{ pkgs ? import <nixpkgs> {} }:
let
  crop_ppm = (pkgs.writers.writePython3Bin "crop_ppm" {libraries = [pkgs.python3Packages.opencv4];} ''
    import cv2
    import sys

    for path in sys.argv[1:]:
        img = cv2.imread(path)[30:-30, 30:]
        gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
        _, thresh = cv2.threshold(gray, 1, 255, cv2.THRESH_BINARY)
        contours, hierarchy = cv2.findContours(
            thresh,
            cv2.RETR_EXTERNAL,
            cv2.CHAIN_APPROX_SIMPLE
        )
        cnt = max(contours, key=cv2.contourArea)
        x, y, w, h = cv2.boundingRect(cnt)
        print(path, x, y, w, h)
        crop = img[y:y + h, x:x + w]
        cv2.imwrite(path, crop)
  '');
in
pkgs.mkShell {
  packages = [
    pkgs.poppler_utils
    pkgs.mupdf
    crop_ppm
    (pkgs.writers.writeDashBin "generate" ''
      set -x
      pwd=$(pwd)
      tmpdir=$(mktemp -d)
      cd "$tmpdir"
      [ $# -eq 1 ] || {
        echo Please provide one PDF file as argument. >&2
        exit 1
      }
      [ -f "$1" ] || {
        echo "$1" does not exist. >&2
        exit 1
      }
      basename=$(basename "$1" .pdf)
      ${pkgs.poppler_utils}/bin/pdftoppm "$1" exploded
      ${crop_ppm}/bin/crop_ppm exploded*.ppm
      ${pkgs.imagemagick}/bin/convert exploded*.ppm "$pwd/cropped-$basename.pdf"
      ${pkgs.mupdf}/bin/mutool poster -x 2 "$pwd/cropped-$basename.pdf" "$pwd/split-cropped-$basename.pdf"
    '')
  ];
}