Add `XDG_DATA_DIRS` to MPQ search paths

Fixes #5304

Source/init.cpp

@@ -23,6 +23,7 @@
 #include "utils/language.h"
 #include "utils/log.hpp"
 #include "utils/paths.h"
+#include "utils/str_split.hpp"
 #include "utils/ui_fwd.h"
 #include "utils/utf8.hpp"
 
@@ -93,8 +94,21 @@ std::vector<std::string> GetMPQSearchPaths()
 		paths.pop_back();
 
 #if defined(__unix__) && !defined(__ANDROID__)
-	paths.emplace_back("/usr/share/diasurgical/devilutionx/");
-	paths.emplace_back("/usr/local/share/diasurgical/devilutionx/");
+	// `XDG_DATA_HOME` is usually the root path of `paths::PrefPath()`, so we only
+	// add `XDG_DATA_DIRS`.
+	const char *xdgDataDirs = std::getenv("XDG_DATA_DIRS");
+	if (xdgDataDirs != nullptr) {
+		for (const string_view path : SplitByChar(xdgDataDirs, ':')) {
+			std::string fullPath(path);
+			if (!path.empty() && path.back() != '/')
+				fullPath += '/';
+			fullPath.append("diasurgical/devilutionx/");
+			paths.push_back(std::move(fullPath));
+		}
+	} else {
+		paths.emplace_back("/usr/local/share/diasurgical/devilutionx/");
+		paths.emplace_back("/usr/share/diasurgical/devilutionx/");
+	}
 #elif defined(NXDK)
 	paths.emplace_back("D:\\");
 #elif (defined(_WIN64) || defined(_WIN32)) && !defined(__UWP__) && !defined(NXDK)

Source/utils/str_split.hpp

@@ -0,0 +1,98 @@
+#pragma once
+
+#include <iterator>
+#include <type_traits>
+
+#include "utils/stdcompat/string_view.hpp"
+
+namespace devilution {
+class SplitByCharIterator {
+public:
+	using iterator_category = std::forward_iterator_tag;
+	using value_type = string_view;
+	using reference = std::add_lvalue_reference<value_type>::type;
+	using pointer = std::add_pointer<value_type>::type;
+
+	static SplitByCharIterator begin(string_view text, char split_by) // NOLINT(readability-identifier-naming)
+	{
+		return SplitByCharIterator(split_by, text, text.substr(0, text.find(split_by)));
+	}
+
+	static SplitByCharIterator end(string_view text, char split_by) // NOLINT(readability-identifier-naming)
+	{
+		return SplitByCharIterator(split_by, text, text.substr(text.size()));
+	}
+
+	[[nodiscard]] string_view operator*() const
+	{
+		return slice_;
+	}
+
+	[[nodiscard]] const string_view *operator->() const
+	{
+		return &slice_;
+	}
+
+	SplitByCharIterator &operator++()
+	{
+		slice_ = text_.substr(slice_.data() - text_.data() + slice_.size());
+		if (!slice_.empty())
+			slice_.remove_prefix(1); // skip the split_by char
+		slice_ = slice_.substr(0, slice_.find(split_by_));
+		return *this;
+	}
+
+	SplitByCharIterator operator++(int)
+	{
+		auto copy = *this;
+		++(*this);
+		return copy;
+	}
+
+	bool operator==(const SplitByCharIterator &rhs) const
+	{
+		return slice_.data() == rhs.slice_.data();
+	}
+
+	bool operator!=(const SplitByCharIterator &rhs) const
+	{
+		return !(*this == rhs);
+	}
+
+private:
+	SplitByCharIterator(char split_by, string_view text, string_view slice)
+	    : split_by_(split_by)
+	    , text_(text)
+	    , slice_(slice)
+	{
+	}
+
+	const char split_by_;
+	const string_view text_;
+	string_view slice_;
+};
+
+class SplitByChar {
+public:
+	explicit SplitByChar(string_view text, char split_by)
+	    : text_(text)
+	    , split_by_(split_by)
+	{
+	}
+
+	[[nodiscard]] SplitByCharIterator begin() const // NOLINT(readability-identifier-naming)
+	{
+		return SplitByCharIterator::begin(text_, split_by_);
+	}
+
+	[[nodiscard]] SplitByCharIterator end() const // NOLINT(readability-identifier-naming)
+	{
+		return SplitByCharIterator::end(text_, split_by_);
+	}
+
+private:
+	const string_view text_;
+	const char split_by_;
+};
+
+} // namespace devilution