Add comparison selection and format flags

integrations/ckconfig/input.go

@@ -10,6 +10,7 @@ import (
 
 type configCommandInput[I, S any] struct {
 	selection Selection
+	against   Selection
 	scope     commandkit.Supplied[string]
 	format    commandkit.Supplied[string]
 	path      commandkit.Supplied[string]
@@ -35,7 +36,7 @@ type configOperationPlan[I, S any] struct {
 func newConfigOperationCommand[I, S any](prepared *preparedSuite[I, S], operation Operation) commandkit.Command {
 	defaultScope := "app"
 	defaultFormat := "text"
-	if operation == Show {
+	if operation == Show || operation == Diff {
 		defaultFormat = "yaml"
 	}
 	flags := []commandkit.Flag[*configCommandInput[I, S]]{
@@ -52,6 +53,18 @@ func newConfigOperationCommand[I, S any](prepared *preparedSuite[I, S], operatio
 				(*input).selection.Overlay = value
 			}),
 	}
+	if operation == Diff {
+		flags = append(flags,
+			commandkit.ValueFlag(commandkit.FlagMeta{Name: "against-config", Usage: "Configuration source for the right side."}, (*string)(nil), parseConfigString,
+				func(input **configCommandInput[I, S], value commandkit.Supplied[string]) {
+					(*input).against.Config = value
+				}),
+			commandkit.ValueFlag(commandkit.FlagMeta{Name: "against-overlay", Usage: "Overlay source for the right side."}, (*string)(nil), parseConfigString,
+				func(input **configCommandInput[I, S], value commandkit.Supplied[string]) {
+					(*input).against.Overlay = value
+				}),
+		)
+	}
 	if operation == Explain {
 		flags = append(flags, commandkit.ValueFlag(commandkit.FlagMeta{Name: "path", Usage: "JSON Pointer path to explain.", Required: true}, (*string)(nil), parseConfigString,
 			func(input **configCommandInput[I, S], value commandkit.Supplied[string]) { (*input).path = value }))
@@ -90,6 +103,8 @@ func operationDeclaration(operation Operation) (string, string) {
 		return "sources", "List configuration sources."
 	case Explain:
 		return "explain", "Explain a configuration path."
+	case Diff:
+		return "diff", "Compare two configuration selections."
 	default:
 		return string(operation), "Inspect configuration."
 	}
@@ -103,6 +118,9 @@ func prepareConfigOperation[I, S any](prepared *preparedSuite[I, S], operation O
 	if operation == Show && format != "yaml" {
 		return fmt.Errorf("unsupported configuration show format %q", format)
 	}
+	if operation == Diff && format != "yaml" {
+		return fmt.Errorf("unsupported configuration diff format %q", format)
+	}
 
 	scopeName := "app"
 	if input.scope.Set {
@@ -151,6 +169,11 @@ func prepareConfigOperation[I, S any](prepared *preparedSuite[I, S], operation O
 		path:      path,
 		scopes:    make([]selectedScopeInput[I, S], 0, len(selected)),
 	}
+	if operation == Diff {
+		input.against = inheritComparisonSelection(input.selection, input.against)
+		input.plan = plan
+		return nil
+	}
 	for _, scope := range selected {
 		var value I
 		if prepared.newInput != nil {
@@ -172,6 +195,16 @@ func prepareConfigOperation[I, S any](prepared *preparedSuite[I, S], operation O
 	return nil
 }
 
+func inheritComparisonSelection(left, right Selection) Selection {
+	if !right.Config.Set {
+		right.Config = left.Config
+	}
+	if !right.Overlay.Set {
+		right.Overlay = left.Overlay
+	}
+	return right
+}
+
 func joinOperations(operations []Operation) string {
 	values := make([]string, len(operations))
 	for index, operation := range operations {