Reintroduce block sums, verify full kernel sums per block (#1559)

* block_sums and full kernel sum verification

* rustfmt

* add docs/comments

* docs

* rustfmt

* comment on fact total_kernel_sum is redundant now

* make sure we setup block_sums correctly on a fork

* rustfmt

* replace those asserts with errors

* rustfmt

core/src/core/block_sums.rs

@@ -0,0 +1,82 @@
+// Copyright 2018 The Grin Developers
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! BlockSums per-block running totals for utxo_sum and kernel_sum.
+//! Allows fast "full" verification of kernel sums at a given block height.
+
+use core::committed::Committed;
+use ser::{self, Readable, Reader, Writeable, Writer};
+use util::secp::pedersen::Commitment;
+use util::secp_static;
+
+/// The output_sum and kernel_sum for a given block.
+/// This is used to validate the next block being processed by applying
+/// the inputs, outputs, kernels and kernel_offset from the new block
+/// and checking everything sums correctly.
+#[derive(Debug, Clone)]
+pub struct BlockSums {
+	/// The sum of the unspent outputs.
+	pub utxo_sum: Commitment,
+	/// The sum of all kernels.
+	pub kernel_sum: Commitment,
+}
+
+impl Writeable for BlockSums {
+	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ser::Error> {
+		writer.write_fixed_bytes(&self.utxo_sum)?;
+		writer.write_fixed_bytes(&self.kernel_sum)?;
+		Ok(())
+	}
+}
+
+impl Readable for BlockSums {
+	fn read(reader: &mut Reader) -> Result<BlockSums, ser::Error> {
+		Ok(BlockSums {
+			utxo_sum: Commitment::read(reader)?,
+			kernel_sum: Commitment::read(reader)?,
+		})
+	}
+}
+
+impl Default for BlockSums {
+	fn default() -> BlockSums {
+		let zero_commit = secp_static::commit_to_zero_value();
+		BlockSums {
+			utxo_sum: zero_commit.clone(),
+			kernel_sum: zero_commit.clone(),
+		}
+	}
+}
+
+/// WAT?
+/// It's a tuple but we can verify the "full" kernel sums on it.
+/// This means we can take a previous block_sums, apply a new block to it
+/// and verify the full kernel sums (full UTXO and kernel sets).
+impl<'a> Committed for (BlockSums, &'a Committed) {
+	fn inputs_committed(&self) -> Vec<Commitment> {
+		self.1.inputs_committed()
+	}
+
+	fn outputs_committed(&self) -> Vec<Commitment> {
+		let mut outputs = vec![self.0.utxo_sum];
+		outputs.extend(&self.1.outputs_committed());
+		outputs
+	}
+
+	fn kernels_committed(&self) -> Vec<Commitment> {
+		let mut kernels = vec![self.0.kernel_sum];
+		kernels.extend(&self.1.kernels_committed());
+		kernels
+	}
+}

core/src/core/mod.rs

@@ -15,6 +15,7 @@
 //! Core types
 
 pub mod block;
+pub mod block_sums;
 pub mod committed;
 pub mod compact_block;
 pub mod compact_transaction;
@@ -30,6 +31,7 @@ use consensus::GRIN_BASE;
 use util::secp::pedersen::Commitment;
 
 pub use self::block::*;
+pub use self::block_sums::*;
 pub use self::committed::Committed;
 pub use self::compact_block::*;
 pub use self::compact_transaction::*;