From 10a1ddf5e8288b251b01f7f101e691f961d44a79 Mon Sep 17 00:00:00 2001
From: AntiochP <30642645+antiochp@users.noreply.github.com>
Date: Mon, 13 Nov 2017 19:45:10 -0500
Subject: [PATCH] Use consensus rule that minimum difficulty is 10 (#251)

---
 chain/src/pipe.rs       |  7 +++++++
 core/src/consensus.rs   | 18 ++++++++++++++----
 core/src/core/target.rs |  9 ++++++++-
 3 files changed, 29 insertions(+), 5 deletions(-)

diff --git a/chain/src/pipe.rs b/chain/src/pipe.rs
index 2e8e04c85..e436acae6 100644
--- a/chain/src/pipe.rs
+++ b/chain/src/pipe.rs
@@ -21,6 +21,7 @@ use time;
 use core::consensus;
 use core::core::hash::{Hash, Hashed};
 use core::core::{Block, BlockHeader};
+use core::core::target::Difficulty;
 use core::core::transaction;
 use types::*;
 use store;
@@ -182,6 +183,12 @@ fn validate_header(header: &BlockHeader, ctx: &mut BlockContext) -> Result<(), E
 	if !ctx.opts.intersects(SKIP_POW) {
 		// verify the proof of work and related parameters
 
+		// explicit check to ensure we are not below the minimum difficulty
+		// we will also check difficulty based on next_difficulty later on
+		if header.difficulty < Difficulty::minimum() {
+			return Err(Error::DifficultyTooLow);
+		}
+
 		if header.total_difficulty != prev.total_difficulty.clone() + prev.pow.to_difficulty() {
 			return Err(Error::WrongTotalDifficulty);
 		}
diff --git a/core/src/consensus.rs b/core/src/consensus.rs
index d06a6f5f8..1624f61c6 100644
--- a/core/src/consensus.rs
+++ b/core/src/consensus.rs
@@ -20,6 +20,7 @@
 //! here.
 
 use std::fmt;
+use std::cmp::max;
 
 use ser;
 use core::target::Difficulty;
@@ -119,10 +120,10 @@ pub const DIFFICULTY_ADJUST_WINDOW: u64 = 23;
 /// Average time span of the difficulty adjustment window
 pub const BLOCK_TIME_WINDOW: u64 = DIFFICULTY_ADJUST_WINDOW * BLOCK_TIME_SEC;
 
-/// Maximum size time window used for difficutly adjustments
+/// Maximum size time window used for difficulty adjustments
 pub const UPPER_TIME_BOUND: u64 = BLOCK_TIME_WINDOW * 4 / 3;
 
-/// Minimum size time window used for difficutly adjustments
+/// Minimum size time window used for difficulty adjustments
 pub const LOWER_TIME_BOUND: u64 = BLOCK_TIME_WINDOW * 5 / 6;
 
 /// Error when computing the next difficulty adjustment.
@@ -181,7 +182,7 @@ where
 
 	// Check we have enough blocks
 	if window_end.len() < (MEDIAN_TIME_WINDOW as usize) {
-		return Ok(Difficulty::from_num(MINIMUM_DIFFICULTY));
+		return Ok(Difficulty::minimum());
 	}
 
 	// Calculating time medians at the beginning and end of the window.
@@ -203,7 +204,10 @@ where
 		ts_damp
 	};
 
-	Ok(diff_avg * Difficulty::from_num(BLOCK_TIME_WINDOW) / Difficulty::from_num(adj_ts))
+	let difficulty =
+		diff_avg * Difficulty::from_num(BLOCK_TIME_WINDOW) / Difficulty::from_num(adj_ts);
+
+	Ok(max(difficulty, Difficulty::minimum()))
 }
 
 /// Consensus rule that collections of items are sorted lexicographically over the wire.
@@ -319,6 +323,12 @@ mod test {
 			next_difficulty(repeat(200, 1000, just_enough)).unwrap(),
 			Difficulty::from_num(750)
 		);
+
+		// We should never drop below MINIMUM_DIFFICULTY (10)
+		assert_eq!(
+			next_difficulty(repeat(90, 10, just_enough)).unwrap(),
+			Difficulty::from_num(10)
+		);
 	}
 
 	#[test]
diff --git a/core/src/core/target.rs b/core/src/core/target.rs
index 673f6e559..87a31120f 100644
--- a/core/src/core/target.rs
+++ b/core/src/core/target.rs
@@ -25,6 +25,7 @@ use std::ops::{Add, Div, Mul, Sub};
 use serde::{de, Deserialize, Deserializer, Serialize, Serializer};
 use byteorder::{BigEndian, ByteOrder};
 
+use consensus;
 use core::hash::Hash;
 use ser::{self, Readable, Reader, Writeable, Writer};
 
@@ -38,7 +39,7 @@ pub struct Difficulty {
 }
 
 impl Difficulty {
-	/// Difficulty of zero, which is practically invalid (not target can be
+	/// Difficulty of zero, which is invalid (no target can be
 	/// calculated from it) but very useful as a start for additions.
 	pub fn zero() -> Difficulty {
 		Difficulty { num: 0 }
@@ -46,10 +47,16 @@ impl Difficulty {
 
 	/// Difficulty of one, which is the minumum difficulty (when the hash
 	/// equals the max target)
+	/// TODO - is this the minimum dificulty or is consensus::MINIMUM_DIFFICULTY the minimum?
 	pub fn one() -> Difficulty {
 		Difficulty { num: 1 }
 	}
 
+	/// Minimum difficulty according to our consensus rules.
+	pub fn minimum() -> Difficulty {
+		Difficulty { num: consensus::MINIMUM_DIFFICULTY }
+	}
+
 	/// Convert a `u32` into a `Difficulty`
 	pub fn from_num(num: u64) -> Difficulty {
 		Difficulty { num: num }