Add links_and_dests.py.

This can be used to print outgoing links and targets in the PDF, and detect a subset of errors.
It depends on the PyPDF2 library (pip3 install PyPDF2).

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

protocol/links_and_dests.py

@@ -0,0 +1,66 @@
+#!/usr/bin/env python3
+
+try:
+    from PyPDF2 import PdfFileReader
+except ImportError:
+    print("Please install the PyPDF2 library using `pip3 install PyPDF2`.\n")
+    raise
+
+from collections import deque
+import sys
+
+def get_links_and_destinations(f):
+    # Based on <https://stackoverflow.com/a/5978161/393146>
+    pdf = PdfFileReader(f)
+
+    links = deque()
+    dests = deque()
+    errors = deque()
+
+    for pg in range(pdf.getNumPages()):
+        obj = pdf.getPage(pg).getObject()
+
+        for annotation in obj.get('/Annots', []):
+            uri = annotation.getObject().get('/A', {}).get('/URI', None)
+            if uri is not None and uri not in links:
+                links.append(uri)
+
+    dests = pdf.getNamedDestinations()
+
+    for l in links:
+        if not l.startswith("https:"):
+            errors.append("Insecure or unrecognized protocol in link: " + l)
+
+        if l.startswith("https://zips.z.cash/protocol/"):
+            fragment = l.partition("#")[2]
+            if fragment and fragment not in dests:
+                errors.append("Missing link target: " + l)
+
+    return (links, dests, errors)
+
+
+def main(args):
+    if len(args) < 2:
+        print("Usage: ./links_and_dests.py <file.pdf>")
+        return 1
+
+    with open(args[1], 'rb') as f:
+        (links, dests, errors) = get_links_and_destinations(f)
+
+    print("Links:")
+    for l in links:
+        print(l)
+
+    print("\nDestinations:")
+    for d in dests:
+        print(d)
+
+    if errors:
+        print("\nErrors:")
+        for e in errors:
+            print(e)
+
+    return 0
+
+if __name__ == '__main__':
+    sys.exit(main(sys.argv))