feat: add disk I/O stats (read/write throughput bars, hotkey 5, auto-scale)

Add a new disk stats category that reads /proc/diskstats, shows read/write
throughput as colored bars (purple/darker purple like network RX/TX), with
optional utilization % overlay in extended mode. Hotkey 5 cycles: aggregate
(sum all whole-disk devices) → per-device → off. Auto-scale with decay and
optional fixed override via --diskmax / diskmax config key.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

2 files changed, 94 insertions, 3 deletions

diff --git a/internal/app/store.go b/internal/app/store.go
index ffe00c8..5864d8d 100644
--- a/internal/app/store.go
+++ b/internal/app/store.go
@@ -19,6 +19,7 @@ type hostData struct {
 	mem                  map[string]int64
 	net                  map[string]stats.NetStamp
 	cpu                  map[string]collector.CPULine
+	disk                 map[string]stats.DiskStamp
 }
 
 // Compile-time interface satisfaction checks.
@@ -62,6 +63,19 @@ func (s *Store) SetNet(host, iface string, net collector.NetLine, stamp float64)
 	d.net[iface] = stats.NetStamp{B: net.B, Tb: net.Tb, Stamp: stamp}
 }
 
+// SetDisk sets the disk stamp for the given host and device.
+func (s *Store) SetDisk(host, device string, disk collector.DiskLine, stamp float64) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	d := s.getOrCreate(host)
+	d.disk[device] = stats.DiskStamp{
+		SectorsRead:  disk.SectorsRead,
+		SectorsWrite: disk.SectorsWrite,
+		IoTicks:      disk.IoTicks,
+		Stamp:        stamp,
+	}
+}
+
 // Snapshot returns a copy of current stats for all hosts for the display.
 func (s *Store) Snapshot() map[string]*stats.HostStats {
 	s.mu.RLock()
@@ -80,6 +94,10 @@ func (s *Store) Snapshot() map[string]*stats.HostStats {
 		for k, v := range d.cpu {
 			cpu[k] = v
 		}
+		disk := make(map[string]stats.DiskStamp, len(d.disk))
+		for k, v := range d.disk {
+			disk[k] = v
+		}
 		out[h] = &stats.HostStats{
 			LoadAvg1:  d.load1,
 			LoadAvg5:  d.load5,
@@ -87,6 +105,7 @@ func (s *Store) Snapshot() map[string]*stats.HostStats {
 			Mem:       mem,
 			Net:       net,
 			CPU:       cpu,
+			Disk:      disk,
 		}
 	}
 	return out
@@ -95,9 +114,10 @@ func (s *Store) Snapshot() map[string]*stats.HostStats {
 func (s *Store) getOrCreate(host string) *hostData {
 	if s.hosts[host] == nil {
 		s.hosts[host] = &hostData{
-			mem: make(map[string]int64),
-			net: make(map[string]stats.NetStamp),
-			cpu: make(map[string]collector.CPULine),
+			mem:  make(map[string]int64),
+			net:  make(map[string]stats.NetStamp),
+			cpu:  make(map[string]collector.CPULine),
+			disk: make(map[string]stats.DiskStamp),
 		}
 	}
 	return s.hosts[host]
diff --git a/internal/app/store_test.go b/internal/app/store_test.go
new file mode 100644
index 0000000..037d7d4
--- /dev/null
+++ b/internal/app/store_test.go
@@ -0,0 +1,71 @@
+package app
+
+import (
+	"testing"
+
+	"codeberg.org/snonux/loadbars/internal/collector"
+	"codeberg.org/snonux/loadbars/internal/stats"
+)
+
+func TestSetDisk(t *testing.T) {
+	s := NewStore()
+	disk := collector.DiskLine{
+		Device:       "sda",
+		SectorsRead:  1000,
+		SectorsWrite: 2000,
+		IoTicks:      50,
+	}
+	s.SetDisk("host1", "sda", disk, 1.0)
+
+	snap := s.Snapshot()
+	h := snap["host1"]
+	if h == nil {
+		t.Fatal("expected host1 in snapshot")
+	}
+	ds, ok := h.Disk["sda"]
+	if !ok {
+		t.Fatal("expected sda in Disk map")
+	}
+	if ds.SectorsRead != 1000 || ds.SectorsWrite != 2000 || ds.IoTicks != 50 || ds.Stamp != 1.0 {
+		t.Errorf("disk stamp mismatch: got %+v", ds)
+	}
+}
+
+func TestSnapshotDiskDeepCopy(t *testing.T) {
+	s := NewStore()
+	disk := collector.DiskLine{
+		Device:       "sda",
+		SectorsRead:  100,
+		SectorsWrite: 200,
+	}
+	s.SetDisk("host1", "sda", disk, 1.0)
+
+	snap1 := s.Snapshot()
+
+	// Mutate the store after snapshot
+	disk2 := collector.DiskLine{
+		Device:       "sda",
+		SectorsRead:  999,
+		SectorsWrite: 888,
+	}
+	s.SetDisk("host1", "sda", disk2, 2.0)
+
+	// snap1 should be unchanged (deep copy)
+	ds := snap1["host1"].Disk["sda"]
+	if ds.SectorsRead != 100 || ds.SectorsWrite != 200 {
+		t.Errorf("snapshot was mutated: got sr=%d sw=%d, want sr=100 sw=200",
+			ds.SectorsRead, ds.SectorsWrite)
+	}
+
+	// Verify new snapshot has updated values
+	snap2 := s.Snapshot()
+	ds2 := snap2["host1"].Disk["sda"]
+	if ds2.SectorsRead != 999 || ds2.SectorsWrite != 888 {
+		t.Errorf("snap2: got sr=%d sw=%d, want sr=999 sw=888",
+			ds2.SectorsRead, ds2.SectorsWrite)
+	}
+}
+
+// Ensure Store still satisfies the collector.StatsStore interface (which now includes SetDisk).
+var _ collector.StatsStore = (*Store)(nil)
+var _ stats.Source = (*Store)(nil)