From bb9c4918bf9fed3ff9adccda1bd3d6c3fc5a138f Mon Sep 17 00:00:00 2001
From: teor <teor@riseup.net>
Date: Thu, 19 Nov 2020 12:24:47 +1000
Subject: [PATCH] Implement threshold_bits

---
 zebra-chain/src/work/difficulty.rs          | 25 +++++++++++++++++++++
 zebra-state/src/service/check/difficulty.rs | 16 +++++++++----
 2 files changed, 37 insertions(+), 4 deletions(-)

diff --git a/zebra-chain/src/work/difficulty.rs b/zebra-chain/src/work/difficulty.rs
index 1057e13f2..e6303cee8 100644
--- a/zebra-chain/src/work/difficulty.rs
+++ b/zebra-chain/src/work/difficulty.rs
@@ -19,6 +19,8 @@ use std::{
     fmt,
     iter::Sum,
     ops::Add,
+    ops::Div,
+    ops::Mul,
 };
 
 use primitive_types::U256;
@@ -399,6 +401,29 @@ impl Sum<ExpandedDifficulty> for ExpandedDifficulty {
     }
 }
 
+impl<T> Div<T> for ExpandedDifficulty
+where
+    T: Into<U256>,
+{
+    type Output = ExpandedDifficulty;
+
+    fn div(self, rhs: T) -> Self::Output {
+        ExpandedDifficulty(self.0 / rhs)
+    }
+}
+
+impl<T> Mul<T> for ExpandedDifficulty
+where
+    U256: Mul<T>,
+    <U256 as Mul<T>>::Output: Into<U256>,
+{
+    type Output = ExpandedDifficulty;
+
+    fn mul(self, rhs: T) -> ExpandedDifficulty {
+        ExpandedDifficulty((self.0 * rhs).into())
+    }
+}
+
 impl PartialEq<block::Hash> for ExpandedDifficulty {
     /// Is `self` equal to `other`?
     ///
diff --git a/zebra-state/src/service/check/difficulty.rs b/zebra-state/src/service/check/difficulty.rs
index 5bb622cc3..1a5cd2a79 100644
--- a/zebra-state/src/service/check/difficulty.rs
+++ b/zebra-state/src/service/check/difficulty.rs
@@ -162,11 +162,19 @@ impl AdjustedDifficulty {
     /// Implements `ThresholdBits` from the Zcash specification. (Which excludes the
     /// Testnet minimum difficulty adjustment.)
     fn threshold_bits(&self) -> CompactDifficulty {
-        let mean_target = self.mean_target_difficulty();
-        let _median_timespan = self.median_timespan_bounded();
+        let averaging_window_timespan = NetworkUpgrade::averaging_window_timespan_for_height(
+            self.network,
+            self.candidate_height,
+        );
 
-        // TODO: calculate the actual value
-        mean_target.to_compact()
+        let threshold = (self.mean_target_difficulty() / averaging_window_timespan.num_seconds())
+            * self.median_timespan_bounded().num_seconds();
+        let threshold = min(
+            ExpandedDifficulty::target_difficulty_limit(self.network),
+            threshold,
+        );
+
+        threshold.to_compact()
     }
 
     /// Calculate the arithmetic mean of the averaging window thresholds: the