move Quantity to its own module, add some Traits for convenience

tools/src/quantity.rs

@@ -0,0 +1,64 @@
+#[derive(Clone, Copy, Debug, PartialEq, serde::Deserialize)]
+pub enum Unit {
+    Foot,
+    Gram,
+    Liter,
+    Meter,
+    USDollar,
+}
+
+/// `Quantity` measures the amount of a resource (Input or Output).
+#[derive(Clone, Copy, PartialEq, serde::Deserialize)]
+pub struct Quantity {
+    pub amount: f32,
+    pub unit: Option<Unit>,
+}
+
+impl std::default::Default for Quantity {
+    fn default() -> Self {
+        Self {
+            amount: 1.0,
+            unit: None,
+        }
+    }
+}
+
+impl std::fmt::Debug for Quantity {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match &self.unit {
+            None => write!(f, "{:#}", self.amount),
+            Some(unit) => write!(f, "{:#} {:?}", self.amount, unit),
+        }
+    }
+}
+
+impl std::ops::Mul<f32> for Quantity {
+    type Output = Self;
+    fn mul(self, rhs: f32) -> Self {
+        Self {
+            unit: self.unit,
+            amount: self.amount * rhs,
+        }
+    }
+}
+
+impl std::ops::Mul<usize> for Quantity {
+    type Output = Self;
+    fn mul(self, rhs: usize) -> Self {
+        Self {
+            unit: self.unit,
+            amount: self.amount * (rhs as f32),
+        }
+    }
+}
+
+impl std::ops::Add<&Quantity> for Quantity {
+    type Output = Self;
+    fn add(self, rhs: &Self) -> Self {
+        assert_eq!(self.unit, rhs.unit);
+        Self {
+            unit: self.unit,
+            amount: self.amount + rhs.amount,
+        }
+    }
+}