add helpers - implement get_block_hash

lighthouse/state/helpers.rs

@@ -0,0 +1,71 @@
+/*
+ * Collection of helper functions used in the state transition modules
+ */
+use super::active_state::ActiveState;
+use super::block::Block;
+use super::chain_config::ChainConfig;
+use super::utils::errors::ParameterError;
+use super::utils::types::Hash256;
+
+/*
+   pub fn get_signed_parent_hashes(
+   active_state: &ActiveState,
+   block: &Block,
+   attestation: &AttestationRecord,
+   chain_config: &ChainConfig)
+   -> Vec<Hash256> {
+   }
+   */
+
+pub fn get_block_hash(
+    active_state_recent_block_hashes: &Vec<Hash256>,
+    current_block_slot: &u64,
+    slot: &u64,
+    cycle_length: &u64, // convert from standard u8
+) -> Result<Hash256, ParameterError> {
+    // active_state must have at 2*cycle_length hashes
+    assert_error!(
+        active_state_recent_block_hashes.len() as u64 == cycle_length * 2,
+        ParameterError::InvalidInput(String::from(
+            "active state has incorrect number of block hashes"
+        ))
+    );
+
+    let state_start_slot = (*current_block_slot)
+        .checked_sub(cycle_length * 2)
+        .unwrap_or(0);
+
+    assert_error!(
+        (state_start_slot <= *slot) && (*slot < *current_block_slot),
+        ParameterError::InvalidInput(String::from("incorrect slot number"))
+    );
+
+    let index = 2 * cycle_length + (*slot) - *current_block_slot; // should always be positive
+    Ok(active_state_recent_block_hashes[index as usize])
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_get_block_hash() {
+        let block_slot: u64 = 10;
+        let slot: u64 = 3;
+        let cycle_length: u64 = 8;
+
+        let mut block_hashes: Vec<Hash256> = Vec::new();
+        for _i in 0..2 * cycle_length {
+            block_hashes.push(Hash256::random());
+        }
+
+        let result = get_block_hash(&block_hashes, &block_slot, &slot, &cycle_length).unwrap();
+
+        assert_eq!(
+            result,
+            block_hashes[(2 * cycle_length + slot - block_slot) as usize]
+        );
+
+        println!("{:?}", result);
+    }
+}