Shorten the form URL by compressing and b64 encoding it. In a real-world example, the baseline URL goes from 1086 characters to 620. This is good for the resulting QR barcode.

cara/apps/calculator/__init__.py

@@ -1,10 +1,12 @@
 import datetime
+import base64
 import html
 import json
 import os
 from pathlib import Path
 import traceback
 import uuid
+import zlib
 
 import jinja2
 from tornado.web import Application, RequestHandler, StaticFileHandler
@@ -131,6 +133,14 @@ class CalculatorForm(BaseRequestHandler):
         self.finish(report)
 
 
+class CompressedCalculatorFormInputs(BaseRequestHandler):
+    def get(self, compressed_args: str):
+        # Convert a base64 zlib encoded shortened URL into a non compressed
+        # URL, and redirect.
+        args = zlib.decompress(base64.b64decode(compressed_args)).decode()
+        self.redirect(f'/calculator?{args}')
+
+
 class ReadmeHandler(BaseRequestHandler):
     def get(self):
         template = self.settings['template_environment'].get_template("userguide.html.j2")
@@ -146,6 +156,7 @@ def make_app(debug=False, prefix='/calculator'):
     calculator_static_dir = Path(__file__).absolute().parent / 'static'
     urls = [
         (r'/?', LandingPage),
+        (r'/c/(.*)', CompressedCalculatorFormInputs),
         (r'/static/(.*)', StaticFileHandler, {'path': static_dir}),
         (prefix + r'/?', CalculatorForm),
         (prefix + r'/report', ConcentrationModel),

cara/apps/calculator/report_generator.py

@@ -4,6 +4,7 @@ from datetime import datetime
 import io
 from pathlib import Path
 import typing
+import zlib
 
 import qrcode
 import urllib
@@ -70,7 +71,22 @@ def calculate_report_data(model: models.ExposureModel):
 
 def generate_qr_code(prefix, form: FormData):
     form_dict = FormData.to_dict(form)
-    url = prefix + "?" + urllib.parse.urlencode(form_dict)
+
+    # Generate the calculator URL arguments that would be needed to re-create this
+    # form.
+    args = urllib.parse.urlencode(form_dict)
+
+    # Then zlib compress + base64 encode the string. To be inverted by the
+    # /c/ endpoint.
+    qr_url = prefix + "/c/" + base64.b64encode(
+        zlib.compress(args.encode())
+    ).decode()
+
+    # We show the human-friendly URL when hovering over the link, but for now
+    # let's keep it the same as the QR URL to ensure everything continues to work
+    # as expected (it takes more effort to validate QR barcodes than it does links).
+    # url = prefix + "/calculator/?" + args
+    url = qr_url
 
     qr = qrcode.QRCode(
         version=1,
@@ -78,7 +94,7 @@ def generate_qr_code(prefix, form: FormData):
         box_size=10,
         border=4,
     )
-    qr.add_data(url)
+    qr.add_data(qr_url)
     qr.make(fit=True)
     img = qr.make_image(fill_color="black", back_color="white").convert('RGB')
 
@@ -272,7 +288,7 @@ def build_report(base_url: str, model: models.ExposureModel, form: FormData):
     context.update(calculate_report_data(model))
     alternative_scenarios = manufacture_alternative_scenarios(form)
     context['alternative_scenarios'] = comparison_report(alternative_scenarios)
-    context['qr_code'] = generate_qr_code(f'{base_url}/calculator', form)
+    context['qr_code'] = generate_qr_code(base_url, form)
 
     cara_templates = Path(__file__).parent.parent / "templates"
     calculator_templates = Path(__file__).parent / "templates"