From 7fab01aeca63d471a3fe51e727fa6d8799d0ddd1 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?S=C3=A9l=C3=A8ne=20Corbineau?= <selene.corbineau@ens.fr>
Date: Wed, 10 Jun 2026 08:18:07 +0200
Subject: [PATCH] Add support for max_folio_size

---
 foliator.py | 55 +++++++++++++++++++++++++++++++++++++----------------
 1 file changed, 39 insertions(+), 16 deletions(-)

diff --git a/foliator.py b/foliator.py
index 6dc2676..17e0132 100755
--- a/foliator.py
+++ b/foliator.py
@@ -6,21 +6,16 @@ from pypdf.annotations import Line
 from pypdf.generic import ArrayObject, FloatObject, NameObject
 import math
 
-# 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
-# FIXME: currently max_folio_size is ignored, and the output is one large folio.
-def foliate_A6(input_pdf, max_folio_size=-1, append_to=PdfWriter()):
-    num_A5 = math.ceil(len(input_pdf.pages)/4.0) * 2
-    writer = append_to
 
+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]
+        ipage = input_pdf.pages[i+offset]
         ipage.mediabox = dpage.mediabox
 
         if i % 2 == 0: # Even pages are filled right to left
@@ -28,10 +23,10 @@ def foliate_A6(input_pdf, max_folio_size=-1, append_to=PdfWriter()):
                                                                 0))
         dpage.merge_page(ipage)
 
-        # Now for the fun part: work backwards to fill the remaining pages
-    for j in range(len(input_pdf.pages) - num_A5):
-        dpage = writer.pages[num_A5 - 1 - j]
-        ipage = input_pdf.pages[num_A5+j]
+    # 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,
@@ -39,6 +34,32 @@ def foliate_A6(input_pdf, max_folio_size=-1, append_to=PdfWriter()):
         dpage.merge_page(ipage)
     return writer
 
+
+# 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, max_folio_size=-1, append_to=PdfWriter()):
+    current_page = 0
+    doc_length = len(input_pdf.pages)
+    writer = append_to
+    if max_folio_size < 0:
+        max_folio_size = doc_length
+
+    folio_page_size = 0
+    while(current_page < doc_length):
+        # max_folio_size is a number of _sheets_, each containing 4 pages
+        folio_page_size = min(doc_length - current_page, 4*max_folio_size)
+        append_folio_A6(input_pdf, current_page, folio_page_size, append_to)
+        current_page += folio_page_size
+
+    print("Dangling folio has",folio_page_size,"pages",
+          "(",int(math.ceil(folio_page_size/4.0)),"sheets).")
+
+    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.
@@ -79,10 +100,12 @@ 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)
 args = parser.parse_args()
 
 input_pdf = PdfReader(args.infile)
-a5 = foliate_A6(input_pdf)
+a5 = foliate_A6(input_pdf, args.fs)
 if (args.a5):
     a5.write(args.outfile)
 else: