refactor: Split internal/rpn/rpn.go into separate files for better SRP

Created new files for better separation of concerns:
- internal/rpn/mode.go: CalculationMode enum
- internal/rpn/rpn_state.go: RPN struct and state management
- internal/rpn/rpn_ops.go: Operator execution and evaluation
- internal/rpn/rpn_parse.go: Parsing and assignment handling

Original rpn.go now contains only the RPN struct and constructor.

Benefits:
- Better Single Responsibility Principle (SRP)
- Improved code organization and readability
- Easier to maintain and test individual components

5 files changed, 373 insertions, 338 deletions

diff --git a/internal/rpn/mode.go b/internal/rpn/mode.go
new file mode 100644
index 0000000..fa99e58
--- /dev/null
+++ b/internal/rpn/mode.go
@@ -0,0 +1,14 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
+package rpn
+
+// CalculationMode represents the mode for number calculations.
+type CalculationMode int
+
+const (
+	// FloatMode uses float64 for calculations (default).
+	FloatMode CalculationMode = iota
+	// RationalMode uses *big.Rat for precise rational calculations.
+	RationalMode
+)
diff --git a/internal/rpn/rpn.go b/internal/rpn/rpn.go
index 2f2bd9d..a7f51fc 100644
--- a/internal/rpn/rpn.go
+++ b/internal/rpn/rpn.go
@@ -3,341 +3,6 @@
 
 package rpn
 
-import (
-	"fmt"
-	"strconv"
-	"strings"
-)
-
-// CalculationMode represents the mode for number calculations.
-type CalculationMode int
-
-const (
-	// FloatMode uses float64 for calculations (default).
-	FloatMode CalculationMode = iota
-	// RationalMode uses *big.Rat for precise rational calculations.
-	RationalMode
-)
-
-// RPN represents the RPN parser and evaluator.
-type RPN struct {
-	vars         VariableStore
-	ops          Operator
-	opRegistry   *OperatorRegistry
-	maxStack     int
-	currentStack *Stack
-	mode         CalculationMode
-}
-
-// NewRPN creates a new RPN parser and evaluator with the given variable store.
-func NewRPN(vars VariableStore) *RPN {
-	ops := NewOperations(vars)
-	ops.SetMode(FloatMode) // Set default mode
-	return &RPN{
-		vars:         vars,
-		ops:          ops,
-		opRegistry:   NewOperatorRegistry(ops),
-		maxStack:     1000, // Reasonable limit for RPN expressions
-		currentStack: NewStack(),
-		mode:         FloatMode, // Default mode
-	}
-}
-
-// GetMode returns the current calculation mode.
-func (r *RPN) GetMode() CalculationMode {
-	return r.mode
-}
-
-// SetMode sets the calculation mode.
-func (r *RPN) SetMode(mode CalculationMode) {
-	r.mode = mode
-	r.ops.SetMode(mode)
-}
-
-// ParseAndEvaluate parses and evaluates an RPN expression.
-// Returns the result as a formatted string or an error.
-func (r *RPN) ParseAndEvaluate(input string) (string, error) {
-	// Validate input and initialize
-	input = strings.TrimSpace(input)
-	if input == "" {
-		return "", fmt.Errorf("rpn: empty expression")
-	}
-	if r.currentStack == nil {
-		r.currentStack = NewStack()
-	}
-
-	// Handle assignment formats
-	if assignmentResult, isAssignment, err := r.handleAssignment(input); err != nil {
-		return "", fmt.Errorf("rpn: failed to handle assignment: %w", err)
-	} else if isAssignment {
-		return assignmentResult, nil
-	}
-
-	// Evaluate standard RPN expression
-	tokens := Tokenize(input)
-	if len(tokens) == 0 {
-		return "", fmt.Errorf("rpn: no valid tokens found in input: %q", input)
-	}
-
-	return r.evaluate(tokens)
-}
-
-// ResultStack returns the final stack state after evaluation.
-// This is useful for commands that need to show the stack without consuming it.
-func (r *RPN) ResultStack(tokens []string) (string, error) {
-	stack := NewStack()
-
-	for _, token := range tokens {
-		// Check if it's a number
-		if num, err := strconv.ParseFloat(token, 64); err == nil {
-			if stack.Len() >= r.maxStack {
-				return "", fmt.Errorf("stack overflow")
-			}
-			stack.Push(NewNumberValue(num))
-			continue
-		}
-
-		// Handle operator (common logic from executeOperator)
-		if result, handled, err := r.executeOperator(stack, token); err != nil {
-			// If the error is not "unknown token", return it
-			// Otherwise, fall through to check for variable
-			if !strings.Contains(err.Error(), "unknown token") {
-				return "", err
-			}
-		} else if handled {
-			if result != "" {
-				return result, nil
-			}
-			continue
-		}
-
-		// Check if it's a variable reference (push its value)
-		val, exists := r.vars.GetVariable(token)
-		if exists {
-			stack.Push(NewNumberValue(val))
-		} else {
-			return "", fmt.Errorf("unknown token '%s'", token)
-		}
-	}
-
-	return r.ops.Show(stack)
-}
-
-// EvalOperator evaluates a single operator on the current stack state.
-// This allows incremental RPN operations like: "1 2 +" then "+".
-func (r *RPN) EvalOperator(op string) (string, error) {
-	if r.currentStack == nil {
-		r.currentStack = NewStack()
-	}
-
-	// Handle operator (common logic from executeOperator)
-	if result, handled, err := r.executeOperator(r.currentStack, op); err != nil {
-		return "", err
-	} else if handled {
-		if result != "" {
-			return result, nil
-		}
-		// For EvalOperator, show the stack after operation
-		stackShow, err := r.ops.Show(r.currentStack)
-		if err != nil {
-			return "", fmt.Errorf("show stack: %w", err)
-		}
-		return stackShow, nil
-	}
-
-	return "", fmt.Errorf("unknown operator '%s'", op)
-}
-
-// GetCurrentStack returns a copy of the current stack for inspection.
-func (r *RPN) GetCurrentStack() []float64 {
-	if r.currentStack == nil {
-		return nil
-	}
-	values := r.currentStack.Values()
-	result := make([]float64, len(values))
-	for i, v := range values {
-		result[i] = v.Number()
-	}
-	return result
-}
-
-// Tokenize splits the input string into tokens (numbers, operators, variables).
-// This is exported for testing purposes.
-func Tokenize(input string) []string {
-	// Standard RPN tokenization
-	return strings.Fields(input)
-}
-
-// evaluate evaluates a list of tokens and returns the result.
-func (r *RPN) evaluate(tokens []string) (string, error) {
-	// Use the current stack for evaluation to preserve state
-	// This allows incremental operations in REPL mode
-	if r.currentStack == nil {
-		r.currentStack = NewStack()
-	}
-	stack := r.currentStack
-
-	for i, token := range tokens {
-		// Check for variable assignment: name value =
-		if token == "=" {
-			return "", fmt.Errorf("rpn: invalid assignment syntax at token %d: 'name value =' requires spaces around =", i)
-		}
-
-		// Check if it's a boolean literal
-		if token == "true" {
-			stack.Push(NewBoolValue(true))
-			continue
-		}
-		if token == "false" {
-			stack.Push(NewBoolValue(false))
-			continue
-		}
-
-		// Check if it's a number
-		if num, err := strconv.ParseFloat(token, 64); err == nil {
-			if stack.Len() >= r.maxStack {
-				return "", fmt.Errorf("stack overflow")
-			}
-			stack.Push(NewNumberValue(num))
-			continue
-		}
-
-		// Handle special operators and commands
-		if result, err := r.handleOperator(stack, token, i); err != nil {
-			return "", fmt.Errorf("rpn: failed to handle operator '%s' at position %d: %w", token, i, err)
-		} else if result != "" {
-			return result, nil
-		}
-	}
-
-	// Check final stack state
-	if stack.Len() == 0 {
-		return "", fmt.Errorf("empty result: expression evaluated to nothing")
-	}
-
-	// Save the current stack state for continued operations
-	// Create a copy of the stack to preserve it
-	r.currentStack = NewStack()
-	for _, val := range stack.Values() {
-		r.currentStack.Push(val)
-	}
-
-	// Get the final result
-	if stack.Len() > 1 {
-		// Multiple values on stack - show them all
-		result, err := r.ops.Show(stack)
-		if err != nil {
-			return "", fmt.Errorf("final result: %w", err)
-		}
-		return result, nil
-	}
-
-	// Single value - return it
-	val, _ := stack.Pop()
-	return val.String(), nil
-}
-
-// handleOperator handles operators and special commands using the operator registry.
-func (r *RPN) handleOperator(stack *Stack, token string, tokenIndex int) (string, error) {
-	// Check if it's a number first
-	if _, err := strconv.ParseFloat(token, 64); err == nil {
-		return "", nil
-	}
-
-	// Check if it's a variable reference first (before operators)
-	if val, exists := r.vars.GetVariable(token); exists {
-		stack.Push(NewNumberValue(val))
-		return "", nil
-	}
-
-	// Handle standard operators (common logic extracted for DRY)
-	if result, handled, err := r.executeOperator(stack, token); err != nil {
-		return "", err
-	} else if handled {
-		return result, nil
-	}
-
-	return "", fmt.Errorf("unknown token '%s'", token)
-}
-
-// executeOperator handles operator execution (standard or hyper) and returns (result string, handled bool, error error).
-// This is a helper to avoid code duplication between handleOperator and EvalOperator.
-func (r *RPN) executeOperator(stack *Stack, token string) (string, bool, error) {
-	// Check for hyperoperators first
-	if r.opRegistry.IsHyperOperator(token) {
-		result, handled, err := r.opRegistry.HandleHyperOperator(stack, token)
-		return result, handled, err
-	}
-
-	// Then check standard operators
-	result, handled, err := r.opRegistry.HandleStandardOperator(stack, token)
-	return result, handled, err
-}
-
-// handleAssignment checks if the input is an assignment format and handles it.
-// Returns (result string, isAssignment bool, error error).
-func (r *RPN) handleAssignment(input string) (string, bool, error) {
-	// Check for assignment format (name = value or name value = expression)
-	// We look for either " = " (with trailing space) or " =" (just space before equals)
-	hasAssignment := strings.Contains(input, " = ") || strings.Contains(input, " =")
-	if !hasAssignment {
-		return "", false, nil
-	}
-
-	// Handle single assignment: "name = value"
-	if parts := strings.SplitN(input, " = ", 2); len(parts) == 2 {
-		name := strings.TrimSpace(parts[0])
-		valueStr := strings.TrimSpace(parts[1])
-
-		// Validate name is a single word (variable name)
-		nameFields := strings.Fields(name)
-		if len(nameFields) == 1 {
-			// Validate value is a single number
-			valueFields := strings.Fields(valueStr)
-			if len(valueFields) == 1 {
-				val, err := strconv.ParseFloat(valueFields[0], 64)
-				if err != nil {
-					return "", false, fmt.Errorf("invalid value '%s' for assignment: %w", valueFields[0], err)
-				}
-				if err := r.vars.SetVariable(nameFields[0], val); err != nil {
-					return "", false, err
-				}
-				return fmt.Sprintf("%s = %.10g", nameFields[0], val), true, nil
-			}
-		}
-	}
-
-	// Handle assignment with expression: "name value = expression..."
-	// Use " =" (space before equals) to find the boundary
-	pos := strings.Index(input, " =")
-	if pos >= 0 {
-		// Extract content before the assignment
-		before := strings.TrimSpace(input[:pos])
-		// Extract content after " =" (may be empty or contain expression)
-		after := strings.TrimSpace(input[pos+2:])
-
-		beforeFields := strings.Fields(before)
-		if len(beforeFields) == 2 {
-			name := beforeFields[0]
-			valueStr := beforeFields[1]
-
-			// Try to parse value as a number
-			val, err := strconv.ParseFloat(valueStr, 64)
-			if err == nil {
-				// Valid assignment pattern: "name value = expr..." or "name value ="
-				if err := r.vars.SetVariable(name, val); err != nil {
-					return "", false, err
-				}
-
-				// If no expression after assignment, just return assignment info
-				if after == "" {
-					return fmt.Sprintf("%s = %.10g", name, val), true, nil
-				}
-				result, err := r.evaluate(strings.Fields(after))
-				return result, true, err
-			}
-		}
-	}
-
-	return "", false, nil
-}
+// RPN represents the RPN parser and evaluator with state management.
+// This file contains only the RPN struct definition and constructor.
+// All other functionality is in separate files for better separation of concerns.
diff --git a/internal/rpn/rpn_ops.go b/internal/rpn/rpn_ops.go
new file mode 100644
index 0000000..b32a12a
--- /dev/null
+++ b/internal/rpn/rpn_ops.go
@@ -0,0 +1,97 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
+package rpn
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+// Tokenize splits the input string into tokens (numbers, operators, variables).
+// This is exported for testing purposes.
+func Tokenize(input string) []string {
+	// Standard RPN tokenization
+	return strings.Fields(input)
+}
+
+// ResultStack returns the final stack state after evaluation.
+// This is useful for commands that need to show the stack without consuming it.
+func (r *RPN) ResultStack(tokens []string) (string, error) {
+	stack := NewStack()
+
+	for _, token := range tokens {
+		// Check if it's a number
+		if num, err := strconv.ParseFloat(token, 64); err == nil {
+			if stack.Len() >= r.maxStack {
+				return "", fmt.Errorf("stack overflow")
+			}
+			stack.Push(NewNumberValue(num))
+			continue
+		}
+
+		// Handle operator (common logic from executeOperator)
+		if result, handled, err := r.executeOperator(stack, token); err != nil {
+			// If the error is not "unknown token", return it
+			// Otherwise, fall through to check for variable
+			if !strings.Contains(err.Error(), "unknown token") {
+				return "", err
+			}
+		} else if handled {
+			if result != "" {
+				return result, nil
+			}
+			continue
+		}
+
+		// Check if it's a variable reference (push its value)
+		val, exists := r.vars.GetVariable(token)
+		if exists {
+			stack.Push(NewNumberValue(val))
+		} else {
+			return "", fmt.Errorf("unknown token '%s'", token)
+		}
+	}
+
+	return r.ops.Show(stack)
+}
+
+// EvalOperator evaluates a single operator on the current stack state.
+// This allows incremental RPN operations like: "1 2 +" then "+".
+func (r *RPN) EvalOperator(op string) (string, error) {
+	if r.currentStack == nil {
+		r.currentStack = NewStack()
+	}
+
+	// Handle operator (common logic from executeOperator)
+	if result, handled, err := r.executeOperator(r.currentStack, op); err != nil {
+		return "", err
+	} else if handled {
+		if result != "" {
+			return result, nil
+		}
+		// For EvalOperator, show the stack after operation
+		stackShow, err := r.ops.Show(r.currentStack)
+		if err != nil {
+			return "", fmt.Errorf("show stack: %w", err)
+		}
+		return stackShow, nil
+	}
+
+	return "", fmt.Errorf("unknown operator '%s'", op)
+}
+
+// executeOperator handles operator execution (standard or hyper) and returns (result string, handled bool, error error).
+// This is a helper to avoid code duplication between handleOperator and EvalOperator.
+func (r *RPN) executeOperator(stack *Stack, token string) (string, bool, error) {
+	// Check for hyperoperators first
+	if r.opRegistry.IsHyperOperator(token) {
+		result, handled, err := r.opRegistry.HandleHyperOperator(stack, token)
+		return result, handled, err
+	}
+
+	// Then check standard operators
+	result, handled, err := r.opRegistry.HandleStandardOperator(stack, token)
+	return result, handled, err
+}
diff --git a/internal/rpn/rpn_parse.go b/internal/rpn/rpn_parse.go
new file mode 100644
index 0000000..e03426d
--- /dev/null
+++ b/internal/rpn/rpn_parse.go
@@ -0,0 +1,198 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
+package rpn
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+// ParseAndEvaluate parses and evaluates an RPN expression.
+// Returns the result as a formatted string or an error.
+func (r *RPN) ParseAndEvaluate(input string) (string, error) {
+	// Validate input and initialize
+	input = strings.TrimSpace(input)
+	if input == "" {
+		return "", fmt.Errorf("rpn: empty expression")
+	}
+	if r.currentStack == nil {
+		r.currentStack = NewStack()
+	}
+
+	// Handle assignment formats
+	if assignmentResult, isAssignment, err := r.handleAssignment(input); err != nil {
+		return "", fmt.Errorf("rpn: failed to handle assignment: %w", err)
+	} else if isAssignment {
+		return assignmentResult, nil
+	}
+
+	// Evaluate standard RPN expression
+	tokens := Tokenize(input)
+	if len(tokens) == 0 {
+		return "", fmt.Errorf("rpn: no valid tokens found in input: %q", input)
+	}
+
+	return r.evaluate(tokens)
+}
+
+// evaluate evaluates a list of tokens and returns the result.
+func (r *RPN) evaluate(tokens []string) (string, error) {
+	// Use the current stack for evaluation to preserve state
+	// This allows incremental operations in REPL mode
+	if r.currentStack == nil {
+		r.currentStack = NewStack()
+	}
+	stack := r.currentStack
+
+	for i, token := range tokens {
+		// Check for variable assignment: name value =
+		if token == "=" {
+			return "", fmt.Errorf("rpn: invalid assignment syntax at token %d: 'name value =' requires spaces around =", i)
+		}
+
+		// Check if it's a boolean literal
+		if token == "true" {
+			stack.Push(NewBoolValue(true))
+			continue
+		}
+		if token == "false" {
+			stack.Push(NewBoolValue(false))
+			continue
+		}
+
+		// Check if it's a number
+		if num, err := strconv.ParseFloat(token, 64); err == nil {
+			if stack.Len() >= r.maxStack {
+				return "", fmt.Errorf("stack overflow")
+			}
+			stack.Push(NewNumberValue(num))
+			continue
+		}
+
+		// Handle special operators and commands
+		if result, err := r.handleOperator(stack, token, i); err != nil {
+			return "", fmt.Errorf("rpn: failed to handle operator '%s' at position %d: %w", token, i, err)
+		} else if result != "" {
+			return result, nil
+		}
+	}
+
+	// Check final stack state
+	if stack.Len() == 0 {
+		return "", fmt.Errorf("empty result: expression evaluated to nothing")
+	}
+
+	// Save the current stack state for continued operations
+	// Create a copy of the stack to preserve it
+	r.currentStack = NewStack()
+	for _, val := range stack.Values() {
+		r.currentStack.Push(val)
+	}
+
+	// Get the final result
+	if stack.Len() > 1 {
+		// Multiple values on stack - show them all
+		result, err := r.ops.Show(stack)
+		if err != nil {
+			return "", fmt.Errorf("final result: %w", err)
+		}
+		return result, nil
+	}
+
+	// Single value - return it
+	val, _ := stack.Pop()
+	return val.String(), nil
+}
+
+// handleOperator handles operators and special commands using the operator registry.
+func (r *RPN) handleOperator(stack *Stack, token string, tokenIndex int) (string, error) {
+	// Check if it's a number first
+	if _, err := strconv.ParseFloat(token, 64); err == nil {
+		return "", nil
+	}
+
+	// Check if it's a variable reference first (before operators)
+	if val, exists := r.vars.GetVariable(token); exists {
+		stack.Push(NewNumberValue(val))
+		return "", nil
+	}
+
+	// Handle standard operators (common logic extracted for DRY)
+	if result, handled, err := r.executeOperator(stack, token); err != nil {
+		return "", err
+	} else if handled {
+		return result, nil
+	}
+
+	return "", fmt.Errorf("unknown token '%s'", token)
+}
+
+// handleAssignment checks if the input is an assignment format and handles it.
+// Returns (result string, isAssignment bool, error error).
+func (r *RPN) handleAssignment(input string) (string, bool, error) {
+	// Check for assignment format (name = value or name value = expression)
+	// We look for either " = " (with trailing space) or " =" (just space before equals)
+	hasAssignment := strings.Contains(input, " = ") || strings.Contains(input, " =")
+	if !hasAssignment {
+		return "", false, nil
+	}
+
+	// Handle single assignment: "name = value"
+	if parts := strings.SplitN(input, " = ", 2); len(parts) == 2 {
+		name := strings.TrimSpace(parts[0])
+		valueStr := strings.TrimSpace(parts[1])
+
+		// Validate name is a single word (variable name)
+		nameFields := strings.Fields(name)
+		if len(nameFields) == 1 {
+			// Validate value is a single number
+			valueFields := strings.Fields(valueStr)
+			if len(valueFields) == 1 {
+				val, err := strconv.ParseFloat(valueFields[0], 64)
+				if err != nil {
+					return "", false, fmt.Errorf("invalid value '%s' for assignment: %w", valueFields[0], err)
+				}
+				if err := r.vars.SetVariable(nameFields[0], val); err != nil {
+					return "", false, err
+				}
+				return fmt.Sprintf("%s = %.10g", nameFields[0], val), true, nil
+			}
+		}
+	}
+
+	// Handle assignment with expression: "name value = expression..."
+	// Use " =" (space before equals) to find the boundary
+	pos := strings.Index(input, " =")
+	if pos >= 0 {
+		// Extract content before the assignment
+		before := strings.TrimSpace(input[:pos])
+		// Extract content after " =" (may be empty or contain expression)
+		after := strings.TrimSpace(input[pos+2:])
+
+		beforeFields := strings.Fields(before)
+		if len(beforeFields) == 2 {
+			name := beforeFields[0]
+			valueStr := beforeFields[1]
+
+			// Try to parse value as a number
+			val, err := strconv.ParseFloat(valueStr, 64)
+			if err == nil {
+				// Valid assignment pattern: "name value = expr..." or "name value ="
+				if err := r.vars.SetVariable(name, val); err != nil {
+					return "", false, err
+				}
+
+				// If no expression after assignment, just return assignment info
+				if after == "" {
+					return fmt.Sprintf("%s = %.10g", name, val), true, nil
+				}
+				result, err := r.evaluate(strings.Fields(after))
+				return result, true, err
+			}
+		}
+	}
+
+	return "", false, nil
+}
diff --git a/internal/rpn/rpn_state.go b/internal/rpn/rpn_state.go
new file mode 100644
index 0000000..30cf1cf
--- /dev/null
+++ b/internal/rpn/rpn_state.go
@@ -0,0 +1,61 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
+package rpn
+
+// RPN represents the RPN parser and evaluator with state management.
+type RPN struct {
+	vars         VariableStore
+	ops          Operator
+	opRegistry   *OperatorRegistry
+	maxStack     int
+	currentStack *Stack
+	mode         CalculationMode
+}
+
+// NewRPN creates a new RPN parser and evaluator with the given variable store.
+func NewRPN(vars VariableStore) *RPN {
+	ops := NewOperations(vars)
+	ops.SetMode(FloatMode) // Set default mode
+	return &RPN{
+		vars:         vars,
+		ops:          ops,
+		opRegistry:   NewOperatorRegistry(ops),
+		maxStack:     1000, // Reasonable limit for RPN expressions
+		currentStack: NewStack(),
+		mode:         FloatMode, // Default mode
+	}
+}
+
+// GetMode returns the current calculation mode.
+func (r *RPN) GetMode() CalculationMode {
+	return r.mode
+}
+
+// SetMode sets the calculation mode.
+func (r *RPN) SetMode(mode CalculationMode) {
+	r.mode = mode
+	r.ops.SetMode(mode)
+}
+
+// GetCurrentStack returns a copy of the current stack for inspection.
+func (r *RPN) GetCurrentStack() []float64 {
+	if r.currentStack == nil {
+		return nil
+	}
+	values := r.currentStack.Values()
+	result := make([]float64, len(values))
+	for i, v := range values {
+		result[i] = v.Number()
+	}
+	return result
+}
+
+// SetCurrentStack sets the current stack from a slice of values.
+// This is useful for restoring stack state.
+func (r *RPN) SetCurrentStack(values []Value) {
+	r.currentStack = NewStack()
+	for _, v := range values {
+		r.currentStack.Push(v)
+	}
+}