mvar: add test for sparse global metrics

fontbe/src/mvar.rs

@@ -169,6 +169,7 @@ mod tests {
             tables::{mvar as read_mvar, variations::ItemVariationData},
             FontData, FontRead,
         },
+        types::F2Dot14,
     };
 
     use super::*;
@@ -267,4 +268,62 @@ mod tests {
         // hence no MVAR needed
         assert!(builder.build().is_none());
     }
+
+    struct MvarReader<'a> {
+        mvar: read_mvar::Mvar<'a>,
+    }
+
+    impl<'a> MvarReader<'a> {
+        fn new(mvar: read_mvar::Mvar<'a>) -> Self {
+            Self { mvar }
+        }
+
+        fn metric_delta(&self, mvar_tag: &str, coords: &[f32]) -> f64 {
+            let mvar_tag = Tag::from_str(mvar_tag).unwrap();
+            let coords: Vec<F2Dot14> = coords
+                .iter()
+                .map(|coord| F2Dot14::from_f32(*coord))
+                .collect();
+            self.mvar.metric_delta(mvar_tag, &coords).unwrap().to_f64()
+        }
+    }
+
+    #[test]
+    fn sparse_global_metrics() {
+        let regular = NormalizedLocation::for_pos(&[("wght", 0.0)]);
+        let medium = NormalizedLocation::for_pos(&[("wght", 0.5)]);
+        let bold = NormalizedLocation::for_pos(&[("wght", 1.0)]);
+        let mut builder = new_mvar_builder(
+            vec![&regular, &medium, &bold],
+            vec![axis("wght", 400.0, 400.0, 700.0)],
+        );
+        // 'xhgt' defines a value for all three locations
+        add_sources(
+            &mut builder,
+            "xhgt",
+            &[(&regular, 500.0), (&medium, 530.0), (&bold, 550.0)],
+        );
+        // 'strs' is sparse: defines a value for regular and bold, not medium
+        add_sources(&mut builder, "strs", &[(&regular, 50.0), (&bold, 100.0)]);
+
+        let Some(mvar) = builder.build() else {
+            panic!("no MVAR?!");
+        };
+
+        let bytes = dump_table(&mvar).unwrap();
+        let mvar = read_mvar::Mvar::read(FontData::new(&bytes)).unwrap();
+
+        assert_eq!(mvar.value_records().len(), 2);
+        assert_eq!(mvar.value_records()[0].value_tag(), Tag::new(b"strs"));
+        assert_eq!(mvar.value_records()[1].value_tag(), Tag::new(b"xhgt"));
+
+        let mvar = MvarReader::new(mvar);
+        assert_eq!(mvar.metric_delta("xhgt", &[0.0]), 0.0);
+        assert_eq!(mvar.metric_delta("xhgt", &[0.5]), 30.0); // not 25.0
+        assert_eq!(mvar.metric_delta("xhgt", &[1.0]), 50.0);
+
+        assert_eq!(mvar.metric_delta("strs", &[0.0]), 0.0);
+        assert_eq!(mvar.metric_delta("strs", &[0.5]), 25.0); // interpolated
+        assert_eq!(mvar.metric_delta("strs", &[1.0]), 50.0);
+    }
 }