Tidy random_alnum_string.

Signed-off-by: Pete Stevenson <jps@pixielabs.ai>

Author: Pete Stevenson

src/cc/api/BPF.cc

@@ -856,14 +856,14 @@ bool BPF::add_module(std::string module)
 
 namespace {
 
-std::string random_alnum_string(int len) {
-  static constexpr char kDict[] = "0123456789abcdefghijklmnopqrstuvwxyz";
+std::string random_alnum_string(const int len) {
+  static constexpr std::string_view kDict = "0123456789abcdefghijklmnopqrstuvwxyz";
   static std::mt19937 gen;
-  static std::uniform_int_distribution<size_t> dist(0, sizeof(kDict)-2);
+  static std::uniform_int_distribution<size_t> dist(0, kDict.length() - 1);
   std::string res;
-  res.reserve(len);
+  res.resize(len);
   for (int i = 0; i < len; ++i) {
-    res.push_back(kDict[dist(gen)]);
+    res[i] = kDict[dist(gen)];
   }
   return res;
 }