#!/bin/bash

#################
# PREREQUISITES #
#################

# executables in PATH
for prereq in "python3" "gs" "pdftk"; do
    if ! command -v "${prereq}" &> /dev/null; then
        echo "required executable '${prereq}' not found!"
        exit 1
    fi
done

# python virtual env with PyMuPDF
VENV_DIR="pdfsign.venv"
if [ -d "${VENV_DIR}" ]; then
    # venv exists, activate it
    source "${VENV_DIR}/bin/activate"
else
    for prereq in "venv"; do
        if ! python3 -c "import ${prereq}" &> /dev/null; then
            echo "python module '${prereq}' not found!"
            exit 1
        fi
    done

    # create venv and activate
    python3 -m "venv" "${VENV_DIR}"
    source "${VENV_DIR}/bin/activate"
    # install PyMuPDF
    python3 -m "pip" install PyMuPDF
fi


############
# CLI ARGS #
############

if [ $# -lt 2 ]; then
    echo "Usage: ${0} <input.pdf> <signed.png> [PAGE]"
    echo
    echo "Replaces page PAGE with the signed version."
    echo "If PAGE is not given, the last page is replaced."
    exit 1
fi

IN_TEXT_PDF="${1}"
IN_SCAN_IMG="${2}"

OUT_PDF="${IN_TEXT_PDF%%.*}_pdfsign.pdf"

IN_TEXT_PDF_LEN="$( pdftk "${IN_TEXT_PDF}" dump_data | grep NumberOfPages | awk '{print $2}' )"
IN_PAGE="${3:-${IN_TEXT_PDF_LEN}}"

RANGES="B1"
if [ ${IN_PAGE} -gt 1 ]; then
    RANGES="A1-$(( IN_PAGE - 1 )) ${RANGES}"
fi

if [ ${IN_PAGE} -lt ${IN_TEXT_PDF_LEN} ]; then
    RANGES="${RANGES} A$(( IN_PAGE + 1 ))-${IN_TEXT_PDF_LEN}"
fi


################
# MAIN PROGRAM #
################

GSFLAGS="-sDEVICE=pdfwrite -dSAFER -dNOPAUSE -dQUIET -dBATCH"

set -e

# extract IN_PAGE (text-only) from IN_TEXT_PDF
gs ${GSFLAGS} \
    -dFirstPage="${IN_PAGE}" -dLastPage="${IN_PAGE}" \
    -dFILTERIMAGE -dFILTERVECTOR -dFILTERTEXT=false \
    -sOutputFile=".pdfsign_textonly.pdf" \
    "${IN_TEXT_PDF}"

# use IN_SCAN_IMG as background, layering text from extracted page on top
python3 ./pdfsign.py \
    ".pdfsign_textonly.pdf" "${IN_SCAN_IMG}" \
    ".pdfsign_signed.pdf"

rm ".pdfsign_textonly.pdf"

# replace IN_PAGE in IN_TEXT_PDF layered page
pdftk A="${IN_TEXT_PDF}" B=".pdfsign_signed.pdf" \
    cat ${RANGES} \
    output ".pdfsign_concat.pdf"

rm ".pdfsign_signed.pdf"

# postprocess/compress pdf
gs ${GSFLAGS} \
    -sPAPERSIZE=a4 -dFIXEDMEDIA -dPDFFitPage \
    -dCompatibilityLevel=1.7 -dPDFSETTINGS=/ebook \
    -sOutputFile="${OUT_PDF}" \
    ".pdfsign_concat.pdf"

rm ".pdfsign_concat.pdf"