1 files changed, 89 insertions, 0 deletions
diff --git a/include/MultigroupCodec.h b/include/MultigroupCodec.h
new file mode 100644
index 0000000..50f7bb8
--- /dev/null
+++ b/include/MultigroupCodec.h
@@ -0,0 +1,89 @@
+#pragma once
+
+#include "IStorageBackend.h"
+
+#include <string>
+#include <utility>
+#include <vector>
+
+/**
+ * @file MultigroupCodec.h
+ * @brief Thin facade over Merkle<std::string> for multigroup snapshots.
+ *
+ * A multigroup snapshot is a `Merkle<std::string>` B-tree mapping each
+ * relation name to that relation's merkle root. The snapshot identifier is
+ * the hex hash of the tree's root node — the same path-localised content
+ * addressing used at the tuple level inside each relation.
+ *
+ * Stored relations and ephemeral relations are represented uniformly: only
+ * the relation's name and its current merkle_root participate in the tree,
+ * regardless of how that root was derived.
+ *
+ * The codec used to be a flat `SHA256(sorted [(name, root)])` over a single
+ * blob. That collapsed all relations into one rehash on any mutation. With
+ * the merkle B-tree, a single relation update touches `O(log_B(n_rels))`
+ * nodes on the way to the new root — sibling subtrees stay byte-identical.
+ *
+ * `RelationEntry` is `(relation_name, relation_root_hex)`. Both forms are
+ * hex strings on the wire; the codec hex-decodes internally before handing
+ * payloads to `Merkle<std::string>`.
+ */
+
+namespace nt::MultigroupCodec
+{
+    /** @brief One entry in a multigroup snapshot: (relation_name, root_hex). */
+    using RelationEntry = std::pair<std::string, std::string>;
+
+    /**
+     * @brief Build a multigroup tree from a list of (name, root) entries.
+     *
+     * Each `Insert` advances the root through `O(log_B)` nodes. The final
+     * root hash is the snapshot identifier. The list need not be pre-sorted —
+     * the merkle B-tree imposes order intrinsically.
+     *
+     * @param store     KV backend; receives all node blobs.
+     * @param relations Entries to load. Duplicates collapse to the last value.
+     * @return Root node hash (empty string when `relations` is empty).
+     */
+    std::string Build(IStorageBackend& store,
+                      const std::vector<RelationEntry>& relations);
+
+    /**
+     * @brief Enumerate all (name, root_hex) pairs at a multigroup root.
+     *
+     * Pages the entire tree (offset 0, no limit). Returned entries are in
+     * key-sorted order. Empty input root yields an empty list.
+     */
+    std::vector<RelationEntry> List(IStorageBackend& store,
+                                     const std::string& root_hex);
+
+    /**
+     * @brief Insert or overwrite one (relation_name → root_hex) entry.
+     *
+     * Equivalent to `Merkle<std::string>::Insert` after hex-decoding the
+     * payload. The caller threads the returned root hash through subsequent
+     * mutations. Pass an empty `root_hex` for the prior-state argument to
+     * start from an empty tree.
+     */
+    std::string InsertOne(IStorageBackend& store,
+                          const std::string& root_hex,
+                          const std::string& relation_name,
+                          const std::string& relation_root_hex);
+
+    /**
+     * @brief Remove one relation entry from the multigroup tree.
+     *
+     * Returns the new root hash; empty when the tree becomes empty.
+     */
+    std::string RemoveOne(IStorageBackend& store,
+                          const std::string& root_hex,
+                          const std::string& relation_name);
+
+    /**
+     * @brief Look up one relation's root hex by name.
+     * @return Hex root, or empty string when the name is absent.
+     */
+    std::string Lookup(IStorageBackend& store,
+                       const std::string& root_hex,
+                       const std::string& relation_name);
+}