From c81c8bfa3cdcf805bce8a7fa5f3b24633a617edc Mon Sep 17 00:00:00 2001
From: Evan Burkey <evan@burkey.co>
Date: Sat, 4 May 2024 11:35:54 -0700
Subject: [PATCH] hamming distance

---
 docs/crypto.md     |  8 ++++++++
 include/lfcrypto.h |  2 ++
 src/crypto.c       | 22 ++++++++++++++++++++++
 tests/tests.c      |  3 +++
 4 files changed, 35 insertions(+)

diff --git a/docs/crypto.md b/docs/crypto.md
index 2d81ab2..d993427 100644
--- a/docs/crypto.md
+++ b/docs/crypto.md
@@ -89,3 +89,11 @@ responsible for freeing the returned array
 ```c
 const unsigned char *repeating_key_xor_s(const char* s, const char* key);
 ```
+
+### hamming_distance
+
+Calculates the Hamming Distance (the number of differing bits) between two strings
+
+```c
+unsigned int hamming_distance(const char *a, const char *b);
+```
diff --git a/include/lfcrypto.h b/include/lfcrypto.h
index 624e776..7492841 100644
--- a/include/lfcrypto.h
+++ b/include/lfcrypto.h
@@ -13,4 +13,6 @@ const char *hex_to_str(const unsigned char *hex, size_t sz);
 const unsigned char* repeating_key_xor(const unsigned char* s, size_t s_sz, const unsigned char* key, size_t k_sz);
 const unsigned char *repeating_key_xor_s(const char* s, const char* key);
 
+unsigned int hamming_distance(const char *a, const char *b);
+
 #endif // LIBFLINT_CRYPTO_H
diff --git a/src/crypto.c b/src/crypto.c
index 6763656..ac04bdf 100644
--- a/src/crypto.c
+++ b/src/crypto.c
@@ -271,3 +271,25 @@ const unsigned char* repeating_key_xor(const unsigned char* s, size_t s_sz, cons
 const unsigned char *repeating_key_xor_s(const char* s, const char* key) {
     return repeating_key_xor((unsigned char*)s, strlen(s), (unsigned char*)key, strlen(key));
 }
+
+unsigned int hamming_distance(const char *a, const char *b) {
+    size_t sz = strlen(a);
+    if (sz != strlen(b)) {
+        return -1;
+    }
+
+    unsigned int hamming = 0;
+    for (size_t i = 0; i < sz; ++i) {
+        if (a[i] == b[i]) {
+            continue;
+        }
+        unsigned char c = a[i] ^ b[i];
+        unsigned int count = 0;
+        for (; c; count++) {
+            c &= c - 1;
+        }
+        hamming += count;
+    }
+
+    return hamming;
+}
diff --git a/tests/tests.c b/tests/tests.c
index c18badf..e8d4ddf 100644
--- a/tests/tests.c
+++ b/tests/tests.c
@@ -374,6 +374,9 @@ void test_crypto() {
     free(enc);
     free(s);
 
+    unsigned int hamming = hamming_distance("this is a test", "wokka wokka!!!");
+    assert(hamming == 37);
+
     printf("Passes all crypto tests\n");
 }