Implement truncated_string().

It will either return the full string if its length was
maxlen or less, or up to maxlen characters with
a "..." appended at the end. It will also replace any
\n or \r character with a literal "\n" or "\r"
respectively. Only line breaks are being replaced, and
this happens inside truncated_string() because truncating
only makes sense to me if you want to use it to
keep stuff short and in one single line.

src/tawashi/CMakeLists.txt

@@ -33,6 +33,7 @@ add_library(${PROJECT_NAME} STATIC
 	ip_utils.cpp
 	mime_split.cpp
 	storage.cpp
+	truncated_string.cpp
 )
 
 target_include_directories(${PROJECT_NAME}

src/tawashi/truncated_string.cpp

@@ -0,0 +1,48 @@
+/* Copyright 2017, Michele Santullo
+ * This file is part of "tawashi".
+ *
+ * "tawashi" is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * "tawashi" is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with "tawashi".  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "truncated_string.hpp"
+#include <cassert>
+#include <algorithm>
+#include <ciso646>
+
+namespace tawashi {
+	std::string truncated_string (const boost::string_view& parString, std::size_t parMaxLen) {
+		const std::size_t char_length = std::min(parMaxLen, parString.size());
+		std::string retval;
+		retval.reserve(char_length);
+		std::size_t doubled_up = 0;
+		for (auto c : parString) {
+			const std::size_t len = ('\r' == c or '\n' == c ? 2 : 1);
+			const char replaced_chara = ('\r' == c ? 'r' : ('\n' == c ? 'n' : c));
+			const char addition[2] = { '\\', replaced_chara };
+			if (retval.size() + len > parMaxLen)
+				break;
+
+			retval.append(addition + (2 - len), len);
+			doubled_up += (len - 1);
+		}
+		assert(
+			retval.size()  == std::min(parString.size() + doubled_up, parMaxLen) or
+			retval.size() + 1  == std::min(parString.size() + doubled_up, parMaxLen)
+		);
+		if (retval.size() - doubled_up < parString.size())
+			retval += "...";
+
+		return retval;
+	}
+} //namespace tawashi

src/tawashi/truncated_string.hpp

@@ -0,0 +1,26 @@
+/* Copyright 2017, Michele Santullo
+ * This file is part of "tawashi".
+ *
+ * "tawashi" is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * "tawashi" is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with "tawashi".  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include <string>
+#include <boost/utility/string_view.hpp>
+#include <cstddef>
+
+namespace tawashi {
+	std::string truncated_string (const boost::string_view& parString, std::size_t parMaxLen);
+} //namespace tawashi