Avoid repeated truncation in difficulty adjust + minor refactoring (#915)

* avoid repeated truncation in difficulty adjust + minor refactoring
* reinstate needed low-diffculty exception on dampening
* leave consensus breaking change for later one line change; obsolete lower testnet1 max target

core/src/consensus.rs

@@ -191,22 +191,17 @@ where
 	// adjustment if there isn't enough window data
 	// length will be DIFFICULTY_ADJUST_WINDOW+MEDIAN_TIME_WINDOW
 	let diff_data = global::difficulty_data_to_vector(cursor);
-	// Get the difficulty sum for averaging later
-	// Which in this case is the sum of the last
-	// DIFFICULTY_ADJUST_WINDOW elements
-	let diff_sum = diff_data
-		.iter()
-		.skip(MEDIAN_TIME_WINDOW as usize)
-		.take(DIFFICULTY_ADJUST_WINDOW as usize)
-		.fold(Difficulty::zero(), |sum, d| sum + d.clone().unwrap().1);
 
 	// Obtain the median window for the earlier time period
-	// which is just the first MEDIAN_TIME_WINDOW elements
+	// the first MEDIAN_TIME_WINDOW elements
 	let mut window_earliest: Vec<u64> = diff_data
 		.iter()
 		.take(MEDIAN_TIME_WINDOW as usize)
 		.map(|n| n.clone().unwrap().0)
 		.collect();
+	// pick median
+	window_earliest.sort();
+	let earliest_ts = window_earliest[MEDIAN_TIME_INDEX as usize];
 
 	// Obtain the median window for the latest time period
 	// i.e. the last  MEDIAN_TIME_WINDOW elements
@@ -215,23 +210,24 @@ where
 		.skip(DIFFICULTY_ADJUST_WINDOW as usize)
 		.map(|n| n.clone().unwrap().0)
 		.collect();
-
-	// And obtain our median values
-	window_earliest.sort();
+	// pick median
 	window_latest.sort();
 	let latest_ts = window_latest[MEDIAN_TIME_INDEX as usize];
-	let earliest_ts = window_earliest[MEDIAN_TIME_INDEX as usize];
 
-	// Calculate the average difficulty
-	let diff_avg = diff_sum.into_num() / Difficulty::from_num(DIFFICULTY_ADJUST_WINDOW).into_num();
-
-	// Actual undampened time delta
+	// median time delta
 	let ts_delta = latest_ts - earliest_ts;
 
-	// Apply dampening
-	let ts_damp = match diff_avg {
-		n if n >= DAMP_FACTOR => ((DAMP_FACTOR - 1) * BLOCK_TIME_WINDOW + ts_delta) / DAMP_FACTOR,
-		_ => ts_delta,
+	// Get the difficulty sum of the last DIFFICULTY_ADJUST_WINDOW elements
+	let diff_sum = diff_data
+		.iter()
+		.skip(MEDIAN_TIME_WINDOW as usize)
+		.fold(0, |sum, d| sum + d.clone().unwrap().1.into_num());
+
+	// Apply dampening except when difficulty is near 1
+	let ts_damp = if diff_sum < DAMP_FACTOR * DIFFICULTY_ADJUST_WINDOW {
+		ts_delta
+	} else {
+		(1 * ts_delta + (DAMP_FACTOR-1) * BLOCK_TIME_WINDOW) / DAMP_FACTOR
 	};
 
 	// Apply time bounds
@@ -243,10 +239,12 @@ where
 		ts_damp
 	};
 
-	let difficulty = diff_avg * Difficulty::from_num(BLOCK_TIME_WINDOW).into_num()
-		/ Difficulty::from_num(adj_ts).into_num();
+	// AVOID BREAKING CONSENSUS FOR NOW WITH OLD DOUBLE TRUNCATION CALC
+	let difficulty = (diff_sum / DIFFICULTY_ADJUST_WINDOW) * BLOCK_TIME_WINDOW / adj_ts;
+	// EVENTUALLY BREAK CONSENSUS WITH THIS IMPROVED SINGLE TRUNCATION DIFF CALC
+	// let difficulty = diff_sum * BLOCK_TIME_SEC / adj_ts;
 
-	Ok(max(Difficulty::from_num(difficulty), Difficulty::one()))
+	Ok(Difficulty::from_num(max(difficulty, 1)))
 }
 
 /// Consensus rule that collections of items are sorted lexicographically.

core/src/core/hash.rs

@@ -21,6 +21,7 @@ use std::cmp::min;
 use std::{fmt, ops};
 use std::convert::AsRef;
 use std::ops::Add;
+use byteorder::{BigEndian, ByteOrder};
 
 use blake2::blake2b::Blake2b;
 
@@ -77,6 +78,11 @@ impl Hash {
 		let bytes = util::from_hex(hex.to_string()).unwrap();
 		Ok(Hash::from_vec(bytes))
 	}
+
+	/// Most significant 64 bits
+	pub fn to_u64(&self) -> u64 {
+		BigEndian::read_u64(&self.0)
+	}
 }
 
 impl ops::Index<usize> for Hash {

core/src/core/target.rs

@@ -23,11 +23,10 @@ use std::fmt;
 use std::ops::{Add, Div, Mul, Sub};
 
 use serde::{de, Deserialize, Deserializer, Serialize, Serializer};
-use byteorder::{BigEndian, ByteOrder};
+use std::cmp::max;
 
 use core::hash::Hash;
 use ser::{self, Readable, Reader, Writeable, Writer};
-use core::global;
 
 /// The difficulty is defined as the maximum target divided by the block hash.
 #[derive(Debug, Clone, PartialEq, PartialOrd, Eq, Ord)]
@@ -56,13 +55,10 @@ impl Difficulty {
 	/// Computes the difficulty from a hash. Divides the maximum target by the
 	/// provided hash.
 	pub fn from_hash(h: &Hash) -> Difficulty {
-		let max_target = BigEndian::read_u64(&global::max_proof_target());
-		// Use the first 64 bits of the given hash
-		let mut in_vec = h.to_vec();
-		in_vec.truncate(8);
-		let num = BigEndian::read_u64(&in_vec);
+		let max_target = <u64>::max_value();
+		let num = h.to_u64();
 		Difficulty {
-			num: max_target / num,
+			num: max_target / max(num, 1),
 		}
 	}
 

core/src/global.rs

@@ -60,13 +60,6 @@ pub const TESTING_INITIAL_DIFFICULTY: u64 = 1;
 /// Testnet 2 initial block difficulty, high to see how it goes
 pub const TESTNET2_INITIAL_DIFFICULTY: u64 = 1000;
 
-/// The target is the 32-bytes hash block hashes must be lower than.
-pub const MAX_PROOF_TARGET: [u8; 8] = [0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff];
-
-/// We want to slow this right down for user testing at cuckoo 16, so pick a
-/// smaller max
-pub const MAX_PROOF_TARGET_TESTING: [u8; 8] = [0x05, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff];
-
 /// Types of chain a server can run with, dictates the genesis block and
 /// and mining parameters used.
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
@@ -141,18 +134,6 @@ pub fn coinbase_maturity() -> u64 {
 	}
 }
 
-/// Max Proof Target
-pub fn max_proof_target() -> [u8; 8] {
-	let param_ref = CHAIN_TYPE.read().unwrap();
-	match *param_ref {
-		ChainTypes::AutomatedTesting => MAX_PROOF_TARGET_TESTING,
-		ChainTypes::UserTesting => MAX_PROOF_TARGET_TESTING,
-		ChainTypes::Testnet1 => MAX_PROOF_TARGET_TESTING,
-		ChainTypes::Testnet2 => MAX_PROOF_TARGET,
-		ChainTypes::Mainnet => MAX_PROOF_TARGET,
-	}
-}
-
 /// Initial mining difficulty
 pub fn initial_block_difficulty() -> u64 {
 	let param_ref = CHAIN_TYPE.read().unwrap();