Clean up SearchPath

- Better types

- Own header / C++ file pair

- Test factored out methods

- Pass parsed thing around more than strings

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/libexpr/search-path.cc

@@ -0,0 +1,56 @@
+#include "search-path.hh"
+#include "util.hh"
+
+namespace nix {
+
+std::optional<std::string_view> SearchPath::Prefix::suffixIfPotentialMatch(
+    std::string_view path) const
+{
+    auto n = s.size();
+
+    /* Non-empty prefix and suffix must be separated by a /, or the
+       prefix is not a valid path prefix. */
+    bool needSeparator = n > 0 && (path.size() - n) > 0;
+
+    if (needSeparator && path[n] != '/') {
+        return std::nullopt;
+    }
+
+    /* Prefix must be prefix of this path. */
+    if (path.compare(0, n, s) != 0) {
+        return std::nullopt;
+    }
+
+    /* Skip next path separator. */
+    return {
+        path.substr(needSeparator ? n + 1 : n)
+    };
+}
+
+
+SearchPath::Elem SearchPath::Elem::parse(std::string_view rawElem)
+{
+    size_t pos = rawElem.find('=');
+
+    return SearchPath::Elem {
+        .prefix = Prefix {
+            .s = pos == std::string::npos
+                ? std::string { "" }
+                : std::string { rawElem.substr(0, pos) },
+        },
+        .path = Path {
+            .s = std::string { rawElem.substr(pos + 1) },
+        },
+    };
+}
+
+
+SearchPath parseSearchPath(const Strings & rawElems)
+{
+    SearchPath res;
+    for (auto & rawElem : rawElems)
+        res.elements.emplace_back(SearchPath::Elem::parse(rawElem));
+    return res;
+}
+
+}