Problem: magic numbers in mechanism_base.cpp

Solution: extract constants

src/mechanism_base.cpp

@@ -55,12 +55,24 @@ int zmq::mechanism_base_t::check_basic_command_structure (msg_t *msg_)
 void zmq::mechanism_base_t::handle_error_reason (const char *error_reason_,
                                                  size_t error_reason_len_)
 {
-    if (error_reason_len_ == 3 && error_reason_[1] == '0'
-        && error_reason_[2] == '0' && error_reason_[0] >= '3'
-        && error_reason_[0] <= '5') {
-        // it is a ZAP status code, so emit an authentication failure event
+    const size_t status_code_len = 3;
+    const char zero_digit = '0';
+    const size_t significant_digit_index = 0;
+    const size_t first_zero_digit_index = 1;
+    const size_t second_zero_digit_index = 2;
+    const int factor = 100;
+    if (error_reason_len_ == status_code_len
+        && error_reason_[first_zero_digit_index] == zero_digit
+        && error_reason_[second_zero_digit_index] == zero_digit
+        && error_reason_[significant_digit_index] >= '3'
+        && error_reason_[significant_digit_index] <= '5') {
+        // it is a ZAP error status code (300, 400 or 500), so emit an authentication failure event
         session->get_socket ()->event_handshake_failed_auth (
-          session->get_endpoint (), (error_reason_[0] - '0') * 100);
+          session->get_endpoint (),
+          (error_reason_[significant_digit_index] - zero_digit) * factor);
+    } else {
+        // this is a violation of the ZAP protocol
+        // TODO zmq_assert in this case?
     }
 }