Ok, LGTM.

test_lastseen.cpp

@@ -4,30 +4,62 @@
 //
 // googletest/googletest/docs/primer.md
 
-#include "gtest/gtest.h"
 #include "lastseen.h"
+#include "gtest/gtest.h"
 
 namespace {
 
+TEST(LastSeenTest, negative) {
+  LastSeen zero(2);
+
+  // buffer: -1, -1
+  EXPECT_TRUE(zero.seen_before(-1));
+  EXPECT_TRUE(zero.seen_before(-1));
+}
+
 TEST(LastSeenTest, basic) {
-    LastSeen one(1);
-
-    EXPECT_FALSE(one.seen_before(0));
-    EXPECT_TRUE(one.seen_before(0));
-    EXPECT_TRUE(one.seen_before(0));
-    EXPECT_TRUE(one.seen_before(0));
-    EXPECT_FALSE(one.seen_before(1));
-    EXPECT_TRUE(one.seen_before(1));
+  LastSeen one(1);
+
+  EXPECT_FALSE(one.seen_before(0));
+  EXPECT_TRUE(one.seen_before(0));
+  EXPECT_TRUE(one.seen_before(0));
+  EXPECT_FALSE(one.seen_before(1));
+  EXPECT_TRUE(one.seen_before(1));
+  EXPECT_FALSE(one.seen_before(0));
 }
 
 TEST(LastSeenTest, Deep2) {
-    LastSeen two(2);
+  LastSeen two(2);
+
+  EXPECT_FALSE(two.seen_before(0));
+  EXPECT_FALSE(two.seen_before(1));
+  // buffer: 0, 1
+  EXPECT_TRUE(two.seen_before(0));
+  EXPECT_TRUE(two.seen_before(1));
+  EXPECT_TRUE(two.seen_before(0));
+  EXPECT_TRUE(two.seen_before(1));
+
+  EXPECT_FALSE(two.seen_before(2));
+  // buffer: 1, 2
+  EXPECT_TRUE(two.seen_before(2));
+  EXPECT_TRUE(two.seen_before(1));
+  EXPECT_TRUE(two.seen_before(2));
+  EXPECT_TRUE(two.seen_before(1));
 
-    EXPECT_FALSE(two.seen_before(0));
-    EXPECT_FALSE(two.seen_before(1));
-    EXPECT_TRUE(two.seen_before(0));
-    EXPECT_TRUE(two.seen_before(1));
+  EXPECT_FALSE(two.seen_before(0));
 }
 
-}  // namespace
+TEST(LastSeenTest, More) {
+  int x;
+
+  LastSeen seven(7);
+  for (x = 0; x < 7; x++) {
+    EXPECT_FALSE(seven.seen_before(x));
+  }
+  for (x = 0; x < 7; x++) {
+    EXPECT_TRUE(seven.seen_before(x));
+  }
+  EXPECT_FALSE(seven.seen_before(8));
+}
 
+} // namespace