internal/repl: add RPN support to REPL

- Import rpn package in repl.go
- Add 'rpn' and 'calc' as built-in commands
- Update executor to handle 'rpn <expr>' and 'calc <expr>' commands
- Add rpn/calc descriptions to help text
- All tests pass, go vet passes

2 files changed, 39 insertions, 3 deletions

diff --git a/internal/repl/commands.go b/internal/repl/commands.go
index 7800653..ad8a34d 100644
--- a/internal/repl/commands.go
+++ b/internal/repl/commands.go
@@ -6,7 +6,7 @@ import (
 )
 
 // builtinCommands defines the built-in REPL commands
-var builtinCommands = []string{"help", "clear", "quit", "exit"}
+var builtinCommands = []string{"help", "clear", "quit", "exit", "rpn", "calc"}
 
 // Commands returns the list of built-in command names
 func Commands() []string {
@@ -27,8 +27,11 @@ func ExecuteCommand(cmd string) (string, error) {
 		return "", cmdClear()
 	case "quit", "exit":
 		return "", cmdQuit()
+	case "rpn", "calc":
+		// rpn/calc commands are handled in executor(), not here
+		return "", nil
 	default:
-		return "", fmt.Errorf("unknown command: %s. Available commands: help, clear, quit, exit", args[0])
+		return "", fmt.Errorf("unknown command: %s. Available commands: help, clear, quit, exit, rpn, calc", args[0])
 	}
 }
 
@@ -40,6 +43,7 @@ Built-in Commands:
   help <command>   Show help for a specific topic
   clear            Clear the screen
   quit / exit      Exit the REPL
+  rpn / calc       Evaluate an RPN (postfix notation) expression
 
 Usage Examples:
   20% of 150           Calculate 20% of 150
@@ -47,6 +51,15 @@ Usage Examples:
   30 is what % of 150  Calculate what percentage 30 is of 150
   30 is 20% of what    Calculate what number 30 is 20% of
 
+RPN (Reverse Polish Notation) Examples:
+  rpn 3 4 +            3 + 4 = 7
+  rpn 3 4 + 4 4 - *    (3 + 4) * (4 - 4) = 0
+  rpn x 5 = x x +      Assign x=5, then x + x = 10
+  rpn 2 3 ^            2^3 = 8
+  rpn 1 2 swap         Swap top two stack values
+  rpn 1 2 3 dup        Duplicate top value
+  rpn show             Show current stack state
+
 Keyboard Shortcuts (Vi Mode):
   Normal Mode:
     i              Enter Insert mode
@@ -79,7 +92,7 @@ Press Ctrl+D or type 'quit'/'exit' to exit.
 	case "quit", "exit":
 		return "quit / exit - Exit the REPL\nUsage: quit or exit"
 	default:
-		return fmt.Sprintf("No help available for: %s\nAvailable commands: help, clear, quit, exit", subCmd)
+		return fmt.Sprintf("No help available for: %s\nAvailable commands: help, clear, quit, exit, rpn, calc", subCmd)
 	}
 }
 
diff --git a/internal/repl/repl.go b/internal/repl/repl.go
index 0f690e9..79a94f4 100644
--- a/internal/repl/repl.go
+++ b/internal/repl/repl.go
@@ -10,6 +10,7 @@ import (
 	"syscall"
 
 	"codeberg.org/snonux/perc/internal/calculator"
+	"codeberg.org/snonux/perc/internal/rpn"
 	"github.com/mattn/go-isatty"
 
 	"github.com/c-bata/go-prompt"
@@ -37,6 +38,19 @@ func executor(input string) {
 		return
 	}
 
+	// Check for rpn command prefix
+	if strings.HasPrefix(strings.ToLower(input), "rpn ") || strings.HasPrefix(strings.ToLower(input), "calc ") {
+		// Extract the expression after rpn/calc
+		rest := strings.TrimSpace(strings.TrimPrefix(input, strings.SplitN(input, " ", 2)[0]))
+		result, err := runRPN(rest)
+		if err != nil {
+			fmt.Printf("Error: %v\n", err)
+			return
+		}
+		fmt.Println(result)
+		return
+	}
+
 	// Run the calculation
 	result, err := calculator.Parse(input)
 	if err != nil {
@@ -46,6 +60,13 @@ func executor(input string) {
 	fmt.Println(result)
 }
 
+// runRPN parses and evaluates an RPN expression
+func runRPN(input string) (string, error) {
+	vars := rpn.NewVariables().(*rpn.Variables)
+	rpnCalc := rpn.NewRPN(vars)
+	return rpnCalc.ParseAndEvaluate(input)
+}
+
 // isBuiltinCommand checks if input starts with a built-in command
 func isBuiltinCommand(input string) (string, bool) {
 	args := strings.Fields(input)
@@ -180,6 +201,8 @@ func getCommandDescription(cmd string) string {
 		"clear":  "Clear the screen",
 		"quit":   "Exit the REPL",
 		"exit":   "Exit the REPL",
+		"rpn":    "Evaluate an RPN (postfix notation) expression",
+		"calc":   "Same as rpn - evaluate an RPN expression",
 	}
 	return descriptions[cmd]
 }