommands .= "$line\n" if defined $find_subcommands;
        $find_subcommands = "" if not defined $find_subcommands
    }

    # process find template stanza, assuming subcommands under first line
    if ($find_subcommands =~ /\S/) {

        # parse template indent level of subcommands
        my $indent_subcommands = "";
        $indent_subcommands = $1 if $template =~ /\n(\s*)/;

        # parse config looking for stanza that matched first line of template
        my $config_stanza = Mnet::Stanza::parse($config, qr/^\Q$find_line\E$/);

        # look for matching stanza subcommands from config
        my $config_subcommands = undef;
        foreach my $line (split(/\n/, $config_stanza)) {
            $config_subcommands .= "$line\n" if defined $config_subcommands;
            $config_subcommands = "" if not defined $config_subcommands
        }

        # resursive compare found template subcommands to config subcommands
        my $output_subcommands = Mnet::Stanza::ios(
            Mnet::Stanza::trim($find_subcommands),
            Mnet::Stanza::trim($config_subcommands),
        );

        # append found stanza with stanza output sub-commands
        if ($output_subcommands =~ /\S/) {
            $output .= $indent.$find_line."\n";
            foreach my $line (split(/\n/, $output_subcommands)) {
                $output .= $indent_subcommands.$line."\n";
            }
        }

    # finished processing find template stanza
    }

    # note that template find stanza first line was checked
    $checked->{$indent.$find_line}++;

    # finished _ios_find, return output config overlay
    return $output;
}



sub _ios_match {

# $output = _ios_match($template, $config, \%checked, $indent)
# purpose: ios template '=' match stanza
# $output: missing config commands that need to be added
# $template: current template line to match, expected to be on left margin
# $config: ios config to work on, expected to be on left margin
# \%checked: keys for each add/find/match/remove in current stanza
# $indent: current template indent, zero or more spaces

    # read input args and initialize output config overlay
    my ($template, $config, $checked, $indent) = (shift, shift, shift, shift);
    my $output = "";

    # parse first line to match
    _ios_error_in_template($template) if $template !~ /^\=(\S.*)$/m;
    my $match_line = $1;

    # parse entire stanza to match
    my $match_stanza = "";
    foreach my $line (split(/\n/, $template)) {
        _ios_error_in_template($line) if $line !~ /^(\s*)=(\S.*)$/;
        $match_stanza .= $1.$2."\n";
    }

    # look for matching stanza in config
    my $config_stanza = Mnet::Stanza::parse($config,qr/^\Q$match_line\E$/)."\n";

    # append match stanza to output if not already present with same indent
    $output .= $indent.$match_stanza if $config_stanza ne $match_stanza;

    # note that this line was checked
    $checked->{$indent.$match_line}++;

    # finished _ios_stanza, return output config overlay
    return $output;
}



sub _ios_remove {

# $output = _ios_remove($template, $config, \%checked, $indent)
# purpose: ios template '-' remove line or stanza
# $output: extra config commands that need to be removed
# $template: current template line to remove, expected to be on left margin
# $config: ios config to work on, expected to be on left margin
# \%checked: keys for each add/find/match/remove in current stanza
# $indent: current template indent, zero or more spaces

    # read input args and initialize output config overlay
    my ($template, $config, $checked, $indent) = (shift, shift, shift, shift);
    my $output = "";

    # parse single line to remove
    _ios_error_in_template($template) if $template !~ /^\-(\S.*)$/;
    my $remove_line = $1;

    # set regex to match line, with optional '*!*' wildcard at end
    my ($regex_line, $regex_wildcard) = ($remove_line, "");
    ($regex_line, $regex_wildcard) = ($1, ".*")
        if $remove_line =~ /^(.*)\*\!\*$/;

    # check each config line for a match to remove line regex/wildcard
    #   ensure line was not already checked by add/find/match/remove operations
    #   skip line removal if it was already checked, otherwise note as checked
    #   lines matched in the config are output with a 'no' in front of them
    #   lines matched already starting with 'no' are output without the 'no'
    foreach my $config_line (split(/\n/, $config)) {
        if ($config_line =~ /^$indent(\Q$regex_line\E$regex_wildcard)$/m) {
            my $match_line = $1;
            if (not $checked->{$indent.$match_line}) {
                $checked->{$indent.$match_line}++;
                my $no_line = "no $match_line";
                $no_line =~ s/^no no //;
                $output .= $indent.$no_line."\n";
            }
        }
    }

    # finished _ios_remove, return output config overlay
    return $output;
}



=head1 SEE ALSO

L<Mnet>

L<Mnet::Log>

=cut

# normal end of package
1;