#! /bin/sh
#
# hc - hard copy helper
#
# SYNOPSIS
#   hc {-e|--encode} [ENCODE_OPTION...] < FILE
#   hc {-d|--decode} IMAGE_FILE...
#
# DESCRIPTION
#   hc encodes arbitrary data for printing, and decodes scanned images back
#   into data.
#
# ENCODING
#   When invoked with the --encode flag, then hc will read data from stdin, and
#   write a PDF file to stdout.  The data will be encoded using QR codes.
#
#   Following options are available to customize the encoding process:
#
#     --lhead=TeX (default: empty)
#     --rhead=TeX (default: empty)
#     --lfoot=TeX (default: current date in ISO 8601 format.
#     --rfoot=TeX (default: page numbering)
#         TeX code to use for headers and footers.
#
#     --show-texlog
#         Print TeX log.
#
# DECODING
#   When invoked with the --decode flag, then hc will read the specified
#   (image) files, and write the decoded data to stdout.  Images can be
#   specified in any order, and might be listed multiple times.
#
#   Sometimes hc won't be able to locate and/or decode all QR codes found in
#   the images.  Problematic QR codes can be identified with
#
#     zbarimg --display IMAGE_FILE...
#
#   Following techniques might help to make problematic QR codes readable:
#
#   - rescan the image
#
#   - rotate the image (try various degrees)
#     example: convert old.tif -rotate 5 new.tif
#
#   - increase brightness and/or contrast
#     example: convert old.tif -brightness-contrast 10x20 new.tif
#
#   - extract the QR code manually
#     e.g. by opening the image containing the problematic QR code with some
#     image viewer, zoom in, and take a screenshot (e.g. using scrot -s)
#
# EXAMPLES
#     echo secret | gpg -c | hc -e --lhead=mypass > /tmp/mypass.pdf
#     convert -density 288 /tmp/mypass.pdf tif:- | hc -d /dev/stdin | gpg -d
#

set -efu

main() {(

  _args=$(getopt -n "$0" -s sh \
      -o de \
      -l decode,encode,lhead:,rhead:,lfoot:,rfoot:,show-texlog \
      -- "$@")
  if \test $? != 0; then exit 1; fi
  eval set -- "$_args"
  unset _args

  unset command

  lhead=
  rhead=
  lfoot=$(date -Id)
  rfoot='\thepage/\pageref{LastPage}'

  show_texlog=false

  set_command() {
    if test -z "${command+1}"; then
      command=$1
    else
      echo "hc: error: only one command can be used" >&2
      exit 1
    fi
  }

  while :; do case $1 in
    -d|--decode) set_command decode; shift;;
    -e|--encode) set_command encode; shift;;
    --lhead) lhead=$2; shift 2;;
    --rhead) rhead=$2; shift 2;;
    --lfoot) lfoot=$2; shift 2;;
    --rfoot) rfoot=$2; shift 2;;
    --show-texlog) show_texlog=true; shift;;
    --) shift; break;;
  esac; done

  # Separates a part's head from its body.
  body_separator=' '

  # Arbitrary limit.  This number is really only needed for its width, to
  # calculate max_body_size.  The value could be made configurable.
  max_part_number=9999

  # QR Code version 40, ECC Level H, alphanumeric capacity: 1852
  max_body_size=$(expr \
      1852 \
      - ${#body_separator} \
      - ${#max_part_number}
  )

  split_parts() {
    base32 -w"$max_body_size" |
    nl -s"$body_separator" -w1
  }
  join_parts() {
    sort -t"$body_separator" -nu -k1 |
    awk --field-separator "$body_separator" '
      $1 ~ /^[1-9][0-9]*$/{
        print "found part " $1 > "/dev/stderr"
        for (i=2; i<=NF; i++) print $i
      }
    ' |
    base32 -d
  }

  cmd_"$command" "$@"
)}

cmd_encode() {(
  unset workdir
  trap cleanup EXIT
  cleanup () {
    { echo "$part_files" | xargs --no-run-if-empty rm -v || :
      rm -v "$workdir/out.aux" || :
      rm -v "$workdir/out.log" || :
      rm -v "$workdir/out.pdf" || :
      rm -v "$workdir/out.tex" || :
      rmdir -v "$workdir" || :
      trap - EXIT
    } >&2
  }
  workdir=$(mktemp --tmpdir -d hc.decode.XXXXXXXX)
  cd "$workdir"

  part_files=$(split_parts | nl -w1 | {
    while read -r i part; do
      f=$workdir/part-$i.png
      printf %s "$part" |
      qrencode \
          --level=H \
          --margin=5 \
          --output="$f" \
          --verbose
      echo "$f"
    done
  })

  {
    echo '\documentclass[paper=a4]{scrartcl}'
    echo '\usepackage{lastpage}'
    echo '\usepackage{graphicx}'
    echo '\setlength{\textheight}{640pt}'
    echo '\setlength{\parindent}{0mm}'
    echo '\usepackage[headsepline,footsepline,plainfootsepline]{scrlayer-scrpage}'
    echo '\def\chead#1{\cohead{#1}\cehead{#1}}'
    echo '\def\lhead#1{\lohead{#1}\lehead{#1}}'
    echo '\def\rhead#1{\rohead{#1}\rehead{#1}}'
    echo '\def\cfoot#1{\cofoot{#1}\cefoot{#1}}'
    echo '\def\lfoot#1{\lofoot{#1}\lefoot{#1}}'
    echo '\def\rfoot#1{\rofoot{#1}\refoot{#1}}'
    echo '\chead{}'
    echo '\lhead{\texttt{'"$lhead"'}}'
    echo '\rhead{\texttt{'"$rhead"'}}'
    echo '\cfoot{}'
    echo '\lfoot{\texttt{'"$lfoot"'}}'
    echo '\rfoot{\texttt{'"$rfoot"'}}'
    echo '\begin{document}'
    echo '\begin{flushleft}'
    echo "$part_files" | while read -r f; do
      echo '\includegraphics[scale=.368]{'"$f"'}'
    done
    echo '\end{flushleft}'
    echo '\end{document}'
  } > out.tex

  while :; do
    pdflatex \
        -no-shell-escape \
        -interaction=batchmode \
        out.tex \
      >&2
    if test "$show_texlog" = true; then
      cat out.log >&2
    fi
    if grep -q "LaTeX Warning: Reference \`LastPage' on page 1 undefined on input line \\([0-9]*\\)." out.log; then
      continue
    fi
    break
  done

  cat out.pdf
)}

cmd_decode() {(
  zbarimg --raw "$@" | join_parts
)}

main "$@"