Prepare both configuration comparison inputs

integrations/ckconfig/input.go

@@ -23,14 +23,20 @@ type selectedScopeInput[I, S any] struct {
 	input I
 }
 
+type preparedComparison[I, S any] struct {
+	scope   *preparedScope[I, S]
+	request ComparisonRequest[I]
+}
+
 type configOperationPlan[I, S any] struct {
-	prepared  *preparedSuite[I, S]
-	operation Operation
-	selection Selection
-	scopeName string
-	format    string
-	path      configkit.Path
-	scopes    []selectedScopeInput[I, S]
+	prepared   *preparedSuite[I, S]
+	operation  Operation
+	selection  Selection
+	scopeName  string
+	format     string
+	path       configkit.Path
+	scopes     []selectedScopeInput[I, S]
+	comparison *preparedComparison[I, S]
 }
 
 func newConfigOperationCommand[I, S any](prepared *preparedSuite[I, S], operation Operation) commandkit.Command {
@@ -171,30 +177,54 @@ func prepareConfigOperation[I, S any](prepared *preparedSuite[I, S], operation O
 	}
 	if operation == Diff {
 		input.against = inheritComparisonSelection(input.selection, input.against)
+		scope := selected[0]
+		left, err := prepareScopeInput(prepared, input.values, scope, input.selection)
+		if err != nil {
+			return fmt.Errorf("configuration scope %q left input: %w", scope.scope.Name, err)
+		}
+		right, err := prepareScopeInput(prepared, input.values, scope, input.against)
+		if err != nil {
+			return fmt.Errorf("configuration scope %q right input: %w", scope.scope.Name, err)
+		}
+		plan.comparison = &preparedComparison[I, S]{
+			scope: scope,
+			request: ComparisonRequest[I]{
+				Left:  ComparisonSide[I]{Selection: input.selection, Input: left},
+				Right: ComparisonSide[I]{Selection: input.against, Input: right},
+			},
+		}
 		input.plan = plan
 		return nil
 	}
 	for _, scope := range selected {
-		var value I
-		if prepared.newInput != nil {
-			value = prepared.newInput()
-		}
-		// Availability was checked above, so every binding for this scope
-		// belongs to the operation's captured union.
-		if err := input.values.ApplyTo(&value, scope.bindings...); err != nil {
+		value, err := prepareScopeInput(prepared, input.values, scope, input.selection)
+		if err != nil {
 			return fmt.Errorf("configuration scope %q: %w", scope.scope.Name, err)
 		}
-		if prepared.checkInput != nil {
-			if err := prepared.checkInput(scope.scope.Name, input.selection, value); err != nil {
-				return fmt.Errorf("configuration scope %q: %w", scope.scope.Name, err)
-			}
-		}
 		plan.scopes = append(plan.scopes, selectedScopeInput[I, S]{scope: scope, input: value})
 	}
 	input.plan = plan
 	return nil
 }
 
+func prepareScopeInput[I, S any](prepared *preparedSuite[I, S], values commandkit.FlagValues[I], scope *preparedScope[I, S], selection Selection) (I, error) {
+	var value I
+	if prepared.newInput != nil {
+		value = prepared.newInput()
+	}
+	// Availability was checked before this helper, so every binding for the
+	// selected scope belongs to the operation's captured union.
+	if err := values.ApplyTo(&value, scope.bindings...); err != nil {
+		return value, err
+	}
+	if prepared.checkInput != nil {
+		if err := prepared.checkInput(scope.scope.Name, selection, value); err != nil {
+			return value, err
+		}
+	}
+	return value, nil
+}
+
 func inheritComparisonSelection(left, right Selection) Selection {
 	if !right.Config.Set {
 		right.Config = left.Config