From a8a4f82675ed2df538e9fbf95504d5674a732c52 Mon Sep 17 00:00:00 2001
From: Paul Buetow <paul@buetow.org>
Date: Mon, 23 Feb 2026 23:19:04 +0200
Subject: task 303: add per-process accumulator with pid-reuse guard

---
 internal/statsengine/process.go | 89 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 89 insertions(+)
 create mode 100644 internal/statsengine/process.go

(limited to 'internal/statsengine/process.go')

diff --git a/internal/statsengine/process.go b/internal/statsengine/process.go
new file mode 100644
index 0000000..296312b
--- /dev/null
+++ b/internal/statsengine/process.go
@@ -0,0 +1,89 @@
+package statsengine
+
+import (
+	"ior/internal/event"
+	"sort"
+	"time"
+)
+
+type processAccumulator struct {
+	byPID map[uint32]*processStats
+}
+
+type processStats struct {
+	pid          uint32
+	comm         string
+	count        uint64
+	totalBytes   uint64
+	totalLatency uint64
+}
+
+func newProcessAccumulator() *processAccumulator {
+	return &processAccumulator{byPID: make(map[uint32]*processStats)}
+}
+
+func (a *processAccumulator) Add(pair *event.Pair) {
+	if a == nil || pair == nil || pair.EnterEv == nil {
+		return
+	}
+
+	pid := pair.EnterEv.GetPid()
+	stats := a.byPID[pid]
+	if stats == nil {
+		stats = &processStats{pid: pid}
+		a.byPID[pid] = stats
+	}
+	if pair.Comm != "" && stats.comm != "" && stats.comm != pair.Comm {
+		// Best-effort PID reuse handling: when command name changes for an
+		// existing PID, treat it as a new process lifetime and reset counters.
+		stats = &processStats{pid: pid}
+		a.byPID[pid] = stats
+	}
+
+	stats.count++
+	stats.totalBytes += pair.Bytes
+	stats.totalLatency += pair.Duration
+	if pair.Comm != "" {
+		stats.comm = pair.Comm
+	}
+}
+
+func (a *processAccumulator) Snapshot(elapsed time.Duration) []ProcessSnapshot {
+	if a == nil {
+		return nil
+	}
+
+	rateDiv := elapsed.Seconds()
+	result := make([]ProcessSnapshot, 0, len(a.byPID))
+	for _, stats := range a.byPID {
+		result = append(result, stats.toSnapshot(rateDiv))
+	}
+
+	sort.Slice(result, func(i, j int) bool {
+		if result[i].Syscalls != result[j].Syscalls {
+			return result[i].Syscalls > result[j].Syscalls
+		}
+		if result[i].Bytes != result[j].Bytes {
+			return result[i].Bytes > result[j].Bytes
+		}
+		return result[i].PID < result[j].PID
+	})
+
+	return result
+}
+
+func (s *processStats) toSnapshot(rateDiv float64) ProcessSnapshot {
+	avg := 0.0
+	if s.count > 0 {
+		avg = float64(s.totalLatency) / float64(s.count)
+	}
+
+	return ProcessSnapshot{
+		PID:          s.pid,
+		Comm:         s.comm,
+		Syscalls:     s.count,
+		RatePerSec:   safeRate(s.count, rateDiv),
+		Bytes:        s.totalBytes,
+		AvgLatencyNs: avg,
+	}
+}
-- 
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