Implement testnet minimum difficulty

zebra-chain/src/parameters/network_upgrade.rs

@@ -8,7 +8,7 @@ use crate::parameters::{Network, Network::*};
 use std::collections::{BTreeMap, HashMap};
 use std::ops::Bound::*;
 
-use chrono::Duration;
+use chrono::{DateTime, Duration, Utc};
 
 /// A Zcash network upgrade.
 ///
@@ -229,6 +229,37 @@ impl NetworkUpgrade {
         }
     }
 
+    /// Returns true if the gap between `block_time` and `previous_block_time` is
+    /// greater than the Testnet minimum difficulty time gap. This time gap
+    /// depends on the `network` and `block_height`.
+    ///
+    /// Returns false on Mainnet, when `block_height` is less than the minimum
+    /// difficulty start height, and when the time gap is too small.
+    ///
+    /// `block_time` can be less than, equal to, or greater than
+    /// `previous_block_time`, because block times are provided by miners.
+    ///
+    /// Implements the Testnet minimum difficulty adjustment from ZIPs 205 and 208.
+    ///
+    /// Spec Note: Some parts of ZIPs 205 and 208 previously specified an incorrect
+    /// check for the time gap. This function implements the correct "greater than"
+    /// check.
+    pub fn is_testnet_min_difficulty_block(
+        network: Network,
+        block_height: block::Height,
+        block_time: DateTime<Utc>,
+        previous_block_time: DateTime<Utc>,
+    ) -> bool {
+        let block_time_gap = block_time - previous_block_time;
+        if let Some(min_difficulty_gap) =
+            NetworkUpgrade::minimum_difficulty_spacing_for_height(network, block_height)
+        {
+            block_time_gap > min_difficulty_gap
+        } else {
+            false
+        }
+    }
+
     /// Returns the averaging window timespan for the network upgrade.
     ///
     /// `AveragingWindowTimespan` from the Zcash specification.

zebra-state/src/service/check/difficulty.rs

@@ -32,7 +32,6 @@ pub const POW_MAX_ADJUST_UP_PERCENT: i32 = 16;
 pub const POW_MAX_ADJUST_DOWN_PERCENT: i32 = 32;
 
 /// Contains the context needed to calculate the adjusted difficulty for a block.
-#[allow(dead_code)]
 pub(super) struct AdjustedDifficulty {
     /// The `header.time` field from the candidate block
     candidate_time: DateTime<Utc>,
@@ -149,8 +148,21 @@ impl AdjustedDifficulty {
     /// Implements `ThresholdBits` from the Zcash specification, and the Testnet
     /// minimum difficulty adjustment from ZIPs 205 and 208.
     pub fn expected_difficulty_threshold(&self) -> CompactDifficulty {
-        // TODO: Testnet minimum difficulty
-        self.threshold_bits()
+        if NetworkUpgrade::is_testnet_min_difficulty_block(
+            self.network,
+            self.candidate_height,
+            self.candidate_time,
+            self.relevant_times[0],
+        ) {
+            assert_eq!(
+                self.network,
+                Network::Testnet,
+                "invalid network: the minimum difficulty rule only applies on testnet"
+            );
+            ExpandedDifficulty::target_difficulty_limit(self.network).to_compact()
+        } else {
+            self.threshold_bits()
+        }
     }
 
     /// Calculate the `difficulty_threshold` for a candidate block, based on the