#!/usr/bin/env python3
import os
import sys

from pdfrw import PdfReader, PdfWriter


def main(input_files, output_file):
    cwd = os.getcwd()

    all_pdf_pages = {}
    lengths = []
    print(f'loading input files: {input_files}')
    for pdf_filename in input_files:
        reader = PdfReader(os.path.join(cwd, pdf_filename))
        all_pdf_pages[pdf_filename] = reader.pages
        lengths.append(len(reader.pages))

    assert all([x == lengths[0] for x in lengths])

    interleaved_pdf = PdfWriter()
    for i in range(lengths[0]):
        for pdf in all_pdf_pages.values():
            interleaved_pdf.addpage(pdf[i])

    interleaved_pdf.write(os.path.join(cwd, output_file))


def usage():
    print('Usage: interleave.py output.pdf input1.pdf input2.pdf [input3.pdf ...]')


if __name__ == '__main__':
    if len(sys.argv) < 4:
        usage()
        sys.exit(1)

    main(sys.argv[2:], sys.argv[1])