feat: add more robust tests

- add test to cover corner cases and known failure points

Signed-off-by: Avinal Kumar <avinal.xlvii@gmail.com>

tests/bf2_test_avalanche.cpp

@@ -0,0 +1,34 @@
+// SPDX-FileCopyrightText: 2025 Avinal Kumar avinal.xlvii@gmail.com
+// SPDX-License-Identifier: MIT
+
+#include "test_framework.h"
+#include <blowfish/blowfish2.h>
+
+static int hamming128(uint64_t a1, uint64_t b1, uint64_t a2, uint64_t b2) {
+  return __builtin_popcountll(a1 ^ a2) + __builtin_popcountll(b1 ^ b2);
+}
+
+// Check that flipping one bit in plaintext or key
+// causes large, unpredictable changes in ciphertext.
+TEST("Blowfish2 Avalanche Effect") {
+  Blowfish2 bf("key-for-avalanche");
+
+  uint64_t L = 0x1122334455667788ULL;
+  uint64_t R = 0x99AABBCCDDEEFF00ULL;
+
+  uint64_t L0 = L, R0 = R;
+  bf.encrypt(L0, R0);
+
+  for (int bit = 0; bit < 64; ++bit) {
+    uint64_t Lf = L ^ (1ULL << bit);
+    uint64_t Rf = R;
+
+    uint64_t L1 = Lf, R1 = Rf;
+    bf.encrypt(L1, R1);
+
+    int hd = hamming128(L0, R0, L1, R1);
+
+    // Expect large hamming distance: ideally >40 for Blowfish2
+    EXPECT_TRUE(hd > 40);
+  }
+}