Write a new impl of the merkle algorithm

This one tries to be understandable.

coins/monero/src/block.rs

@@ -5,12 +5,11 @@ use std_shims::{
 
 use crate::{
   hash,
+  merkle::merkle_root,
   serialize::*,
   transaction::{Input, Transaction},
 };
 
-mod merkle_root;
-
 #[derive(Clone, PartialEq, Eq, Debug)]
 pub struct BlockHeader {
   pub major_version: u64,
@@ -71,11 +70,11 @@ impl Block {
     Ok(())
   }
 
-  pub fn tx_merkle_root(&self) -> [u8; 32] {
-    merkle_root::tree_hash(self.miner_tx.hash(), &self.txs)
+  fn tx_merkle_root(&self) -> [u8; 32] {
+    merkle_root(self.miner_tx.hash(), &self.txs)
   }
 
-  pub fn serialize_hashable(&self) -> Vec<u8> {
+  fn serialize_hashable(&self) -> Vec<u8> {
     let mut blob = self.header.serialize();
     blob.extend_from_slice(&self.tx_merkle_root());
     write_varint(&(1 + u64::try_from(self.txs.len()).unwrap()), &mut blob).unwrap();
@@ -87,7 +86,7 @@ impl Block {
     out
   }
 
-  pub fn id(&self) -> [u8; 32] {
+  pub fn hash(&self) -> [u8; 32] {
     // TODO: Handle block 202612
     // https://monero.stackexchange.com/questions/421/what-happened-at-block-202612
     // If this block's header is fully-equivalent to 202612, return the malformed hash instead

coins/monero/src/block/merkle_root.rs

@@ -1,68 +0,0 @@
-use crate::hash;
-
-/// Round to power of two, for count>=3 and for count being not too large (<= 2^28)
-/// (as reasonable for tree hash calculations)
-///
-fn tree_hash_cnt(count: usize) -> usize {
-  // This algo has some bad history but all we are doing is 1 << floor(log2(count))
-  // There are _many_ ways to do log2, for some reason the one selected was the most obscure one,
-  // and fixing it made it even more obscure.
-  //
-  // Iterative method implemented below aims for clarity over speed, if performance is needed
-  // then my advice is to use the BSR instruction on x86
-  //
-  // All the paranoid asserts have been removed since it is trivial to mathematically prove that
-  // the return will always be a power of 2.
-  // Problem space has been defined as 3 <= count <= 2^28. Of course quarter of a billion
-  // transactions is not a sane upper limit for a block, so there will be tighter limits
-  // in other parts of the code
-
-  assert!(count >= 3); // cases for 0,1,2 are handled elsewhere
-  assert!(count <= 0x10000000); // sanity limit to 2^28, MSB=1 will cause an inf loop
-
-  let mut pow = 2_usize;
-  while pow < count {
-    pow <<= 1
-  }
-
-  pow >> 1
-}
-
-fn hash_concat(a: [u8; 32], b: [u8; 32]) -> [u8; 32] {
-  let v = [a, b].concat();
-  hash(&v)
-}
-
-/// Compute tree hash as defined by Cryptonote
-pub fn tree_hash(root_hash: [u8; 32], extra_hashes: &[[u8; 32]]) -> [u8; 32] {
-  match extra_hashes.len() {
-    0 => root_hash,
-    1 => hash_concat(root_hash, extra_hashes[0]),
-    other => {
-      let count = other + 1;
-
-      let mut cnt = tree_hash_cnt(count);
-
-      let mut hashes =
-        std::iter::once(root_hash).chain(extra_hashes.iter().copied()).collect::<Vec<_>>();
-
-      let mut i = 2 * cnt - count;
-      let mut j = 2 * cnt - count;
-      while j < cnt {
-        hashes[j] = hash_concat(hashes[i], hashes[i + 1]);
-        i += 2;
-        j += 1;
-      }
-      assert_eq!(i, count);
-
-      while cnt > 2 {
-        cnt >>= 1;
-        for i in 0 .. cnt {
-          hashes[i] = hash_concat(hashes[2 * i], hashes[2 * i + 1]);
-        }
-      }
-
-      hash_concat(hashes[0], hashes[1])
-    }
-  }
-}

coins/monero/src/lib.rs

@@ -18,6 +18,8 @@ use curve25519_dalek::{constants::ED25519_BASEPOINT_TABLE, scalar::Scalar, edwar
 
 pub use monero_generators::H;
 
+mod merkle;
+
 mod serialize;
 use serialize::{read_byte, read_u16};
 

coins/monero/src/merkle.rs

@@ -0,0 +1,45 @@
+use crate::hash;
+
+pub fn merkle_root(root: [u8; 32], leafs: &[[u8; 32]]) -> [u8; 32] {
+  match leafs.len() {
+    0 => root,
+    1 => hash(&[root, leafs[0]].concat()),
+    _ => {
+      let mut hashes = Vec::with_capacity(1 + leafs.len());
+      hashes.push(root);
+      hashes.extend(leafs);
+
+      // Monero preprocess this so the length is a power of 2
+      let mut high_pow_2 = 4; // 4 is the lowest value this can be
+      while high_pow_2 < hashes.len() {
+        high_pow_2 *= 2;
+      }
+      let low_pow_2 = high_pow_2 / 2;
+
+      // Merge hashes until we're at the low_pow_2
+      let mut i = high_pow_2 - hashes.len();
+      while hashes.len() != low_pow_2 {
+        let l = hashes.remove(i);
+        let r = hashes.remove(i);
+        hashes.insert(i, hash(&[l.as_ref(), &r].concat()));
+        i += 1;
+      }
+      assert_eq!(hashes.len(), i);
+      assert_eq!(hashes.len(), low_pow_2);
+
+      // Do a traditional pairing off
+      let mut new_hashes = Vec::with_capacity(hashes.len() / 2);
+      while hashes.len() > 2 {
+        let mut i = 0;
+        while i < hashes.len() {
+          new_hashes.push(hash(&[hashes[i], hashes[i + 1]].concat()));
+          i += 2;
+        }
+
+        hashes = new_hashes;
+        new_hashes = Vec::with_capacity(hashes.len() / 2);
+      }
+      hashes[0]
+    }
+  }
+}

coins/monero/src/rpc/mod.rs

@@ -279,7 +279,12 @@ impl<R: RpcConnection> Rpc<R> {
       self.json_rpc_call("get_block", Some(json!({ "hash": hex::encode(hash) }))).await?;
 
     // TODO: Verify the TXs included are actually committed to by the header
-    Block::read::<&[u8]>(&mut rpc_hex(&res.blob)?.as_ref()).map_err(|_| RpcError::InvalidNode)
+    let block =
+      Block::read::<&[u8]>(&mut rpc_hex(&res.blob)?.as_ref()).map_err(|_| RpcError::InvalidNode)?;
+    if block.hash() != hash {
+      Err(RpcError::InvalidNode)?;
+    }
+    Ok(block)
   }
 
   pub async fn get_block_by_number(&self, number: usize) -> Result<Block, RpcError> {