add Recipe::unit_cost()

tools/src/recipe.rs

@@ -8,6 +8,16 @@ pub enum RecipeLoadError {
     TomlDeserializeError(#[from] toml::de::Error),
 }
 
+#[derive(Debug, thiserror::Error, PartialEq)]
+pub enum VitaminError {
+    #[error("not a vitamin")]
+    NotAVitamin,
+    #[error("vitamin has bogus inputs")]
+    InputsError,
+    #[error("vitamon has no output")]
+    OutputError,
+}
+
 #[derive(Debug, serde::Deserialize, PartialEq)]
 pub struct Input {
     // Quantity defaults to "amount=1" if omitted.
@@ -85,6 +95,45 @@ impl Recipe {
         Ok(recipe)
     }
 
+    /// Compute the cost (of quantity 1).  Currently only work on vitamins.
+    pub fn unit_cost(&self) -> Result<f32, VitaminError> {
+        if !self.is_vitamin() {
+            return Err(VitaminError::NotAVitamin);
+        }
+
+        // Vitamins must have exactly one Input, and it must be Capital.
+        if self.inputs.len() != 1 {
+            return Err(VitaminError::InputsError);
+        }
+
+        let capital = match self.inputs.get("capital") {
+            Some(capital) => capital,
+            None => return Err(VitaminError::InputsError),
+        };
+
+        if capital.quantity.unit != Some(crate::quantity::Unit::USDollar) {
+            return Err(VitaminError::InputsError);
+        }
+
+        let total_cost = capital.quantity.amount;
+
+        let outputs = match &self.outputs {
+            Some(outputs) => outputs,
+            None => return Err(VitaminError::OutputError),
+        };
+
+        // FIXME: For now Vitamins must produce exactly one output.
+        if outputs.len() != 1 {
+            return Err(VitaminError::OutputError);
+        }
+
+        let (_output_name, output_info) = outputs.iter().next().unwrap();
+        let output_quantity = output_info.quantity;
+
+        // compute the "unit cost" of this input
+        return Ok(total_cost / output_quantity.amount);
+    }
+
     // A "Vitamin" is a recipe whose only input is "capital".
     pub fn is_vitamin(&self) -> bool {
         if self.inputs.len() != 1 {

tools/src/repos.rs

@@ -52,6 +52,8 @@ pub enum RecipeCompileError {
     VitaminCapitalHasWrongUnit(String),
     #[error("input produces no output: {0}")]
     InputProducesNoOutput(String),
+    #[error(transparent)]
+    VitaminError(#[from] crate::recipe::VitaminError),
 }
 
 impl Repos {
@@ -130,17 +132,8 @@ impl Repos {
                 true => {
                     let amount_needed = input_info.quantity * quantity;
 
-                    // FIXME: for now Vitamins must have exactly one Input, and it must be Capital.
-                    assert_eq!(input_recipe.inputs.len(), 1);
-                    let input_capital = input_recipe.inputs.get("capital").ok_or_else(|| {
-                        RecipeCompileError::VitaminLacksCapital(input_name.into())
-                    })?;
-                    if input_capital.quantity.unit != Some(crate::quantity::Unit::USDollar) {
-                        return Err(RecipeCompileError::VitaminCapitalHasWrongUnit(
-                            input_name.into(),
-                        ));
-                    }
-                    let cost = input_capital.quantity.amount;
+                    let cost_each = input_recipe.unit_cost()?;
+                    let total_cost = amount_needed.amount * cost_each;
 
                     let outputs = match &input_recipe.outputs {
                         None => {
@@ -155,11 +148,6 @@ impl Repos {
                     assert_eq!(outputs.len(), 1);
 
                     let (_output_name, output_info) = outputs.iter().next().unwrap();
-                    let output_quantity = output_info.quantity;
-
-                    // compute the "unit cost" of this input
-                    let cost_each = cost / output_quantity.amount;
-                    let total_cost = amount_needed.amount * cost_each;
 
                     // FIXME: Try to coerce the units here - if we buy
                     // hose in rolls measured in feet, but we use lengths