Block now uses the new Target type. Consensus next_target calculation adjusting every block based on deviation and increasing Cuckoo size gradually. Block time of a minute until we learn more from mining ecosystem, so enough PoW attempts can be made even on CPUs.

core/src/consensus.rs

@@ -19,40 +19,146 @@
 //! enough, consensus-relevant constants and short functions should be kept
 //! here.
 
+use std::cmp;
+
+use core::target::Target;
+
 /// The block subsidy amount
 pub const REWARD: u64 = 1_000_000_000;
 
-/// Block interval, in seconds, the network will tune its difficulty for. Note
-/// that contrarily to bitcoin, we may reduce this value in the future as
-/// networks improve and block propagation is optimized (adjusting the reward
-/// accordingly).
-pub const BLOCK_TIME_SEC: u8 = 30;
+/// Block interval, in seconds, the network will tune its next_target for. Note
+/// that we may reduce this value in the future as we get more data on mining
+/// with Cuckoo Cycle, networks improve and block propagation is optimized
+/// (adjusting the reward accordingly).
+pub const BLOCK_TIME_SEC: u8 = 60;
 
 /// Cuckoo-cycle proof size (cycle length)
 pub const PROOFSIZE: usize = 42;
 
-/// Default Cuckoo Cycle size shift used is 28. We may decide to increase it
-/// when hashrate increases. May also start lower.
-pub const SIZESHIFT: u32 = 28;
+/// Origin Cuckoo Cycle size shift used by the genesis block.
+pub const DEFAULT_SIZESHIFT: u8 = 25;
+
+/// Maximum Cuckoo Cycle size shift we'll ever use. We adopt a schedule that
+/// progressively increases the size as the target becomes lower.
+///   Start => 25
+///   MAX_TARGET >> 12 => 26
+///   MAX_TARGET >> 20 => 27
+///   MAX_TARGET >> 28 => 28
+///   MAX_TARGET >> 36 => 29
+pub const MAX_SIZESHIFT: u8 = 29;
 
 /// Default Cuckoo Cycle easiness, high enough to have good likeliness to find
 /// a solution.
 pub const EASINESS: u32 = 50;
 
+/// Difficulty adjustment somewhat inspired by Ethereum's. Tuned to add or
+/// remove 1/1024th of the target for each 10 seconds of deviation from the 30
+/// seconds block time. Increases Cuckoo size shift by one when next_target
+/// reaches soft max.
+pub fn next_target(ts: i64, prev_ts: i64, prev_target: Target, prev_cuckoo_sz: u8) -> (Target, u8) {
+	// increase the cuckoo size when the target gets lower than the soft min as
+	// long as we're not at the max size already; target gets 2x to compensate for
+	// increased next_target
+	let soft_min = SOFT_MIN_TARGET >> (((prev_cuckoo_sz - DEFAULT_SIZESHIFT) * 8) as usize);
+	let (ptarget, clen) = if prev_target < soft_min && prev_cuckoo_sz < MAX_SIZESHIFT {
+		(prev_target << 1, prev_cuckoo_sz + 1)
+	} else {
+		(prev_target, prev_cuckoo_sz)
+	};
+
+	// target is increased/decreased by multiples of 1/1024th of itself
+	let incdec = ptarget >> 10;
+	// signed deviation from desired value divided by ten and bounded in [-3, 3]
+	let delta = cmp::max(cmp::min((ts - prev_ts - (BLOCK_TIME_SEC as i64)) / 10, 3),
+	                     -3);
+	// increase or decrease the target based on the sign of delta by a shift of
+	// |delta|-1; keep as-is for delta of zero
+	let new_target = match delta {
+		1...3 => ptarget + (incdec << ((delta - 1) as usize)),
+		-3...-1 => ptarget - (incdec << ((-delta - 1) as usize)),
+		_ => ptarget,
+	};
+
+	// cannot exceed the maximum target
+	if new_target > MAX_TARGET {
+		(MAX_TARGET, clen)
+	} else {
+		(new_target, clen)
+	}
+}
+
+/// Max target hash, lowest next_target
+pub const MAX_TARGET: Target = Target([0xf, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+                                       0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+                                       0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+                                       0xff, 0xff, 0xff, 0xff]);
+
+/// Target limit under which we start increasing the size shift on Cuckoo cycle.
+pub const SOFT_MIN_TARGET: Target = Target([0, 0, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+                                            0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+                                            0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+                                            0xff, 0xff, 0xff, 0xff]);
+
 /// Default number of blocks in the past when cross-block cut-through will start
-/// happening. Needs to be long enough to not overlap with a long reorg. Rational
+/// happening. Needs to be long enough to not overlap with a long reorg.
+/// Rational
 /// behind the value is the longest bitcoin fork was about 30 blocks, so 5h. We
 /// add an order of magnitude to be safe and round to 48h of blocks to make it
 /// easier to reason about.
 pub const CUT_THROUGH_HORIZON: u32 = 48 * 3600 / (BLOCK_TIME_SEC as u32);
 
-/// Max target hash, lowest difficulty
-pub const MAX_TARGET: [u32; PROOFSIZE] =
-	[0xfff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff,
-	 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff,
-	 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff,
-	 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff];
-
 /// The maximum number of inputs or outputs a transaction may have
 /// and be deserializable. Only for DoS protection.
 pub const MAX_IN_OUT_LEN: u64 = 50000;
+
+#[cfg(test)]
+mod test {
+	use super::*;
+
+	#[test]
+	/// Checks different next_target adjustments and difficulty boundaries
+	fn next_target_adjustment() {
+		// can't do lower than min
+		assert_eq!(next_target(60, 0, MAX_TARGET, 26), (MAX_TARGET, 26));
+		assert_eq!(next_target(90, 30, MAX_TARGET, 26), (MAX_TARGET, 26));
+		assert_eq!(next_target(60, 0, MAX_TARGET, 26), (MAX_TARGET, 26));
+
+		// lower next_target if gap too short, even negative
+		assert_eq!(next_target(50, 0, MAX_TARGET, 26).0,
+		           MAX_TARGET - (MAX_TARGET >> 10));
+		assert_eq!(next_target(40, 0, MAX_TARGET, 26).0,
+		           MAX_TARGET - ((MAX_TARGET >> 10) << 1));
+		assert_eq!(next_target(0, 20, MAX_TARGET, 26).0,
+		           MAX_TARGET - ((MAX_TARGET >> 10) << 2));
+
+		// raise next_target if gap too wide
+		let lower_target = MAX_TARGET >> 8;
+		assert_eq!(next_target(70, 0, lower_target, 26).0,
+		           lower_target + (lower_target >> 10));
+		assert_eq!(next_target(80, 0, lower_target, 26).0,
+		           lower_target + ((lower_target >> 10) << 1));
+		assert_eq!(next_target(200, 0, lower_target, 26).0,
+		           lower_target + ((lower_target >> 10) << 2));
+
+		// close enough, no adjustment
+		assert_eq!(next_target(65, 0, lower_target, 26).0, lower_target);
+		assert_eq!(next_target(55, 0, lower_target, 26).0, lower_target);
+
+		// increase cuckoo size if next_target goes above soft max, target is doubled,
+		// up to 29
+		assert_eq!(next_target(60, 0, SOFT_MIN_TARGET >> 1, 25),
+		           ((SOFT_MIN_TARGET >> 1) << 1, 26));
+		assert_eq!(next_target(60, 0, SOFT_MIN_TARGET >> 9, 26),
+		           ((SOFT_MIN_TARGET >> 9) << 1, 27));
+		assert_eq!(next_target(60, 0, SOFT_MIN_TARGET >> 17, 27),
+		           ((SOFT_MIN_TARGET >> 17) << 1, 28));
+		assert_eq!(next_target(60, 0, SOFT_MIN_TARGET >> 25, 28),
+		           ((SOFT_MIN_TARGET >> 25) << 1, 29));
+		assert_eq!(next_target(60, 0, SOFT_MIN_TARGET >> 33, 29),
+		           (SOFT_MIN_TARGET >> 33, 29));
+
+		// should only increase on the according previous size
+		assert_eq!(next_target(60, 0, SOFT_MIN_TARGET >> 1, 26),
+		           (SOFT_MIN_TARGET >> 1, 26));
+	}
+}

core/src/core/block.rs

@@ -23,19 +23,28 @@ use std::collections::HashSet;
 use core::Committed;
 use core::{Input, Output, Proof, TxProof, Transaction};
 use core::transaction::merkle_inputs_outputs;
-use consensus::{PROOFSIZE, REWARD, MAX_IN_OUT_LEN};
+use consensus::{PROOFSIZE, REWARD, DEFAULT_SIZESHIFT, MAX_IN_OUT_LEN, MAX_TARGET};
 use core::hash::{Hash, Hashed, ZERO_HASH};
+use core::target::Target;
 use ser::{self, Readable, Reader, Writeable, Writer};
 
 /// Block header, fairly standard compared to other blockchains.
 pub struct BlockHeader {
+	/// Height of this block since the genesis block (height 0)
 	pub height: u64,
+	/// Hash of the block previous to this in the chain.
 	pub previous: Hash,
+	/// Timestamp at which the block was built.
 	pub timestamp: time::Tm,
-	pub td: u64, // total difficulty up to this block
+	/// Length of the cuckoo cycle used to mine this block.
+	pub cuckoo_len: u8,
+	/// Difficulty used to mine the block.
+	pub target: Target,
 	pub utxo_merkle: Hash,
 	pub tx_merkle: Hash,
+	/// Nonce increment used to mine this block.
 	pub nonce: u64,
+	/// Proof of work data.
 	pub pow: Proof,
 }
 
@@ -45,7 +54,8 @@ impl Default for BlockHeader {
 			height: 0,
 			previous: ZERO_HASH,
 			timestamp: time::at_utc(time::Timespec { sec: 0, nsec: 0 }),
-			td: 0,
+			cuckoo_len: 20, // only for tests
+			target: MAX_TARGET,
 			utxo_merkle: ZERO_HASH,
 			tx_merkle: ZERO_HASH,
 			nonce: 0,
@@ -62,6 +72,9 @@ impl Writeable for BlockHeader {
 		                [write_u64, self.height],
 		                [write_fixed_bytes, &self.previous],
 		                [write_i64, self.timestamp.to_timespec().sec],
+		                [write_u8, self.cuckoo_len]);
+		try!(self.target.write(writer));
+		ser_multiwrite!(writer,
 		                [write_fixed_bytes, &self.utxo_merkle],
 		                [write_fixed_bytes, &self.tx_merkle]);
 		// make sure to not introduce any variable length data before the nonce to
@@ -71,7 +84,7 @@ impl Writeable for BlockHeader {
 		for n in 0..42 {
 			try!(writer.write_u32(self.pow.0[n]));
 		}
-		writer.write_u64(self.td)
+		Ok(())
 	}
 }
 
@@ -114,13 +127,11 @@ impl Writeable for Block {
 /// from a binary stream.
 impl Readable<Block> for Block {
 	fn read(reader: &mut Reader) -> Result<Block, ser::Error> {
-		let (height, previous, timestamp, utxo_merkle, tx_merkle, nonce) = ser_multiread!(reader,
-			               read_u64,
-			               read_32_bytes,
-			               read_i64,
-			               read_32_bytes,
-			               read_32_bytes,
-			               read_u64);
+		let (height, previous, timestamp, cuckoo_len) =
+			ser_multiread!(reader, read_u64, read_32_bytes, read_i64, read_u8);
+		let target = try!(Target::read(reader));
+		let (utxo_merkle, tx_merkle, nonce) =
+			ser_multiread!(reader, read_32_bytes, read_32_bytes, read_u64);
 
 		// cuckoo cycle of 42 nodes
 		let mut pow = [0; PROOFSIZE];
@@ -148,7 +159,8 @@ impl Readable<Block> for Block {
 					sec: timestamp,
 					nsec: 0,
 				}),
-				td: td,
+				cuckoo_len: cuckoo_len,
+				target: target,
 				utxo_merkle: Hash::from_vec(utxo_merkle),
 				tx_merkle: Hash::from_vec(tx_merkle),
 				pow: Proof(pow),

core/src/core/hash.rs

@@ -23,6 +23,8 @@ use tiny_keccak::Keccak;
 
 use ser::{self, AsFixedBytes};
 
+pub const ZERO_HASH: Hash = Hash([0; 32]);
+
 /// A hash to uniquely (or close enough) identify one of the main blockchain
 /// constructs. Used pervasively for blocks, transactions and ouputs.
 #[derive(Debug, Copy, Clone, PartialEq, PartialOrd, Eq, Ord, Hash)]
@@ -56,15 +58,13 @@ impl Hash {
 	}
 }
 
-pub const ZERO_HASH: Hash = Hash([0; 32]);
-
 /// Serializer that outputs a hash of the serialized object
 pub struct HashWriter {
 	state: Keccak,
 }
 
 impl HashWriter {
-	fn finalize(self, output: &mut [u8]) {
+	pub fn finalize(self, output: &mut [u8]) {
 		self.state.finalize(output);
 	}
 }

core/src/core/mod.rs

@@ -29,7 +29,7 @@ use secp::pedersen::*;
 use consensus::PROOFSIZE;
 pub use self::block::{Block, BlockHeader};
 pub use self::transaction::{Transaction, Input, Output, TxProof};
-use self::hash::{Hash, Hashed, ZERO_HASH};
+use self::hash::{Hash, Hashed, HashWriter, ZERO_HASH};
 use ser::{Writeable, Writer, Error};
 
 /// Implemented by types that hold inputs and outputs including Pedersen
@@ -109,11 +109,24 @@ impl Clone for Proof {
 	}
 }
 
+impl Hashed for Proof {
+	fn hash(&self) -> Hash {
+		let mut hasher = HashWriter::default();
+		let mut ret = [0; 32];
+		for p in self.0.iter() {
+			hasher.write_u32(*p).unwrap();
+		}
+		hasher.finalize(&mut ret);
+		Hash(ret)
+	}
+}
+
 impl Proof {
 	/// Builds a proof with all bytes zeroed out
 	pub fn zero() -> Proof {
 		Proof([0; 42])
 	}
+
 	/// Builds a proof from a vector of exactly PROOFSIZE (42) u32.
 	pub fn from_vec(v: Vec<u32>) -> Proof {
 		assert!(v.len() == PROOFSIZE);
@@ -123,6 +136,7 @@ impl Proof {
 		}
 		Proof(p)
 	}
+
 	/// Converts the proof to a vector of u64s
 	pub fn to_u64s(&self) -> Vec<u64> {
 		let mut nonces = Vec::with_capacity(PROOFSIZE);
@@ -131,10 +145,17 @@ impl Proof {
 		}
 		nonces
 	}
+
 	/// Converts the proof to a vector of u32s
 	pub fn to_u32s(&self) -> Vec<u32> {
 		self.0.to_vec()
 	}
+
+	/// Converts the proof to a proof-of-work Target so they can be compared.
+	/// Hashes the Cuckoo Proof data.
+	pub fn to_target(self) -> target::Target {
+		target::Target(self.hash().0)
+	}
 }
 
 /// Two hashes that will get hashed together in a Merkle tree to build the next

core/src/core/target.rs

@@ -244,14 +244,15 @@ mod test {
 
 	#[test]
 	fn shift_truncate() {
-		assert_eq!((Target::join(0, 0xffff).unwrap() >> 8) << 8, Target::join(0, 0xff00).unwrap());
+		assert_eq!((Target::join(0, 0xffff).unwrap() >> 8) << 8,
+		           Target::join(0, 0xff00).unwrap());
 	}
 
 	#[test]
 	fn split_fit() {
 		let t = Target::join(10 * 8, ::std::u32::MAX).unwrap();
 		let (exp, mant) = t.split();
-		assert_eq!(exp, 10*8);
+		assert_eq!(exp, 10 * 8);
 		assert_eq!(mant, ::std::u32::MAX);
 	}
 

core/src/genesis.rs

@@ -17,6 +17,7 @@
 use time;
 
 use core;
+use consensus::{DEFAULT_SIZESHIFT, MAX_TARGET};
 
 use tiny_keccak::Keccak;
 
@@ -38,7 +39,8 @@ pub fn genesis() -> core::Block {
 				tm_mday: 4,
 				..time::empty_tm()
 			},
-			td: 0,
+			cuckoo_len: DEFAULT_SIZESHIFT,
+			target: MAX_TARGET,
 			utxo_merkle: core::hash::Hash::from_vec(empty_h.to_vec()),
 			tx_merkle: core::hash::Hash::from_vec(empty_h.to_vec()),
 			nonce: 0,

core/src/pow/mod.rs

@@ -27,9 +27,10 @@ mod cuckoo;
 
 use time;
 
-use consensus::{SIZESHIFT, EASINESS};
+use consensus::EASINESS;
 use core::{Block, Proof};
 use core::hash::{Hash, Hashed};
+use core::target::Target;
 use pow::cuckoo::{Cuckoo, Miner, Error};
 
 use ser;
@@ -87,21 +88,21 @@ impl PowHeader {
 }
 
 /// Validates the proof of work of a given header.
-pub fn verify(b: &Block, target: Proof) -> bool {
-	verify_size(b, target, SIZESHIFT)
+pub fn verify(b: &Block, target: Target) -> bool {
+	verify_size(b, target, b.header.cuckoo_len as u32)
 }
 
 /// Same as default verify function but uses the much easier Cuckoo20 (mostly
 /// for tests).
-pub fn verify20(b: &Block, target: Proof) -> bool {
+pub fn verify20(b: &Block, target: Target) -> bool {
 	verify_size(b, target, 20)
 }
 
-pub fn verify_size(b: &Block, target: Proof, sizeshift: u32) -> bool {
+pub fn verify_size(b: &Block, target: Target, sizeshift: u32) -> bool {
 	let hash = PowHeader::from_block(b).hash();
 	// make sure the hash is smaller than our target before going into more
 	// expensive validation
-	if target < b.header.pow {
+	if target < b.header.pow.to_target() {
 		return false;
 	}
 	Cuckoo::new(hash.to_slice(), sizeshift).verify(b.header.pow, EASINESS as u64)
@@ -110,17 +111,17 @@ pub fn verify_size(b: &Block, target: Proof, sizeshift: u32) -> bool {
 /// Runs a naive single-threaded proof of work computation over the provided
 /// block, until the required difficulty target is reached. May take a
 /// while for a low target...
-pub fn pow(b: &Block, target: Proof) -> Result<(Proof, u64), Error> {
-	pow_size(b, target, SIZESHIFT)
+pub fn pow(b: &Block, target: Target) -> Result<(Proof, u64), Error> {
+	pow_size(b, target, b.header.cuckoo_len as u32)
 }
 
 /// Same as default pow function but uses the much easier Cuckoo20 (mostly for
 /// tests).
-pub fn pow20(b: &Block, target: Proof) -> Result<(Proof, u64), Error> {
+pub fn pow20(b: &Block, target: Target) -> Result<(Proof, u64), Error> {
 	pow_size(b, target, 20)
 }
 
-fn pow_size(b: &Block, target: Proof, sizeshift: u32) -> Result<(Proof, u64), Error> {
+fn pow_size(b: &Block, target: Target, sizeshift: u32) -> Result<(Proof, u64), Error> {
 	let mut pow_header = PowHeader::from_block(b);
 	let start_nonce = pow_header.nonce;
 
@@ -133,7 +134,7 @@ fn pow_size(b: &Block, target: Proof, sizeshift: u32) -> Result<(Proof, u64), Er
 		// if we found a cycle (not guaranteed) and the proof is lower that the target,
 		// we're all good
 		if let Ok(proof) = Miner::new(pow_hash.to_slice(), EASINESS, sizeshift).mine() {
-			if proof <= target {
+			if proof.to_target() <= target {
 				return Ok((proof, pow_header.nonce));
 			}
 		}
@@ -159,11 +160,11 @@ mod test {
 	#[test]
 	fn genesis_pow() {
 		let mut b = genesis::genesis();
-		let (proof, nonce) = pow20(&b, Proof(MAX_TARGET)).unwrap();
+		let (proof, nonce) = pow20(&b, MAX_TARGET).unwrap();
 		assert!(nonce > 0);
-		assert!(proof < Proof(MAX_TARGET));
+		assert!(proof.to_target() < MAX_TARGET);
 		b.header.pow = proof;
 		b.header.nonce = nonce;
-		assert!(verify20(&b, Proof(MAX_TARGET)));
+		assert!(verify20(&b, MAX_TARGET));
 	}
 }