#!/bin/sh

# Initialize variables
input="$1"
inputMul4="/tmp/${input%.pdf}-Mul4.pdf"
outputImpaginato="${input%.pdf}-Impaginato.pdf"
empty="/tmp/empty.pdf"
n="$(pdftk "$input" dump_data | grep NumberOfPages | cut -d' ' -f2)"

# Add empty pages
echo .bp | groff -T pdf > "$empty"

pdftk F=$input E="$empty" cat F1-$n $(for x in $(seq 1 $((4-n%4))); do printf 'E1 '; done) output $inputMul4

# Generate alternating page ranges
n=$((n+(4-n%4)))
range=
k=1
while [ "$k" -lt $((n/2)) ]; do
	range="$range$k $((n/2+k)) $((n/2+k+1)) $((k+1)) "
	k=$((k+2))
done

# Output final pdf
pdftk $inputMul4 cat $range output $outputImpaginato

rm "$empty" $inputMul4