fix(rpn): introduce MetricReader interface for testability (task 7k)

4 files changed, 14 insertions, 3 deletions

diff --git a/internal/rpn/metric_parse.go b/internal/rpn/metric_parse.go
index 28c08de..e7a4611 100644
--- a/internal/rpn/metric_parse.go
+++ b/internal/rpn/metric_parse.go
@@ -10,7 +10,7 @@ import "strconv"
 // The metric suffix is looked up in the given registry (exact, alias, then case-insensitive).
 // Returns (num, metric, true) if successful, or (0, nil, false) if the token
 // does not contain a number+metric combination.
-func parseNumberWithMetric(token string, reg *MetricRegistry) (float64, *Metric, bool) {
+func parseNumberWithMetric(token string, reg MetricReader) (float64, *Metric, bool) {
 	if len(token) == 0 {
 		return 0, nil, false
 	}
diff --git a/internal/rpn/metric_registry.go b/internal/rpn/metric_registry.go
index 2c89b11..a221e56 100644
--- a/internal/rpn/metric_registry.go
+++ b/internal/rpn/metric_registry.go
@@ -17,6 +17,17 @@ type MetricRegistry struct {
 	exactMatchNames map[string]bool   // names that must match exactly (no case-insensitive fallback)
 }
 
+// MetricReader defines the read-only operations on a metric registry.
+// This interface allows tests to mock the registry without depending on the
+// concrete *MetricRegistry type.
+type MetricReader interface {
+	Find(name string) (*Metric, bool)
+	FindCaseInsensitive(name string) (*Metric, bool)
+	FindWithAliases(name string) (*Metric, bool)
+	List() []*Metric
+	ListByCategory(cat Category) []*Metric
+}
+
 // Global registry instance.
 var defaultRegistry *MetricRegistry
 var registryOnce sync.Once
diff --git a/internal/rpn/operations.go b/internal/rpn/operations.go
index 832ac77..f79457c 100644
--- a/internal/rpn/operations.go
+++ b/internal/rpn/operations.go
@@ -90,6 +90,6 @@ func (o *Operations) SetPrefixMode(mode PrefixMode) {
 }
 
 // MetricRegistry returns the metric registry used by this Operations instance.
-func (o *Operations) MetricRegistry() *MetricRegistry {
+func (o *Operations) MetricRegistry() MetricReader {
 	return o.metricRegistry
 }
diff --git a/internal/rpn/operations_interfaces.go b/internal/rpn/operations_interfaces.go
index 784ac76..8ad80ed 100644
--- a/internal/rpn/operations_interfaces.go
+++ b/internal/rpn/operations_interfaces.go
@@ -87,7 +87,7 @@ type ModeController interface {
 
 // MetricCommander defines the interface for metric query commands.
 type MetricCommander interface {
-	MetricRegistry() *MetricRegistry
+	MetricRegistry() MetricReader
 	MetricShow(stack *Stack) (string, error)
 	MetricList(stack *Stack) (string, error)
 	MetricCategory(stack *Stack, categoryName string) (string, error)