[c++/atbash-cypher] Create mentoring.md

tracks/cpp/exercises/atbash-cipher/mentoring.md

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+# Mentoring
+
+## Reasonable solutions
+```cpp
+#include <string>
+
+namespace atbash_cipher
+{
+
+std::string apply_cypher(std::string aStr);
+std::string apply_spaces(const std::string& aStr);
+
+std::string encode(const std::string& aStr)
+{
+	auto str = apply_cypher(aStr);
+	return apply_spaces(str);
+}
+
+std::string apply_spaces(const std::string& aStr)
+{
+	std::string out;
+	out.reserve(aStr.length() + aStr.length() / 5);
+	for(size_t i = 0; i < aStr.length(); ++i)
+	{
+		if(0 != i && 0 == i % 5)
+			out.push_back(' ');
+		out.push_back(aStr[i]);
+	}
+
+	return out;
+}
+
+std::string decode(const std::string& aStr)
+{
+	return apply_cypher(aStr);
+}
+
+std::string apply_cypher(std::string aStr)
+{
+	auto wr_it = std::begin(aStr);
+	for(const unsigned char letter : aStr)
+	           // ^ implicit cast bounded by cctype header functions ex. std::tolower and std::isdigit
+	{
+		if(std::isalpha(letter))
+		{
+			const auto a = static_cast<int>('a'); 
+			const auto z = static_cast<int>('z');
+			*wr_it = static_cast<char>(z - std::tolower(letter) + a);
+			++wr_it;
+		}
+		else if(std::isdigit(letter))
+		{
+			*wr_it = letter;
+			++wr_it;
+		}
+	}
+
+	aStr.erase(wr_it, aStr.end());
+
+	return aStr;
+}  // namespace atbash_cipher
+
+```
+## Common suggestions
+
+- Use `const std::string&` or `std::string_view` instead of `std::string` as a parameter, unless the string is re-used and returned.
+- If students use structures representing alphabet for linear find, explain what is ASCII table, char type
+
+## Talking points
+
+- Be aware of hidden copies being made by using certain types as function parameters. (`std::string` in this case)
+- When appending to a std::string, call std::string::reserve is a good optimization.
+- Be aware of undefined behaviour using <cctype> if the argument's value is neither representable as unsigned char nor equal to EOF