Add duplex PDF build targets with pdftk page interleaving

- Add pdftk-java to Docker image for PDF page manipulation
- Extend generate_backside() to repeat back content num_pages times so
  front and back PDFs have matching page counts for pdftk shuffle
- Add build-duplex and build-blanko-duplex Makefile targets that combine
  front and back PDFs with alternating pages (front1, back1, front2, back2)
- Document duplex targets in README

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -7,6 +7,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     make \
     python3 \
     python3-yaml \
+    pdftk-java \
     && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /workspace

Makefile

@@ -7,9 +7,11 @@ TEX_FILE_BACK := brevetkarte-rueckseite.tex
 PDF_FILE_PERSONALIZED := brevetkarte-personalized.pdf
 PDF_FILE_BLANKO := brevetkarte-blanko.pdf
 PDF_FILE_BACK := brevetkarte-rueckseite.pdf
+PDF_FILE_DUPLEX := brevetkarte-duplex.pdf
+PDF_FILE_BLANKO_DUPLEX := brevetkarte-blanko-duplex.pdf
 CSV_FILE := export_brevetcard.csv
 
-.PHONY: all build clean build-image build-back generate build-personalized build-blanko run shell help
+.PHONY: all build clean build-image build-back generate build-personalized build-blanko build-duplex build-blanko-duplex run shell help
 
 # Default target
 all: build-personalized build-blanko
@@ -75,6 +77,24 @@ build-blanko: build-image
 		pdflatex -interaction=nonstopmode $(TEX_FILE_BACK)
 	@echo "PDF generated: $(PDF_FILE_BACK)"
 
+# Build duplex PDF (personalized front + back interleaved for duplex printing)
+build-duplex: build-personalized
+	@echo "Combining front and back for duplex printing..."
+	docker run --rm \
+		-v $(PWD):/workspace \
+		$(IMAGE_NAME) \
+		pdftk A=$(PDF_FILE_PERSONALIZED) B=$(PDF_FILE_BACK) shuffle A B output $(PDF_FILE_DUPLEX)
+	@echo "PDF generated: $(PDF_FILE_DUPLEX)"
+
+# Build duplex PDF (blank front + back interleaved for duplex printing)
+build-blanko-duplex: build-blanko
+	@echo "Combining blank front and back for duplex printing..."
+	docker run --rm \
+		-v $(PWD):/workspace \
+		$(IMAGE_NAME) \
+		pdftk A=$(PDF_FILE_BLANKO) B=$(PDF_FILE_BACK) shuffle A B output $(PDF_FILE_BLANKO_DUPLEX)
+	@echo "PDF generated: $(PDF_FILE_BLANKO_DUPLEX)"
+
 # Run container interactively
 run:
 	docker run --rm -it \
@@ -92,7 +112,7 @@ shell:
 # Clean generated files
 clean:
 	@echo "Cleaning generated files..."
-	rm -f *.aux *.log *.out *.toc *.pdf brevetkarte-personalized.tex brevetkarte-blanko.tex brevetkarte-rueckseite.tex
+	rm -f *.aux *.log *.out *.toc *.pdf brevetkarte-personalized.tex brevetkarte-blanko.tex brevetkarte-rueckseite.tex $(PDF_FILE_DUPLEX) $(PDF_FILE_BLANKO_DUPLEX)
 
 # Clean everything including Docker image
 clean-all: clean
@@ -112,6 +132,8 @@ help:
 	@echo "  make generate           - Generate tex files from CSV + event.yml"
 	@echo "  make build-personalized - Generate and compile front + back side PDFs"
 	@echo "  make build-blanko       - Generate and compile blank card (no CSV needed)"
+	@echo "  make build-duplex       - Build duplex PDF (front+back interleaved)"
+	@echo "  make build-blanko-duplex - Build blank duplex PDF"
 	@echo "  make build-back         - Compile back side PDF only (after generate)"
 	@echo "  make shell              - Open interactive shell in container"
 	@echo "  make clean              - Remove generated files (aux, log, pdf)"

README.md

@@ -67,6 +67,15 @@ Führt folgende Schritte aus:
 3. Erzeugt `brevetkarte-rueckseite.tex` (Rückseite mit Kontrollpunkten aus event.yml)
 4. Kompiliert beide .tex-Dateien zu PDFs
 
+### Duplex-PDF erzeugen
+
+```bash
+make build-duplex        # personalisierte Karten
+make build-blanko-duplex # Blanko-Karten
+```
+
+Erzeugt ein einzelnes PDF mit abwechselnden Vorder- und Rückseiten für den direkten Duplexdruck: Seite 1 Vorderseite, Seite 2 Rückseite, Seite 3 nächste Vorderseite, usw. Erfordert kein manuelles Zusammenführen zweier Dateien.
+
 ### Blanko-Karten erzeugen und bauen
 
 ```bash

generate_cards.py

@@ -59,7 +59,7 @@ def apply_event_placeholders(text, config):
     return text
 
 
-def generate_backside(template, config):
+def generate_backside(template, config, num_pages=1):
     """Fill back side template with cell content from event config."""
     cells = config.get('backside', {})
     result = template
@@ -71,8 +71,19 @@ def generate_backside(template, config):
             if content is None:
                 content = ""
             result = result.replace(placeholder, content.strip())
+
+    if num_pages <= 1:
         return result
 
+    # Repeat body between \begin{document} and \end{document} num_pages times
+    begin_marker = '\\begin{document}\n'
+    end_marker = '\\end{document}'
+    begin_idx = result.find(begin_marker) + len(begin_marker)
+    end_idx = result.rfind(end_marker)
+    preamble = result[:begin_idx]
+    body = result[begin_idx:end_idx].rstrip()
+    return preamble + ('\n\n\\newpage\n\n').join([body] * num_pages) + '\n' + result[end_idx:]
+
 
 def generate_card_from_template(template, data):
     """Replace participant placeholders in template with participant data."""
@@ -142,7 +153,7 @@ def main():
         blanko_output_file.write_text(blanko_output, encoding='utf-8')
         print(f"Generated {blanko_output_file}")
         backside_template = backside_template_file.read_text(encoding='utf-8')
-        backside_output = generate_backside(backside_template, event_config)
+        backside_output = generate_backside(backside_template, event_config, num_pages=1)
         backside_output_file.write_text(backside_output, encoding='utf-8')
         print(f"Generated {backside_output_file}")
         return
@@ -179,10 +190,11 @@ def main():
     output_file.write_text(''.join(document_parts), encoding='utf-8')
     print(f"Generated {output_file}")
 
-    # Generate personalized back side
+    # Generate personalized back side (one page per front page for duplex)
+    num_pages = (len(participants) + 1) // 2
     print(f"Reading back side template from {backside_template_file}...")
     backside_template = backside_template_file.read_text(encoding='utf-8')
-    backside_output = generate_backside(backside_template, event_config)
+    backside_output = generate_backside(backside_template, event_config, num_pages=num_pages)
     backside_output_file.write_text(backside_output, encoding='utf-8')
     print(f"Generated {backside_output_file}")