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/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/blowfish.h>
+
+static int hamming(uint64_t a, uint64_t b) {
+  return __builtin_popcountll(a ^ b);
+}
+
+// Check that flipping one bit in plaintext or key
+// causes large, unpredictable changes in ciphertext.
+TEST("Blowfish Avalanche Effect") {
+  Blowfish bf("key");
+
+  uint32_t L = 0x11223344, R = 0x55667788;
+  uint32_t Lc = L, Rc = R;
+
+  bf.encrypt(Lc, Rc);
+  uint64_t C1 = (uint64_t(Lc) << 32) | Rc;
+
+  for (int bit = 0; bit < 32; ++bit) {
+    uint32_t Lflip = L ^ (1u << bit);
+    uint32_t Rflip = R;
+
+    uint32_t L2 = Lflip, R2 = Rflip;
+    bf.encrypt(L2, R2);
+
+    uint64_t C2 = (uint64_t(L2) << 32) | R2;
+
+    int hd = hamming(C1, C2);
+    EXPECT_TRUE(hd > 20); // Strong avalanche threshold
+  }
+}