LrmScale: add get_measure and get_position to work on scale positions

Author: Tristramg

src/lrm_scale.rs

@@ -231,6 +231,34 @@ impl LrmScale {
         })
     }
 
+    /// Returns a measure given a distance along the `LrmScale`.
+    /// The corresponding [Anchor] is the named `Anchor` that gives the smallest positive `offset`.
+    /// If such an `Anchor` does not exists, the first named `Anchor` is used.
+    pub fn get_measure(
+        &self,
+        scale_position: ScalePosition,
+    ) -> Result<LrmScaleMeasure, LrmScaleError> {
+        let named_anchor = self
+            .scale_nearest_named(scale_position)
+            .ok_or(LrmScaleError::NoAnchorFound)?;
+
+        Ok(LrmScaleMeasure {
+            anchor_name: named_anchor.id,
+            scale_offset: scale_position - named_anchor.scale_position,
+        })
+    }
+
+    /// Locates a point along the scale given an [Anchor] and an `offset`,
+    /// which might be negative.
+    pub fn get_position(&self, measure: LrmScaleMeasure) -> Result<ScalePosition, LrmScaleError> {
+        let named_anchor = self
+            .iter_named()
+            .find(|anchor| anchor.id == measure.anchor_name)
+            .ok_or(LrmScaleError::UnknownAnchorName)?;
+
+        Ok(named_anchor.scale_position + measure.scale_offset)
+    }
+
     fn nearest_named(&self, curve_position: CurvePosition) -> Option<NamedAnchor> {
         // Tries to find the Anchor whose curve_position is the biggest possible, yet smaller than Curve position
         // Otherwise take the first named
@@ -246,6 +274,14 @@ impl LrmScale {
             .or_else(|| self.iter_named().next())
     }
 
+    fn scale_nearest_named(&self, scale_position: ScalePosition) -> Option<NamedAnchor> {
+        // Like nearest_named, but our position is along the scale
+        self.iter_named()
+            .rev()
+            .find(|anchor| anchor.scale_position <= scale_position)
+            .or_else(|| self.iter_named().next())
+    }
+
     // Finds the closest Anchor before the Anchor having the name `name`
     fn previous_anchor(&self, name: &str) -> Option<&Anchor> {
         self.anchors
@@ -383,4 +419,34 @@ mod tests {
         assert_eq!(measure.anchor_name, "b");
         assert_eq!(measure.scale_offset, 2.);
     }
+
+    #[test]
+    fn get_measure() {
+        let measure = scale().get_measure(5.).unwrap();
+        assert_eq!(measure.anchor_name, "a");
+        assert_eq!(measure.scale_offset, 5.);
+
+        let measure = scale().get_measure(25.).unwrap();
+        assert_eq!(measure.anchor_name, "b");
+        assert_eq!(measure.scale_offset, 15.);
+    }
+
+    #[test]
+    fn get_position() {
+        let position = scale()
+            .get_position(LrmScaleMeasure {
+                anchor_name: "a".to_string(),
+                scale_offset: 5.,
+            })
+            .unwrap();
+        assert_eq!(position, 5.);
+
+        let position = scale()
+            .get_position(LrmScaleMeasure {
+                anchor_name: "b".to_string(),
+                scale_offset: 15.,
+            })
+            .unwrap();
+        assert_eq!(position, 25.);
+    }
 }