feat(metricstore): maintain running buffer stats on write

pkg/metricstore/stats_test.go

@@ -8,6 +8,8 @@ package metricstore
 import (
 	"math"
 	"testing"
+
+	"github.com/ClusterCockpit/cc-lib/v2/schema"
 )
 
 func TestNewBufferStatsInitialized(t *testing.T) {
@@ -28,3 +30,57 @@ func TestNewBufferStatsInitialized(t *testing.T) {
 		t.Error("statsValid = false, want true for a fresh empty buffer")
 	}
 }
+
+func TestWriteUpdatesRunningStats(t *testing.T) {
+	b := newBuffer(100, 10)
+	b.write(100, schema.Float(3.0))
+	b.write(110, schema.Float(1.0))
+	b.write(120, schema.Float(5.0))
+
+	if b.statSamples != 3 {
+		t.Errorf("statSamples = %d, want 3", b.statSamples)
+	}
+	if b.statSum != 9.0 {
+		t.Errorf("statSum = %v, want 9.0", b.statSum)
+	}
+	if b.statMin != 1.0 {
+		t.Errorf("statMin = %v, want 1.0", b.statMin)
+	}
+	if b.statMax != 5.0 {
+		t.Errorf("statMax = %v, want 5.0", b.statMax)
+	}
+	if !b.statsValid {
+		t.Error("statsValid = false, want true after appends only")
+	}
+}
+
+func TestWriteNaNExcludedFromStats(t *testing.T) {
+	b := newBuffer(100, 10)
+	b.write(100, schema.Float(2.0))
+	b.write(110, schema.NaN)  // explicit NaN
+	b.write(130, schema.Float(4.0)) // leaves a NaN-filled gap at t=120
+
+	if b.statSamples != 2 {
+		t.Errorf("statSamples = %d, want 2 (NaN and gap excluded)", b.statSamples)
+	}
+	if b.statSum != 6.0 {
+		t.Errorf("statSum = %v, want 6.0", b.statSum)
+	}
+	if b.statMin != 2.0 || b.statMax != 4.0 {
+		t.Errorf("statMin/statMax = %v/%v, want 2.0/4.0", b.statMin, b.statMax)
+	}
+}
+
+func TestWriteOverwriteInvalidatesStats(t *testing.T) {
+	b := newBuffer(100, 10)
+	b.write(100, schema.Float(1.0))
+	b.write(110, schema.Float(2.0))
+	if !b.statsValid {
+		t.Fatal("precondition: statsValid should be true after appends")
+	}
+
+	b.write(100, schema.Float(99.0)) // overwrite existing index
+	if b.statsValid {
+		t.Error("statsValid = true, want false after overwrite")
+	}
+}