From 859c48d5e58e5cb218c6c8e7836bebb6aa29607d Mon Sep 17 00:00:00 2001
From: Paul Hauner <paul@paulhauner.com>
Date: Fri, 24 Aug 2018 14:33:05 +1000
Subject: [PATCH] Progress on parent hash selection. Add test.

---
 lighthouse/state/transition/helpers.rs | 91 ++++++++++++++++++--------
 lighthouse/state/transition/mod.rs     |  2 +-
 2 files changed, 63 insertions(+), 30 deletions(-)

diff --git a/lighthouse/state/transition/helpers.rs b/lighthouse/state/transition/helpers.rs
index c8e98316e..94de8a3d9 100644
--- a/lighthouse/state/transition/helpers.rs
+++ b/lighthouse/state/transition/helpers.rs
@@ -2,38 +2,62 @@ use super::Hash256;
 use super::TransitionError;
 
 pub fn get_signed_parent_hashes(
-    cycle_length: u64,
+    cycle_length: u8,
     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;
+    // This cast places a limit on cycle_length. If you change it, check math
+    // for overflow.
+    let cycle_length: u64 = cycle_length as u64;
 
-    let end = cycle_length.checked_mul(2)
-        .and_then(|x| x.checked_add(attestation_slot))
-        .and_then(|x| x.checked_sub(block_slot))
+    if current_hashes.len() as u64 != (cycle_length * 2) {
+        return Err(TransitionError::InvalidInput(String::from(
+                    "current_hashes.len() must equal cycle_length * 2")));
+    }
+    if attestation_slot >= block_slot {
+        return Err(TransitionError::InvalidInput(String::from(
+                    "attestation_slot must be less than block_slot")));
+    }
+    if oblique_hashes.len() as u64 > cycle_length {
+        return Err(TransitionError::InvalidInput(String::from(
+                    "oblique_hashes.len() must be <= cycle_length * 2")));
+    }
+
+    /*
+     * Cannot underflow as block_slot cannot be less
+     * than attestation_slot.
+     */
+    let attestation_distance = block_slot - attestation_slot;
+
+    if attestation_distance > cycle_length {
+        return Err(TransitionError::InvalidInput(String::from(
+                    "attestation_slot must be withing one cycle of block_slot")));
+    }
+
+    /*
+     * Cannot underflow because attestation_distance cannot
+     * be larger than cycle_length.
+     */
+    let start = cycle_length - attestation_distance;
+
+    /*
+     * Overflow is potentially impossible, but proof is complicated
+     * enough to just use checked math.
+     *
+     * Math is:
+     * start + cycle_length - oblique_hashes.len()
+     */
+    let end = start.checked_add(cycle_length)
         .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]);
+        &current_hashes[(start as usize)..(end as usize)]);
     hashes.append(&mut oblique_hashes.clone());
 
     Ok(hashes)
@@ -44,25 +68,34 @@ pub fn get_signed_parent_hashes(
 mod tests {
     use super::*;
 
-    fn get_n_hashes(value: &[u8], n: usize) -> Vec<Hash256> {
-        (0..n).map(|_| Hash256::from_slice(value)).collect()
+    fn get_range_of_hashes(from: usize, to: usize) -> Vec<Hash256> {
+        (from..to).map(|i| get_hash(&vec![i as u8])).collect()
+    }
+
+    fn get_hash(value: &[u8]) -> Hash256 {
+        Hash256::from_slice(value)
     }
 
     #[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);
+    fn test_get_signed_hashes_no_oblique() {
+        /*
+         * Google Slides example.
+         */
+        let cycle_length: u8 = 8;
+        let block_slot: u64 = 19;
+        let attestation_slot: u64 = 15;
+        let current_hashes = get_range_of_hashes(3, 19);
+        let oblique_hashes = vec![];
         let result = get_signed_parent_hashes(
             cycle_length,
             block_slot,
             attestation_slot,
             current_hashes,
             oblique_hashes);
-        // TODO: complete testing
         assert!(result.is_ok());
+        let result = result.unwrap();
+        assert_eq!(result.len(), cycle_length as usize);
+        let expected_result = get_range_of_hashes(7, 15);
+        assert_eq!(result, expected_result);
     }
 }
diff --git a/lighthouse/state/transition/mod.rs b/lighthouse/state/transition/mod.rs
index 676266d95..e958f7831 100644
--- a/lighthouse/state/transition/mod.rs
+++ b/lighthouse/state/transition/mod.rs
@@ -6,6 +6,6 @@ pub mod helpers;
 pub enum TransitionError {
     IntWrapping,
     OutOfBounds,
-    InvalidInput(&str),
+    InvalidInput(String),
 }