Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libutil/file-path-impl.hh

@@ -0,0 +1,173 @@
+#pragma once
+/**
+ * @file
+ *
+ * Pure (no IO) infrastructure just for defining other path types;
+ * should not be used directly outside of utilities.
+ */
+#include <string>
+#include <string_view>
+
+namespace nix {
+
+/**
+ * Unix-style path primives.
+ *
+ * Nix'result own "logical" paths are always Unix-style. So this is always
+ * used for that, and additionally used for native paths on Unix.
+ */
+struct UnixPathTrait
+{
+    using CharT = char;
+
+    using String = std::string;
+
+    using StringView = std::string_view;
+
+    constexpr static char preferredSep = '/';
+
+    static inline bool isPathSep(char c)
+    {
+        return c == '/';
+    }
+
+    static inline size_t findPathSep(StringView path, size_t from = 0)
+    {
+        return path.find('/', from);
+    }
+
+    static inline size_t rfindPathSep(StringView path, size_t from = StringView::npos)
+    {
+        return path.rfind('/', from);
+    }
+};
+
+
+/**
+ * Windows-style path primitives.
+ *
+ * The character type is a parameter because while windows paths rightly
+ * work over UTF-16 (*) using `wchar_t`, at the current time we are
+ * often manipulating them converted to UTF-8 (*) using `char`.
+ *
+ * (Actually neither are guaranteed to be valid unicode; both are
+ * arbitrary non-0 8- or 16-bit bytes. But for charcters with specifical
+ * meaning like '/', '\\', ':', etc., we refer to an encoding scheme,
+ * and also for sake of UIs that display paths a text.)
+ */
+template<class CharT0>
+struct WindowsPathTrait
+{
+    using CharT = CharT0;
+
+    using String = std::basic_string<CharT>;
+
+    using StringView = std::basic_string_view<CharT>;
+
+    constexpr static CharT preferredSep = '\\';
+
+    static inline bool isPathSep(CharT c)
+    {
+        return c == '/' || c == preferredSep;
+    }
+
+    static size_t findPathSep(StringView path, size_t from = 0)
+    {
+        size_t p1 = path.find('/', from);
+        size_t p2 = path.find(preferredSep, from);
+        return p1 == String::npos ? p2 :
+               p2 == String::npos ? p1 :
+               std::min(p1, p2);
+    }
+
+    static size_t rfindPathSep(StringView path, size_t from = String::npos)
+    {
+        size_t p1 = path.rfind('/', from);
+        size_t p2 = path.rfind(preferredSep, from);
+        return p1 == String::npos ? p2 :
+               p2 == String::npos ? p1 :
+               std::max(p1, p2);
+    }
+};
+
+
+template<typename CharT>
+using OsPathTrait =
+#ifdef _WIN32
+    WindowsPathTrait<CharT>
+#else
+    UnixPathTrait
+#endif
+    ;
+
+
+/**
+ * Core pure path canonicalization algorithm.
+ *
+ * @param hookComponent
+ *   A callback which is passed two arguments,
+ *   references to
+ *
+ *   1. the result so far
+ *
+ *   2. the remaining path to resolve
+ *
+ *   This is a chance to modify those two paths in arbitrary way, e.g. if
+ *   "result" points to a symlink.
+ */
+template<class PathDict>
+typename PathDict::String canonPathInner(
+    typename PathDict::StringView remaining,
+    auto && hookComponent)
+{
+    assert(remaining != "");
+
+    typename PathDict::String result;
+    result.reserve(256);
+
+    while (true) {
+
+        /* Skip slashes. */
+        while (!remaining.empty() && PathDict::isPathSep(remaining[0]))
+            remaining.remove_prefix(1);
+
+        if (remaining.empty()) break;
+
+        auto nextComp = ({
+            auto nextPathSep = PathDict::findPathSep(remaining);
+            nextPathSep == remaining.npos ? remaining : remaining.substr(0, nextPathSep);
+        });
+
+        /* Ignore `.'. */
+        if (nextComp == ".")
+            remaining.remove_prefix(1);
+
+        /* If `..', delete the last component. */
+        else if (nextComp == "..")
+        {
+            if (!result.empty()) result.erase(PathDict::rfindPathSep(result));
+            remaining.remove_prefix(2);
+        }
+
+        /* Normal component; copy it. */
+        else {
+            result += PathDict::preferredSep;
+            if (const auto slash = PathDict::findPathSep(remaining); slash == result.npos) {
+                result += remaining;
+                remaining = {};
+            } else {
+                result += remaining.substr(0, slash);
+                remaining = remaining.substr(slash);
+            }
+
+            hookComponent(result, remaining);
+        }
+    }
+
+    if (result.empty())
+        result = typename PathDict::String { PathDict::preferredSep };
+
+    return result;
+}
+
+}