From 931a2a6e79ad72fc019ded31ef037b1df698d943 Mon Sep 17 00:00:00 2001
From: Aditya Ujeniya <adityauj@gmail.com>
Date: Mon, 20 Jul 2026 10:22:16 +0200
Subject: [PATCH] feat(metricstore): maintain running buffer stats on write

---
 pkg/metricstore/buffer.go     | 21 +++++++++++++
 pkg/metricstore/stats_test.go | 56 +++++++++++++++++++++++++++++++++++
 2 files changed, 77 insertions(+)

diff --git a/pkg/metricstore/buffer.go b/pkg/metricstore/buffer.go
index 52e3b287..69b401f3 100644
--- a/pkg/metricstore/buffer.go
+++ b/pkg/metricstore/buffer.go
@@ -207,6 +207,23 @@ func newBuffer(ts, freq int64) *buffer {
 	return b
 }
 
+// updateStats folds a single value into the buffer's running statistics.
+// NaN values are ignored so they never affect min/max/sum/samples.
+func (b *buffer) updateStats(value schema.Float) {
+	xf := float64(value)
+	if math.IsNaN(xf) {
+		return
+	}
+	b.statSum += xf
+	b.statSamples++
+	if xf < b.statMin {
+		b.statMin = xf
+	}
+	if xf > b.statMax {
+		b.statMax = xf
+	}
+}
+
 // write appends a timestamped value to the buffer chain.
 //
 // Returns the head buffer (which may be newly created if capacity was reached).
@@ -241,6 +258,9 @@ func (b *buffer) write(ts int64, value schema.Float) (*buffer, error) {
 	// Overwriting value or writing value from past
 	if idx < len(b.data) {
 		b.data[idx] = value
+		// The replaced value may have been the running min/max; the cached
+		// aggregate can no longer be trusted. Force a recompute on next query.
+		b.statsValid = false
 		return b, nil
 	}
 
@@ -250,6 +270,7 @@ func (b *buffer) write(ts int64, value schema.Float) (*buffer, error) {
 	}
 
 	b.data = append(b.data, value)
+	b.updateStats(value)
 	return b, nil
 }
 
diff --git a/pkg/metricstore/stats_test.go b/pkg/metricstore/stats_test.go
index f437f0bd..f19d6853 100644
--- a/pkg/metricstore/stats_test.go
+++ b/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")
+	}
+}