Move /src to /subprojects

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

subprojects/libstore/derived-path-map.hh

@@ -0,0 +1,110 @@
+#pragma once
+///@file
+
+#include "types.hh"
+#include "derived-path.hh"
+
+namespace nix {
+
+/**
+ * A simple Trie, of sorts. Conceptually a map of `SingleDerivedPath` to
+ * values.
+ *
+ * Concretely, an n-ary tree, as described below. A
+ * `SingleDerivedPath::Opaque` maps to the value of an immediate child
+ * of the root node. A `SingleDerivedPath::Built` maps to a deeper child
+ * node: the `SingleDerivedPath::Built::drvPath` is first mapped to a a
+ * child node (inductively), and then the
+ * `SingleDerivedPath::Built::output` is used to look up that child's
+ * child via its map. In this manner, every `SingleDerivedPath` is
+ * mapped to a child node.
+ *
+ * @param V A type to instantiate for each output. It should probably
+ * should be an "optional" type so not every interior node has to have a
+ * value. `* const Something` or `std::optional<Something>` would be
+ * good choices for "optional" types.
+ */
+template<typename V>
+struct DerivedPathMap {
+    /**
+     * A child node (non-root node).
+     */
+    struct ChildNode {
+        /**
+         * Value of this child node.
+         *
+         * @see DerivedPathMap for what `V` should be.
+         */
+        V value;
+
+        /**
+         * The map type for the root node.
+         */
+        using Map = std::map<OutputName, ChildNode>;
+
+        /**
+         * The map of the root node.
+         */
+        Map childMap;
+
+        bool operator == (const ChildNode &) const noexcept;
+
+        // TODO libc++ 16 (used by darwin) missing `std::map::operator <=>`, can't do yet.
+        // decltype(std::declval<V>() <=> std::declval<V>())
+        // operator <=> (const ChildNode &) const noexcept;
+    };
+
+    /**
+     * The map type for the root node.
+     */
+    using Map = std::map<StorePath, ChildNode>;
+
+    /**
+     * The map of root node.
+     */
+    Map map;
+
+    bool operator == (const DerivedPathMap &) const = default;
+
+    // TODO libc++ 16 (used by darwin) missing `std::map::operator <=>`, can't do yet.
+    // auto operator <=> (const DerivedPathMap &) const noexcept;
+
+    /**
+     * Find the node for `k`, creating it if needed.
+     *
+     * The node is referred to as a "slot" on the assumption that `V` is
+     * some sort of optional type, so the given key can be set or unset
+     * by changing this node.
+     */
+    ChildNode & ensureSlot(const SingleDerivedPath & k);
+
+    /**
+     * Like `ensureSlot` but does not create the slot if it doesn't exist.
+     *
+     * Read the entire description of `ensureSlot` to understand an
+     * important caveat here that "have slot" does *not* imply "key is
+     * set in map". To ensure a key is set one would need to get the
+     * child node (with `findSlot` or `ensureSlot`) *and* check the
+     * `ChildNode::value`.
+     */
+    ChildNode * findSlot(const SingleDerivedPath & k);
+};
+
+template<>
+bool DerivedPathMap<std::set<std::string>>::ChildNode::operator == (
+    const DerivedPathMap<std::set<std::string>>::ChildNode &) const noexcept;
+
+// TODO libc++ 16 (used by darwin) missing `std::map::operator <=>`, can't do yet.
+#if 0
+template<>
+std::strong_ordering DerivedPathMap<std::set<std::string>>::ChildNode::operator <=> (
+    const DerivedPathMap<std::set<std::string>>::ChildNode &) const noexcept;
+
+template<>
+inline auto DerivedPathMap<std::set<std::string>>::operator <=> (const DerivedPathMap<std::set<std::string>> &) const noexcept = default;
+#endif
+
+extern template struct DerivedPathMap<std::set<std::string>>::ChildNode;
+extern template struct DerivedPathMap<std::set<std::string>>;
+
+}