proc die {reason} {
  global argv0
  puts stderr "$argv0: fatal: $reason"
  exit -2
}

proc nextrec {fd} {
  set hdr [read $fd 512]
  binary scan $hdr c _
  if {[string length $hdr] != 512} { die "got partial header: [string length $hdr]b" }

  binary scan $hdr a100c24a11c1c12c8a1c* name blind1 size _ blind2 _ type blind3
  set name [lindex [split $name "\0"] 0]

  if {$name == {}} return

  set data [read $fd [expr {$size}]]
  if {$size % 512 != 0} {
    read $fd [expr {512 - ($size % 512)}]
  }

  return [list name $name type $type data [binary encode hex $data]  blind1 $blind1 blind2 $blind2 blind3 $blind3]
}

proc putrec {fd rec} {
  set name [dict get $rec name]
  set type [dict get $rec type]
  set size [expr {[string length [dict get $rec data]] / 2}]
  set size [format %011o $size]

#  puts "$name - $size"

  set blind1 [dict get $rec blind1]
  set blind2 [dict get $rec blind2]
  set blind3 [dict get $rec blind3]

  set hdr [binary format a100c24a12c12A8a1c* $name $blind1 $size $blind2 "" $type $blind3]
  binary scan $hdr c* bytes
  set checksum 0
  foreach byte $bytes {
    incr checksum [expr {$byte & 0xFF}]
  }
  set checksum [format %07o $checksum]
  set hdr [binary format a100c24a12c12a8a1c* $name $blind1 $size $blind2 $checksum $type $blind3]

  puts -nonewline $fd $hdr

  puts -nonewline $fd [binary decode hex [dict get $rec data]]
  if {$size % 512 != 0} {
    puts -nonewline $fd [binary format a[expr {512 - ($size % 512)}] ""]
  }
}

set fd [open [lindex $argv 0] rb]
set fd2 [open [lindex $argv 1] wb]

while {[set record [nextrec $fd]] != {}} {
  if {[dict get $record type] == "0"} {
    puts "type of [dict get $record name] is 0, overwriting"
    dict set record data [binary encode hex "Hello, World!"]
#    puts $record
  }
  putrec $fd2 $record
}

puts -nonewline $fd2 [binary format a[expr {16384 - ([tell $fd2] % 16384)}] ""]