Move difficulty threshold checks into their own function

zebra-consensus/src/block.rs

@@ -135,18 +135,7 @@ where
 
             // Do the difficulty checks first, to raise the threshold for
             // attacks that use any other fields.
-            let difficulty_threshold = block
+            check::difficulty_is_valid(&block.header, &height, &hash)?;
-                .header
-                .difficulty_threshold
-                .to_expanded()
-                .ok_or(BlockError::InvalidDifficulty(height, hash))?;
-            if hash > difficulty_threshold {
-                Err(BlockError::DifficultyFilter(
-                    height,
-                    hash,
-                    difficulty_threshold,
-                ))?;
-            }
             check::equihash_solution_is_valid(&block.header)?;
 
             // Since errors cause an early exit, try to do the

zebra-consensus/src/block/check.rs

@@ -3,6 +3,8 @@
 use chrono::{DateTime, Utc};
 
 use zebra_chain::{
+    block::Hash,
+    block::Height,
     block::{Block, Header},
     parameters::NetworkUpgrade,
     work::equihash,
@@ -39,6 +41,35 @@ pub fn coinbase_is_first(block: &Block) -> Result<(), BlockError> {
     Ok(())
 }
 
+/// Returns `Ok(())` if `hash` passes the difficulty filter and PoW limit,
+/// based on the fields in `header`.
+///
+/// If the block is invalid, returns an error containing `height` and `hash`.
+pub fn difficulty_is_valid(
+    header: &Header,
+    height: &Height,
+    hash: &Hash,
+) -> Result<(), BlockError> {
+    // TODO:
+    //   - PoW limit
+
+    let difficulty_threshold = header
+        .difficulty_threshold
+        .to_expanded()
+        .ok_or(BlockError::InvalidDifficulty(*height, *hash))?;
+
+    // Difficulty filter
+    if hash > &difficulty_threshold {
+        Err(BlockError::DifficultyFilter(
+            *height,
+            *hash,
+            difficulty_threshold,
+        ))?;
+    }
+
+    Ok(())
+}
+
 /// Returns `Ok(())` if the `EquihashSolution` is valid for `header`
 pub fn equihash_solution_is_valid(header: &Header) -> Result<(), equihash::Error> {
     header.solution.check(&header)