#!/usr/bin/perl -w

use strict;
use warnings;
use feature 'unicode_strings';

use Carp;
use Digest;
use File::Find;
use File::Spec;
use Scalar::Util;
use Text::Tabs;

BEGIN {
    require '../regen/regen_lib.pl';
}

sub DEBUG { 0 };

=pod

=head1 NAME

podcheck.t - Look for possible problems in the Perl pods

=head1 SYNOPSIS

 cd t
 ./perl -I../lib porting/podcheck.t [--show_all] [--cpan] [--deltas]
                                                  [--counts] [ FILE ...]
 ./perl -I../lib porting/podcheck.t --add_link MODULE ...

 ./perl -I../lib porting/podcheck.t --regen

=head1 DESCRIPTION

podcheck.t is an extension of Pod::Checker.  It looks for pod errors and
potential errors in the files given as arguments, or if none specified, in all
pods in the distribution workspace, except certain known special ones
(specified below).  It does additional checking beyond that done by
Pod::Checker, and keeps a database of known potential problems, and will
fail a pod only if the number of such problems differs from that given in the
database.  It also suppresses the C<(section) deprecated> message from
Pod::Checker, since specifying the man page section number is quite proper to do.

The additional checks it makes are:

=over

=item Cross-pod link checking

Pod::Checker verifies that links to an internal target in a pod are not
broken.  podcheck.t extends that (when called without FILE arguments) to
external links.  It does this by gathering up all the possible targets in the
workspace, and cross-checking them.  It also checks that a non-broken link
points to just one target.  (The destination pod could have two targets with
the same name.)

The way that the C<LE<lt>E<gt>> pod command works (for links outside the pod)
is to actually create a link to C<search.cpan.org> with an embedded query for
the desired pod or man page.  That means that links outside the distribution
are valid.  podcheck.t doesn't verify the validity of such links, but instead
keeps a data base of those known to be valid.  This means that if a link to a
target not on the list is created, the target needs to be added to the data
base.  This is accomplished via the L<--add_link|/--add_link MODULE ...>
option to podcheck.t, described below.

=item An internal link that isn't so specified

If a link is broken, but there is an existing internal target of the same
name, it is likely that the internal target was meant, and the C<"/"> is
missing from the C<LE<lt>E<gt>> pod command.

=item Verbatim paragraphs that wrap in an 80 (including 1 spare) column window

It's annoying to have lines wrap when displaying pod documentation in a
terminal window.  This checks that all verbatim lines fit in a standard 80
column window, even when using a pager that reserves a column for its own use.
(Thus the check is for a net of 79 columns.)
For those that lines that don't fit, it tells you how much needs to be cut in
order to fit.

Often, the easiest thing to do to gain space for these is to lower the indent
to just one space.

=item Missing or duplicate NAME or missing NAME short description

A pod can't be linked to unless it has a unique name.
And a NAME should have a dash and short description after it.

=item =encoding statement issues

This indicates if an C<=encoding> statement should be present, or moved to the
front of the pod.

=item Items that perhaps should be links

There are mentions of apparent files in the pods that perhaps should be links
instead, using C<LE<lt>...E<gt>>

=item Items that perhaps should be C<FE<lt>...E<gt>>

What look like path names enclosed in C<CE<lt>...E<gt>> should perhaps have
C<FE<lt>...E<gt>> mark-up instead.

=back

A number of issues raised by podcheck.t and by the base Pod::Checker are not
really problems, but merely potential problems, that is, false positives.
After inspecting them and
deciding that they aren't real problems, it is possible to shut up this program
about them, unlike base Pod::Checker.  To do this, call podcheck.t with the
C<--regen> option to regenerate the database.  This tells it that all existing
issues are to not be mentioned again.

This isn't fool-proof.  The database merely keeps track of the number of these
potential problems of each type for each pod.  If a new problem of a given
type is introduced into the pod, podcheck.t will spit out all of them.  You
then have to figure out which is the new one, and should it be changed or not.
But doing it this way insulates the database from having to keep track of line
numbers of problems, which may change, or the exact wording of each problem
which might also change without affecting whether it is a problem or not.

Also, if the count of potential problems of a given type for a pod decreases,
the database must be regenerated so that it knows the new number.  The program
gives instructions when this happens.

Some pods will have varying numbers of problems of a given type.  This can
be handled by manually editing the database file (see L</FILES>), and setting
the number of those problems for that pod to a negative number.  This will
cause the corresponding error to always be suppressed no matter how many there
actually are.

There is currently no check that modules listed as valid in the data base
actually are.  Thus any errors introduced there will remain there.

=head2 Specially handled pods

=over

=item perltoc

This pod is generated by pasting bits from other pods.  Errors in those bits
will show up as errors here, as well as for those other pods.  Therefore
errors here are suppressed, and the pod is checked only to verify that nodes
within it that are externally linked to actually exist.

=item perldelta

The current perldelta pod is initialized from a template that contains
placeholder text.  Some of this text is in the form of links that don't really
exist.  Any such links that are listed in C<@perldelta_ignore_links> will not
generate messages.  It is presumed that these links will be cleaned up when
the perldelta is cleaned up for release since they should be marked with
C<XXX>.

=item Porting/perldelta_template.pod

This is not a pod, but a template for C<perldelta>.  Any errors introduced
here will show up when C<perldelta> is created from it.

=item cpan-upstream pods

See the L</--cpan> option documentation

=item old perldeltas

See the L</--deltas> option documentation

=back

=head1 OPTIONS

=over

=item --add_link MODULE ...

Use this option to teach podcheck.t that the C<MODULE>s or man pages actually
exist, and to silence any messages that links to them are broken.

podcheck.t checks that links within the Perl core distribution are valid, but
it doesn't check links to man pages or external modules.  When it finds
a broken link, it checks its data base of external modules and man pages,
and only if not found there does it raise a message.  This option just adds
the list of modules and man page references that follow it on the command line
to that data base.

For example,

    cd t
    ./perl -I../lib porting/podcheck.t --add_link Unicode::Casing

causes the external module "Unicode::Casing" to be added to the data base, so
C<LE<lt>Unicode::Casing<gt>> will be considered valid.

=item --regen

Regenerate the data base used by podcheck.t to include all the existing
potential problems.  Future runs of the program will not then flag any of
these.

=item --cpan

Normally, all pods in the cpan directory are skipped, except to make sure that
any blead-upstream links to such pods are valid.
This option will cause cpan upstream pods to be fully checked.

=item --deltas

Normally, all old perldelta pods are skipped, except to make sure that
any links to such pods are valid.  This is because they are considered
stable, and perhaps trying to fix them will cause changes that will
misrepresent Perl's history.  But, this option will cause them to be fully
checked.

=item --show_all

Normally, if the number of potential problems of a given type found for a
pod matches the expected value in the database, they will not be displayed.
This option forces the database to be ignored during the run, so all potential
problems are displayed and will fail their respective pod test.  Specifying
any particular FILES to operate on automatically selects this option.

=item --counts

Instead of testing, this just dumps the counts of the occurrences of the
various types of potential problems in the data base.

=back

=head1 FILES

The database is stored in F<t/porting/known_pod_issues.dat>

=head1 SEE ALSO

L<Pod::Checker>

=cut

#####################################################
# HOW IT WORKS (in general)
#
# If not called with specific files to check, the directory structure is
# examined for files that have pods in them.  Files that might not have to be
# fully parsed (e.g. in cpan) are parsed enough at this time to find their
# pod's NAME, and to get a checksum.
#
# Those kinds of files are sorted last, but otherwise the pods are parsed with
# the package coded here, My::Pod::Checker, which is an extension to
# Pod::Checker that adds some tests and suppresses others that aren't
# appropriate.  The latter module has no provision for capturing diagnostics,
# so a package, Tie_Array_to_FH, is used to force them to be placed into an
# array instead of printed.
#
# Parsing the files builds up a list of links.  The files are gone through
# again, doing cross-link checking and outputting all saved-up problems with
# each pod.
#
# Sorting the files last that potentially don't need to be fully parsed allows
# us to not parse them unless there is a link to an internal anchor in them
# from something that we have already parsed.  Keeping checksums allows us to
# not parse copies of other pods.
#
#####################################################

# 1 => Exclude low priority messages that aren't likely to be problems, and
# has many false positives; higher numbers give more messages.
my $Warnings_Level = 200;

# perldelta during construction may have place holder links.
our @perldelta_ignore_links = ( "XXX", "perl5YYYdelta", "perldiag/message" );

# To see if two pods with the same NAME are actually copies of the same pod,
# which is not an error, it uses a checksum to save work.
my $digest_type = "SHA-1";

my $original_dir = File::Spec->rel2abs(File::Spec->curdir);
my $data_dir = File::Spec->catdir($original_dir, 'porting');
my $known_issues = File::Spec->catfile($data_dir, 'known_pod_issues.dat');
my $copy_fh;

my $MAX_LINE_LENGTH = 79;   # 79 columns
my $INDENT = 7;             # default nroff indent

# Our warning messages.  Better not have [('"] in them, as those are used as
# delimiters for variable parts of the messages by poderror.
my $line_length = "Verbatim line length including indents exceeds $MAX_LINE_LENGTH by";
my $broken_link = "Apparent broken link";
my $broken_internal_link = "Apparent internal link is missing its forward slash";
my $see_not_linked = "? Should you be using L<...> instead of";
my $C_with_slash = "? Should you be using F<...> or maybe L<...> instead of";
my $multiple_targets = "There is more than one target";
my $duplicate_name = "Pod NAME already used";
my $need_encoding = "Should have =encoding statement because have non-ASCII";
my $encoding_first = "=encoding must be first command (if present)";
my $no_name = "There is no NAME";
my $missing_name_description = "The NAME should have a dash and short description after it";

# objects, tests, etc can't be pods, so don't look for them. Also skip
# files output by the patch program.  Could also ignore most of .gitignore
# files, but not all, so don't.
my $non_pods = qr/ (?: \.
                       (?: [achot]  | zip | gz | bz2 | jar | tar | tgz | PL | so
                           | orig | rej | patch   # Patch program output
                           | sw[op] | \#.*  # Editor droppings
                           | old      # buildtoc output
                       )
                       $
                    ) | ~$ | \ \(Autosaved\)\.txt$ # Other editor droppings
                /x;


# Pod::Checker messages to suppress
my @suppressed_messages = (
    "(section) in",                         # Checker is wrong to flag this
    "multiple occurrence of link target",   # We catch independently the ones
                                            # that are real problems.
    "unescaped <>",
    "Entity number out of range",   # Checker outputs this for anything above
                                    # 255, and all Unicode is valid
);

sub suppressed {
    # Returns bool as to if input message is one that is to be suppressed

    my $message = shift;
    return grep { $message =~ /^\Q$_/i } @suppressed_messages;
}

{   # Closure to contain a simple subset of test.pl.  This is to get rid of the
    # unnecessary 'failed at' messages that would otherwise be output pointing
    # to a particular line in this file.

    my $current_test = 0;
    my $planned;

    sub plan {
        my %plan = @_;
        $planned = $plan{tests};
        print "1..$planned\n";
        return;
    }

    sub ok {
        my $success = shift;
        my $message = shift;

        chomp $message;

        $current_test++;
        print "not " unless $success;
        print "ok $current_test - $message\n";
        return;
    }

    sub skip {
        my $why = shift;
        my $n    = @_ ? shift : 1;
        for (1..$n) {
            $current_test++;
            print "ok $current_test # skip $why\n";
        }
        no warnings 'exiting';
        last SKIP;
    }

    sub note {
        my $message = shift;

        chomp $message;

        print $message =~ s/^/# /mgr;
        print "\n";
        return;
    }

    END {
        if ($planned && $planned != $current_test) {
            print STDERR
            "# Looks like you planned $planned tests but ran $current_test.\n";
        }
    }
}

# This is to get this to work across multiple file systems, including those
# that are not case sensitive.  The db is stored in lower case, Un*x style,
# and all file name comparisons are done that way.
sub canonicalize($) {
    my $input = shift;
    my ($volume, $directories, $file)
                    = File::Spec->splitpath(File::Spec->canonpath($input));
    # Assumes $volume is constant for everything in this directory structure
    $directories = "" if ! $directories;
    $file = "" if ! $file;
    my $output = lc join '/', File::Spec->splitdir($directories), $file;
    $output =~ s! / /+ !/!gx;       # Multiple slashes => single slash
    return $output;
}


# List of known potential problems by pod and type.
my %known_problems;

# Pods given by the keys contain an interior node that is referred to from
# outside it.
my %has_referred_to_node;

my $show_counts = 0;
my $regen = 0;
my $add_link = 0;
my $show_all = 0;

my $do_upstream_cpan = 0; # Assume that are to skip anything in /cpan
my $do_deltas = 0;        # And stable perldeltas

while (@ARGV && substr($ARGV[0], 0, 1) eq '-') {
    my $arg = shift @ARGV;

    $arg =~ s/^--/-/; # Treat '--' the same as a single '-'
    if ($arg eq '-regen') {
        $regen = 1;
    }
    elsif ($arg eq '-add_link') {
        $add_link = 1;
    }
    elsif ($arg eq '-cpan') {
        $do_upstream_cpan = 1;
    }
    elsif ($arg eq '-deltas') {
        $do_deltas = 1;
    }
    elsif ($arg eq '-show_all') {
        $show_all = 1;
    }
    elsif ($arg eq '-counts') {
        $show_counts = 1;
    }
    else {
        die <<EOF;
Unknown option '$arg'

Usage: $0 [ --regen | --cpan | --show_all | FILE ... | --add_link MODULE ... ]\n"
    --add_link -> Add the MODULE and man page references to the data base
    --regen    -> Regenerate the data file for $0
    --cpan     -> Include files in the cpan subdirectory.
    --deltas   -> Include stable perldeltas
    --show_all -> Show all known potential problems
    --counts   -> Don't test, but give summary counts of the currently
                  existing database
EOF
    }
}

my @files = @ARGV;

my $cpan_or_deltas = $do_upstream_cpan || $do_deltas;
if (($regen + $show_all + $show_counts + $add_link + $cpan_or_deltas ) > 1) {
    croak "--regen, --show_all, --counts, and --add_link are mutually exclusive\n and none can be run with --cpan nor --deltas";
}

my $has_input_files = @files;

if ($has_input_files
    && ($regen || $show_counts || $do_upstream_cpan || $do_deltas))
{
    croak "--regen, --counts, --deltas, and --cpan can't be used since using specific files";
}

if ($add_link && ! $has_input_files) {
    croak "--add_link requires at least one module or man page reference";
}

our %problems;  # potential problems found in this run

package My::Pod::Checker {      # Extend Pod::Checker
    use parent 'Pod::Checker';

    # Uses inside out hash to protect from typos
    # For new fields, remember to add to destructor DESTROY()
    my %indents;            # Stack of indents from =over's in effect for
                            # current line
    my %current_indent;     # Current line's indent
    my %filename;           # The pod is store in this file
    my %skip;               # is SKIP set for this pod
    my %in_NAME;            # true if within NAME section
    my %in_begin;           # true if within =begin section
    my %linkable_item;      # Bool: if the latest =item is linkable.  It isn't
                            # for bullet and number lists
    my %linkable_nodes;     # Pod::Checker adds all =items to its node list,
                            # but not all =items are linkable to
    my %seen_encoding_cmd;  # true if have =encoding earlier
    my %command_count;      # Number of commands seen
    my %seen_pod_cmd;       # true if have =pod earlier
    my %warned_encoding;    # true if already have warned about =encoding
                            # problems

    sub DESTROY {
        my $addr = Scalar::Util::refaddr $_[0];
        delete $command_count{$addr};
        delete $current_indent{$addr};
        delete $filename{$addr};
        delete $in_begin{$addr};
        delete $indents{$addr};
        delete $in_NAME{$addr};
        delete $linkable_item{$addr};
        delete $linkable_nodes{$addr};
        delete $seen_encoding_cmd{$addr};
        delete $seen_pod_cmd{$addr};
        delete $skip{$addr};
        delete $warned_encoding{$addr};
        return;
    }

    sub new {
        my $class = shift;
        my $filename = shift;

        my $self = $class->SUPER::new(-quiet => 1,
                                     -warnings => $Warnings_Level);
        my $addr = Scalar::Util::refaddr $self;
        $command_count{$addr} = 0;
        $current_indent{$addr} = 0;
        $filename{$addr} = $filename;
        $in_begin{$addr} = 0;
        $in_NAME{$addr} = 0;
        $linkable_item{$addr} = 0;
        $seen_encoding_cmd{$addr} = 0;
        $seen_pod_cmd{$addr} = 0;
        $warned_encoding{$addr} = 0;
        return $self;
    }

    # re's for messages that Pod::Checker outputs
    my $location = qr/ \b (?:in|at|on|near) \s+ /xi;
    my $optional_location = qr/ (?: $location )? /xi;
    my $line_reference = qr/ [('"]? $optional_location \b line \s+
                             (?: \d+ | EOF | \Q???\E | - )
                             [)'"]? /xi;

    sub poderror {  # Called to register a potential problem

        # This adds an extra field to the parent hash, 'parameter'.  It is
        # used to extract the variable parts of a message leaving just the
        # constant skeleton.  This in turn allows the message to be
        # categorized better, so that it shows up as a single type in our
        # database, with the specifics of each occurrence not being stored with
        # it.

        my $self = shift;
        my $opts = shift;

        my $addr = Scalar::Util::refaddr $self;
        return if $skip{$addr};

        # Input can be a string or hash.  If a string, parse it to separate
        # out the line number and convert to a hash for easier further
        # processing
        my $message;
        if (ref $opts ne 'HASH') {
            $message = join "", $opts, @_;
            my $line_number;
            if ($message =~ s/\s*($line_reference)//) {
                ($line_number = $1) =~ s/\s*$optional_location//;
            }
            else {
                $line_number = '???';
            }
            $opts = { -msg => $message, -line => $line_number };
        } else {
            $message = $opts->{'-msg'};

        }

        $message =~ s/^\d+\s+//;
        return if main::suppressed($message);

        $self->SUPER::poderror($opts, @_);

        $opts->{parameter} = "" unless $opts->{parameter};

        # The variable parts of the message tend to be enclosed in '...',
        # "....", or (...).  Extract them and put them in an extra field,
        # 'parameter'.  This is trickier because the matching delimiter to a
        # '(' is its mirror, and not itself.  Text::Balanced could be used
        # instead.
        while ($message =~ m/ \s* $optional_location ( [('"] )/xg) {
            my $delimiter = $1;
            my $start = $-[0];
            $delimiter = ')' if $delimiter eq '(';

            # If there is no ending delimiter, don't consider it to be a
            # variable part.  Most likely it is a contraction like "Don't"
            last unless $message =~ m/\G .+? \Q$delimiter/xg;

            my $length = $+[0] - $start;

            # Get the part up through the closing delimiter
            my $special = substr($message, $start, $length);
            $special =~ s/^\s+//;   # No leading whitespace

            # And add that variable part to the parameter, while removing it
            # from the message.  This isn't a foolproof way of finding the
            # variable part.  For example '(s)' can occur in e.g.,
            # 'paragraph(s)'
            if ($special ne '(s)') {
                substr($message, $start, $length) = "";
                pos $message = $start;
                $opts->{-msg} = $message;
                $opts->{parameter} .= " " if $opts->{parameter};
                $opts->{parameter} .= $special;
            }
        }

        # Extract any additional line number given.  This is often the
        # beginning location of something whereas the main line number gives
        # the ending one.
        if ($message =~ /( $line_reference )/xi) {
            my $line_ref = $1;
            while ($message =~ s/\s*\Q$line_ref//) {
                $opts->{-msg} = $message;
                $opts->{parameter} .= " " if $opts->{parameter};
                $opts->{parameter} .= $line_ref;
            }
        }

        Carp::carp("Couldn't extract line number from '$message'") if $message =~ /line \d+/;
        push @{$problems{$filename{$addr}}{$message}}, $opts;
        #push @{$problems{$self->get_filename}{$message}}, $opts;
    }

    sub check_encoding {    # Does it need an =encoding statement?
        my ($self, $paragraph, $line_num, $pod_para) = @_;

        # Do nothing if there is an =encoding in the file, or if the line
        # doesn't require an =encoding, or have already warned.
        my $addr = Scalar::Util::refaddr $self;
        return if $seen_encoding_cmd{$addr}
                    || $warned_encoding{$addr}
                    || $paragraph !~ /\P{ASCII}/;

        $warned_encoding{$addr} = 1;
        my ($file, $line) = $pod_para->file_line;
        $self->poderror({ -line => $line, -file => $file,
                          -msg => $need_encoding
                        });
        return;
    }

    sub verbatim {
        my ($self, $paragraph, $line_num, $pod_para) = @_;
        $self->check_encoding($paragraph, $line_num, $pod_para);

        $self->SUPER::verbatim($paragraph, $line_num, $pod_para);

        my $addr = Scalar::Util::refaddr $self;

        # Pick up the name, since the parent class doesn't in verbatim
        # NAMEs; so treat as non-verbatim.  The parent class only allows one
        # paragraph in a NAME section, so if there is an extra blank line, it
        # will trigger a message, but such a blank line is harmless, so skip
        # in that case.
        if ($in_NAME{$addr} && $paragraph =~ /\S/) {
            $self->textblock($paragraph, $line_num, $pod_para);
        }

        my @lines = split /^/, $paragraph;
        for my $i (0 .. @lines - 1) {
            if ( my $encoding = $seen_encoding_cmd{$addr} ) {
              require Encode;
              $lines[$i] = Encode::decode($encoding, $lines[$i]);
            }
            $lines[$i] =~ s/\s+$//;
            my $indent = $self->get_current_indent;
            my $exceeds = length(Text::Tabs::expand($lines[$i]))
                          + $indent - $MAX_LINE_LENGTH;
            next unless $exceeds > 0;
            my ($file, $line) = $pod_para->file_line;
            $self->poderror({ -line => $line + $i, -file => $file,
                -msg => $line_length,
                parameter => "+$exceeds (including " . ($indent - $INDENT) . " from =over's)",
            });
        }
    }

    sub textblock {
        my ($self, $paragraph, $line_num, $pod_para) = @_;
        $self->check_encoding($paragraph, $line_num, $pod_para);

        $self->SUPER::textblock($paragraph, $line_num, $pod_para);

        my ($file, $line) = $pod_para->file_line;
        my $addr = Scalar::Util::refaddr $self;
        if ($in_NAME{$addr}) {
            if (! $self->name) {
                my $text = $self->interpolate($paragraph, $line_num);
                if ($text =~ /^\s*(\S+?)\s*$/) {
                    $self->name($1);
                    $self->poderror({ -line => $line, -file => $file,
                        -msg => $missing_name_description,
                        parameter => $1});
                }
            }
        }
        $paragraph = join " ", split /^/, $paragraph;

        # Matches something that looks like a file name, but is enclosed in
        # C<...>
        my $C_path_re = qr{ \b ( C<
                                # exclude various things that have slashes
                                # in them but aren't paths
                                (?!
                                    (?: (?: s | qr | m) / ) # regexes
                                    | \d+/\d+>       # probable fractions
                                    | OS/2>
                                    | Perl/Tk>
                                    | origin/blead>
                                    | origin/maint
                                    | -    # File names don't begin with "-"
                                 )
                                 [-\w]+ (?: / [-\w]+ )+ (?: \. \w+ )? > )
                          }x;

        # If looks like a reference to other documentation by containing the
        # word 'See' and then a likely pod directive, warn.
        while ($paragraph =~ m{
                                ( (?: \w+ \s+ )* )  # The phrase before, if any
                                \b [Ss]ee \s+
                                ( ( [^L] )
                                  <
                                  ( [^<]*? )  # The not < excludes nested C<L<...
                                  >
                                )
                                ( \s+ (?: under | in ) \s+ L< )?
                            }xg) {
            my $prefix = $1 // "";
            my $construct = $2;     # The whole thing, like C<...>
            my $type = $3;
            my $interior = $4;
            my $trailing = $5;      # After the whole thing ending in "L<"

            # If the full phrase is something like, "you might see C<", or
            # similar, it really isn't a reference to a link.  The ones I saw
            # all had the word "you" in them; and the "you" wasn't the
            # beginning of a sentence.
            if ($prefix !~ / \b you \b /x) {

                # Now, find what the module or man page name within the
                # construct would be if it actually has L<> syntax.  If it
                # doesn't have that syntax, will set the module to the entire
                # interior.
                $interior =~ m/ ^
                                (?: [^|]+ \| )? # Optional arbitrary text ending
                                                # in "|"
                                ( .+? )         # module, etc. name
                                (?: \/ .+ )?    # target within module
                                $
                            /xs;
                my $module = $1;
                if (! defined $trailing # not referring to something in another
                                        # section
                    && $interior !~ /$non_pods/

                    # C<> that look like files have their own message below, so
                    # exclude them
                    && $construct !~ /$C_path_re/g

                    # There can't be spaces (I think) in module names or man
                    # pages
                    && $module !~ / \s /x

                    # F<> that end in eg \.pl are almost certainly ok, as are
                    # those that look like a path with multiple "/" chars
                    && ($type ne "F"
                        || (! -e $interior
                            && $interior !~ /\.\w+$/
                            && $interior !~ /\/.+\//)
                    )
                ) {
                    $self->poderror({ -line => $line, -file => $file,
                        -msg => $see_not_linked,
                        parameter => $construct
                    });
                }
            }
        }
        while ($paragraph =~ m/$C_path_re/g) {
            my $construct = $1;
            $self->poderror({ -line => $line, -file => $file,
                -msg => $C_with_slash,
                parameter => $construct
            });
        }
        return;
    }

    sub command {
        my ($self, $cmd, $paragraph, $line_num, $pod_para) = @_;
        my $addr = Scalar::Util::refaddr $self;
        if ($cmd eq "pod") {
            $seen_pod_cmd{$addr}++;
        }
        elsif ($cmd eq "encoding") {
            my ($file, $line) = $pod_para->file_line;
            $seen_encoding_cmd{$addr} = $paragraph; # for later decoding
            if ($command_count{$addr} != 1 && $seen_pod_cmd{$addr}) {
                $self->poderror({ -line => $line, -file => $file,
                                  -msg => $encoding_first
                                });
            }
        }
        $self->check_encoding($paragraph, $line_num, $pod_para);

        # Pod::Check treats all =items as linkable, but the bullet and
        # numbered lists really aren't.  So keep our own list.  This has to be
        # processed before SUPER is called so that the list is started before
        # the rest of it gets parsed.
        if ($cmd eq 'item') { # Not linkable if item begins with * or a digit
            $linkable_item{$addr} = ($paragraph !~ / ^ \s*
                                                   (?: [*]
                                                   | \d+ \.? (?: \$ | \s+ )
                                                   )/x)
                                  ? 1
                                  : 0;

        }
        $self->SUPER::command($cmd, $paragraph, $line_num, $pod_para);

        $command_count{$addr}++;

        $in_NAME{$addr} = 0;    # Will change to 1 below if necessary
        $in_begin{$addr} = 0;   # ibid
        if ($cmd eq 'over') {
            my $text = $self->interpolate($paragraph, $line_num);
            my $indent = 4; # default
            $indent = $1 if $text && $text =~ /^\s*(\d+)\s*$/;
            push @{$indents{$addr}}, $indent;
            $current_indent{$addr} += $indent;
        }
        elsif ($cmd eq 'back') {
            if (@{$indents{$addr}}) {
                $current_indent{$addr} -= pop @{$indents{$addr}};
            }
            else {
                 # =back without corresponding =over, but should have
                 # warned already
                $current_indent{$addr} = 0;
            }
        }
        elsif ($cmd =~ /^head/) {
            if (! $in_begin{$addr}) {

                # If a particular formatter, then this command doesn't really
                # apply
                $current_indent{$addr} = 0;
                undef @{$indents{$addr}};
            }

            my $text = $self->interpolate($paragraph, $line_num);
            $in_NAME{$addr} = 1 if $cmd eq 'head1'
                                   && $text && $text =~ /^NAME\b/;
        }
        elsif ($cmd eq 'begin') {
            $in_begin{$addr} = 1;
        }

        return;
    }

    sub hyperlink {
        my $self = shift;

        my $page;
        if ($_[0] && ($page = $_[0][1]{'-page'})) {
            my $node = $_[0][1]{'-node'};

            # If the hyperlink is to an interior node of another page, save it
            # so that we can see if we need to parse normally skipped files.
            $has_referred_to_node{$page} = 1 if $node;

            # Ignore certain placeholder links in perldelta.  Check if the
            # link is page-level, and also check if to a node within the page
            if ($self->name && $self->name eq "perldelta"
                && ((grep { $page eq $_ } @perldelta_ignore_links)
                    || ($node
                        && (grep { "$page/$node" eq $_ } @perldelta_ignore_links)
            ))) {
                return;
            }
        }
        return $self->SUPER::hyperlink($_[0]);
    }

    sub node {
        my $self = shift;
        my $text = $_[0];
        if($text) {
            $text =~ s/\s+$//s; # strip trailing whitespace
            $text =~ s/\s+/ /gs; # collapse whitespace
            my $addr = Scalar::Util::refaddr $self;
            push(@{$linkable_nodes{$addr}}, $text) if
                                    ! $current_indent{$addr}
                                    || $linkable_item{$addr};
        }
        return $self->SUPER::node($_[0]);
    }

    sub get_current_indent {
        return $INDENT + $current_indent{Scalar::Util::refaddr $_[0]};
    }

    sub get_filename {
        return $filename{Scalar::Util::refaddr $_[0]};
    }

    sub linkable_nodes {
        my $linkables = $linkable_nodes{Scalar::Util::refaddr $_[0]};
        return undef unless $linkables;
        return @$linkables;
    }

    sub get_skip {
        return $skip{Scalar::Util::refaddr $_[0]} // 0;
    }

    sub set_skip {
        my $self = shift;
        $skip{Scalar::Util::refaddr $self} = shift;

        # If skipping, no need to keep the problems for it
        delete $problems{$self->get_filename};
        return;
    }
}

package Tie_Array_to_FH {  # So printing actually goes to an array

    my %array;

    sub TIEHANDLE {
        my $class = shift;
        my $array_ref = shift;

        my $self = bless \do{ my $anonymous_scalar }, $class;
        $array{Scalar::Util::refaddr $self} = $array_ref;

        return $self;
    }

    sub PRINT {
        my $self = shift;
        push @{$array{Scalar::Util::refaddr $self}}, @_;
        return 1;
    }
}


my %filename_to_checker; # Map a filename to it's pod checker object
my %id_to_checker;      # Map a checksum to it's pod checker object
my %nodes;              # key is filename, values are nodes in that file.
my %nodes_first_word;   # same, but value is first word of each node
my %valid_modules;      # List of modules known to exist outside us.
my %digests;            # checksums of files, whose names are the keys
my %filename_to_pod;    # Map a filename to its pod NAME
my %files_with_unknown_issues;
my %files_with_fixes;

my $data_fh;
open $data_fh, '<:bytes', $known_issues or die "Can't open $known_issues";

my %counts; # For --counts param, count of each issue type
my %suppressed_files;   # Files with at least one issue type to suppress
my $HEADER = <<END;
# This file is the data file for $0.
# There are three types of lines.
# Comment lines are white-space only or begin with a '#', like this one.  Any
#   changes you make to the comment lines will be lost when the file is
#   regen'd.
# Lines without tab characters are simply NAMES of pods that the program knows
#   will have links to them and the program does not check if those links are
#   valid.
# All other lines should have three fields, each separated by a tab.  The
#   first field is the name of a pod; the second field is an error message
#   generated by this program; and the third field is a count of how many
#   known instances of that message there are in the pod.  -1 means that the
#   program can expect any number of this type of message.
END

my @existing_issues;


while (<$data_fh>) {    # Read the data base
    chomp;
    next if /^\s*(?:#|$)/;  # Skip comment and empty lines
    if (/\t/) {
        next if $show_all;
        if ($add_link) {    # The issues are saved and later output unchanged
            push @existing_issues, $_;
            next;
        }

        # Keep track of counts of each issue type for each file
        my ($filename, $message, $count) = split /\t/;
        $known_problems{$filename}{$message} = $count;

        if ($show_counts) {
            if ($count < 0) {   # -1 means to suppress this issue type
                $suppressed_files{$filename} = $filename;
            }
            else {
                $counts{$message} += $count;
            }
        }
    }
    else {  # Lines without a tab are modules known to be valid
        $valid_modules{$_} = 1
    }
}
close $data_fh;

if ($add_link) {
    $copy_fh = open_new($known_issues);

    # Check for basic sanity, and add each command line argument
    foreach my $module (@files) {
        die "\"$module\" does not look like a module or man page"
            # Must look like (A or A::B or A::B::C ..., or foo(3C)
            if $module !~ /^ (?: \w+ (?: :: \w+ )* | \w+ \( \d \w* \) ) $/x;
        $valid_modules{$module} = 1
    }
    my_safer_print($copy_fh, $HEADER);
    foreach (sort { lc $a cmp lc $b } keys %valid_modules) {
        my_safer_print($copy_fh, $_, "\n");
    }

    # The rest of the db file is output unchanged.
    my_safer_print($copy_fh, join "\n", @existing_issues, "");

    close_and_rename($copy_fh);
    exit;
}

if ($show_counts) {
    my $total = 0;
    foreach my $message (sort keys %counts) {
        $total += $counts{$message};
        note(Text::Tabs::expand("$counts{$message}\t$message"));
    }
    note("-----\n" . Text::Tabs::expand("$total\tknown potential issues"));
    if (%suppressed_files) {
        note("\nFiles that have all messages of at least one type suppressed:");
        note(join ",", keys %suppressed_files);
    }
    exit 0;
}


# Not really pods, but can look like them.
my %excluded_files = (
                        "lib/unicore/mktables" => 1,
                        "Porting/perldelta_template.pod" => 1,
                        "autodoc.pl" => 1,
                        "configpm" => 1,
                        "miniperl" => 1,
                        "perl" => 1,
                    );

# Convert to more generic form.
foreach my $file (keys %excluded_files) {
    delete $excluded_files{$file};
    $excluded_files{canonicalize($file)} = 1;
}

# re to match files that are to be parsed only if there is an internal link
# to them.  It does not include cpan, as whether those are parsed depends
# on a switch.  Currently, only perltoc and the stable perldelta.pod's
# are included.  The latter all have characters between 'perl' and
# 'delta'.  (Actually the currently developed one matches as well, but
# is a duplicate of perldelta.pod, so can be skipped, so fine for it to
# match this.
my $only_for_interior_links_re = qr/ ^ pod\/perltoc.pod $
                                   /x;
unless ($do_deltas) {
    $only_for_interior_links_re = qr/$only_for_interior_links_re |
                                    \b perl \d+ delta \. pod \b
                                /x;
}

{ # Closure
    my $first_time = 1;

    sub output_thanks ($$$$) {  # Called when an issue has been fixed
        my $filename = shift;
        my $original_count = shift;
        my $current_count = shift;
        my $message = shift;

        $files_with_fixes{$filename} = 1;
        my $return;
        my $fixed_count = $original_count - $current_count;
        my $a_problem = ($fixed_count == 1) ? "a problem" : "multiple problems";
        my $another_problem = ($fixed_count == 1) ? "another problem" : "another set of problems";
        my $diff;
        if ($message) {
            $diff = <<EOF;
There were $original_count occurrences (now $current_count) in this pod of type
"$message",
EOF
        } else {
            $diff = <<EOF;
There are no longer any problems found in this pod!
EOF
        }

        if ($first_time) {
            $first_time = 0;
            $return = <<EOF;
Thanks for fixing $a_problem!
$diff
Now you must teach $0 that this was fixed.
EOF
        }
        else {
            $return = <<EOF
Thanks for fixing $another_problem.
$diff
EOF
        }

        return $return;
    }
}

sub my_safer_print {    # print, with error checking for outputting to db
    my ($fh, @lines) = @_;

    if (! print $fh @lines) {
        my $save_error = $!;
        close($fh);
        die "Write failure: $save_error";
    }
}

sub extract_pod {   # Extracts just the pod from a file
    my $filename = shift;

    my @pod;

    # Arrange for the output of Pod::Parser to be collected in an array we can
    # look at instead of being printed
    tie *ALREADY_FH, 'Tie_Array_to_FH', \@pod;
    open my $in_fh, '<:bytes', $filename

        # The file should already have been opened once to get here, so if
        # fails, just die.  It's possible that a transitory file containing a
        # pod would get here, but not bothering to add code for that very
        # unlikely event.
        or die "Can't open '$filename': $!\n";

    my $parser = Pod::Parser->new();
    $parser->parse_from_filehandle($in_fh, *ALREADY_FH);
    close $in_fh;

    return join "", @pod
}

my $digest = Digest->new($digest_type);

sub is_pod_file {
    # If $_ is a pod file, add it to the lists and do other prep work.

    if (-d $_) {
        # Don't look at files in directories that are for tests, nor those
        # beginning with a dot
        if ($_ eq 't' || $_ =~ /^\../) {
            $File::Find::prune = 1;
        }
        return;
    }

    return if $_ =~ /^\./;           # No hidden Unix files
    return if $_ =~ $non_pods;

    my $filename = $File::Find::name;

    # Assumes that the path separator is exactly one character.
    $filename =~ s/^\..//;

    return if $excluded_files{canonicalize($filename)};

    my $contents = do {
        local $/;
        my $candidate;
        if (! open $candidate, '<:bytes', $_) {

            # If a transitory file was found earlier, the open could fail
            # legitimately and we just skip the file; also skip it if it is a
            # broken symbolic link, as it is probably just a build problem;
            # certainly not a file that we would want to check the pod of.
            # Otherwise fail it here and no reason to process it further.
            # (But the test count will be off too)
            ok(0, "Can't open '$filename': $!")
                                            if -e $filename && ! -l $filename;
            return;
        }
        <$candidate>;
    };

    # If the file is a .pm or .pod, having any initial '=' on a line is
    # grounds for testing it.  Otherwise, require a head1 NAME line to view it
    # as a potential pod
    if ($filename =~ /\.(?:pm|pod)/) {
        return unless $contents =~ /^=/m;
    } else {
        return unless $contents =~ /^=head1 +NAME/m;
    }

    # Here, we know that the file is a pod.  Add it to the list of files
    # to check and create a checker object for it.

    push @files, $filename;
    my $checker = My::Pod::Checker->new($filename);
    $filename_to_checker{$filename} = $checker;

    # In order to detect duplicate pods and only analyze them once, we
    # compute checksums for the file, so don't have to do an exact
    # compare.  Note that if the pod is just part of the file, the
    # checksums can differ for the same pod.  That special case is handled
    # later, since if the checksums of the whole file are the same, that
    # case won't even come up.  We don't need the checksums for files that
    # we parse only if there is a link to its interior, but we do need its
    # NAME, which is also retrieved in the code below.

    if ($filename =~ / (?: ^(cpan|lib|ext|dist)\/ )
                        | $only_for_interior_links_re
                    /x) {
        $digest->add($contents);
        $digests{$filename} = $digest->digest;

        # lib files aren't analyzed if they are duplicates of files copied
        # there from some other directory.  But to determine this, we need
        # to know their NAMEs.  We might as well find the NAME now while
        # the file is open.  Similarly, cpan files aren't analyzed unless
        # we're analyzing all of them, or this particular file is linked
        # to by a file we are analyzing, and thus we will want to verify
        # that the target exists in it.  We need to know at least the NAME
        # to see if it's worth analyzing, or so we can determine if a lib
        # file is a copy of a cpan one.
        if ($filename =~ m{ (?: ^ (?: cpan | lib ) / )
                            | $only_for_interior_links_re
                            }x) {
            if ($contents =~ /^=head1 +NAME.*/mg) {
                # The NAME is the first non-spaces on the line up to a
                # comma, dash or end of line.  Otherwise, it's invalid and
                # this pod doesn't have a legal name that we're smart
                # enough to find currently.  But the  parser will later
                # find it if it thinks there is a legal name, and set the
                # name
                if ($contents =~ /\G    # continue from the line after =head1
                                  \s*   # ignore any empty lines
                                  ^ \s* ( \S+?) \s* (?: [,-] | $ )/mx) {
                    my $name = $1;
                    $checker->name($name);
                    $id_to_checker{$name} = $checker
                        if $filename =~ m{^cpan/};
                }
            }
            elsif ($filename =~ m{^cpan/}) {
                $id_to_checker{$digests{$filename}} = $checker;
            }
        }
    }

    return;
} # End of is_pod_file()

# Start of real code that isn't processing the command line (except the
# db is read in above, as is processing of the --add_link option).
# Here, @files contains list of files on the command line.  If have any of
# these, unconditionally test them, and show all the errors, even the known
# ones, and, since not testing other pods, don't do cross-pod link tests.
# (Could add extra code to do cross-pod tests for the ones in the list.)

if ($has_input_files) {
    undef %known_problems;
    $do_upstream_cpan = $do_deltas = 1;  # In case one of the inputs is one
                                         # of these types
}
else { # No input files -- go find all the possibilities.
    if ($regen) {
        $copy_fh = open_new($known_issues);
        note("Regenerating $known_issues, please be patient...");
        print $copy_fh $HEADER;
    }

    # Move to the directory above us, but have to adjust @INC to account for
    # that.
    s{^\.\./lib$}{lib} for @INC;
    chdir File::Spec->updir;

    # And look in this directory and all its subdirectories
    find( \&is_pod_file, '.');

    # Add ourselves to the test
    push @files, "t/porting/podcheck.t";
}

# Now we know how many tests there will be.
plan (tests => scalar @files) if ! $regen;


 # Sort file names so we get consistent results, and to put cpan last,
 # preceeded by the ones that we don't generally parse.  This is because both
 # these classes are generally parsed only if there is a link to the interior
 # of them, and we have to parse all others first to guarantee that they don't
 # have such a link. 'lib' files come just before these, as some of these are
 # duplicates of others.  We already have figured this out when gathering the
 # data as a special case for all such files, but this, while unnecessary,
 # puts the derived file last in the output.  'readme' files come before those,
 # as those also could be duplicates of others, which are considered the
 # primary ones.  These currently aren't figured out when gathering data, so
 # are done here.
 @files = sort { if ($a =~ /^cpan/) {
                    return 1 if $b !~ /^cpan/;
                    return $a cmp $b;
                }
                elsif ($b =~ /^cpan/) {
                    return -1;
                }
                elsif ($a =~ /$only_for_interior_links_re/) {
                    return 1 if $b !~ /$only_for_interior_links_re/;
                    return $a cmp $b;
                }
                elsif ($b =~ /$only_for_interior_links_re/) {
                    return -1;
                }
                elsif ($a =~ /^lib/) {
                    return 1 if $b !~ /^lib/;
                    return $a cmp $b;
                }
                elsif ($b =~ /^lib/) {
                    return -1;
                } elsif ($a =~ /\breadme\b/i) {
                    return 1 if $b !~ /\breadme\b/i;
                    return $a cmp $b;
                }
                elsif ($b =~ /\breadme\b/i) {
                    return -1;
                }
                else {
                    return lc $a cmp lc $b;
                }
            }
            @files;

# Now go through all the files and parse them
foreach my $filename (@files) {
    my $parsed = 0;
    note("parsing $filename") if DEBUG;

    # We may have already figured out some things in the process of generating
    # the file list.  If so, have a $checker object already.  But if not,
    # generate one now.
    my $checker = $filename_to_checker{$filename};
    if (! $checker) {
        $checker = My::Pod::Checker->new($filename);
        $filename_to_checker{$filename} = $checker;
    }

    # We have set the name in the checker object if there is a possibility
    # that no further parsing is necessary, but otherwise do the parsing now.
    if (! $checker->name) {
        $parsed = 1;
        $checker->parse_from_file($filename, undef);
    }

    if ($checker->num_errors() < 0) {   # Returns negative if not a pod
        $checker->set_skip("$filename is not a pod");
    }
    else {

        # Here, is a pod.  See if it is one that has already been tested,
        # or should be tested under another directory.  Use either its NAME
        # if it has one, or a checksum if not.
        my $name = $checker->name;
        my $id;

        if ($name) {
            $id = $name;
        }
        else {
            my $digest = Digest->new($digest_type);
            $digest->add(extract_pod($filename));
            $id = $digest->digest;
        }

        # If there is a match for this pod with something that we've already
        # processed, don't process it, and output why.
        my $prior_checker;
        if (defined ($prior_checker = $id_to_checker{$id})
            && $prior_checker != $checker)  # Could have defined the checker
                                            # earlier without pursuing it
        {

            # If the pods are identical, then it's just a copy, and isn't an
            # error.  First use the checksums we have already computed to see
            # if the entire files are identical, which means that the pods are
            # identical too.
            my $prior_filename = $prior_checker->get_filename;
            my $same = (! $name
                        || ($digests{$prior_filename}
                            && $digests{$filename}
                            && $digests{$prior_filename} eq $digests{$filename}));

            # If they differ, it could be that the files differ for some
            # reason, but the pods they contain are identical.  Extract the
            # pods and do the comparisons on just those.
            if (! $same && $name) {
                $same = extract_pod($prior_filename) eq extract_pod($filename);
            }

            if ($same) {
                $checker->set_skip("The pod of $filename is a duplicate of "
                                    . "the pod for $prior_filename");
            } elsif ($prior_filename =~ /\breadme\b/i) {
                $checker->set_skip("$prior_filename is a README apparently for $filename");
            } elsif ($filename =~ /\breadme\b/i) {
                $checker->set_skip("$filename is a README apparently for $prior_filename");
            } elsif (! $do_upstream_cpan && $filename =~ /^cpan/) {
                $checker->set_skip("CPAN is upstream for $filename");
            } else { # Here have two pods with identical names that differ
                $prior_checker->poderror(
                        { -msg => $duplicate_name,
                            -line => "???",
                            parameter => "'$filename' also has NAME '$name'"
                        });
                $checker->poderror(
                    { -msg => $duplicate_name,
                        -line => "???",
                        parameter => "'$prior_filename' also has NAME '$name'"
                    });

                # Changing the names helps later.
                $prior_checker->name("$name version arbitrarily numbered 1");
                $checker->name("$name version arbitrarily numbered 2");
            }

            # In any event, don't process this pod that has the same name as
            # another.
            next;
        }

        # A unique pod.
        $id_to_checker{$id} = $checker;

        my $parsed_for_links = ", but parsed for its interior links";
        if ((! $do_upstream_cpan && $filename =~ /^cpan/)
             || $filename =~ $only_for_interior_links_re)
        {
            if ($filename =~ /^cpan/) {
                $checker->set_skip("CPAN is upstream for $filename");
            }
            elsif ($filename =~ /perl\d+delta/ && ! $do_deltas) {
                $checker->set_skip("$filename is a stable perldelta");
            }
            elsif ($filename =~ /perltoc/) {
                $checker->set_skip("$filename dependent on component pods");
            }
            else {
                croak("Unexpected file '$filename' encountered that has parsing for interior-linking only");
            }

            if ($name && $has_referred_to_node{$name}) {
                $checker->set_skip($checker->get_skip() . $parsed_for_links);
            }
        }

        # Need a name in order to process it, because not meaningful
        # otherwise, and also can't test links to this without a name.
        if (!defined $name) {
            $checker->poderror( { -msg => $no_name,
                                  -line => '???'
                                });
            next;
        }

        # For skipped files, just get its NAME
        my $skip;
        if (($skip = $checker->get_skip()) && $skip !~ /$parsed_for_links/)
        {
            $checker->node($name) if $name;
        }
        else {
            $checker->parse_from_file($filename, undef) if ! $parsed;
        }

        # Go through everything in the file that could be an anchor that
        # could be a link target.  Count how many there are of the same name.
        foreach my $node ($checker->linkable_nodes) {
            next if ! $node;        # Can be empty is like '=item *'
            if (exists $nodes{$name}{$node}) {
                $nodes{$name}{$node}++;
            }
            else {
                $nodes{$name}{$node} = 1;
            }

            # Experiments have shown that cpan search can figure out the
            # target of a link even if the exact wording is incorrect, as long
            # as the first word is.  This happens frequently in perlfunc.pod,
            # where the link will be just to the function, but the target
            # entry also includes parameters to the function.
            my $first_word = $node;
            if ($first_word =~ s/^(\S+)\s+\S.*/$1/) {
                $nodes_first_word{$name}{$first_word} = $node;
            }
        }
        $filename_to_pod{$filename} = $name;
    }
}

# Here, all files have been parsed, and all links and link targets are stored.
# Now go through the files again and see which don't have matches.
if (! $has_input_files) {
    foreach my $filename (@files) {
        next if $filename_to_checker{$filename}->get_skip;
        my $checker = $filename_to_checker{$filename};
        foreach my $link ($checker->hyperlink) {
            my $linked_to_page = $link->[1]->page;
            next unless $linked_to_page;   # intra-file checks are handled by std
                                           # Pod::Checker

            # Initialize the potential message.
            my %problem = ( -msg => $broken_link,
                            -line => $link->[0],
                            parameter => "to \"$linked_to_page\"",
                        );

            # See if we have found the linked-to_file in our parse
            if (exists $nodes{$linked_to_page}) {
                my $node = $link->[1]->node;

                # If link is only to the page-level, already have it
                next if ! $node;

                # Transform pod language to what we are expecting
                $node =~ s,E<sol>,/,g;
                $node =~ s/E<verbar>/|/g;

                # If link is to a node that exists in the file, is ok
                if ($nodes{$linked_to_page}{$node}) {

                    # But if the page has multiple targets with the same name,
                    # it's ambiguous which one this should be to.
                    if ($nodes{$linked_to_page}{$node} > 1) {
                        $problem{-msg} = $multiple_targets;
                        $problem{parameter} = "in $linked_to_page that $node could be pointing to";
                        $checker->poderror(\%problem);
                    }
                } elsif (! $nodes_first_word{$linked_to_page}{$node}) {

                    # Here the link target was not found, either exactly or to
                    # the first word.  Is an error.
                    $problem{parameter} =~ s,"$,/$node",;
                    $checker->poderror(\%problem);
                }

            } # Linked-to-file not in parse; maybe is in exception list
            elsif (! exists $valid_modules{$link->[1]->page}) {

                # Here, is a link to a target that we can't find.  Check if
                # there is an internal link on the page with the target name.
                # If so, it could be that they just forgot the initial '/'
                # But perldelta is handled specially: only do this if the
                # broken link isn't one of the known bad ones (that are
                # placemarkers and should be removed for the final)
                my $NAME = $filename_to_pod{$filename};
                if (! defined $NAME) {
                    $checker->poderror(\%problem);
                }
                else {
                    if ($nodes{$NAME}{$linked_to_page}) {
                        $problem{-msg} =  $broken_internal_link;
                    }
                    $checker->poderror(\%problem);
                }
            }
        }
    }
}

# If regenerating the data file, start with the modules for which we don't
# check targets
if ($regen) {
    foreach (sort { lc $a cmp lc $b } keys %valid_modules) {
        my_safer_print($copy_fh, $_, "\n");
    }
}

# Now ready to output the messages.
foreach my $filename (@files) {
    my $test_name = "POD of $filename";
    my $canonical = canonicalize($filename);
    SKIP: {
        my $skip = $filename_to_checker{$filename}->get_skip // "";

        if ($regen) {
            foreach my $message ( sort keys %{$problems{$filename}}) {
                my $count;

                # Preserve a negative setting.
                if ($known_problems{$canonical}{$message}
                    && $known_problems{$canonical}{$message} < 0)
                {
                    $count = $known_problems{$canonical}{$message};
                }
                else {
                    $count = @{$problems{$filename}{$message}};
                }
                my_safer_print($copy_fh, canonicalize($filename) . "\t$message\t$count\n");
            }
            next;
        }

        skip($skip, 1) if $skip;
        my @diagnostics;
        my $indent = '  ';

        my $total_known = 0;
        foreach my $message ( sort keys %{$problems{$filename}}) {
            $known_problems{$canonical}{$message} = 0
                                    if ! $known_problems{$canonical}{$message};
            my $diagnostic = "";
            my $problem_count = scalar @{$problems{$filename}{$message}};
            $total_known += $problem_count;
            next if $known_problems{$canonical}{$message} < 0;
            if ($problem_count > $known_problems{$canonical}{$message}) {

                # Here we are about to output all the messages for this type,
                # subtract back this number we previously added in.
                $total_known -= $problem_count;

                $diagnostic .= $indent . $message;
                if ($problem_count > 2) {
                    $diagnostic .= "  ($problem_count occurrences)";
                }
                foreach my $problem (@{$problems{$filename}{$message}}) {
                    $diagnostic .= " " if $problem_count == 1;
                    $diagnostic .= "\n$indent$indent";
                    $diagnostic .= "$problem->{parameter}" if $problem->{parameter};
                    $diagnostic .= " near line $problem->{-line}";
                    $diagnostic .= " $problem->{comment}" if $problem->{comment};
                }
                $diagnostic .= "\n";
                $files_with_unknown_issues{$filename} = 1;
            } elsif ($problem_count < $known_problems{$canonical}{$message}) {
               $diagnostic = output_thanks($filename, $known_problems{$canonical}{$message}, $problem_count, $message);
            }
            push @diagnostics, $diagnostic if $diagnostic;
        }

        # The above loop has output messages where there are current potential
        # issues.  But it misses where there were some that have been entirely
        # fixed.  For those, we need to look through the old issues
        foreach my $message ( sort keys %{$known_problems{$canonical}}) {
            next if $problems{$filename}{$message};
            next if ! $known_problems{$canonical}{$message};
            next if $known_problems{$canonical}{$message} < 0; # Preserve negs
            my $diagnostic = output_thanks($filename, $known_problems{$canonical}{$message}, 0, $message);
            push @diagnostics, $diagnostic if $diagnostic;
        }

        my $output = "POD of $filename";
        $output .= ", excluding $total_known not shown known potential problems"
                                                                if $total_known;
        ok(@diagnostics == 0, $output);
        if (@diagnostics) {
            note(join "", @diagnostics,
            "See end of this test output for your options on silencing this");
        }
    }
}

my $how_to = <<EOF;
   run this test script by hand, using the following formula (on
   Un*x-like machines):
        cd t
        ./perl -I../lib porting/podcheck.t --regen
EOF

if (%files_with_unknown_issues) {
    my $were_count_files = scalar keys %files_with_unknown_issues;
    $were_count_files = ($were_count_files == 1)
                        ? "was $were_count_files file"
                        : "were $were_count_files files";
    my $message = <<EOF;

HOW TO GET THIS .t TO PASS

There $were_count_files that had new potential problems identified.
Some of them may be real, and some of them may be false positives because
this program isn't as smart as it likes to think it is.  You can teach this
program to ignore the issues it has identified, and hence pass, by doing the
following:

1) If a problem is about a link to an unknown module or man page that
   you know exists, re-run the command something like:
      ./perl -I../lib porting/podcheck.t --add_link MODULE man_page ...
   (MODULEs should look like Foo::Bar, and man_pages should look like
   bar(3c); don't do this for a module or man page that you aren't sure
   about; instead treat as another type of issue and follow the
   instructions below.)

2) For other issues, decide if each should be fixed now or not.  Fix the
   ones you decided to, and rerun this test to verify that the fixes
   worked.

3) If there remain false positive or problems that you don't plan to fix right
   now,
$how_to
   That should cause all current potential problems to be accepted by
   the program, so that the next time it runs, they won't be flagged.
EOF
    if (%files_with_fixes) {
        $message .= "   This step will also take care of the files that have fixes in them\n";
    }

    $message .= <<EOF;
   For a few files, such as perltoc, certain issues will always be
   expected, and more of the same will be added over time.  For those,
   before you do the regen, you can edit
   $known_issues
   and find the entry for the module's file and specific error message,
   and change the count of known potential problems to -1.
EOF

    note($message);
} elsif (%files_with_fixes) {
    note(<<EOF
To teach this test script that the potential problems have been fixed,
$how_to
EOF
    );
}

if ($regen) {
    chdir $original_dir || die "Can't change directories to $original_dir";
    close_and_rename($copy_fh);
}