1 files changed, 73 insertions, 0 deletions
diff --git a/src/libstore/derived-path-map.hh b/src/libstore/derived-path-map.hh
new file mode 100644
index 000000000..9ce58206e
--- /dev/null
+++ b/src/libstore/derived-path-map.hh
@@ -0,0 +1,73 @@
+#pragma once
+
+#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. For example, the scheduler uses
+ * `DerivedPathMap<std::weak_ptr<CreateDerivationAndRealiseGoal>>` to
+ * remember which goals correspond to which outputs. `* const Something`
+ * or `std::optional<Something>` would also 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;
+    };
+
+    /**
+     * The map type for the root node.
+     */
+    using Map = std::map<StorePath, ChildNode>;
+
+    /**
+     * The map of root node.
+     */
+    Map map;
+
+    /**
+     * 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);
+};
+
+}