2015-04, add md5 helper function

src/2015/04.c

@@ -1,10 +1,51 @@
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 
 #include "input.h"
+#include "advent_utility.h"
+
+static int is_five(char *test) {
+  char *t = test;
+  for (int i = 0; i < 5; ++i, ++t) {
+    if (*t != '0') {
+      return 0;
+    }
+  }
+  return 1;
+}
+
+static int is_six(char *test) {
+  char *t = test;
+  for (int i = 0; i < 6; ++i, ++t) {
+    if (*t != '0') {
+      return 0;
+    }
+  }
+  return 1;
+}
 
 void advent2015day04(void) {
   char *input = get_input("input/2015/04");
-  printf("Solution for Day 04 of 2015 is not completed yet\n");
+  int c = 0, five = 0, six = 0;
+
+  while (!five || !six) {
+    char test[strlen(input) + 8];
+    sprintf(test, "%s%d", input, c);
+    char *md5 = md5_str(test);
+
+    if (!five && is_five(md5)) {
+      five = c;
+      printf("%d\n", five);
+    }
+    if (is_six(md5)) {
+      six = c;
+    }
+
+    free(md5);
+    ++c;
+  }
+
+  printf("%d\n", six);
   free(input);
 }