fix(shard): ensure correct evicted entries count (#30)

When evicting all entries (100% eviction), store the count before
clearing the map. Previously we were reporting the length of an empty
map, resulting in incorrect eviction metrics. The fix ensures accurate
reporting of mass evictions for monitoring cache behavior.

Added test coverage to verify both the evicted entries count and
forced eviction events are correct.

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

Author: thevilledev

cache_test.go

@@ -181,20 +181,40 @@ func TestForceEvictAllEntries(t *testing.T) {
 	ttl := time.Hour
 	evictionpercentage := 100
 	clock := sturdyc.NewTestClock(time.Now())
+	metricRecorder := newTestMetricsRecorder(numShards)
 	c := sturdyc.New[string](capacity, numShards, ttl, evictionpercentage,
 		sturdyc.WithClock(clock),
+		sturdyc.WithMetrics(metricRecorder),
 	)
 
-	// Now we're going to write 101 records to the cache which should
-	// exceed its capacity and trigger a forced eviction.
-	for i := 0; i < 101; i++ {
+	// Fill the cache to capacity
+	for i := 0; i < capacity; i++ {
 		c.Set(strconv.Itoa(i), strconv.Itoa(i))
 	}
 
+	// Record metrics before eviction
+	preEvictionCount := metricRecorder.evictedEntries
+
+	// Trigger eviction by adding one more entry
 	// When the eviction is triggered by the 100th write, we expect the cache to
 	// be emptied. Therefore, the 101th write should mean that the size is now 1.
+	c.Set("trigger", "value")
+
+	// When the eviction is triggered, we expect the cache to be emptied
+	// and only contain the trigger value
 	if c.Size() != 1 {
-		t.Errorf("expected cache size to be 0, got %d", c.Size())
+		t.Errorf("expected cache size to be 1, got %d", c.Size())
+	}
+
+	// Verify eviction metrics
+	metricRecorder.Lock()
+	defer metricRecorder.Unlock()
+	evictedEntries := metricRecorder.evictedEntries - preEvictionCount
+	if evictedEntries != capacity {
+		t.Errorf("got %d evicted entries, want %d", evictedEntries, capacity)
+	}
+	if metricRecorder.forcedEvictions != 1 {
+		t.Errorf("got %d forced eviction events, want 1", metricRecorder.forcedEvictions)
 	}
 }
 

shard.go

@@ -67,8 +67,9 @@ func (s *shard[T]) forceEvict() {
 
 	// Check if we should evict all entries.
 	if s.evictionPercentage == 100 {
+		evictedCount := len(s.entries)
 		s.entries = make(map[string]*entry[T])
-		s.reportEntriesEvicted(len(s.entries))
+		s.reportEntriesEvicted(evictedCount)
 		return
 	}