Add first pass of grow cache algo

eth2/utils/ssz/src/cached_tree_hash.rs

@@ -6,6 +6,7 @@ use std::ops::Range;
 use std::vec::Splice;
 
 mod impls;
+mod resize;
 mod tests;
 
 const BYTES_PER_CHUNK: usize = 32;

eth2/utils/ssz/src/cached_tree_hash/impls.rs

@@ -170,23 +170,6 @@ where
     }
 }
 
-/// New vec is bigger than old vec.
-fn grow_merkle_cache(cache: Vec<u8>, to: usize) -> Vec<u8> {
-    let new = Vec::with_capacity(to * HASHSIZE);
-
-    let i = cache.len() / HASHSIZE;
-    let j = to;
-
-    assert_eq!(i.next_power_of_two(), i);
-    assert_eq!(j.next_power_of_two(), j);
-
-    while i > 0 {
-        
-    }
-
-    new
-}
-
 fn get_packed_leaves<T>(vec: &Vec<T>) -> Vec<u8>
 where
     T: CachedTreeHash<T>,

eth2/utils/ssz/src/cached_tree_hash/resize.rs

@@ -0,0 +1,33 @@
+use super::*;
+
+/// New vec is bigger than old vec.
+fn grow_merkle_cache(old_bytes: &[u8], old_flags: &[bool], to: usize) -> Option<Vec<u8>> {
+    let mut bytes = Vec::with_capacity(to * HASHSIZE);
+    let mut flags = Vec::with_capacity(to);
+
+    let from = old_bytes.len() / HASHSIZE;
+    let to = to;
+
+    let distance = (from.leading_zeros() - to.leading_zeros()) as usize;
+
+    let leading_zero_chunks = 1 >> distance;
+
+    bytes.resize(leading_zero_chunks * HASHSIZE, 0);
+    flags.resize(leading_zero_chunks, true); // all new chunks are modified by default.
+
+    for i in 0..to.leading_zeros() as usize {
+        let new_slice = bytes.get_mut(1 >> i + distance..1 >> i + distance + 1)?;
+        let old_slice = old_bytes.get(1 >> i..1 >> i + 1)?;
+        new_slice.copy_from_slice(old_slice);
+    }
+
+    Some(bytes)
+}
+
+#[cfg(test)]
+mod test {
+    #[test]
+    fn can_grow() {
+        // TODO
+    }
+}