From 43bc759359e47b15e80dad074c4ec3939ce345c6 Mon Sep 17 00:00:00 2001
From: Himanshu Sharma <himanshu.kumarr07@gmail.com>
Date: Tue, 11 Mar 2025 09:33:43 +0530
Subject: [PATCH 1/2] Adding function in the types

---
 api/v1alpha1/types.go | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/api/v1alpha1/types.go b/api/v1alpha1/types.go
index 77aa01f7..efe90b3b 100644
--- a/api/v1alpha1/types.go
+++ b/api/v1alpha1/types.go
@@ -36,6 +36,10 @@ type ResourceGraphDefinitionSpec struct {
 	//
 	// +kubebuilder:validation:Optional
 	Resources []*Resource `json:"resources,omitempty"`
+
+	// Functions defines custom CEL functions that can be used in expressions
+	// +kubebuilder:validation:Optional
+	Functions []Function `json:"functions,omitempty"`
 	// ServiceAccount configuration for controller impersonation.
 	// Key is the namespace, value is the service account name to use.
 	// Special key "*" defines the default service account for any
@@ -45,6 +49,18 @@ type ResourceGraphDefinitionSpec struct {
 	DefaultServiceAccounts map[string]string `json:"defaultServiceAccounts,omitempty"`
 }
 
+type Function struct {
+	// The unique identifier for the function
+	ID string `json:"id"`
+
+	// Inputs defines the expected input types
+	// +kubebuilder:validation:MinItems=1
+	Inputs []string `json:"inputs"`
+
+	// The CEL expression template that defines the function logic
+	Template string `json:"template"`
+}
+
 // Schema represents the attributes that define an instance of
 // a resourcegraphdefinition.
 type Schema struct {

From 34c40bf816906faa9d9fd899a49bed1e4d4aa245 Mon Sep 17 00:00:00 2001
From: Himanshu Sharma <himanshu.kumarr07@gmail.com>
Date: Tue, 11 Mar 2025 09:34:32 +0530
Subject: [PATCH 2/2] Adding custom functions support

---
 pkg/cel/environment.go       |   4 ++
 pkg/cel/function/function.go | 108 +++++++++++++++++++++++++++++++++++
 2 files changed, 112 insertions(+)
 create mode 100644 pkg/cel/function/function.go

diff --git a/pkg/cel/environment.go b/pkg/cel/environment.go
index 5cf278d4..e4c80740 100644
--- a/pkg/cel/environment.go
+++ b/pkg/cel/environment.go
@@ -59,8 +59,12 @@ func DefaultEnvironment(options ...EnvOption) (*cel.Env, error) {
 		ext.Strings(),
 	}
 
+	// Add resource IDs
 	for _, name := range opts.resourceIDs {
 		declarations = append(declarations, cel.Variable(name, cel.AnyType))
 	}
+
+	declarations = append(declarations, opts.customDeclarations...)
+
 	return cel.NewEnv(declarations...)
 }
diff --git a/pkg/cel/function/function.go b/pkg/cel/function/function.go
new file mode 100644
index 00000000..3b075835
--- /dev/null
+++ b/pkg/cel/function/function.go
@@ -0,0 +1,108 @@
+package function
+
+import (
+	"fmt"
+
+	"github.com/google/cel-go/cel"
+	"github.com/google/cel-go/common/types"
+	"github.com/google/cel-go/common/types/ref"
+	xv1alpha1 "github.com/kro-run/kro/api/v1alpha1"
+)
+
+// FunctionRegistry manages custom CEL functions
+type FunctionRegistry struct {
+	functions map[string]*xv1alpha1.Function
+}
+
+// NewFunctionRegistry creates a new function registry
+func NewFunctionRegistry() *FunctionRegistry {
+	return &FunctionRegistry{
+		functions: make(map[string]*xv1alpha1.Function),
+	}
+}
+
+// RegisterFunction adds a function to the registry
+func (r *FunctionRegistry) RegisterFunction(fn *xv1alpha1.Function) error {
+	if _, exists := r.functions[fn.ID]; exists {
+		return fmt.Errorf("function %s already registered", fn.ID)
+	}
+	r.functions[fn.ID] = fn
+	return nil
+}
+
+// GetCELFunction converts a Function to a CEL function declaration
+func (r *FunctionRegistry) GetCELFunction(fn *xv1alpha1.Function) (cel.EnvOption, error) {
+
+	//convert input types to CEL types
+	celTypes := make([]*cel.Type, len((fn.Inputs)))
+	envOpts := make([]cel.EnvOption, len(fn.Inputs))
+
+	for i, inputType := range fn.Inputs {
+		celType, err := getCelType(inputType)
+		if err != nil {
+			return nil, err
+		}
+		celTypes[i] = celType
+		envOpts[i] = cel.Variable(fmt.Sprintf("_%d", i), celType)
+	}
+
+	// Create CEL environment for function template
+	env, err := cel.NewEnv(envOpts...)
+	if err != nil {
+		return nil, fmt.Errorf("environment creation failed: %w", err)
+	}
+
+	// Compile function template
+	ast, iss := env.Compile(fn.Template)
+	if iss.Err() != nil {
+		return nil, fmt.Errorf("template compilation failed: %w", iss.Err())
+	}
+
+	prg, err := env.Program(ast)
+	if err != nil {
+		return nil, fmt.Errorf("program creation failed: %w", err)
+	}
+
+	// Return CEL function declaration
+	return cel.Function(fn.ID,
+		cel.Overload(fn.ID,
+			celTypes,
+			ast.OutputType(),
+			cel.FunctionBinding(func(args ...ref.Val) ref.Val {
+				vars := map[string]any{}
+				for i, arg := range args {
+					vars[fmt.Sprintf("_%d", i)] = arg
+				}
+				out, _, err := prg.Eval(vars)
+				if err != nil {
+					return types.WrapErr(err)
+				}
+				return out
+			}),
+		),
+	), nil
+
+}
+
+func getCelType(typeName string) (*cel.Type, error) {
+	switch typeName {
+	case "string":
+		return cel.StringType, nil
+	case "int":
+		return cel.IntType, nil
+	case "bool":
+		return cel.BoolType, nil
+	case "double", "float":
+		return cel.DoubleType, nil
+	case "bytes":
+		return cel.BytesType, nil
+	case "any":
+		return cel.AnyType, nil
+	case "list":
+		return cel.ListType(cel.AnyType), nil
+	case "map":
+		return cel.MapType(cel.StringType, cel.AnyType), nil
+	default:
+		return nil, fmt.Errorf("unsupported type: %s", typeName)
+	}
+}