Re-work formatting to take a list of possible units.

Author: Jonathan Pallant

src/length.rs

@@ -246,21 +246,19 @@ impl Measurement for Length {
     }
 
     fn get_appropriate_units(&self) -> (&'static str, f64) {
-        if self.meters >= 1_000_000_000.0 {
-            ("million km", self.meters / 1000_000_000.0)
-        } else if self.meters >= 1_000.0 {
-            ("km", self.meters / 1000.0)
-        } else if self.meters < 0.000_000_1 {
-            ("nm", self.meters * 1_000_000_000.0)
-        } else if self.meters < 0.000_1 {
-            ("\u{00B5}m", self.meters * 1_000_000.0)
-        } else if self.meters < 0.1 {
-            ("mm", self.meters * 1000.0)
-        } else if self.meters < 1.0 {
-            ("cm", self.meters * 100.0)
-        } else {
-            ("m", self.meters)
-        }
+        // Smallest to largest
+        let list = [
+            ("pm", 1e-12),
+            ("nm", 1e-9),
+            ("\u{00B5}m", 1e-6),
+            ("mm", 1e-3),
+            ("cm", 1e-2),
+            ("m", 1e0),
+            ("km", 1e3),
+            ("thousand km", 1e6),
+            ("million km", 1e9),
+        ];
+        self.pick_appropriate_units(&list)
     }
 }
 

src/measurement.rs

@@ -38,8 +38,26 @@ pub trait Measurement {
     fn get_appropriate_units(&self) -> (&'static str, f64) {
         (self.get_base_units_name(), self.get_base_units())
     }
+
+    /// Given a list of units and their scale relative to the base unit,
+    /// select the most appropriate one.
+    ///
+    /// The list must be smallest to largest, e.g. ("nanometre", 10-9) to
+    /// ("kilometre", 10e3)
+    fn pick_appropriate_units(&self, list: &[(&'static str, f64)]) -> (&'static str, f64) {
+        for &(ref unit, ref scale) in list.iter().rev() {
+            let value = self.get_base_units() / scale;
+            if value.abs() > 1.0 {
+                return (unit, value);
+            }
+        };
+        (list[0].0, self.get_base_units() / list[0].1)
+    }
+
     fn get_base_units_name(&self) -> &'static str;
+
     fn get_base_units(&self) -> f64;
+
     fn from_base_units(units: f64) -> Self;
 }
 

src/pressure.rs

@@ -89,11 +89,13 @@ impl Measurement for Pressure {
     }
 
     fn get_appropriate_units(&self) -> (&'static str, f64) {
-        if self.millibars >= 1_000.0 {
-            ("bar", self.millibars / 1000.0)
-        } else {
-            ("mbar", self.millibars)
-        }
+        let list = [
+            ("millibars", 1e0),
+            ("bar", 1e3),
+            ("thousand bar", 1e6),
+            ("million bar", 1e9),
+        ];
+        self.pick_appropriate_units(&list)
     }
 }
 

src/volume.rs

@@ -243,15 +243,17 @@ impl Measurement for Volume {
     }
 
     fn get_appropriate_units(&self) -> (&'static str, f64) {
-        if self.liters >= 1_000.0 {
-            ("m\u{00B3}", self.liters / 1000.0)
-        } else if self.liters < 0.000_1 {
-            ("\u{00B5}l", self.liters * 1_000_000.0)
-        } else if self.liters < 1.0 {
-            ("ml", self.liters * 1000.0)
-        } else {
-            ("l", self.liters)
-        }
+        // Smallest to largest
+        let list = [
+            ("pl", 1e-12),
+            ("nl", 1e-9),
+            ("\u{00B5}l", 1e-6),
+            ("ml", 1e-3),
+            ("l", 1e0),
+            ("m\u{00B3}", 1e3),
+            ("km\u{00B3}", 1e12),
+        ];
+        self.pick_appropriate_units(&list)
     }
 }
 

src/weight.rs

@@ -159,17 +159,18 @@ impl Measurement for Weight {
     }
 
     fn get_appropriate_units(&self) -> (&'static str, f64) {
-        if self.kilograms >= 1_000.0 {
-            ("tonnes", self.kilograms / 1000.0)
-        } else if self.kilograms < 0.000_000_1 {
-            ("\u{00B5}g", self.kilograms * 1_000_000_000.0)
-        } else if self.kilograms < 0.000_1 {
-            ("mg", self.kilograms * 1_000_000.0)
-        } else if self.kilograms < 1.0 {
-            ("g", self.kilograms * 1000.0)
-        } else {
-            ("kg", self.kilograms)
-        }
+        // Smallest to largest
+        let list = [
+            ("ng", 1e-12),
+            ("\u{00B5}g", 1e-9),
+            ("mg", 1e-6),
+            ("g", 1e-3),
+            ("kg", 1e0),
+            ("tonnes", 1e3),
+            ("thousand tonnes", 1e6),
+            ("million tonnes", 1e9),
+        ];
+        self.pick_appropriate_units(&list)
     }
 }