Add function to decode base64

tests/main.c

@@ -37,6 +37,26 @@ static MunitResult test_hex_to_base64(const MunitParameter params[],
   return MUNIT_OK;
 }
 
+static MunitResult test_base64_decode(const MunitParameter params[],
+                                      void *data) {
+  (void)params;
+  (void)data;
+
+  const char *input = "TWFu";
+  unsigned char buf[512] = {0};
+  size_t len = base64_to_bytes(buf, input);
+  assert_string_equal((char *)buf, "Man");
+  assert_size(len, ==, 3);
+
+  const char *input2 = "WUVMTE9XIFNVQk1BUklORQ==";
+  memset(buf, 0, 512);
+  len = base64_to_bytes(buf, input2);
+  assert_string_equal((char *)buf, "YELLOW SUBMARINE");
+  assert_size(len, ==, 18); // the 2 = create two bytes of padding at the end
+
+  return MUNIT_OK;
+}
+
 static MunitResult test_frequency_score(const MunitParameter params[],
                                         void *data) {
   (void)params;
@@ -73,6 +93,8 @@ static MunitTest test_suite_tests[] = {
      NULL},
     {"/hex_to_base64", test_hex_to_base64, NULL, NULL, MUNIT_TEST_OPTION_NONE,
      NULL},
+    {"/base64_decode", test_base64_decode, NULL, NULL, MUNIT_TEST_OPTION_NONE,
+     NULL},
     {"/frequency_score", test_frequency_score, NULL, NULL,
      MUNIT_TEST_OPTION_NONE, NULL},
     {"/hamming", test_hamming, NULL, NULL, MUNIT_TEST_OPTION_NONE, NULL},

utils.c

@@ -36,10 +36,10 @@ size_t hex_to_bytes(unsigned char *out, const char hex[static 1]) {
   return strlen(hex) / 2 + strlen(hex) % 2;
 }
 
-char *bytes_to_base64(char *out, const unsigned char in[static 1], size_t len) {
 static const char base64_table[65] =
     "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
 
+char *bytes_to_base64(char *out, const unsigned char in[static 1], size_t len) {
   for (size_t i = 0; i < len - 2; i += 3) {
     size_t j = i / 3 * 4;
     out[j] = base64_table[in[i] >> 2];
@@ -50,6 +50,42 @@ char *bytes_to_base64(char *out, const unsigned char in[static 1], size_t len) {
   return out;
 }
 
+size_t base64_to_bytes(unsigned char *out, const char *in) {
+  char decoding_table[256] = {0};
+  for (size_t i = 0; i < 64; ++i) {
+    decoding_table[(unsigned char)base64_table[i]] = i;
+  }
+
+  size_t in_length = strlen(in);
+  if (in_length % 4 != 0) {
+    return 0;
+  }
+
+  size_t i = 0;
+  for (size_t j = 0; j < in_length - 3;) {
+    unsigned char sextet1 =
+        in[j] == '=' ? 0 : decoding_table[(unsigned char)in[j]];
+    unsigned char sextet2 =
+        in[j + 1] == '=' ? 0 : decoding_table[(unsigned char)in[j + 1]];
+    unsigned char sextet3 =
+        in[j + 2] == '=' ? 0 : decoding_table[(unsigned char)in[j + 2]];
+    unsigned char sextet4 =
+        in[j + 3] == '=' ? 0 : decoding_table[(unsigned char)in[j + 3]];
+
+    out[i] = sextet1 << 2;
+    out[i] += sextet2 >> 4;
+    out[i + 1] = sextet2 << 4;
+    out[i + 1] += sextet3 >> 2;
+    out[i + 2] = sextet3 << 6;
+    out[i + 2] += sextet4;
+
+    j += 4;
+    i += 3;
+  }
+
+  return i;
+}
+
 double frequency_score(const char *buf, size_t len) {
   static const double english_freqs[27] = {
       0.08167, 0.01492, 0.02782, 0.04253, 0.12702, 0.02228, 0.02015, // A-G

utils.h

@@ -6,6 +6,7 @@
 unsigned char hex_to_byte(const char c);
 size_t hex_to_bytes(unsigned char *out, const char hex[static 1]);
 char *bytes_to_base64(char *out, const unsigned char in[static 1], size_t len);
+size_t base64_to_bytes(unsigned char *out, const char *in);
 double frequency_score(const char *buf, size_t len);
 unsigned int hamming(const char *s1, const char *s2);