From ad5fc0eda118ce843f52354652f1aaf9fa968963 Mon Sep 17 00:00:00 2001
From: Nico Weber <thakis@chromium.org>
Date: Wed, 3 Jan 2024 19:43:06 -0500
Subject: [PATCH] LibPDF: An Encoding's /Differences entry is optional

Per "TABLE 5.11 Entries in an encoding dictionary", /Differences is
optional.

(Per "Encodings for TrueType Fonts" in 5.5.5 Character Encoding,
nonsymbolic truetype fonts are even recommended to have "no Differences
array." But in practice, most seem to have it.)

Fixes crashes on:
* 0000001.pdf
* 0000574.pdf
* 0000337.pdf

All three don't render super great, but at least they no longer crash.
---
 Userland/Libraries/LibPDF/Encoding.cpp | 30 ++++++++++++++------------
 1 file changed, 16 insertions(+), 14 deletions(-)

diff --git a/Userland/Libraries/LibPDF/Encoding.cpp b/Userland/Libraries/LibPDF/Encoding.cpp
index 95f7b364ef..0ce2da76c8 100644
--- a/Userland/Libraries/LibPDF/Encoding.cpp
+++ b/Userland/Libraries/LibPDF/Encoding.cpp
@@ -50,22 +50,24 @@ PDFErrorOr<NonnullRefPtr<Encoding>> Encoding::from_object(Document* document, No
     encoding->m_descriptors = TRY(base_encoding->m_descriptors.clone());
     encoding->m_name_mapping = TRY(base_encoding->m_name_mapping.clone());
 
-    auto differences_array = TRY(dict->get_array(document, CommonNames::Differences));
+    if (dict->contains(CommonNames::Differences)) {
+        auto differences_array = TRY(dict->get_array(document, CommonNames::Differences));
 
-    u16 current_code_point = 0;
-    bool first = true;
+        u16 current_code_point = 0;
+        bool first = true;
 
-    for (auto& item : *differences_array) {
-        if (item.has_u32()) {
-            current_code_point = item.to_int();
-            first = false;
-        } else {
-            VERIFY(item.has<NonnullRefPtr<Object>>());
-            VERIFY(!first);
-            auto& object = item.get<NonnullRefPtr<Object>>();
-            auto name = object->cast<NameObject>()->name();
-            encoding->set(current_code_point, name);
-            current_code_point++;
+        for (auto& item : *differences_array) {
+            if (item.has_u32()) {
+                current_code_point = item.to_int();
+                first = false;
+            } else {
+                VERIFY(item.has<NonnullRefPtr<Object>>());
+                VERIFY(!first);
+                auto& object = item.get<NonnullRefPtr<Object>>();
+                auto name = object->cast<NameObject>()->name();
+                encoding->set(current_code_point, name);
+                current_code_point++;
+            }
         }
     }