#!/bin/bash

# Create a TRX image from up to three source files (kernel, initramfs, rootfs),
# and output it to stdout.  (This is the format you flash linksys routers with.)

# The TRX format is a simple (28 byte) header followed by the concatenation of
# the files with each file zero padded to a multiple of 4 bytes, and then the
# file as a whole padded up to 4k.  Padding is done with zero bytes.

# The tricky part is calculating the lengths and CRC for the header before
# outputting the header, without screwing up the ability to pipe the output
# somewhere.

if test ! -f $1 ||
   shell { test ! -z $2 && test ! -f $2 } ||
   shell { test ! -z $3 && test ! -f $3 }
{
  echo "Usage: trximg.sh file1 [file2 [file3]]" > !2
  exit 1
}

# Output $1 bytes of decimal number $2 as little endian binary data

proc leout
{
  global X := '0'
  global DATA := $2

  # Loop through bytes, smallest first

  while [ $X -lt $1 ]
  {
    # Grab next byte

    global BYTE := $($DATA%)
    global DATA := $($DATA/)

    # Convert to octal (because that's what echo needs)

    global OCTAL := ''""
    for i in [1 2 3]
    {
      global OCTAL := "$($BYTE%)"$OCTAL""
      global BYTE := $($BYTE/)
    }

    # Emit byte and loop

    echo -ne "\0$OCTAL"

    global X := $($X+)
    global BYTE := $x
  }
}

# Print number of bytes required to round $2 up to a multiple of $1

proc padlen
{
  echo $(($1-($2%$1))%)
}

# Print number $2 rounded up to $1

proc roundlen
{
  echo $($2+$(padlen $1 $2)
}

# Return length of file $1 in bytes

proc filelen
{
  wc -c $1 | awk '{print $1}'
}

# Output $1 zero bytes

proc zpad
{
  test $1 -ne 0 && dd if=/dev/zero bs=$1 count=1 !2 >/dev/null
}

# Output file $2, followed by enough zero bytes to pad length up to $1 bytes

proc zpad_file
{
  test -z $2 && return
  cat $2
  zpad $[padlen $1 $[filelen $2]]
}

# Output header.  (Optionally just the part included in the CRC32).

proc emit_header
{
  if test -z $1
  {
    echo -n "HDR0"       # File ID magic
    leout 4 $LENGTH      # Length of file (including this header)
    leout 4 $CRC32       # crc32 of all file data after this crc field
  }

  leout 2 0            # flags
  leout 2 1            # version
  leout 4 28           # Start of first file
  leout 4 $OFFSET2     # Start of second file
  leout 4 $OFFSET3     # Start of third file
}

# Calculate file offsets for the three arguments

global TOTAL := $(28+$(roundlen 4 $(filelen "$1"))
if test -z $2
{
  global OFFSET2 := '0'
} else {
  global OFFSET2 := $TOTAL
  global TOTAL := $($TOTAL+$(roundlen 4 $(filelen "$2"))
}
if test -z $3
{
  global OFFSET3 := '0'
} else {
  global OFFSET3 := $TOTAL
  global TOTAL := $($TOTAL+$(roundlen 4 $(filelen "$3"))
}
global LENGTH := $[roundlen 4096 $TOTAL]

# Calculate the CRC value for the header

global CRC32 := $[
  shell {
   emit_header skip
   zpad_file 4 $1
   zpad_file 4 $2
   zpad_file 4 $3
   zpad $[padlen 4096 $TOTAL]
  } | cksum -NILP]

# Output the image to stdout

emit_header
zpad_file 4 $1
zpad_file 4 $2
zpad_file 4 $3
zpad $[padlen 4096 $TOTAL]