#!/usr/bin/env perl
# html-toc
# Michael Ernst <mernst@cs.washington.edu>
# Thanks to C. Scott Ananian <cscott@cscott.net>
# Time-stamp: <2020-12-24 11:03:08 mernst>

# Generate (to standard out) table of contents for an HTML document.
# The table of contents is based on the header tags (<h1>, <h2>, etc.).

# See comments in html-update-toc for documentation.
# (Typically, you will run html-update-toc rather than this program.)

use strict;
use English;
$WARNING = 1;

my $level         = 0;
my $num_named_1   = 0;    # number of named items in <h1>...</h1>
my $num_unnamed_1 = 0;    # number of unnamed items in <h1>...</h1>
my $output        = "";
my $errors        = "";
my $eol;                  # end-of-line character in input file
my $debug = 0;

# $debug = 1;

sub enter_level
{

    # called with cursor at start of line.
    $output .= ((" " x ($level * 4)) . "<ul>$eol");
    $level++;

    # returns with cursor at start of line.
}

sub exit_level
{

    # always called with an open <li>
    $level--;
    $output .= "</li>$eol";
    $output .= ((" " x ($level * 4)) . "</ul>");

    # returns with an open <li>
}

sub found_header ( $$$$ )
{
    my ($thishlevel, $anchorname, $secname1, $secname2) = @_;

    # Remove leading and trailing whitespace in section title.
    $secname1 =~ s/^[ \t]+//;
    $secname1 =~ s/[ \t]+$//;
    $secname2 =~ s/^[ \t]+//;
    $secname2 =~ s/[ \t]+$//;
    my $sectitle;
    if ($secname1 eq "")
    {
        $sectitle = $secname2;
    }
    elsif ($secname2 eq "")
    {
        $sectitle = $secname1;
    }
    else
    {
        # $errors .= "Merging two section titles (\"$secname1\" and \"$secname2\") found on one line: $_";
        # next;
        $sectitle = "$secname1 $secname2";
    }

    # if $level >= $thishlevel, we have an open <li>
    while ($level > $thishlevel)
    {
        exit_level();
    }

    # close this <li>, unless we're going deeper
    $output .= "</li>" if ($level == $thishlevel);
    $output .= "$eol" unless $level == 0;
    while ($level < $thishlevel)
    {
        enter_level();
        $output .= (" " x ($level * 4 - 2)) . "<li>$eol"
          if $level < $thishlevel;
    }
    $output .=
      ((" " x ($level * 4 - 2)) . "<li><a href=\"#$anchorname\">$sectitle</a>");
    if ($level == 1) { $num_named_1++; }

}

# Main loop

while (<>)
{
    if (!defined($eol))
    {
        if (/\r\n$/)
        {
            $eol = "\r\n";
        }
        elsif (/\n$/)
        {
            $eol = "\n";
        }
        elsif (/\r$/)
        {
            $eol = "\r";
        }
        else
        {
            die "Bad line ending: $_";
        }
    }
    if (/^<!--/)
    {
        next;
    }
    elsif (/<!-- omit from toc -->|<omit from toc>/)
    {

        # Check <omit from toc> because sometimes, a HTML comment is not appropriate.
        next;
    }
    elsif (
        /<h([1-9])(?: [^>]*)?>
           (?:<!--[^>]*-->)?      # permit a comment
           <a   # open the a start tag
           (?:\s+\w+\s*=\s*\"[^\"]*\")*? # allow random attributes
           \s+(?:name|id)\s*=\s*\"([^\"]*)\" # look for name or id attribute
           (?:\s+\w+\s*=\s*\"[^\"]*\")*? # allow random attributes
           \s*> # close the a start tag
           (.*?)  # the text of the anchor
           <\/a> # the a end tag
           (.*)   # sometimes the anchor text is found here instead
           <\/h([1-9])>
           /ix
      )
    {
        if ($debug) { print STDERR "match 3: <$1><$2><$3><$4><$5>\n"; }
        if ($1 ne $5)
        {
            $errors .= "Non-matching heading tags $1 and $5: $_";
        }
        my $thishlevel = $1;
        my $anchorname = $2;
        my $secname1   = $3;
        my $secname2   = $4;
        found_header($thishlevel, $anchorname, $secname1, $secname2);
    }
    elsif (
        /<h([1-9])   # open the h start tag
           (?:\s+\w+\s*=\s*\"[^\"]*\")*? # allow random attributes
           \s+(?:name|id)\s*=\s*\"([^\"]*)\" # look for name or id attribute
           (?:\s+\w+\s*=\s*\"[^\"]*\")*? # allow random attributes
           \s*> # close the h start tag
           (.*?)  # the text of the anchor
           <\/h([1-9])>
           /ix
      )
    {
        if ($debug) { print STDERR "match 3: <$1><$2><$3><$4>\n"; }
        if ($1 ne $4)
        {
            $errors .= "Non-matching heading tags $1 and $4: $_";
        }
        my $thishlevel = $1;
        my $anchorname = $2;
        my $secname1   = $3;
        my $secname2   = "";
        found_header($thishlevel, $anchorname, $secname1, $secname2);
    }
    elsif (/<h([1-9])>(.*)<\/h([1-9])>/)
    {
        if ($1 == 1)
        {
            $num_unnamed_1++;
        }
        $errors .= "$ARGV:$.: No <a name=...> tag: $_";
    }
    elsif (/<h([1-9])>/)
    {
        $errors .= "$ARGV:$.: No </h$1> tag: $_";
    }
}
continue
{
    # Reset line numbers; example from "eof" text in "man perlfunc".
    close ARGV if eof;    # Not eof()!
}

while ($level > 0)
{
    exit_level();
}
$output .= "$eol";
if ($num_named_1 + $num_unnamed_1 < 2)
{

    # There is only one top-level (<h1>) item.
    # Assume <h1> is the page title, and the h2, h3, h4 etc are the ones
    # we should be making a toc for.
    $output =~ s/^<ul>${eol}//;
    $output =~ s/<\/ul>${eol}$//;

    # remove top-level item.
    $output =~ s/^  <li>.*${eol}//;
    $output =~ s/<\/li>${eol}$/${eol}/;
    if ($num_unnamed_1 == 1)
    {

        # Remove error from output
        $errors =~ s/^.*: No <a name=\.\.\.> tag: <h1>[^${eol}]*${eol}//;
    }
}
print STDERR $errors;
print $output;

exit(0);