Merge pull request paritytech#173 from subspace/heaviest-chain

Fix block weight calculation

Author: nazar-pc

crates/sp-consensus-subspace/src/digests.rs

@@ -72,7 +72,13 @@ impl PreDigest {
     /// Returns the weight _added_ by this digest, not the cumulative weight
     /// of the chain.
     pub fn added_weight(&self) -> SubspaceBlockWeight {
-        1
+        let target = u64::from_be_bytes(self.solution.local_challenge.derive_target());
+        let tag = u64::from_be_bytes(self.solution.tag);
+        let diff = target.wrapping_sub(tag);
+        let diff2 = tag.wrapping_sub(target);
+        // Find smaller diff between 2 directions.
+        let bidirectional_diff = (diff).min(diff2);
+        u128::from(u64::MAX - bidirectional_diff)
     }
 }
 

crates/sp-consensus-subspace/src/lib.rs

@@ -69,9 +69,8 @@ pub type EquivocationProof<H> = sp_consensus_slots::EquivocationProof<H, FarmerP
 /// The cumulative weight of a Subspace block, i.e. sum of block weights starting
 /// at this block until the genesis block.
 ///
-/// Primary blocks have a weight of 1 whereas secondary blocks have a weight
-/// of 0 (regardless of whether they are plain or vrf secondary blocks).
-pub type SubspaceBlockWeight = u32;
+/// The closer solution's tag is to the target, the heavier it is.
+pub type SubspaceBlockWeight = u128;
 
 /// An consensus log item for Subspace.
 #[derive(Decode, Encode, Clone, PartialEq, Eq, RuntimeDebug)]