1 files changed, 82 insertions, 0 deletions
diff --git a/internal/server/handlers/lineprocessor_test.go b/internal/server/handlers/lineprocessor_test.go
new file mode 100644
index 0000000..0c8adf3
--- /dev/null
+++ b/internal/server/handlers/lineprocessor_test.go
@@ -0,0 +1,82 @@
+package handlers
+
+import (
+	"bytes"
+	"testing"
+)
+
+// TestGrepLineProcessor_StatsBytesWrittenMatchesOutput verifies that the
+// bytesWritten stat counts each line's formatted bytes exactly once even
+// though writeBuf accumulates many lines before flushing. Regression test:
+// the stat used to add the whole accumulated buffer length per line, so it
+// grew quadratically with the number of buffered lines.
+func TestGrepLineProcessor_StatsBytesWrittenMatchesOutput(t *testing.T) {
+	tests := []struct {
+		name       string
+		plain      bool
+		serverless bool
+	}{
+		{name: "plain", plain: true, serverless: false},
+		{name: "network formatted", plain: false, serverless: false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var buf bytes.Buffer
+			p := NewGrepLineProcessor(&buf, "testhost", tt.plain, tt.serverless)
+
+			// All lines stay buffered (well below the 64KB flush threshold)
+			// until Flush, so bytesWritten must equal the flushed output size.
+			const numLines = 50
+			for i := uint64(1); i <= numLines; i++ {
+				line := bytes.NewBufferString("grep match line")
+				if err := p.ProcessLine(line, i, "source.log"); err != nil {
+					t.Fatalf("ProcessLine failed on line %d: %v", i, err)
+				}
+			}
+			if err := p.Flush(); err != nil {
+				t.Fatalf("Flush failed: %v", err)
+			}
+
+			lines, bytesWritten := p.Stats()
+			if lines != numLines {
+				t.Errorf("Expected %d lines processed, got %d", numLines, lines)
+			}
+			if want := uint64(buf.Len()); bytesWritten != want {
+				t.Errorf("Expected bytesWritten %d (flushed output size), got %d", want, bytesWritten)
+			}
+		})
+	}
+}
+
+// TestGrepLineProcessor_StatsBytesWrittenAcrossFlushThreshold verifies the
+// bytesWritten stat stays exact when the buffer crosses the flush threshold
+// several times during processing.
+func TestGrepLineProcessor_StatsBytesWrittenAcrossFlushThreshold(t *testing.T) {
+	var buf bytes.Buffer
+	p := NewGrepLineProcessor(&buf, "testhost", true, false)
+	p.bufSize = 32 // Force several intermediate flushes.
+
+	const numLines = 20
+	content := "0123456789" // 11 bytes with the message delimiter.
+	for i := uint64(1); i <= numLines; i++ {
+		if err := p.ProcessLine(bytes.NewBufferString(content), i, "source.log"); err != nil {
+			t.Fatalf("ProcessLine failed on line %d: %v", i, err)
+		}
+	}
+	if err := p.Flush(); err != nil {
+		t.Fatalf("Flush failed: %v", err)
+	}
+
+	lines, bytesWritten := p.Stats()
+	if lines != numLines {
+		t.Errorf("Expected %d lines processed, got %d", numLines, lines)
+	}
+	want := uint64(numLines * (len(content) + 1))
+	if bytesWritten != want {
+		t.Errorf("Expected bytesWritten %d, got %d", want, bytesWritten)
+	}
+	if got := uint64(buf.Len()); got != want {
+		t.Errorf("Expected flushed output size %d, got %d", want, got)
+	}
+}