chanfitness: cool down flap count for rate limiting

Since we store all-time flap count for a peer, we add a cooldown factor
which will discount poor flap counts in the past. This is only applied
to peers that have not flapped for at least a cooldown period, so that
we do not downgrade our rate limiting for badly behaved peers.

chanfitness/rate_limit_test.go

@@ -49,3 +49,57 @@ func TestGetRateLimit(t *testing.T) {
 		})
 	}
 }
+
+// TestCooldownFlapCount tests cooldown of all time flap counts.
+func TestCooldownFlapCount(t *testing.T) {
+	tests := []struct {
+		name      string
+		flapCount int
+		lastFlap  time.Time
+		expected  int
+	}{
+		{
+			name:      "just flapped, do not cooldown",
+			flapCount: 1,
+			lastFlap:  testNow,
+			expected:  1,
+		},
+		{
+			name:      "period not elapsed, do not cooldown",
+			flapCount: 1,
+			lastFlap:  testNow.Add(flapCountCooldownPeriod / 2 * -1),
+			expected:  1,
+		},
+		{
+			name:      "rounded to 0",
+			flapCount: 1,
+			lastFlap:  testNow.Add(flapCountCooldownPeriod * -1),
+			expected:  0,
+		},
+		{
+			name:      "decreased to integer value",
+			flapCount: 10,
+			lastFlap:  testNow.Add(flapCountCooldownPeriod * -1),
+			expected:  9,
+		},
+		{
+			name:      "multiple cooldown periods",
+			flapCount: 10,
+			lastFlap:  testNow.Add(flapCountCooldownPeriod * -3),
+			expected:  8,
+		},
+	}
+
+	for _, test := range tests {
+		test := test
+
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+
+			flapCount := cooldownFlapCount(
+				testNow, test.flapCount, test.lastFlap,
+			)
+			require.Equal(t, test.expected, flapCount)
+		})
+	}
+}