[ivs_builder] add test for all-zero delta sets not being de-duplicated

this test currently fails because the VarStore ends up with two identical [0] delta-sets instead of one. The builder is treating glyphs without any variations (add_deltas([]) and glyphs with explicitly defined 0 deltas (add_deltas([0]) as different, whereas it should treat them the same and have all point to the same no-op delta-set row.

write-fonts/src/tables/variations/ivs_builder.rs

@@ -1086,4 +1086,45 @@ mod tests {
             .iter()
             .all(|(key, idx)| *key == idx.delta_set_inner_index as u32))
     }
+
+    #[test]
+    fn no_duplicate_zero_delta_sets() {
+        let r0 = VariationRegion::new(vec![reg_coords(0.0, 1.0, 1.0)]);
+        let mut builder = VariationStoreBuilder::new();
+        let varidxes = vec![
+            // first glyph has no variations (e.g. .notdef only defined at default location)
+            // but we still need to add it to the variation store to reserve an index so
+            // we add an empty delta set
+            builder.add_deltas::<i32>(Vec::new()),
+            builder.add_deltas(vec![(r0.clone(), 100)]),
+            // this glyph has an explicit master that is the same as the default (delta is 0);
+            // we expect the builder to reuse the same no-op delta set as the first glyph
+            builder.add_deltas(vec![(r0.clone(), 0)]),
+            // this glyph repeats the same delta set as the second glyph, thus we expect
+            // the builder to map it to the same delta set index
+            builder.add_deltas(vec![(r0.clone(), 100)]),
+        ];
+        let (store, key_map) = builder.build();
+
+        let varidx_map: Vec<u32> = varidxes
+            .into_iter()
+            .map(|idx| key_map.get(idx).unwrap().into())
+            .collect::<Vec<_>>();
+
+        println!("{:#?}", store);
+        println!("{:#?}", varidx_map);
+
+        assert_eq!(store.variation_region_list.variation_regions.len(), 1);
+        assert_eq!(store.item_variation_data.len(), 1);
+
+        let var_data = store.item_variation_data[0].as_ref().unwrap();
+
+        assert_eq!(var_data.item_count, 2);
+        assert_eq!(var_data.word_delta_count, 0);
+        assert_eq!(var_data.region_indexes, vec![0]);
+        assert_eq!(var_data.delta_sets, vec![0, 100],);
+        // glyph 0 and 2 should map to the same no-op 0 delta, while
+        // glyph 1 and 3 should map to delta 100
+        assert_eq!(varidx_map, vec![0, 1, 0, 1]);
+    }
 }