fix(rpn): replace parseCategory iota loop with O(1) map lookup (task zj)

2 files changed, 14 insertions, 8 deletions

diff --git a/internal/rpn/metric_type.go b/internal/rpn/metric_type.go
index 9d92fb1..49e3236 100644
--- a/internal/rpn/metric_type.go
+++ b/internal/rpn/metric_type.go
@@ -49,6 +49,15 @@ var categoryNames = []string{
 	"Custom",
 }
 
+// categoryByName provides O(1) lookup from category name to Category value.
+var categoryByName = func() map[string]Category {
+	m := make(map[string]Category, len(categoryNames))
+	for i, name := range categoryNames {
+		m[name] = Category(i)
+	}
+	return m
+}()
+
 // String returns the human-readable name of the category.
 func (c Category) String() string {
 	if c >= 0 && c < Category(len(categoryNames)) {
diff --git a/internal/rpn/operations_metric_cmd.go b/internal/rpn/operations_metric_cmd.go
index bde134b..255daed 100644
--- a/internal/rpn/operations_metric_cmd.go
+++ b/internal/rpn/operations_metric_cmd.go
@@ -81,16 +81,13 @@ func (o *Operations) MetricCompatible(stack *Stack) (string, error) {
 }
 
 // parseCategory converts a category name string to a Category constant.
-// Iterates over all valid Category values using a for loop bounded by _sentinel,
-// so adding a new Category constant (between Universal and _sentinel) automatically
-// makes it available here without modifying this function.
+// Uses a pre-built map for O(1) lookup.
 func parseCategory(name string) (Category, bool) {
-	for cat := Category(0); cat < _sentinel; cat++ {
-		if cat.String() == name {
-			return cat, true
-		}
+	cat, ok := categoryByName[name]
+	if !ok {
+		return invalidCategory, false
 	}
-	return invalidCategory, false
+	return cat, true
 }
 
 // CustomShow returns detailed info for custom metrics.