feature: Add the MinimumDifficulty consensus parameter

zebra-consensus/src/parameters.rs

@@ -10,9 +10,11 @@
 //! `Network` and `BlockHeight`.
 
 pub mod genesis;
+pub mod minimum_difficulty;
 pub mod network_upgrade;
 
 pub use genesis::*;
+pub use minimum_difficulty::*;
 pub use network_upgrade::*;
 
 #[cfg(test)]

zebra-consensus/src/parameters/minimum_difficulty.rs

@@ -0,0 +1,39 @@
+//! The minimum difficulty block rule for Zcash.
+
+use zebra_chain::types::BlockHeight;
+use zebra_chain::{Network, Network::*};
+
+/// The testnet block height when minimum difficulty blocks start being
+/// accepted.
+pub(crate) const TESTNET_MINIMUM_DIFFICULTY_HEIGHT: BlockHeight = BlockHeight(299_188);
+
+/// The Zcash Testnet consensus rules were changed to allow
+/// minimum-difficulty blocks, shortly after Testnet Sapling activation.
+/// See ZIP-205 and ZIP-208 for details.
+///
+/// This change represents a hard-fork on Testnet, but it doesn't appear on
+/// Mainnet, so we handle it as an independent consensus rule change.
+#[derive(Copy, Clone, Debug, Eq, Hash, PartialEq)]
+pub enum MinimumDifficulty {
+    /// Minimum difficulty blocks are rejected.
+    ///
+    /// Always returned for Mainnet blocks.
+    Rejected,
+    /// Minimum difficulty blocks are allowed.
+    ///
+    /// Only allowed for Testnet blocks.
+    AllowedOnTestnet,
+}
+
+impl MinimumDifficulty {
+    /// Returns the current minimum difficulty rule for `network` and `height`.
+    pub fn current(network: Network, height: BlockHeight) -> MinimumDifficulty {
+        use MinimumDifficulty::*;
+
+        match network {
+            Mainnet => Rejected,
+            Testnet if (height >= TESTNET_MINIMUM_DIFFICULTY_HEIGHT) => AllowedOnTestnet,
+            Testnet => Rejected,
+        }
+    }
+}

zebra-consensus/src/parameters/tests.rs

@@ -183,3 +183,44 @@ fn branch_id_consistent(network: Network) {
         }
     }
 }
+
+#[test]
+fn minimum_difficulty_mainnet() {
+    minimum_difficulty(Mainnet)
+}
+
+#[test]
+fn minimum_difficulty_testnet() {
+    minimum_difficulty(Testnet)
+}
+
+/// Test MinimumDifficulty
+fn minimum_difficulty(network: Network) {
+    use MinimumDifficulty::*;
+
+    let allowed_if_testnet = match network {
+        Mainnet => Rejected,
+        Testnet => AllowedOnTestnet,
+    };
+
+    assert_eq!(
+        MinimumDifficulty::current(network, BlockHeight(0)),
+        Rejected
+    );
+    assert_eq!(
+        MinimumDifficulty::current(network, BlockHeight(299_187)),
+        Rejected
+    );
+    assert_eq!(
+        MinimumDifficulty::current(network, BlockHeight(299_188)),
+        allowed_if_testnet
+    );
+    assert_eq!(
+        MinimumDifficulty::current(network, BlockHeight(299_189)),
+        allowed_if_testnet
+    );
+    assert_eq!(
+        MinimumDifficulty::current(network, BlockHeight::MAX),
+        allowed_if_testnet
+    );
+}