internal/rpn: define Operator interface and make RPN depend on it

- Added Operator interface in operations.go with all required methods
- Updated RPN struct to use Operator interface instead of concrete *Operations type
- This fixes the SOLID Dependency Inversion Principle violation

2 files changed, 32 insertions, 2 deletions

diff --git a/internal/rpn/operations.go b/internal/rpn/operations.go
index 2f4b84c..1ebd78b 100644
--- a/internal/rpn/operations.go
+++ b/internal/rpn/operations.go
@@ -5,6 +5,36 @@ import (
 	"math"
 )
 
+// Operator defines the interface for operator implementations and stack manipulation.
+// This allows RPN to depend on an abstraction instead of the concrete Operations type.
+type Operator interface {
+	// Arithmetic operators
+	Add(stack *Stack) error
+	Subtract(stack *Stack) error
+	Multiply(stack *Stack) error
+	Divide(stack *Stack) error
+	Power(stack *Stack) error
+	Modulo(stack *Stack) error
+
+	// Hyper operators
+	HyperAdd(stack *Stack) error
+	HyperSubtract(stack *Stack) error
+	HyperMultiply(stack *Stack) error
+	HyperDivide(stack *Stack) error
+	HyperPower(stack *Stack) error
+	HyperModulo(stack *Stack) error
+
+	// Stack manipulation operators
+	Dup(stack *Stack) error
+	Swap(stack *Stack) error
+	Pop(stack *Stack) error
+	Show(stack *Stack) (string, error)
+
+	// Variable operations
+	ListVariables() (string, error)
+	ClearVariables()
+}
+
 // Operations provides operator implementations and stack manipulation.
 type Operations struct {
 	vars VariableStore
diff --git a/internal/rpn/rpn.go b/internal/rpn/rpn.go
index 5c2b844..348da96 100644
--- a/internal/rpn/rpn.go
+++ b/internal/rpn/rpn.go
@@ -8,8 +8,8 @@ import (
 
 // RPN represents the RPN parser and evaluator.
 type RPN struct {
-	vars    VariableStore
-	ops     *Operations
+	vars     VariableStore
+	ops      Operator
 	maxStack int
 	currentStack *Stack
 }