Fix checksum calculation for TrueType tables

The current way of calculating the checksum is wrong because the last 1,
2 or 3 bytes are not respected.

CHANGELOG.md

@@ -1,3 +1,10 @@
+## Unreleased
+
+### Fixed
+
+* Checksum calculation for TrueType tables
+
+
 ## 1.0.1 - 2024-11-04
 
 ### Changed

lib/hexapdf/font/true_type/table.rb

@@ -63,7 +63,12 @@ class Table
 
         # Calculates the checksum for the given data.
         def self.calculate_checksum(data)
-          data.unpack('N*').inject(0) {|sum, long| sum + long } % 2**32
+          checksum = 0
+          if (remainder_length = data.length % 4) != 0
+            checksum = (data[-remainder_length, remainder_length] << "\0" * (4 - remainder_length)).
+              unpack1('N')
+          end
+          checksum + data.unpack('N*').inject(0) {|sum, long| sum + long } % 2**32
         end
 
         # The TrueType font object associated with this table.

test/hexapdf/font/true_type/test_table.rb

@@ -13,6 +13,18 @@
     @entry = HexaPDF::Font::TrueType::Table::Directory::Entry.new('tagg', 0, 0, @file.io.string.length)
   end
 
+  describe "self.calculate_checksum" do
+    it "works for data with a length divisible by four" do
+      klass = HexaPDF::Font::TrueType::Table
+      assert_equal(256, klass.calculate_checksum("\x00\x00\x00\x01\x00\x00\x00\xFF"))
+    end
+
+    it "works for data with a length not divisible by four" do
+      klass = HexaPDF::Font::TrueType::Table
+      assert_equal(512, klass.calculate_checksum("\x00\x00\x00\x01\x00\x00\x00\xFF\x00\x00\x01"))
+    end
+  end
+
   describe "initialize" do
     it "reads the data from the associated file" do
       table = TrueTypeTestTable.new(@file, @entry)