Add functions to get deposit_count and deposit_root from deposit cache

beacon_node/eth1/src/deposit_cache.rs

@@ -4,6 +4,8 @@ use std::ops::Range;
 use tree_hash::TreeHash;
 use types::{Deposit, Hash256};
 
+const DEPOSIT_CONTRACT_TREE_DEPTH: usize = 32;
+
 #[derive(Debug, PartialEq, Clone)]
 pub enum Error {
     /// A deposit log was added when a prior deposit was not already in the cache.
@@ -203,6 +205,30 @@ impl DepositCache {
             Ok((tree.root(), deposits))
         }
     }
+
+    /// Gets the deposit count at block height = block_number.
+    ///
+    /// Fetches the `DepositLog` that was emitted at or just before `block_number`
+    /// and returns the deposit count as `index + 1`.
+    pub fn get_deposit_count_from_cache(&self, block_number: u64) -> Option<u64> {
+        self.logs
+            .iter()
+            .take_while(|log| log.block_number >= block_number)
+            .collect::<Vec<_>>()
+            .last()
+            .map(|x| x.index + 1)
+    }
+
+    /// Gets the deposit root at block height = block_number.
+    ///
+    /// Fetches the `DepositLog` that was emitted at or just before `block_number`
+    /// and returns the deposit root at that state.
+    pub fn get_deposit_root_from_cache(&self, block_number: u64) -> Option<Hash256> {
+        let index = self.get_deposit_count_from_cache(block_number)? - 1;
+        let roots = self.roots.get(0..index as usize)?;
+        let tree = DepositDataTree::create(roots, index as usize, DEPOSIT_CONTRACT_TREE_DEPTH);
+        Some(tree.root())
+    }
 }
 
 /// Returns `int` as little-endian bytes with a length of 32.