feat(monitor/collectors/coinbase): per-product sequence-gap detector

PROJECTS/advanced/monitor-the-situation-dashboard/backend/internal/collectors/coinbase/sequencer.go

@@ -0,0 +1,32 @@
+// ©AngelaMos | 2026
+// sequencer.go
+
+package coinbase
+
+import "sync"
+
+type Sequencer struct {
+	mu   sync.Mutex
+	last map[string]int64
+}
+
+func NewSequencer() *Sequencer {
+	return &Sequencer{last: make(map[string]int64)}
+}
+
+func (s *Sequencer) Observe(productID string, seq int64) bool {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	prev, ok := s.last[productID]
+	s.last[productID] = seq
+	if !ok {
+		return false
+	}
+	return seq != prev+1
+}
+
+func (s *Sequencer) Reset() {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.last = make(map[string]int64)
+}

PROJECTS/advanced/monitor-the-situation-dashboard/backend/internal/collectors/coinbase/sequencer_test.go

@@ -0,0 +1,63 @@
+// ©AngelaMos | 2026
+// sequencer_test.go
+
+package coinbase_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/carterperez-dev/monitor-the-situation/backend/internal/collectors/coinbase"
+)
+
+func TestSequencer_FirstTickIsAlwaysInOrder(t *testing.T) {
+	s := coinbase.NewSequencer()
+	gap := s.Observe("BTC-USD", 100)
+	require.False(t, gap, "first sequence number must not be a gap")
+}
+
+func TestSequencer_ConsecutiveSequencesAreInOrder(t *testing.T) {
+	s := coinbase.NewSequencer()
+	for i := int64(50); i < 60; i++ {
+		gap := s.Observe("BTC-USD", i)
+		require.False(t, gap, "i=%d", i)
+	}
+}
+
+func TestSequencer_GapTriggersReportSignal(t *testing.T) {
+	s := coinbase.NewSequencer()
+	require.False(t, s.Observe("BTC-USD", 50))
+	require.False(t, s.Observe("BTC-USD", 51))
+	require.True(t, s.Observe("BTC-USD", 60), "skipping 52-59 must report gap")
+}
+
+func TestSequencer_GapsArePerProduct(t *testing.T) {
+	s := coinbase.NewSequencer()
+	require.False(t, s.Observe("BTC-USD", 100))
+	require.False(t, s.Observe("ETH-USD", 200))
+	require.False(t, s.Observe("BTC-USD", 101))
+	require.True(t, s.Observe("ETH-USD", 250))
+	require.False(t, s.Observe("BTC-USD", 102))
+}
+
+func TestSequencer_ResetClearsAllProducts(t *testing.T) {
+	s := coinbase.NewSequencer()
+	s.Observe("BTC-USD", 100)
+	s.Observe("ETH-USD", 200)
+	s.Reset()
+	require.False(t, s.Observe("BTC-USD", 9999))
+	require.False(t, s.Observe("ETH-USD", 8888))
+}
+
+func TestSequencer_DuplicateSequenceTreatedAsGap(t *testing.T) {
+	s := coinbase.NewSequencer()
+	require.False(t, s.Observe("BTC-USD", 100))
+	require.True(t, s.Observe("BTC-USD", 100), "replaying the same seq is a gap")
+}
+
+func TestSequencer_BackwardSequenceTreatedAsGap(t *testing.T) {
+	s := coinbase.NewSequencer()
+	require.False(t, s.Observe("BTC-USD", 100))
+	require.True(t, s.Observe("BTC-USD", 90))
+}