use trait to simplify and consolidate cumulative code (#17852)

        (cherry picked from commit 2dc6969)

        # Conflicts:
        #       runtime/src/accounts_hash.rs

runtime/src/accounts_hash.rs

@@ -108,6 +108,22 @@ impl CumulativeOffset {
     }
 }
 
+pub trait ExtractSliceFromRawData<'b, T: 'b> {
+    fn extract<'a>(&'b self, offset: &'a CumulativeOffset, start: usize) -> &'b [T];
+}
+
+impl<'b, T: 'b> ExtractSliceFromRawData<'b, T> for Vec<Vec<T>> {
+    fn extract<'a>(&'b self, offset: &'a CumulativeOffset, start: usize) -> &'b [T] {
+        &self[offset.index[0]][start..]
+    }
+}
+
+impl<'b, T: 'b> ExtractSliceFromRawData<'b, T> for Vec<Vec<Vec<T>>> {
+    fn extract<'a>(&'b self, offset: &'a CumulativeOffset, start: usize) -> &'b [T] {
+        &self[offset.index[0]][offset.index[1]][start..]
+    }
+}
+
 // Allow retrieving &[start..end] from a logical src: Vec<T>, where src is really Vec<Vec<T>> (or later Vec<Vec<Vec<T>>>)
 // This model prevents callers from having to flatten which saves both working memory and time.
 #[derive(Default, Debug)]
@@ -163,32 +179,28 @@ impl CumulativeOffsets {
         }
     }
 
-    fn find(&self, start: usize) -> (usize, &CumulativeOffset) {
+    fn find_index(&self, start: usize) -> usize {
         assert!(!self.cumulative_offsets.is_empty());
-        let index = match self.cumulative_offsets[..]
-            .binary_search_by(|index| index.start_offset.cmp(&start))
-        {
+        match self.cumulative_offsets[..].binary_search_by(|index| index.start_offset.cmp(&start)) {
             Ok(index) => index,
             Err(index) => index - 1, // we would insert at index so we are before the item at index
-        };
+        }
+    }
+
+    fn find(&self, start: usize) -> (usize, &CumulativeOffset) {
+        let index = self.find_index(start);
         let index = &self.cumulative_offsets[index];
         let start = start - index.start_offset;
         (start, index)
     }
 
     // return the biggest slice possible that starts at 'start'
-    pub fn get_slice<'a, T>(&self, raw: &'a [Vec<T>], start: usize) -> &'a [T] {
-        let (start, index) = self.find(start);
-        const DIMENSION: usize = 0;
-        &raw[index.index[DIMENSION]][start..]
-    }
-
-    // return the biggest slice possible that starts at 'start'
-    pub fn get_slice_2d<'a, T>(&self, raw: &'a [Vec<Vec<T>>], start: usize) -> &'a [T] {
+    pub fn get_slice<'a, 'b, T, U>(&'a self, raw: &'b U, start: usize) -> &'b [T]
+    where
+        U: ExtractSliceFromRawData<'b, T> + 'b,
+    {
         let (start, index) = self.find(start);
-        const DIMENSION_0: usize = 0;
-        const DIMENSION_1: usize = 1;
-        &raw[index.index[DIMENSION_0]][index.index[DIMENSION_1]][start..]
+        raw.extract(index, start)
     }
 }
 
@@ -751,7 +763,7 @@ impl AccountsHash {
             // move tail hashes that don't evenly hash into a 1d vector for next time
             let mut i = hash_total - left_over_hashes;
             while i < hash_total {
-                let data = cumulative.get_slice_2d(&hashes, i);
+                let data = cumulative.get_slice(&hashes, i);
                 next_pass.remaining_unhashed.extend(data);
                 i += data.len();
             }
@@ -769,7 +781,7 @@ impl AccountsHash {
                 hash_total, // note this does not include the ones that didn't divide evenly, unless we're in the last iteration
                 MERKLE_FANOUT,
                 Some(TARGET_FANOUT_LEVEL),
-                |start| cumulative.get_slice_2d(&hashes, start),
+                |start| cumulative.get_slice(&hashes, start),
                 Some(TARGET_FANOUT_LEVEL),
             )
             .1;
@@ -1636,7 +1648,7 @@ pub mod tests {
 
     #[test]
     fn test_accountsdb_cumulative_offsets2_d() {
-        let input = vec![vec![vec![0, 1], vec![], vec![2, 3, 4], vec![]]];
+        let input: Vec<Vec<Vec<u64>>> = vec![vec![vec![0, 1], vec![], vec![2, 3, 4], vec![]]];
         let cumulative = CumulativeOffsets::from_raw_2d(&input);
 
         let src: Vec<_> = input
@@ -1661,7 +1673,7 @@ pub mod tests {
         assert_eq!(cumulative.cumulative_offsets[1].start_offset, 2);
 
         for start in 0..len {
-            let slice = cumulative.get_slice_2d(&input, start);
+            let slice: &[u64] = cumulative.get_slice(&input, start);
             let len = slice.len();
             assert!(len > 0);
             assert_eq!(&src[start..(start + len)], slice);
@@ -1690,7 +1702,7 @@ pub mod tests {
         assert_eq!(cumulative.cumulative_offsets[1].start_offset, 2);
 
         for start in 0..len {
-            let slice = cumulative.get_slice_2d(&input, start);
+            let slice: &[u64] = cumulative.get_slice(&input, start);
             let len = slice.len();
             assert!(len > 0);
             assert_eq!(&src[start..(start + len)], slice);
@@ -1731,7 +1743,7 @@ pub mod tests {
         assert_eq!(cumulative.cumulative_offsets[1].start_offset, 2);
 
         for start in 0..len {
-            let slice = cumulative.get_slice_2d(&input, start);
+            let slice: &[u64] = cumulative.get_slice(&input, start);
             let len = slice.len();
             assert!(len > 0);
             assert_eq!(&src[start..(start + len)], slice);
@@ -1903,7 +1915,7 @@ pub mod tests {
                 ];
                 let offsets = CumulativeOffsets::from_raw_2d(&src);
 
-                let get_slice = |start: usize| -> &[Hash] { offsets.get_slice_2d(&src, start) };
+                let get_slice = |start: usize| -> &[Hash] { offsets.get_slice(&src, start) };
                 let result2 = AccountsHash::compute_merkle_root_from_slices(
                     offsets.total_count,
                     fanout,