Retire pruned cache (#3573)

* use range beneath subtree for efficient is_pruned check

core/src/core/pmmr/pmmr.rs

@@ -12,8 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-use std::marker;
-use std::u64;
+use std::{marker, ops::Range, u64};
 
 use croaring::Bitmap;
 
@@ -541,10 +540,18 @@ pub fn peak_map_height(mut pos: u64) -> (u64, u64) {
 /// index. This function is the base on which all others, as well as the MMR,
 /// are built.
 pub fn bintree_postorder_height(num: u64) -> u64 {
-	if num == 0 {
+	let mut pos = num.saturating_sub(1);
+	if pos == 0 {
 		return 0;
 	}
-	peak_map_height(num - 1).1
+	let mut peak_size = ALL_ONES >> pos.leading_zeros();
+	while peak_size != 0 {
+		if pos >= peak_size {
+			pos -= peak_size;
+		}
+		peak_size >>= 1;
+	}
+	pos
 }
 
 /// Is this position a leaf in the MMR?
@@ -658,3 +665,10 @@ pub fn bintree_leftmost(num: u64) -> u64 {
 	let height = bintree_postorder_height(num);
 	num + 2 - (2 << height)
 }
+
+/// All pos in the subtree beneath the provided root, including root itself.
+pub fn bintree_range(num: u64) -> Range<u64> {
+	let height = bintree_postorder_height(num);
+	let leftmost = num + 2 - (2 << height);
+	leftmost..(num + 1)
+}