#!/usr/local/bin/perl 

$|=1;

$firstfile  = shift;
$secondfile = shift;
$resultfile = shift;
$threshold  = shift || 0;  # noise gate threshold at beginning and end

$bytes      = 100;         # amount of header byte to read
$wav1       = $firstfile  . ".copy";
$wav2       = $secondfile . ".copy";

if (!(-e $firstfile)) {
    print "The file \"$firstfile\" does not exist!\n";
    exit;
}

if (!(-e $secondfile)) {
    print "The file \"$secondfile\" does not exist!\n";
    exit;
}

if (($threshold < 0) || ($threshold > 32000) || ($threshold =~ /\D/g)) {
    print "Invalid threshold value \"$threshold\"";
    print "   It has to be between 0 and 32,000. I set it to 0 now.\n"
}

# copy original files
`cp $firstfile  $wav1`;
`cp $secondfile $wav2`;

&modify_second;
&modify_first;

# concatenate them
`cat $wav2 \>\> $wav1`;

# copy to result file
`cp $wav1 $resultfile`;

#delete copy of second;
unlink $wav1;
unlink $wav2;
print "Deleted copies of the files. The resultfile is $resultfile.\nDone! Seeya...\n";



sub modify_second {

  $old_rs = $/;
  open(SECOND, "<$secondfile");
  undef $/;
  $second_content = <SECOND>; #so-called slurp mode...
  close SECOND;
  chomp $second_content;      # do i need that?
  $/ = $old_rs;

  print "OK, let's start...\n\n";
  print "----------SECOND FILE--------\n";
  print "Let's look for the data length.\n";

  $posi = -1;
  for ($i=0; $i<=($bytes-4); $i++) {
    $cha = substr($second_content,$i,1);
    if ($cha eq 'd'){
      $cha = substr($second_content,$i+1,1);
      if ($cha eq 'a') {
        $cha = substr($second_content,$i+2,1);
        if ($cha eq 't') {
          $cha = substr($second_content,$i+3,1);
          if ($cha eq 'a') {
	      $posi = $i + 4;
              last;
	  }
	}
      }
    }
  }
print "  \"data\" starts at position $posi\n";
$second_data_length = substr($second_content,$posi,4);         # read out the 4 bytes
$second_databytes_length =  unpack("l", $second_data_length); #transform to numbers
print "  Data bytes length: ---$second_databytes_length---\n";
$increase = $second_databytes_length;      # that's the amount the second file will be longer
substr($second_content,0,($posi+4)) = '';  # delete all after 'data' +4 bytes

#  &test_prog;
  &trunc_beginning;
  &trunc_end;

print "The total length of the second file is now $increase bytes.\n";

open(COPYSECOND, ">$wav2");
print COPYSECOND $second_content;
close COPYSECOND;

}


sub modify_first {

open(FILE, "+<$wav1");
sysread(FILE, $content, $bytes);

print "---------FIRST FILE, TOTAL-----------\n";
print "Let's look for the total length.\n";
$total_length = substr($content,4,4);
$bytes_length =  unpack("l", $total_length);
print "  Total byte length: ---$bytes_length---\n";
$bytes_length = $bytes_length + $increase;
print "  Total byte length increased to ---$bytes_length---\n";
$substring = pack("l", $bytes_length);
substr($content,4,4) = $substring;  # write back the modified substring
print "  Replaced the total length field!\n";

print "----------FIRST FILE, DATA------------\n";
print "Let's look for the data length.\n";
$pos = -1;
for ($i=0; $i<=($bytes-4); $i++) {
    $ch = substr($content,$i,1);
    #print "-----char $i: $ch\n";
    if ($ch eq 'd'){
      $ch = substr($content,$i+1,1);
      if ($ch eq 'a') {
        $ch = substr($content,$i+2,1);
        if ($ch eq 't') {
          $ch = substr($content,$i+3,1);
          if ($ch eq 'a') {
	      $pos = $i + 4;
              last;
	  }
	}
      }
    }
}
print "  \"data\" starts at position $pos\n";
$data_length = substr($content,$pos,4);
$databytes_length =  unpack("l", $data_length);
print "  Data byte length: ---$databytes_length---\n";
$databytes_length = $databytes_length + $increase;
print "  Data byte length increased to ---$databytes_length---\n";
$substring = pack("l", $databytes_length);
substr($content,$pos,4) = $substring;
print "  Replaced the data length field!\n";

seek(FILE, 0, 0);
syswrite(FILE,$content,$bytes);
close FILE;

} # end of sub modify_first


sub trunc_beginning {

  # Look for the first bytes above threshold
  print "Truncating beginning...\n";
  $cutoff_beginning = 0;
  for ($i=0; $i <=$increase-4; $i = $i+4){
      $sound_1 = substr($second_content,$i,2);
      $sound_2 = substr($second_content,$i+2,2);
      $sound_1 = unpack("v",$sound_1);
      $sound_2 = unpack("v",$sound_2);
      if ($sound_1 >32767) { $sound_1 = 65536 - $sound_1  ;}
      if ($sound_2 >32767) { $sound_2 = 65536 - $sound_2  ;}

      if ((($sound_1 + $sound_2)/2) > $threshold)  { 
	 print "  Found levels above threshold: L $sound_1, R $sound_2, byte $i of $increase\n";
         $cutoff_beginning = $i;
         last;
      }
  }
 
  print "    I will cut off the first $cutoff_beginning bytes.\n";

  substr($second_content,0,$cutoff_beginning) = '';# take off the silent part at the beginning
  $increase = $increase - $cutoff_beginning;       # adjust the length of the string

} # end of sub trunc_beginning


sub trunc_end {

  # Look for the last bytes above threshold
  print "Truncating end...\n";
  $cutoff_end = 0;
  for ($i=$increase-4; $i >=0; $i = $i-4){
      $sound_1 = substr($second_content,$i,2);
      $sound_2 = substr($second_content,$i+2,2);
      $sound_1 = unpack("v",$sound_1);
      $sound_2 = unpack("v",$sound_2);
      if ($sound_1 >32767) { $sound_1 = 65536 - $sound_1  ;}
      if ($sound_2 >32767) { $sound_2 = 65536 - $sound_2  ;}

      if ((($sound_1 + $sound_2)/2) > $threshold)  { 
        print "  Found levels above threshold: L $sound_1, R $sound_2, byte $i of $increase\n";
        $cutoff_end = $i;
         last;
      }
  }

  $how_much_truncate = $increase - $cutoff_end;
  print "    I will cut off the last $how_much_truncate bytes.\n";

  substr($second_content,-$how_much_truncate,$how_much_truncate) = ''; # truncate at end
  $increase = $increase - $how_much_truncate;       # adjust the length of the string

} # end of sub trunc_end

sub test_prog {

  # TEST PROGRAM
  $start =3800;
  $limit = $start + 200;  

  for ($i=$start; $i <=$limit; $i = $i+4){
      $sound_1 = substr($second_content,$i,2);
      $sound_2 = substr($second_content,$i+2,2);
      $sound_1 = unpack("v",$sound_1);
      $sound_2 = unpack("v",$sound_2);
      if ($sound_1 >32767) { $sound_1 = 65536 - $sound_1  ;}
      if ($sound_2 >32767) { $sound_2 = 65536 - $sound_2  ;}
      print "Byte $i: L $sound_1\tR $sound_2\n";
  }

} # end of test_prog