#include <stdlib.h>
#include <string.h>

static const char base64_chars[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

int base64_decode(const char *b64message, unsigned char **buffer, size_t *length) {
  size_t b64len = strlen(b64message);
  size_t declen = (b64len * 3) / 4; // Maximum possible decoded length
  *buffer = (unsigned char *)malloc(declen);
  *length = 0;

  int padding = 0;
  if (b64message[b64len - 1] == '=') padding++;
  if (b64message[b64len - 2] == '=') padding++;

  int i, j;
  unsigned char sextet_a, sextet_b, sextet_c, sextet_d;
  unsigned char triple[3];

  for (i = 0, j = 0; i < b64len; i += 4) {
    sextet_a = strchr(base64_chars, b64message[i]) - base64_chars;
    sextet_b = strchr(base64_chars, b64message[i + 1]) - base64_chars;
    sextet_c = strchr(base64_chars, b64message[i + 2]) - base64_chars;
    sextet_d = strchr(base64_chars, b64message[i + 3]) - base64_chars;

    triple[0] = (sextet_a << 2) | (sextet_b >> 4);
    triple[1] = (sextet_b << 4) | (sextet_c >> 2);
    triple[2] = (sextet_c << 6) | sextet_d;

    if (j < declen) (*buffer)[j++] = triple[0];
    if (j < declen) (*buffer)[j++] = triple[1];
    if (j < declen) (*buffer)[j++] = triple[2];
  }

  *length = j - padding;
  return 1;
}

int main() {
  const char *b64message = "SGVsbG8gV29ybGQh";
  unsigned char *buffer = NULL;
  size_t length = 0;

  if (base64_decode(b64message, &buffer, &length)) {
    printf("Decoded message: %.*s\n", (int)length, buffer);
    free(buffer);
    return 0;
  } else {
    printf("Failed to decode Base64 message\n");
    return 1;
  }
}