add get_line(doc, line_num) utility

As discussed at issue #254

Author: N-Dekker

include/toml++/impl/source_region.hpp

@@ -217,7 +217,74 @@ TOML_NAMESPACE_START
 			return lhs;
 		}
 	};
+
+
+
+	/// \brief	Returns the line at the specified line number, from the specified document.
+	///
+	/// \detail \cpp
+	/// auto doc = "alpha = 1\nbeta = 2"sv;
+	/// auto second_line = toml::get_line(doc, 2);
+	///
+	/// std::cout << "The second line says \"" << second_line << "\"\n";
+	/// \ecpp
+	///
+	/// \out
+	/// The second line says "beta = 2"
+	/// \eout
+	///
+	/// \param 	doc	The document.
+	/// \param 	line_num	The line number (1-based).
+	///
+	/// \returns	The specified line, excluding any possible trailing carriage return or line feed character.
+	/// \remarks Returns an empty string_view when there is no line at the specified line number, in the specified document.
+	inline std::string_view get_line(std::string_view doc, source_index line_num)
+	{
+		if (line_num == 0)
+		{
+			// Invalid line number. Should be greater than zero.
+			return {};
+		}
+
+		// The position of the first character of the specified line.
+		const auto begin_of_line = [doc, line_num]() -> std::size_t
+		{
+			if (line_num == 1)
+			{
+				return 0;
+			}
+
+			const auto num_chars = doc.size();
+			std::size_t current_line_num{ 1 };
+
+			for (std::size_t i{}; i < num_chars; ++i)
+			{
+				if (doc[i] == '\n')
+				{
+					++current_line_num;
+
+					if (current_line_num == line_num)
+					{
+						return i + 1;
+					}
+				}
+			}
+			return std::string_view::npos;
+		}();
+
+		if (begin_of_line >= doc.size())
+		{
+			return {};
+		}
+
+		const auto end_of_line = doc.find_first_of("\r\n", begin_of_line);
+
+		return doc.substr(begin_of_line,
+						  (end_of_line == std::string_view::npos) ? std::string_view::npos
+																  : (end_of_line - begin_of_line));
+	}
 }
+
 TOML_NAMESPACE_END;
 
 #include "header_end.hpp"

tests/user_feedback.cpp

@@ -451,4 +451,25 @@ b = []
 		oss << toml::source_region{ { 1, 2 }, { 3, 4 }, nullptr };
 		CHECK(oss.str() == "line 1, column 2 to line 3, column 4");
 	}
+
+	SECTION("tomlplusplus/issues/254") // https://github.com/marzer/tomlplusplus/issues/254
+	{
+		for (const toml::source_index line_num: { 0, 1, 2 })
+		{
+			CHECK(toml::get_line(""sv, line_num) == std::string_view{});
+		}
+
+		for (const auto input : {
+				 "alpha = 1\nbeta = 2\n # last line # "sv,
+				 "alpha = 1\nbeta = 2\n # last line # \n"sv,
+				 "alpha = 1\r\nbeta = 2\r\n # last line # \r\n"sv,
+			 })
+		{
+			CHECK(toml::get_line(input, 0) == std::string_view{});
+			CHECK(toml::get_line(input, 1) == "alpha = 1"sv);
+			CHECK(toml::get_line(input, 2) == "beta = 2"sv);
+			CHECK(toml::get_line(input, 3) == " # last line # "sv);
+			CHECK(toml::get_line(input, 4) == std::string_view{});
+		}
+	}
 }

toml.hpp

@@ -2634,7 +2634,54 @@ TOML_NAMESPACE_START
 			return lhs;
 		}
 	};
+
+	inline std::string_view get_line(std::string_view doc, source_index line_num)
+	{
+		if (line_num == 0)
+		{
+			// Invalid line number. Should be greater than zero.
+			return {};
+		}
+
+		// The position of the first character of the specified line.
+		const auto begin_of_line = [doc, line_num]() -> std::size_t
+		{
+			if (line_num == 1)
+			{
+				return 0;
+			}
+
+			const auto num_chars = doc.size();
+			std::size_t current_line_num{ 1 };
+
+			for (std::size_t i{}; i < num_chars; ++i)
+			{
+				if (doc[i] == '\n')
+				{
+					++current_line_num;
+
+					if (current_line_num == line_num)
+					{
+						return i + 1;
+					}
+				}
+			}
+			return std::string_view::npos;
+		}();
+
+		if (begin_of_line >= doc.size())
+		{
+			return {};
+		}
+
+		const auto end_of_line = doc.find_first_of("\r\n", begin_of_line);
+
+		return doc.substr(begin_of_line,
+						  (end_of_line == std::string_view::npos) ? std::string_view::npos
+																  : (end_of_line - begin_of_line));
+	}
 }
+
 TOML_NAMESPACE_END;
 
 #ifdef _MSC_VER