#!/bin/sh
# Reorganize PDF for book-alike printing
# -- w/o cover i.e. page 1 is the cover page --
#
# Author:    Nils Freydank <holgersson@posteo.de>
# Copyright: 2020 by Nils Freydank
# License:   MIT
# Version:   2020-11-11
# 
# dependencies:
# - bash
# - coreutils (date, mkdir)
# - groff (for empty page generation)
# - poppler (pdfseperate, pdfunite)

PATH="/bin:/usr/bin"

INPUT_FILE="${1:-input.pdf}"
OUTPUT_FILE="${2:-printable_output.pdf}"

TMPDIR="tmp_$(date --utc +%s)"
mkdir -p "${TMPDIR}"

# Separate the document into single pages.
pdfseparate "${INPUT_FILE}" "${TMPDIR}/%d.pdf"

# Rearrange the pages:
# 1. Find out the highest page number.
# 2. If page number can't be devided by 4 (i.e. modulus operation is not zero),
#    then align/fill with empty pages until it can.
# 3. Create an array with old -> new mappings.
# 4. Move the pages.

# -- step 1 of 4 --
PAGE_COUNTER="$(ls "${TMPDIR}" | wc -l)"

# -- step 2 of 4 --
while [[ $((${PAGE_COUNTER} % 4)) != 0 ]]
do
	# Create an new empty page.
	echo "" | groff -T pdf > ${TMPDIR}/$((${PAGE_COUNTER} + 1)).pdf
	# Re-scan the files.
	PAGE_COUNTER="$(ls "${TMPDIR}" | wc -l)"
done

# -- step 3 of 4 --
NEW_PAGE_LIST=()
ctr_down="${PAGE_COUNTER}"

# Note: Start at 1 as this is our first page.
ctr_up=1
while [[ ${ctr_up} -le $((${PAGE_COUNTER} / 2 )) ]]
do
	NEW_PAGE_LIST+=( ${ctr_down} )
	NEW_PAGE_LIST+=( ${ctr_up} )
	ctr_down=$((${ctr_down} - 1))
	ctr_up=$((${ctr_up} + 1))
	NEW_PAGE_LIST+=( ${ctr_up} )
	NEW_PAGE_LIST+=( ${ctr_down} )
	ctr_down=$((${ctr_down} - 1))
	ctr_up=$((${ctr_up} + 1))
done

# -- step 4 of 4 --
# Warning: Remember to start at zero as an array is accessed!
for ((i=0; i < ${#NEW_PAGE_LIST[*]}; i++))
do
	if [[ $((i + 1)) -le 9 ]]
	then
		mv "${TMPDIR}/${NEW_PAGE_LIST[$i]}.pdf" "${TMPDIR}/newpage-0$((i + 1)).pdf"
	else
		mv "${TMPDIR}/${NEW_PAGE_LIST[$i]}.pdf" "${TMPDIR}/newpage-$((i + 1)).pdf"
	fi
done

# Re-unite the sorted pages for printing.
pdfunite "${TMPDIR}"/newpage-* "${OUTPUT_FILE}"

# Cleanup intermediate data.
rm -rf "${TMPDIR}"

# vim:fileencoding=utf-8:ts=4:syntax=bash:colorcolumn=81:noexpandtab