When the cuckoo size is reduced for tests, pow needs to be recomputed for the genesis block.

chain/src/types.rs

@@ -52,7 +52,7 @@ impl Tip {
 			height: b.header.height,
 			last_block_h: b.hash(),
 			prev_block_h: b.header.previous,
-			total_difficulty: b.header.total_difficulty.clone() + Difficulty::from_hash(&b.hash()),
+			total_difficulty: b.header.total_difficulty.clone(),
 		}
 	}
 }

grin/src/server.rs

@@ -96,7 +96,6 @@ impl Server {
 		let handle = evtlp.handle();
 		evtlp.run(server.start(handle.clone())).unwrap();
 
-
 		warn!("Grin server started.");
 		Ok(Server {
 			config: config,
@@ -173,6 +172,8 @@ fn store_head(config: &ServerConfig)
 			let mut gen = core::genesis::genesis();
 			if config.cuckoo_size > 0 {
 				gen.header.cuckoo_len = config.cuckoo_size;
+				let diff = gen.header.difficulty.clone();
+				core::pow::pow(&mut gen, diff).unwrap();
 			}
 			try!(chain_store.save_block(&gen).map_err(&Error::StoreErr));
 			let tip = chain::types::Tip::new(gen.hash());

grin/tests/simulnet.rs

@@ -77,7 +77,8 @@ fn change<'a>(s: &'a grin::Server) -> HeadChange<'a> {
   }
 }
 
-/// Future that monitors when a server has had its head updated.
+/// Future that monitors when a server has had its head updated. Current
+/// implementation isn't optimized, only use for tests.
 struct HeadChange<'a> {
   server: &'a grin::Server,
   original:  chain::Tip,