#!/usr/bin/env python3

import argparse
import numpy as np
from pypdf import PaperSize, PdfReader, PdfWriter, Transformation, PageObject
from pypdf.generic import ArrayObject, FloatObject, NameObject
from pypdf.annotations import Line
import math


def append_folio_A6(input_pdf, offset, size, append_to):
    # Handles edge case where folio has 1 page
    num_A5 = min(math.ceil(size / 4.0) * 2, size)
    writer = append_to
    w_offset = len(writer.pages)
    for i in range(num_A5):
        dpage = writer.add_blank_page(width=PaperSize.A5.height,
                                      height=PaperSize.A5.width)
        ipage = input_pdf.pages[i+offset]
        ipage.mediabox = dpage.mediabox

        if i % 2 == 0: # Even pages are filled right to left
            ipage.add_transformation(Transformation().translate(PaperSize.A6.width,
                                                                0))
        dpage.merge_page(ipage)

    # Now for the fun part: work backwards to fill the remaining pages
    for j in range(size - num_A5):
        dpage = writer.pages[w_offset + num_A5 - 1 - j]
        ipage = input_pdf.pages[num_A5+j+offset]
        ipage.mediabox = dpage.mediabox
        if (num_A5 - 1 - j) % 2 == 1:
            ipage.add_transformation(Transformation().translate(PaperSize.A6.width,
                                                                0))
        dpage.merge_page(ipage)
    return writer

# Tries to find a partition as
# num_sheets = max_folio_size * a + (max_folio_size - k) * b
# where k is minimal. We let n = max_folio_size and L = num_sheets.
# Note that when L > n^2, then k = 1 must succeed.
def find_partition_mink(num_sheets, max_folio_size):
    max_a = num_sheets // max_folio_size
    for k in range(1, max_folio_size):
        # We want to solve a*k = L (mod n-k). This happens iff
        # a*k' = L' (mod m') where k,L,m are divided by their gcd.
        # Then, if k' and m' have a common factor, L' doesn't have it and there
        # are no solutions. Otherwise, there's one solution mod m'.
        g = math.gcd(max_folio_size - k, k, num_sheets)
        mprime = (max_folio_size - k)//g
        kprime = k//g
        Lprime = num_sheets//g
        if (math.gcd(kprime, mprime) > 1):
            continue
        a_mod_mprime = (pow(kprime, -1, mprime) * Lprime) % mprime
        tosub = (max_a - a_mod_mprime + mprime) % mprime
        # If we're a bit above, substract only a little. But a bit below implies
        # substracting almost mprime.
        candidate = max_a - tosub
        if (candidate >= 0):
            b = (num_sheets - max_folio_size * candidate)//(max_folio_size - k)
            return [max_folio_size] * candidate + [max_folio_size - k] * b

def find_partition_greedy(num_sheets, max_folio_size):
    q = num_sheets//max_folio_size
    return [max_folio_size] * q + [num_sheets - q*max_folio_size]

# Interprets input_pdf as a list of A6 pages, and appends to append_to with
# A5 pages using the following rules:
#   a. Odd/even page pairs correspond to front/back pages of a single sheet, printed
#      with "mirror around **short** edge"
#   b. when grouped as consecutive sequences of max_folio_size or less, the printed
#      sheets can be folded in the middle and assembled as a folio
def foliate_A6(input_pdf, strat_func, max_folio_size, append_to=PdfWriter()):
    doc_length = len(input_pdf.pages)
    writer = append_to

    num_sheets = math.ceil(doc_length/4.0)
    print("Using", num_sheets, "two-sided A5 sheets")

    L = [num_sheets]
    if max_folio_size > 0:
        L = strat_func(num_sheets, max_folio_size)

    if(np.sum(L) != num_sheets):
        print("[ERROR] strategy gave", np.sum(L), " A5 sheets")
        exit(1)

    folio_page_size = 0
    current_page = 0

    print("Folio sizes, in order")
    old_l     = L[0]
    old_l_run = 0
    for l in L:
        if l != old_l:
          print(old_l, "a5 sheet(s), x", old_l_run)
          old_l = l
          old_l_run = 1
        else:
          old_l_run += 1

        # l is a number of _sheets_, each containing 4 pages
        folio_page_size = min(doc_length - current_page, 4*l)
        append_folio_A6(input_pdf, current_page, folio_page_size, append_to)
        current_page += folio_page_size
    print(old_l, "a5 sheet(s), x", old_l_run)

    return writer

# Take an A5 printing plan, with odd/even pages being understood
# as front/back pairs of sheets, short edge mirroring. Produces an A4 printing plan
# containing two A5 panels per sheet, long edge mirroring.
def blit_to_A4(input_pdf, append_to=PdfWriter()):
    L = len(input_pdf.pages)
    writer = append_to

    for k in range(0, L, 2):
        if(k%4 == 0): # Open up a new sheet
            front_page = writer.add_blank_page(width=PaperSize.A4.width,
                                               height=PaperSize.A4.height)
            back_page = writer.add_blank_page(width=PaperSize.A4.width,
                                              height=PaperSize.A4.height)

            midline = Line(rect = (0, PaperSize.A4.height/2, PaperSize.A4.width, PaperSize.A4.height/2),
                           p1=(10, PaperSize.A4.height/2),
                           p2=(PaperSize.A4.width-10, PaperSize.A4.height/2))
            midline.flags = 4 # print the midline
            midline[NameObject("/C")] = ArrayObject([FloatObject(0.5)])
            writer.add_annotation(front_page, midline)

        front = input_pdf.pages[k]
        back = input_pdf.pages[k+1] if (k+1<L) else PageObject.create_blank_page(None, PaperSize.A5.width, PaperSize.A5.height)

        front.mediabox = front_page.mediabox
        back.mediabox  = back_page.mediabox

        if(k%4 == 0): # Translate to bottom half of page
            front.add_transformation(Transformation().translate(0, PaperSize.A5.width))
            back.add_transformation(Transformation().translate(0, PaperSize.A5.width))

        front_page.merge_page(front)
        back_page.merge_page(back)
    return writer

parser = argparse.ArgumentParser()
parser.add_argument("infile",  help="input document, A6 pages")
parser.add_argument("outfile", help="output document")
parser.add_argument("-a5", help="output as a5, reverse on short edge",
                    action="store_true")
parser.add_argument("-fs", help="maximum folio size, in sheets",
                    action="store", default=-1, type=int)
parser.add_argument("--strategy", help="greedy packing of pages into folios",
                    action="store", default="balanced")

stratnames_dict = {"balanced": find_partition_mink, "greedy": find_partition_greedy}

args = parser.parse_args()

input_pdf = PdfReader(args.infile)
strategy = stratnames_dict[args.strategy]
a5 = foliate_A6(input_pdf, find_partition_greedy, args.fs)
if (args.a5):
    a5.write(args.outfile)
else:
    a4 = blit_to_A4(a5)
    a4.write(args.outfile)