add state_at_id and balance_at_id, integrate with RPC

ethcore/src/client/client.rs

@@ -346,6 +346,16 @@ impl<V> Client<V> where V: Verifier {
 		imported
 	}
 
+	/// Attempt to get a copy of a specific block's state.
+	///
+	/// This can fail (but may not) if the DB prunes state.
+	pub fn state_at_id(&self, id: BlockID) -> Option<State> {
+		self.block_header(id).map(|header| {
+			let db = self.state_db.lock().unwrap().boxed_clone();
+			State::from_existing(db, HeaderView::new(&header).state_root(), self.engine.account_start_nonce())
+		})
+	}
+
 	/// Get a copy of the best block's state.
 	pub fn state(&self) -> State {
 		State::from_existing(self.state_db.lock().unwrap().boxed_clone(), HeaderView::new(&self.best_block_header()).state_root(), self.engine.account_start_nonce())
@@ -557,6 +567,13 @@ impl<V> BlockChainClient for Client<V> where V: Verifier {
 		self.state().balance(address)
 	}
 
+	fn balance_at_id(&self, address: &Address, id: BlockID) -> Option<U256> {
+		match id {
+			BlockID::Latest => Some(self.balance(address)),
+			id => self.state_at_id(id).map(|s| s.balance(address)),
+		}
+	}
+
 	fn storage_at(&self, address: &Address, position: &H256) -> H256 {
 		self.state().storage_at(address, position)
 	}

ethcore/src/client/mod.rs

@@ -74,9 +74,21 @@ pub trait BlockChainClient : Sync + Send {
 	/// Get address code.
 	fn code(&self, address: &Address) -> Option<Bytes>;
 
-	/// Get address balance.
+	/// Get address balance at latest state.
 	fn balance(&self, address: &Address) -> U256;
 
+	/// Account balance at a specific block ID.
+	///
+	/// Will fail if the block is not valid or the block's root hash has been
+	/// pruned from the DB.
+	fn balance_at_id(&self, address: &Address, id: BlockID) -> Option<U256> {
+		if let BlockID::Latest = id {
+			Some(self.balance(address))
+		} else {
+			None
+		}
+	}
+
 	/// Get value of the storage at given position.
 	fn storage_at(&self, address: &Address, position: &H256) -> H256;