HexToASCII: display information about invisible characters

- Reduce the number of entered characters to 128, since we can't be sure
  of the display width any more.
- Show "invisible characters" (unprintable + non-space whitespace) in a
  special format

lib/DDG/Goodie/HexToASCII.pm

@@ -5,11 +5,11 @@ use strict;
 
 use DDG::Goodie;
 # Used to restrict long generated outputs
-use constant MAX_OUTPUT_LEN => 256;
+use constant MAX_INPUT_CHARS => 128;
 
 triggers start => 'ascii';
 
-zci is_cached => 1;
+zci is_cached   => 1;
 zci answer_type => 'ascii';
 
 primary_example_queries 'ascii 0x74657374';
@@ -26,23 +26,74 @@ handle remainder => sub {
     my $value = $_;
     $value =~ s/^\s+//;
     $value =~ s/\s+$//;
-    if ($value =~ /^(?:[0\\]x)?([0-9a-f]+)$/i or 
-        $value =~ /^([0-9a-f]+)h?$/i) {
-       my @digits = $1 =~ /(..)/g;
-       my $string = '';
-       foreach my $digit (@digits) {
-           my $hex = hex $digit;
-           return if $hex <= 0x1F || $hex >= 0x7F;
-           $string .= chr $hex;
-       }
-       # Don't let long strings make the output untidy
-       if (length($string) > MAX_OUTPUT_LEN) {
-           $string = substr($string, 0, MAX_OUTPUT_LEN - 3) . '...';
-       }
-       $string =~ s/\n/ /g;
-       return "$string (ASCII)";
+    if (   $value =~ /^(?:[0\\]x)?([0-9a-f]+)$/i
+        or $value =~ /^([0-9a-f]+)h?$/i)
+    {
+        my @digits = $1 =~ /(..)/g;
+        my @chars;
+        foreach my $digit (@digits) {
+            my $hex = hex $digit;
+            return if $hex > 0x7F;
+            push @chars, printable_chr($hex);
+        }
+        # Don't let long strings make the output untidy
+        if (scalar @chars > MAX_INPUT_CHARS) {
+            @chars = (@chars[0 .. MAX_INPUT_CHARS], '...');
+        }
+        return join('', @chars) . " (ASCII)";
     }
     return;
 };
 
+# Some of these are actually printable whitespace, but
+# for consistency, we'll make them discoverable, too.
+my %invisibles = (
+    0x00 => { abbr => 'NUL', desc => 'null' },
+    0x01 => { abbr => 'SOH', desc => 'start of heading' },
+    0x02 => { abbr => 'STX', desc => 'start of text' },
+    0x03 => { abbr => 'ETX', desc => 'end of text' },
+    0x04 => { abbr => 'EOT', desc => 'end of tranmision' },
+    0x05 => { abbr => 'ENQ', desc => 'enquiry' },
+    0x06 => { abbr => 'ACK', desc => 'acknowledge' },
+    0x07 => { abbr => 'BEL', desc => 'bell', },
+    0x08 => { abbr => 'BS',  desc => 'backspace', },
+    0x09 => { abbr => 'TAB', desc => 'horizontal tab', },
+    0x0A => { abbr => 'LF',  desc => 'linefeed', },
+    0x0B => { abbr => 'VT',  desc => 'vertical tab', },
+    0x0C => { abbr => 'FF',  desc => 'form feed', },
+    0x0D => { abbr => 'CR',  desc => 'carriage return', },
+    0x0E => { abbr => 'SO',  desc => 'shift out', },
+    0x0F => { abbr => 'SI',  desc => 'shift in', },
+    0x10 => { abbr => 'DLE', desc => 'data link escape', },
+    0x11 => { abbr => 'DC1', desc => 'device control 1', },
+    0x12 => { abbr => 'DC2', desc => 'device control 2', },
+    0x13 => { abbr => 'DC3', desc => 'device control 3', },
+    0x14 => { abbr => 'DC4', desc => 'device control 4', },
+    0x15 => { abbr => 'NAK', desc => 'negative acknowledge', },
+    0x16 => { abbr => 'SYN', desc => 'synchronous idle', },
+    0x17 => { abbr => 'ETB', desc => 'end of transmission block', },
+    0x18 => { abbr => 'CAN', desc => 'cancel', },
+    0x19 => { abbr => 'EM',  desc => 'end of medium', },
+    0x1A => { abbr => 'SUB', desc => 'substitute', },
+    0x1B => { abbr => 'ESC', desc => 'escape', },
+    0x1C => { abbr => 'FS',  desc => 'file separator', },
+    0x1D => { abbr => 'GS',  desc => 'group separator', },
+    0x1E => { abbr => 'RS',  desc => 'record separator', },
+    0x1F => { abbr => 'US',  desc => 'unit separator', },
+    0x7F => { abbr => 'DEL', desc => 'delete', },
+);
+
+sub printable_chr {
+    my ($hex) = @_;
+
+    my $printable;
+    if (my $char_info = $invisibles{$hex}) {
+        $printable = '<span style="background-color: #ddd" title="' . $char_info->{desc} . '">[' . $char_info->{abbr} . ']</span>';
+    } else {
+        $printable = chr $hex;
+    }
+
+    return $printable;
+}
+
 1;

t/HexToASCII.t

@@ -12,6 +12,7 @@ ddg_goodie_test(
         [qw(
                 DDG::Goodie::HexToASCII
         )],
+        'ascii 0x7465007374'    => test_zci('te<span style="background-color: #ddd" title="null">[NUL]</span>st (ASCII)'),
         'ascii 0x74657374' => test_zci('test (ASCII)'),
         'ascii 0x5468697320697320612074657374' => test_zci('This is a test (ASCII)'),
 );