Merge pull request #141 from SixLabors/js/fix-140

Use bit masking to limit the glyph id to prevent an off by on error

src/SixLabors.Fonts/Tables/General/CMap/Format4SubTable.cs

@@ -34,7 +34,7 @@ public override bool TryGetGlyphId(int codePoint, out ushort glyphId)
                 {
                     if (seg.Offset == 0)
                     {
-                        glyphId = (ushort)((charAsInt + seg.Delta) % ushort.MaxValue); // TODO: bitmask instead?
+                        glyphId = (ushort)((charAsInt + seg.Delta) & ushort.MaxValue);
                         return true;
                     }
                     else

tests/SixLabors.Fonts.Tests/Issues/Issues_104.cs

@@ -0,0 +1,29 @@
+using SixLabors.Fonts.Tables.General.CMap;
+using Xunit;
+using static SixLabors.Fonts.Tables.General.CMap.Format4SubTable;
+
+namespace SixLabors.Fonts.Tests.Issues
+{
+    public class Issues_104
+    {
+        [Fact]
+        public void Format4SubTableWithSegmentsHasOffByOneWhenOverflowing()
+        {
+            var tbl = new Format4SubTable(0, WellKnownIds.PlatformIDs.Windows, 0, new[] {
+                new Segment(0,
+                ushort.MaxValue, // end
+                ushort.MinValue, // start of range
+                (short)(short.MaxValue), //delta
+                0 // zero to force correctly tested codepath
+                )
+            }, null);
+
+            const int delta = short.MaxValue + 2; // extra 2 to handle the difference between ushort and short when offsettings
+
+            const int codePoint = delta + 5;
+            Assert.True(tbl.TryGetGlyphId(codePoint, out ushort gid));
+
+            Assert.Equal(5, gid);
+        }
+    }
+}