feature: Add a LightClientRootHash type

zebra-chain/src/block.rs

@@ -4,6 +4,7 @@
 mod difficulty;
 mod hash;
 mod header;
+mod light_client;
 mod serialize;
 
 #[cfg(test)]

zebra-chain/src/block/header.rs

@@ -34,15 +34,10 @@ pub struct BlockHeader {
     /// header.
     pub merkle_root_hash: MerkleTreeRootHash,
 
-    /// [Pre-Sapling] Reserved. All zeroes.
-    /// [Sapling and Blossom] The root LEBS2OSP256(rt) of the Sapling note
-    /// commitment tree corresponding to the final Sapling treestate of
-    /// this block.
-    /// [Heartwood activation block] All zeroes. See ZIP-221 for details.
-    /// [After Heartwood activation block] The root of a Merkle Mountain
-    /// Range tree, which commits to various features of the chain's
-    /// history, including the Sapling commitment tree. This commitment
-    /// supports the FlyClient protocol. See ZIP-221 for details.
+    /// The light client root hash.
+    ///
+    /// This field is interpreted differently, based on the current
+    /// block height. See LightClientRootHash for details.
     pub light_client_root_hash: [u8; 32],
 
     /// The block timestamp is a Unix epoch time (UTC) when the miner

zebra-chain/src/block/light_client.rs

@@ -0,0 +1,77 @@
+//! The LightClientRootHash enum, used for the corresponding block header field.
+
+use crate::note_commitment_tree::SaplingNoteTreeRootHash;
+use crate::types::BlockHeight;
+use crate::Network;
+
+/// Light client root hashes.
+///
+/// The `BlockHeader.light_client_root_hash` field is interpreted differently,
+/// based on the current block height. The interpretation changes at or after
+/// network upgrades.
+#[derive(Clone, Copy, Debug, Eq, PartialEq, Serialize, Deserialize)]
+pub enum LightClientRootHash {
+    /// [Pre-Sapling] Reserved field.
+    ///
+    /// All zeroes.
+    PreSaplingReserved([u8; 32]),
+
+    /// [Sapling and Blossom] The final Sapling treestate of this block.
+    ///
+    /// The root LEBS2OSP256(rt) of the Sapling note commitment tree
+    /// corresponding to the final Sapling treestate of this block.
+    FinalSaplingRoot(SaplingNoteTreeRootHash),
+
+    /// [Heartwood activation block] Reserved field.
+    ///
+    /// All zeroes. This MUST NOT be interpreted as a root hash.
+    /// See ZIP-221 for details.
+    ChainHistoryActivationReserved([u8; 32]),
+
+    /// [After Heartwood activation block] The root of a Merkle Mountain
+    /// Range chain history tree.
+    ///
+    /// This root hash commits to various features of the chain's history,
+    /// including the Sapling commitment tree. This commitment supports the
+    /// FlyClient protocol. See ZIP-221 for details.
+    ///
+    /// The commitment in each block covers the chain history from the most
+    /// recent network upgrade, through to the previous block. In particular,
+    /// an activation block commits to the entire previous network upgrade, and
+    /// the block after activation commits only to the activation block.
+    ChainHistoryRoot(ChainHistoryMmrRootHash),
+}
+
+impl LightClientRootHash {
+    /// Returns `bytes` as the LightClientRootHash variant for `network` and
+    /// `height`.
+    pub fn from_bytes(
+        bytes: [u8; 32],
+        network: Network,
+        height: BlockHeight,
+    ) -> LightClientRootHash {
+        // TODO(teor): use the correct network upgrade here, after moving the
+        //             network upgrades from zebra-consensus to zebra-chain.
+        LightClientRootHash::PreSaplingReserved(bytes)
+    }
+
+    /// Returns the serialized bytes for this LightClientRootHash.
+    pub fn to_bytes(self) -> [u8; 32] {
+        use LightClientRootHash::*;
+
+        match self {
+            PreSaplingReserved(b) => b,
+            FinalSaplingRoot(v) => v.0,
+            ChainHistoryActivationReserved(b) => b,
+            ChainHistoryRoot(v) => v.0,
+        }
+    }
+}
+
+/// The root hash of a Merkle Mountain Range chain history tree.
+// TODO:
+//    - add methods for maintaining the MMR peaks, and calculating the root
+//      hash from the current set of peaks.
+//    - move to a separate file.
+#[derive(Clone, Copy, Debug, Eq, PartialEq, Serialize, Deserialize)]
+pub struct ChainHistoryMmrRootHash([u8; 32]);

zebra-chain/src/block/tests.rs

@@ -1,12 +1,14 @@
 use super::*;
 
-use crate::block::difficulty::CompactDifficulty;
+use crate::block::{difficulty::CompactDifficulty, light_client::LightClientRootHash};
 use crate::equihash_solution::EquihashSolution;
 use crate::merkle_tree::MerkleTreeRootHash;
 use crate::serialization::{
     SerializationError, ZcashDeserialize, ZcashDeserializeInto, ZcashSerialize,
 };
+use crate::types::BlockHeight;
 use crate::types::LockTime;
+use crate::Network;
 use crate::{sha256d_writer::Sha256dWriter, test::generate};
 
 use chrono::{DateTime, Duration, LocalResult, TimeZone, Utc};
@@ -18,6 +20,20 @@ use proptest::{
 use std::env;
 use std::io::{Cursor, ErrorKind, Write};
 
+impl Arbitrary for LightClientRootHash {
+    type Parameters = ();
+
+    fn arbitrary_with(_args: ()) -> Self::Strategy {
+        (any::<[u8; 32]>(), any::<Network>(), any::<BlockHeight>())
+            .prop_map(|(light_client_root_hash, network, block_height)| {
+                LightClientRootHash::from_bytes(light_client_root_hash, network, block_height)
+            })
+            .boxed()
+    }
+
+    type Strategy = BoxedStrategy<Self>;
+}
+
 impl Arbitrary for BlockHeader {
     type Parameters = ();