feat: reusable rule engine to prevent memory reallocation

internal/rule/engine.go

@@ -1,16 +1,28 @@
 package rule
 
 import (
+	"context"
+
 	"github.com/expr-lang/expr"
 	"github.com/expr-lang/expr/vm"
 	"github.com/pkg/errors"
 )
 
-type Engine[E any] struct {
+type Engine[V any] struct {
 	rules []*vm.Program
 }
 
-func (e *Engine[E]) Apply(env E) ([]any, error) {
+func (e *Engine[V]) Apply(ctx context.Context, vars V) ([]any, error) {
+	type Env[V any] struct {
+		Context context.Context `expr:"ctx"`
+		Vars    V               `expr:"vars"`
+	}
+
+	env := Env[V]{
+		Context: ctx,
+		Vars:    vars,
+	}
+
 	results := make([]any, 0, len(e.rules))
 	for i, r := range e.rules {
 		result, err := expr.Run(r, env)
@@ -42,3 +54,26 @@ func NewEngine[E any](funcs ...OptionFunc) (*Engine[E], error) {
 
 	return engine, nil
 }
+
+func Context[T any](ctx context.Context, key any) (T, bool) {
+	raw := ctx.Value(key)
+	if raw == nil {
+		return *new(T), false
+	}
+
+	value, err := Assert[T](raw)
+	if err != nil {
+		return *new(T), false
+	}
+
+	return value, true
+}
+
+func Assert[T any](raw any) (T, error) {
+	value, ok := raw.(T)
+	if !ok {
+		return *new(T), errors.Errorf("unexpected value '%T'", value)
+	}
+
+	return value, nil
+}