Add get_signed_parent_hashes (untested)

lighthouse/state/mod.rs

@@ -13,4 +13,5 @@ pub mod chain_config;
 pub mod block;
 pub mod crosslink_record;
 pub mod shard_and_committee;
+pub mod transition;
 pub mod validator_record;

lighthouse/state/transition/helpers.rs

@@ -0,0 +1,68 @@
+use super::Hash256;
+use super::TransitionError;
+
+pub fn get_signed_parent_hashes(
+    cycle_length: u64,
+    block_slot: u64,
+    attestation_slot: u64,
+    current_hashes: Vec<Hash256>,
+    oblique_hashes: Vec<Hash256>)
+    -> Result<Vec<Hash256>, TransitionError>
+{
+    let start = cycle_length.checked_add(attestation_slot)
+        .and_then(|x| x.checked_sub(block_slot))
+        .ok_or(TransitionError::IntWrapping)?;
+    let start = start as usize;
+
+    let end = cycle_length.checked_mul(2)
+        .and_then(|x| x.checked_add(attestation_slot))
+        .and_then(|x| x.checked_sub(block_slot))
+        .and_then(|x| x.checked_sub(oblique_hashes.len() as u64))
+        .ok_or(TransitionError::IntWrapping)?;
+    let end = end as usize;
+    
+    println!("start: {}, end: {}", start, end);
+
+    if end >= current_hashes.len() {
+        return Err(TransitionError::OutOfBounds);
+    }
+    if start > end {
+        return Err(TransitionError::InvalidInput("cats"));
+    }
+
+    let mut hashes = Vec::new();
+
+    hashes.extend_from_slice(
+        &current_hashes[start..end]);
+    hashes.append(&mut oblique_hashes.clone());
+
+    Ok(hashes)
+}
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn get_n_hashes(value: &[u8], n: usize) -> Vec<Hash256> {
+        (0..n).map(|_| Hash256::from_slice(value)).collect()
+    }
+
+    #[test]
+    fn test_get_signed_hashes() {
+        let cycle_length: u64 = 8;
+        let block_slot: u64 = 500;
+        let attestation_slot: u64 = 498;
+        let current_hashes =
+            get_n_hashes(b"0", 100);
+        let oblique_hashes = get_n_hashes(b"1", 2);
+        let result = get_signed_parent_hashes(
+            cycle_length,
+            block_slot,
+            attestation_slot,
+            current_hashes,
+            oblique_hashes);
+        // TODO: complete testing
+        assert!(result.is_ok());
+    }
+}

lighthouse/state/transition/mod.rs

@@ -0,0 +1,11 @@
+use super::super::utils::types::Hash256;
+
+pub mod helpers;
+
+#[derive(Debug)]
+pub enum TransitionError {
+    IntWrapping,
+    OutOfBounds,
+    InvalidInput(&str),
+}
+