Allow NULL offsets in ChainedContextLookup v2

src/ggg/chained_context.rs

@@ -1,5 +1,5 @@
 use super::{ClassDefinition, Coverage, SequenceLookupRecord};
-use crate::parser::{FromSlice, LazyArray16, LazyOffsetArray16, Stream};
+use crate::parser::{FromSlice, LazyArray16, LazyOffsetArray16, Offset, Offset16, Stream};
 
 /// A [Chained Contextual Lookup Subtable](
 /// https://docs.microsoft.com/en-us/typography/opentype/spec/chapter2#chseqctxt1).
@@ -44,9 +44,16 @@ impl<'a> ChainedContextLookup<'a> {
             }
             2 => {
                 let coverage = Coverage::parse(s.read_at_offset16(data)?)?;
-                let backtrack_classes = ClassDefinition::parse(s.read_at_offset16(data)?)?;
-                let input_classes = ClassDefinition::parse(s.read_at_offset16(data)?)?;
-                let lookahead_classes = ClassDefinition::parse(s.read_at_offset16(data)?)?;
+
+                let mut parse_func = || match s.read::<Option<Offset16>>()? {
+                    Some(offset) => Some(ClassDefinition::parse(data.get(offset.to_usize()..)?)?),
+                    None => Some(ClassDefinition::Empty),
+                };
+
+                let backtrack_classes = parse_func()?;
+                let input_classes = parse_func()?;
+                let lookahead_classes = parse_func()?;
+
                 let count = s.read::<u16>()?;
                 let offsets = s.read_array16(count)?;
                 Some(Self::Format2 {

src/ggg/mod.rs

@@ -132,6 +132,7 @@ pub enum ClassDefinition<'a> {
     Format2 {
         records: LazyArray16<'a, RangeRecord>,
     },
+    Empty,
 }
 
 impl<'a> ClassDefinition<'a> {
@@ -162,6 +163,7 @@ impl<'a> ClassDefinition<'a> {
                 .checked_sub(start.0)
                 .and_then(|index| classes.get(index)),
             Self::Format2 { records } => records.range(glyph).map(|record| record.value),
+            Self::Empty => Some(0),
         }
         .unwrap_or(0)
     }