Enable faster sync (#3108)

* add bitmap accumulator refactor vec backend so we can use it outside of tests introduce a "hash only" vec backend for the accumulator * get core tests passing * initial test coverage for bitmap_accumulator * better test coverage for bitmap accumulator and cleanup code * refactor txhashset roots, call validate() on roots during block validation * fix store tests * log the "merged" root when validating roots * cleanup, revise based on feedback * cleanup * rework it to pass explicit size into bitmap accumulator when applying
2025-02-01 08:51:08 +03:00 · 2019-11-26 20:21:49 +00:00 · 2019-11-26 20:21:49 +00:00 · 11ac7d827a
commit 11ac7d827a
parent 41896f0ec2
22 changed files with 902 additions and 176 deletions
--- a/Cargo.lock
+++ b/Cargo.lock
@ -130,6 +130,11 @@ dependencies = [
 "which 1.0.5 (registry+https://github.com/rust-lang/crates.io-index)",
 ]

+[[package]]
+name = "bit-vec"
+version = "0.6.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
 [[package]]
 name = "bitflags"
 version = "0.9.1"
@ -719,6 +724,7 @@ dependencies = [
 name = "grin_chain"
 version = "3.0.0-alpha.1"
 dependencies = [
+ "bit-vec 0.6.1 (registry+https://github.com/rust-lang/crates.io-index)",
 "bitflags 1.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
 "byteorder 1.3.2 (registry+https://github.com/rust-lang/crates.io-index)",
 "chrono 0.4.7 (registry+https://github.com/rust-lang/crates.io-index)",
@ -894,6 +900,7 @@ dependencies = [
 name = "grin_store"
 version = "3.0.0-alpha.1"
 dependencies = [
+ "bit-vec 0.6.1 (registry+https://github.com/rust-lang/crates.io-index)",
 "byteorder 1.3.2 (registry+https://github.com/rust-lang/crates.io-index)",
 "chrono 0.4.7 (registry+https://github.com/rust-lang/crates.io-index)",
 "croaring 0.3.9 (registry+https://github.com/rust-lang/crates.io-index)",
@ -2744,6 +2751,7 @@ dependencies = [
 "checksum backtrace-sys 0.1.29 (registry+https://github.com/rust-lang/crates.io-index)" = "12cb9f1eef1d1fc869ad5a26c9fa48516339a15e54a227a25460fc304815fdb3"
 "checksum base64 0.9.3 (registry+https://github.com/rust-lang/crates.io-index)" = "489d6c0ed21b11d038c31b6ceccca973e65d73ba3bd8ecb9a2babf5546164643"
 "checksum bindgen 0.37.4 (registry+https://github.com/rust-lang/crates.io-index)" = "1b25ab82877ea8fe6ce1ce1f8ac54361f0218bad900af9eb11803994bf67c221"
+"checksum bit-vec 0.6.1 (registry+https://github.com/rust-lang/crates.io-index)" = "a4523a10839ffae575fb08aa3423026c8cb4687eef43952afb956229d4f246f7"
 "checksum bitflags 0.9.1 (registry+https://github.com/rust-lang/crates.io-index)" = "4efd02e230a02e18f92fc2735f44597385ed02ad8f831e7c1c1156ee5e1ab3a5"
 "checksum bitflags 1.1.0 (registry+https://github.com/rust-lang/crates.io-index)" = "3d155346769a6855b86399e9bc3814ab343cd3d62c7e985113d46a0ec3c281fd"
 "checksum blake2-rfc 0.2.18 (registry+https://github.com/rust-lang/crates.io-index)" = "5d6d530bdd2d52966a6d03b7a964add7ae1a288d25214066fd4b600f0f796400"
--- a/api/src/types.rs
+++ b/api/src/types.rs
@ -119,7 +119,7 @@ impl TxHashSet {
 	pub fn from_head(head: Arc<chain::Chain>) -> TxHashSet {
 		let roots = head.get_txhashset_roots();
 		TxHashSet {
-			output_root_hash: roots.output_root.to_hex(),
+			output_root_hash: roots.output_root().to_hex(),
 			range_proof_root_hash: roots.rproof_root.to_hex(),
 			kernel_root_hash: roots.kernel_root.to_hex(),
 		}
--- a/chain/Cargo.toml
+++ b/chain/Cargo.toml
@ -10,6 +10,7 @@ workspace = ".."
 edition = "2018"

 [dependencies]
+bit-vec = "0.6"
 bitflags = "1"
 byteorder = "1"
 failure = "0.1"
--- a/chain/src/chain.rs
+++ b/chain/src/chain.rs
@ -594,7 +594,7 @@ impl Chain {
 		b.header.prev_root = prev_root;

 		// Set the output, rangeproof and kernel MMR roots.
-		b.header.output_root = roots.output_root;
+		b.header.output_root = roots.output_root();
 		b.header.range_proof_root = roots.rproof_root;
 		b.header.kernel_root = roots.kernel_root;

--- a/chain/src/txhashset.rs
+++ b/chain/src/txhashset.rs
@ -15,10 +15,12 @@
 //! Utility structs to handle the 3 hashtrees (output, range proof,
 //! kernel) more conveniently and transactionally.

+mod bitmap_accumulator;
 mod rewindable_kernel_view;
 mod txhashset;
 mod utxo_view;

+pub use self::bitmap_accumulator::*;
 pub use self::rewindable_kernel_view::*;
 pub use self::txhashset::*;
 pub use self::utxo_view::*;
--- a/chain/src/txhashset/bitmap_accumulator.rs
+++ b/chain/src/txhashset/bitmap_accumulator.rs
@ -0,0 +1,239 @@
+// Copyright 2019 The Grin Developers
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use std::convert::TryFrom;
+use std::time::Instant;
+
+use bit_vec::BitVec;
+use croaring::Bitmap;
+
+use crate::core::core::hash::{DefaultHashable, Hash};
+use crate::core::core::pmmr::{self, ReadonlyPMMR, VecBackend, PMMR};
+use crate::core::ser::{self, FixedLength, PMMRable, Readable, Reader, Writeable, Writer};
+use crate::error::{Error, ErrorKind};
+
+/// The "bitmap accumulator" allows us to commit to a specific bitmap by splitting it into
+/// fragments and inserting these fragments into an MMR to produce an overall root hash.
+/// Leaves in the MMR are fragments of the bitmap consisting of 1024 contiguous bits
+/// from the overall bitmap. The first (leftmost) leaf in the MMR represents the first 1024 bits
+/// of the bitmap, the next leaf is the next 1024 bits of the bitmap etc.
+///
+/// Flipping a single bit does not require the full bitmap to be rehashed, only the path from the
+/// relevant leaf up to its associated peak.
+///
+/// Flipping multiple bits *within* a single chunk is no more expensive than flipping a single bit
+/// as a leaf node in the MMR represents a sequence of 1024 bits. Flipping multiple bits located
+/// close together is a relatively cheap operation with minimal rehashing required to update the
+/// relevant peaks and the overall MMR root.
+///
+/// It is also possible to generate Merkle proofs for these 1024 bit fragments, proving
+/// both inclusion and location in the overall "accumulator" MMR. We plan to take advantage of
+/// this during fast sync, allowing for validation of partial data.
+///
+#[derive(Clone)]
+pub struct BitmapAccumulator {
+	backend: VecBackend<BitmapChunk>,
+}
+
+impl BitmapAccumulator {
+	/// Crate a new empty bitmap accumulator.
+	pub fn new() -> BitmapAccumulator {
+		BitmapAccumulator {
+			backend: VecBackend::new_hash_only(),
+		}
+	}
+
+	/// Initialize a bitmap accumulator given the provided idx iterator.
+	pub fn init<T: IntoIterator<Item = u64>>(&mut self, idx: T, size: u64) -> Result<(), Error> {
+		self.apply_from(idx, 0, size)
+	}
+
+	/// Find the start of the first "chunk" of 1024 bits from the provided idx.
+	/// Zero the last 10 bits to round down to multiple of 1024.
+	pub fn chunk_start_idx(idx: u64) -> u64 {
+		idx & !0x3ff
+	}
+
+	/// The first 1024 belong to chunk 0, the next 1024 to chunk 1 etc.
+	fn chunk_idx(idx: u64) -> u64 {
+		idx / 1024
+	}
+
+	/// Apply the provided idx iterator to our bitmap accumulator.
+	/// We start at the chunk containing from_idx and rebuild chunks as necessary
+	/// for the bitmap, limiting it to size (in bits).
+	/// If from_idx is 1023 and size is 1024 then we rebuild a single chunk.
+	fn apply_from<T>(&mut self, idx: T, from_idx: u64, size: u64) -> Result<(), Error>
+	where
+		T: IntoIterator<Item = u64>,
+	{
+		let now = Instant::now();
+
+		// Find the (1024 bit chunk) chunk_idx for the (individual bit) from_idx.
+		let from_chunk_idx = BitmapAccumulator::chunk_idx(from_idx);
+		let mut chunk_idx = from_chunk_idx;
+
+		let mut chunk = BitmapChunk::new();
+
+		let mut idx_iter = idx.into_iter().filter(|&x| x < size).peekable();
+		while let Some(x) = idx_iter.peek() {
+			if *x < chunk_idx * 1024 {
+				// skip until we reach our first chunk
+				idx_iter.next();
+			} else if *x < (chunk_idx + 1) * 1024 {
+				let idx = idx_iter.next().expect("next after peek");
+				chunk.set(idx % 1024, true);
+			} else {
+				self.append_chunk(chunk)?;
+				chunk_idx += 1;
+				chunk = BitmapChunk::new();
+			}
+		}
+		if chunk.any() {
+			self.append_chunk(chunk)?;
+		}
+		debug!(
+			"applied {} chunks from idx {} to idx {} ({}ms)",
+			1 + chunk_idx - from_chunk_idx,
+			from_chunk_idx,
+			chunk_idx,
+			now.elapsed().as_millis(),
+		);
+		Ok(())
+	}
+
+	/// Apply updates to the bitmap accumulator given an iterator of invalidated idx and
+	/// an iterator of idx to be set to true.
+	/// We determine the existing chunks to be rebuilt given the invalidated idx.
+	/// We then rebuild given idx, extending the accumulator with new chunk(s) as necessary.
+	/// Resulting bitmap accumulator will contain sufficient bitmap chunks to cover size.
+	/// If size is 1 then we will have a single chunk.
+	/// If size is 1023 then we will have a single chunk (bits 0 to 1023 inclusive).
+	/// If the size is 1024 then we will have two chunks.
+	pub fn apply<T, U>(&mut self, invalidated_idx: T, idx: U, size: u64) -> Result<(), Error>
+	where
+		T: IntoIterator<Item = u64>,
+		U: IntoIterator<Item = u64>,
+	{
+		// Determine the earliest chunk by looking at the min invalidated idx (assume sorted).
+		// Rewind prior to this and reapply new_idx.
+		// Note: We rebuild everything after rewind point but much of the bitmap may be
+		// unchanged. This can be further optimized by only rebuilding necessary chunks and
+		// rehashing.
+		if let Some(from_idx) = invalidated_idx.into_iter().next() {
+			self.rewind_prior(from_idx)?;
+			self.pad_left(from_idx)?;
+			self.apply_from(idx, from_idx, size)?;
+		}
+
+		Ok(())
+	}
+
+	/// Given the provided (bit) idx rewind the bitmap accumulator to the end of the
+	/// previous chunk ready for the updated chunk to be appended.
+	fn rewind_prior(&mut self, from_idx: u64) -> Result<(), Error> {
+		let chunk_idx = BitmapAccumulator::chunk_idx(from_idx);
+		let last_pos = self.backend.size();
+		let mut pmmr = PMMR::at(&mut self.backend, last_pos);
+		let chunk_pos = pmmr::insertion_to_pmmr_index(chunk_idx + 1);
+		let rewind_pos = chunk_pos.saturating_sub(1);
+		pmmr.rewind(rewind_pos, &Bitmap::create())
+			.map_err(|e| ErrorKind::Other(e))?;
+		Ok(())
+	}
+
+	/// Make sure we append empty chunks to fill in any gap before we append the chunk
+	/// we actually care about. This effectively pads the bitmap with 1024 chunks of 0s
+	/// as necessary to put the new chunk at the correct place.
+	fn pad_left(&mut self, from_idx: u64) -> Result<(), Error> {
+		let chunk_idx = BitmapAccumulator::chunk_idx(from_idx);
+		let current_chunk_idx = pmmr::n_leaves(self.backend.size());
+		for _ in current_chunk_idx..chunk_idx {
+			self.append_chunk(BitmapChunk::new())?;
+		}
+		Ok(())
+	}
+
+	/// Append a new chunk to the BitmapAccumulator.
+	/// Append parent hashes (if any) as necessary to build associated peak.
+	pub fn append_chunk(&mut self, chunk: BitmapChunk) -> Result<u64, Error> {
+		let last_pos = self.backend.size();
+		PMMR::at(&mut self.backend, last_pos)
+			.push(&chunk)
+			.map_err(|e| ErrorKind::Other(e).into())
+	}
+
+	/// The root hash of the bitmap accumulator MMR.
+	pub fn root(&self) -> Hash {
+		ReadonlyPMMR::at(&self.backend, self.backend.size()).root()
+	}
+}
+
+/// A bitmap "chunk" representing 1024 contiguous bits of the overall bitmap.
+/// The first 1024 bits belong in one chunk. The next 1024 bits in the next chunk, etc.
+#[derive(Clone, Debug)]
+pub struct BitmapChunk(BitVec);
+
+impl BitmapChunk {
+	const LEN_BITS: usize = 1024;
+	const LEN_BYTES: usize = Self::LEN_BITS / 8;
+
+	/// Create a new bitmap chunk, defaulting all bits in the chunk to false.
+	pub fn new() -> BitmapChunk {
+		BitmapChunk(BitVec::from_elem(Self::LEN_BITS, false))
+	}
+
+	/// Set a single bit in this chunk.
+	/// 0-indexed from start of chunk.
+	/// Panics if idx is outside the valid range of bits in a chunk.
+	pub fn set(&mut self, idx: u64, value: bool) {
+		let idx = usize::try_from(idx).expect("usize from u64");
+		assert!(idx < Self::LEN_BITS);
+		self.0.set(idx, value)
+	}
+
+	/// Does this bitmap chunk have any bits set to 1?
+	pub fn any(&self) -> bool {
+		self.0.any()
+	}
+}
+
+impl PMMRable for BitmapChunk {
+	type E = Self;
+
+	fn as_elmt(&self) -> Self::E {
+		self.clone()
+	}
+}
+
+impl FixedLength for BitmapChunk {
+	const LEN: usize = Self::LEN_BYTES;
+}
+
+impl DefaultHashable for BitmapChunk {}
+
+impl Writeable for BitmapChunk {
+	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ser::Error> {
+		self.0.to_bytes().write(writer)
+	}
+}
+
+impl Readable for BitmapChunk {
+	/// Reading is not currently supported, just return an empty one for now.
+	/// We store the underlying roaring bitmap externally for the bitmap accumulator
+	/// and the "hash only" backend means we never actually read these chunks.
+	fn read(_reader: &mut dyn Reader) -> Result<BitmapChunk, ser::Error> {
+		Ok(BitmapChunk::new())
+	}
+}
--- a/chain/src/txhashset/txhashset.rs
+++ b/chain/src/txhashset/txhashset.rs
@ -23,8 +23,9 @@ use crate::core::core::{Block, BlockHeader, Input, Output, OutputIdentifier, TxK
 use crate::core::ser::{PMMRIndexHashable, PMMRable, ProtocolVersion};
 use crate::error::{Error, ErrorKind};
 use crate::store::{Batch, ChainStore};
+use crate::txhashset::bitmap_accumulator::BitmapAccumulator;
 use crate::txhashset::{RewindableKernelView, UTXOView};
-use crate::types::{OutputMMRPosition, Tip, TxHashSetRoots, TxHashsetWriteStatus};
+use crate::types::{OutputMMRPosition, OutputRoots, Tip, TxHashSetRoots, TxHashsetWriteStatus};
 use crate::util::secp::pedersen::{Commitment, RangeProof};
 use crate::util::{file, secp_static, zip};
 use croaring::Bitmap;
@ -117,6 +118,8 @@ pub struct TxHashSet {
 	rproof_pmmr_h: PMMRHandle<RangeProof>,
 	kernel_pmmr_h: PMMRHandle<TxKernel>,

+	bitmap_accumulator: BitmapAccumulator,
+
 	// chain store used as index of commitments to MMR positions
 	commit_index: Arc<ChainStore>,
 }
@ -148,6 +151,9 @@ impl TxHashSet {
 			header,
 		)?;

+		// Initialize the bitmap accumulator from the current output PMMR.
+		let bitmap_accumulator = TxHashSet::bitmap_accumulator(&output_pmmr_h)?;
+
 		let mut maybe_kernel_handle: Option<PMMRHandle<TxKernel>> = None;
 		let versions = vec![ProtocolVersion(2), ProtocolVersion(1)];
 		for version in versions {
@ -195,6 +201,7 @@ impl TxHashSet {
 				output_pmmr_h,
 				rproof_pmmr_h,
 				kernel_pmmr_h,
+				bitmap_accumulator,
 				commit_index,
 			})
 		} else {
@ -202,6 +209,15 @@ impl TxHashSet {
 		}
 	}

+	// Build a new bitmap accumulator for the provided output PMMR.
+	fn bitmap_accumulator(pmmr_h: &PMMRHandle<Output>) -> Result<BitmapAccumulator, Error> {
+		let pmmr = ReadonlyPMMR::at(&pmmr_h.backend, pmmr_h.last_pos);
+		let size = pmmr::n_leaves(pmmr_h.last_pos);
+		let mut bitmap_accumulator = BitmapAccumulator::new();
+		bitmap_accumulator.init(&mut pmmr.leaf_idx_iter(0), size)?;
+		Ok(bitmap_accumulator)
+	}
+
 	/// Close all backend file handles
 	pub fn release_backend_files(&mut self) {
 		self.output_pmmr_h.backend.release_files();
@ -329,7 +345,10 @@ impl TxHashSet {
 			ReadonlyPMMR::at(&self.kernel_pmmr_h.backend, self.kernel_pmmr_h.last_pos);

 		TxHashSetRoots {
-			output_root: output_pmmr.root(),
+			output_roots: OutputRoots {
+				pmmr_root: output_pmmr.root(),
+				bitmap_root: self.bitmap_accumulator.root(),
+			},
 			rproof_root: rproof_pmmr.root(),
 			kernel_root: kernel_pmmr.root(),
 		}
@ -554,6 +573,7 @@ where
 	let sizes: (u64, u64, u64);
 	let res: Result<T, Error>;
 	let rollback: bool;
+	let bitmap_accumulator: BitmapAccumulator;

 	let head = batch.head()?;

@ -581,6 +601,7 @@ where

 		rollback = extension_pair.extension.rollback;
 		sizes = extension_pair.extension.sizes();
+		bitmap_accumulator = extension_pair.extension.bitmap_accumulator.clone();
 	}

 	// During an extension we do not want to modify the header_extension (and only read from it).
@ -610,6 +631,9 @@ where
 				trees.output_pmmr_h.last_pos = sizes.0;
 				trees.rproof_pmmr_h.last_pos = sizes.1;
 				trees.kernel_pmmr_h.last_pos = sizes.2;
+
+				// Update our bitmap_accumulator based on our extension
+				trees.bitmap_accumulator = bitmap_accumulator;
 			}

 			trace!("TxHashSet extension done.");
@ -826,6 +850,8 @@ pub struct Extension<'a> {
 	rproof_pmmr: PMMR<'a, RangeProof, PMMRBackend<RangeProof>>,
 	kernel_pmmr: PMMR<'a, TxKernel, PMMRBackend<TxKernel>>,

+	bitmap_accumulator: BitmapAccumulator,
+
 	/// Rollback flag.
 	rollback: bool,

@ -879,6 +905,7 @@ impl<'a> Extension<'a> {
 				&mut trees.kernel_pmmr_h.backend,
 				trees.kernel_pmmr_h.last_pos,
 			),
+			bitmap_accumulator: trees.bitmap_accumulator.clone(),
 			rollback: false,
 			batch,
 		}
@ -901,28 +928,53 @@ impl<'a> Extension<'a> {

 	/// Apply a new block to the current txhashet extension (output, rangeproof, kernel MMRs).
 	pub fn apply_block(&mut self, b: &Block) -> Result<(), Error> {
+		let mut affected_pos = vec![];
+
 		for out in b.outputs() {
 			let pos = self.apply_output(out)?;
-			// Update the (output_pos,height) index for the new output.
+			affected_pos.push(pos);
 			self.batch
 				.save_output_pos_height(&out.commitment(), pos, b.header.height)?;
 		}

 		for input in b.inputs() {
-			self.apply_input(input)?;
+			let pos = self.apply_input(input)?;
+			affected_pos.push(pos);
 		}

 		for kernel in b.kernels() {
 			self.apply_kernel(kernel)?;
 		}

+		// Update our BitmapAccumulator based on affected outputs (both spent and created).
+		self.apply_to_bitmap_accumulator(&affected_pos)?;
+
 		// Update the head of the extension to reflect the block we just applied.
 		self.head = Tip::from_header(&b.header);

 		Ok(())
 	}

-	fn apply_input(&mut self, input: &Input) -> Result<(), Error> {
+	fn apply_to_bitmap_accumulator(&mut self, output_pos: &[u64]) -> Result<(), Error> {
+		// if self.output_pmmr.is_empty() || output_pos.is_empty() {
+		// 	return Ok(());
+		// }
+		let mut output_idx: Vec<_> = output_pos
+			.iter()
+			.map(|x| pmmr::n_leaves(*x).saturating_sub(1))
+			.collect();
+		output_idx.sort_unstable();
+		let min_idx = output_idx.first().cloned().unwrap_or(0);
+		let size = pmmr::n_leaves(self.output_pmmr.last_pos);
+		self.bitmap_accumulator.apply(
+			output_idx,
+			self.output_pmmr
+				.leaf_idx_iter(BitmapAccumulator::chunk_start_idx(min_idx)),
+			size,
+		)
+	}
+
+	fn apply_input(&mut self, input: &Input) -> Result<u64, Error> {
 		let commit = input.commitment();
 		let pos_res = self.batch.get_output_pos(&commit);
 		if let Ok(pos) = pos_res {
@ -943,14 +995,14 @@ impl<'a> Extension<'a> {
 					self.rproof_pmmr
 						.prune(pos)
 						.map_err(|e| ErrorKind::TxHashSetErr(e))?;
+					Ok(pos)
 				}
-				Ok(false) => return Err(ErrorKind::AlreadySpent(commit).into()),
-				Err(e) => return Err(ErrorKind::TxHashSetErr(e).into()),
+				Ok(false) => Err(ErrorKind::AlreadySpent(commit).into()),
+				Err(e) => Err(ErrorKind::TxHashSetErr(e).into()),
 			}
 		} else {
-			return Err(ErrorKind::AlreadySpent(commit).into());
+			Err(ErrorKind::AlreadySpent(commit).into())
 		}
-		Ok(())
 	}

 	fn apply_output(&mut self, out: &Output) -> Result<(u64), Error> {
@ -1083,6 +1135,13 @@ impl<'a> Extension<'a> {
 		self.kernel_pmmr
 			.rewind(kernel_pos, &Bitmap::create())
 			.map_err(&ErrorKind::TxHashSetErr)?;
+
+		// Update our BitmapAccumulator based on affected outputs.
+		// We want to "unspend" every rewound spent output.
+		// Treat output_pos as an affected output to ensure we rebuild far enough back.
+		let mut affected_pos: Vec<_> = rewind_rm_pos.iter().map(|x| x as u64).collect();
+		affected_pos.push(output_pos);
+		self.apply_to_bitmap_accumulator(&affected_pos)?;
 		Ok(())
 	}

@ -1090,10 +1149,13 @@ impl<'a> Extension<'a> {
 	/// and kernel sum trees.
 	pub fn roots(&self) -> Result<TxHashSetRoots, Error> {
 		Ok(TxHashSetRoots {
-			output_root: self
-				.output_pmmr
-				.root()
-				.map_err(|_| ErrorKind::InvalidRoot)?,
+			output_roots: OutputRoots {
+				pmmr_root: self
+					.output_pmmr
+					.root()
+					.map_err(|_| ErrorKind::InvalidRoot)?,
+				bitmap_root: self.bitmap_accumulator.root(),
+			},
 			rproof_root: self
 				.rproof_pmmr
 				.root()
@ -1111,16 +1173,7 @@ impl<'a> Extension<'a> {
 			return Ok(());
 		}
 		let head_header = self.batch.get_block_header(&self.head.hash())?;
-		let header_roots = TxHashSetRoots {
-			output_root: head_header.output_root,
-			rproof_root: head_header.range_proof_root,
-			kernel_root: head_header.kernel_root,
-		};
-		if header_roots != self.roots()? {
-			Err(ErrorKind::InvalidRoot.into())
-		} else {
-			Ok(())
-		}
+		self.roots()?.validate(&head_header)
 	}

 	/// Validate the header, output and kernel MMR sizes against the block header.
--- a/chain/src/types.rs
+++ b/chain/src/types.rs
@ -20,8 +20,8 @@ use std::sync::Arc;
 use crate::core::core::hash::{Hash, Hashed, ZERO_HASH};
 use crate::core::core::{Block, BlockHeader};
 use crate::core::pow::Difficulty;
-use crate::core::ser;
-use crate::error::Error;
+use crate::core::ser::{self, PMMRIndexHashable};
+use crate::error::{Error, ErrorKind};
 use crate::util::RwLock;

 bitflags! {
@ -181,18 +181,70 @@ impl TxHashsetWriteStatus for SyncState {
 	}
 }

-/// A helper to hold the roots of the txhashset in order to keep them
-/// readable.
-#[derive(Debug, PartialEq)]
+/// A helper for the various txhashset MMR roots.
+#[derive(Debug)]
 pub struct TxHashSetRoots {
-	/// Output root
-	pub output_root: Hash,
+	/// Output roots
+	pub output_roots: OutputRoots,
 	/// Range Proof root
 	pub rproof_root: Hash,
 	/// Kernel root
 	pub kernel_root: Hash,
 }

+impl TxHashSetRoots {
+	/// Accessor for the underlying output PMMR root
+	pub fn output_root(&self) -> Hash {
+		self.output_roots.output_root()
+	}
+
+	/// Validate roots against the provided block header.
+	pub fn validate(&self, header: &BlockHeader) -> Result<(), Error> {
+		debug!(
+			"validate roots: {} at {}, output_root: {}, output pmmr: {} (bitmap: {}, merged: {})",
+			header.hash(),
+			header.height,
+			header.output_root,
+			self.output_roots.output_root(),
+			self.output_roots.bitmap_root,
+			self.output_roots.merged_root(header),
+		);
+
+		if header.output_root != self.output_roots.pmmr_root {
+			Err(ErrorKind::InvalidRoot.into())
+		} else if header.range_proof_root != self.rproof_root {
+			Err(ErrorKind::InvalidRoot.into())
+		} else if header.kernel_root != self.kernel_root {
+			Err(ErrorKind::InvalidRoot.into())
+		} else {
+			Ok(())
+		}
+	}
+}
+
+/// A helper for the various output roots.
+#[derive(Debug)]
+pub struct OutputRoots {
+	/// The output PMMR root
+	pub pmmr_root: Hash,
+	/// The bitmap accumulator root
+	pub bitmap_root: Hash,
+}
+
+impl OutputRoots {
+	/// The root of the underlying output PMMR.
+	pub fn output_root(&self) -> Hash {
+		self.pmmr_root
+	}
+
+	/// Hash the root of the output PMMR and the root of the bitmap accumulator
+	/// together with the size of the output PMMR (for consistency with existing PMMR impl).
+	/// H(pmmr_size | pmmr_root | bitmap_root)
+	pub fn merged_root(&self, header: &BlockHeader) -> Hash {
+		(self.pmmr_root, self.bitmap_root).hash_with_index(header.output_mmr_size)
+	}
+}
+
 /// A helper to hold the output pmmr position of the txhashset in order to keep them
 /// readable.
 #[derive(Debug)]
--- a/chain/tests/bitmap_accumulator.rs
+++ b/chain/tests/bitmap_accumulator.rs
@ -0,0 +1,188 @@
+// Copyright 2019 The Grin Developers
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use self::chain::txhashset::BitmapAccumulator;
+use self::core::core::hash::Hash;
+use self::core::ser::PMMRIndexHashable;
+use bit_vec::BitVec;
+use grin_chain as chain;
+use grin_core as core;
+use grin_util as util;
+
+#[test]
+fn test_bitmap_accumulator() {
+	util::init_test_logger();
+
+	let mut accumulator = BitmapAccumulator::new();
+	assert_eq!(accumulator.root(), Hash::default());
+
+	// 1000... (rebuild from 0, setting [0] true)
+	accumulator.apply(vec![0], vec![0], 1).unwrap();
+	let expected_hash = {
+		let mut bit_vec = BitVec::from_elem(1024, false);
+		bit_vec.set(0, true);
+		bit_vec.to_bytes().hash_with_index(0)
+	};
+	assert_eq!(accumulator.root(), expected_hash);
+
+	// 1100... (rebuild from 0, setting [0, 1] true)
+	accumulator.apply(vec![0], vec![0, 1], 2).unwrap();
+	let expected_hash = {
+		let mut bit_vec = BitVec::from_elem(1024, false);
+		bit_vec.set(0, true);
+		bit_vec.set(1, true);
+		bit_vec.to_bytes().hash_with_index(0)
+	};
+	assert_eq!(accumulator.root(), expected_hash);
+
+	// 0100... (rebuild from 0, setting [1] true, which will reset [0] false)
+	accumulator.apply(vec![0], vec![1], 2).unwrap();
+	let expected_hash = {
+		let mut bit_vec = BitVec::from_elem(1024, false);
+		bit_vec.set(1, true);
+		let expected_bytes = bit_vec.to_bytes();
+		expected_bytes.hash_with_index(0)
+	};
+	assert_eq!(accumulator.root(), expected_hash);
+
+	// 0100... (rebuild from 1, setting [1] true)
+	accumulator.apply(vec![1], vec![1], 2).unwrap();
+	let expected_hash = {
+		let mut bit_vec = BitVec::from_elem(1024, false);
+		bit_vec.set(1, true);
+		let expected_bytes = bit_vec.to_bytes();
+		expected_bytes.hash_with_index(0)
+	};
+	assert_eq!(accumulator.root(), expected_hash);
+
+	// 0100...0001 (rebuild from 0, setting [1, 1023] true)
+	accumulator.apply(vec![0], vec![1, 1023], 1024).unwrap();
+	let expected_hash = {
+		let mut bit_vec = BitVec::from_elem(1024, false);
+		bit_vec.set(1, true);
+		bit_vec.set(1023, true);
+		let expected_bytes = bit_vec.to_bytes();
+		expected_bytes.hash_with_index(0)
+	};
+	assert_eq!(accumulator.root(), expected_hash);
+
+	// Now set bits such that we extend the bitmap accumulator across multiple 1024 bit chunks.
+	// We need a second bit_vec here to reflect the additional chunk.
+	// 0100...0001, 1000...0000 (rebuild from 0, setting [1, 1023, 1024] true)
+	accumulator
+		.apply(vec![0], vec![1, 1023, 1024], 1025)
+		.unwrap();
+	let expected_hash = {
+		let mut bit_vec = BitVec::from_elem(1024, false);
+		bit_vec.set(1, true);
+		bit_vec.set(1023, true);
+		let mut bit_vec2 = BitVec::from_elem(1024, false);
+		bit_vec2.set(0, true);
+		let expected_bytes_0 = bit_vec.to_bytes();
+		let expected_bytes_1 = bit_vec2.to_bytes();
+		let expected_hash_0 = expected_bytes_0.hash_with_index(0);
+		let expected_hash_1 = expected_bytes_1.hash_with_index(1);
+		(expected_hash_0, expected_hash_1).hash_with_index(2)
+	};
+	assert_eq!(accumulator.root(), expected_hash);
+
+	// Just rebuild the second bitmap chunk.
+	// 0100...0001, 0100...0000 (rebuild from 1025, setting [1025] true)
+	accumulator.apply(vec![1025], vec![1025], 1026).unwrap();
+	let expected_hash = {
+		let mut bit_vec = BitVec::from_elem(1024, false);
+		bit_vec.set(1, true);
+		bit_vec.set(1023, true);
+		let mut bit_vec2 = BitVec::from_elem(1024, false);
+		bit_vec2.set(1, true);
+		let expected_bytes_0 = bit_vec.to_bytes();
+		let expected_bytes_1 = bit_vec2.to_bytes();
+		let expected_hash_0 = expected_bytes_0.hash_with_index(0);
+		let expected_hash_1 = expected_bytes_1.hash_with_index(1);
+		(expected_hash_0, expected_hash_1).hash_with_index(2)
+	};
+	assert_eq!(accumulator.root(), expected_hash);
+
+	// Rebuild the first bitmap chunk and all chunks after it.
+	// 0100...0000, 0100...0000 (rebuild from 1, setting [1, 1025] true)
+	accumulator.apply(vec![1], vec![1, 1025], 1026).unwrap();
+	let expected_hash = {
+		let mut bit_vec = BitVec::from_elem(1024, false);
+		bit_vec.set(1, true);
+		let mut bit_vec2 = BitVec::from_elem(1024, false);
+		bit_vec2.set(1, true);
+		let expected_bytes_0 = bit_vec.to_bytes();
+		let expected_bytes_1 = bit_vec2.to_bytes();
+		let expected_hash_0 = expected_bytes_0.hash_with_index(0);
+		let expected_hash_1 = expected_bytes_1.hash_with_index(1);
+		(expected_hash_0, expected_hash_1).hash_with_index(2)
+	};
+	assert_eq!(accumulator.root(), expected_hash);
+
+	// Make sure we handle the case where the first chunk is all 0s
+	// 0000...0000, 0100...0000 (rebuild from 1, setting [1025] true)
+	accumulator.apply(vec![1], vec![1025], 1026).unwrap();
+	let expected_hash = {
+		let bit_vec = BitVec::from_elem(1024, false);
+		let mut bit_vec2 = BitVec::from_elem(1024, false);
+		bit_vec2.set(1, true);
+		let expected_bytes_0 = bit_vec.to_bytes();
+		let expected_bytes_1 = bit_vec2.to_bytes();
+		let expected_hash_0 = expected_bytes_0.hash_with_index(0);
+		let expected_hash_1 = expected_bytes_1.hash_with_index(1);
+		(expected_hash_0, expected_hash_1).hash_with_index(2)
+	};
+	assert_eq!(accumulator.root(), expected_hash);
+
+	// Check that removing the last bit in a chunk removes the now empty chunk
+	// if it is the rightmost chunk.
+	// 0000...0001 (rebuild from 1023, setting [1023] true)
+	accumulator.apply(vec![1023], vec![1023], 1024).unwrap();
+	let expected_hash = {
+		let mut bit_vec = BitVec::from_elem(1024, false);
+		bit_vec.set(1023, true);
+		let expected_bytes = bit_vec.to_bytes();
+		expected_bytes.hash_with_index(0)
+	};
+	assert_eq!(accumulator.root(), expected_hash);
+
+	// Make sure we pad appropriately with 0s if we set a distant bit to 1.
+	// Start with an empty accumulator.
+	// 0000...0000, 0000...0000, 0000...0000, 0000...0001 (rebuild from 4095, setting [4095] true)
+	let mut accumulator = BitmapAccumulator::new();
+	accumulator.apply(vec![4095], vec![4095], 4096).unwrap();
+	let expected_hash = {
+		let bit_vec0 = BitVec::from_elem(1024, false);
+		let bit_vec1 = BitVec::from_elem(1024, false);
+		let bit_vec2 = BitVec::from_elem(1024, false);
+		let mut bit_vec3 = BitVec::from_elem(1024, false);
+		bit_vec3.set(1023, true);
+
+		let expected_bytes_0 = bit_vec0.to_bytes();
+		let expected_bytes_1 = bit_vec1.to_bytes();
+		let expected_bytes_2 = bit_vec2.to_bytes();
+		let expected_bytes_3 = bit_vec3.to_bytes();
+
+		let expected_hash_0 = expected_bytes_0.hash_with_index(0);
+		let expected_hash_1 = expected_bytes_1.hash_with_index(1);
+		let expected_hash_2 = (expected_hash_0, expected_hash_1).hash_with_index(2);
+
+		let expected_hash_3 = expected_bytes_2.hash_with_index(3);
+		let expected_hash_4 = expected_bytes_3.hash_with_index(4);
+		let expected_hash_5 = (expected_hash_3, expected_hash_4).hash_with_index(5);
+
+		(expected_hash_2, expected_hash_5).hash_with_index(6)
+	};
+	assert_eq!(accumulator.root(), expected_hash);
+}
--- a/core/src/core/pmmr.rs
+++ b/core/src/core/pmmr.rs
@ -40,8 +40,10 @@ mod backend;
 mod pmmr;
 mod readonly_pmmr;
 mod rewindable_pmmr;
+mod vec_backend;

 pub use self::backend::*;
 pub use self::pmmr::*;
 pub use self::readonly_pmmr::*;
 pub use self::rewindable_pmmr::*;
+pub use self::vec_backend::*;
--- a/core/src/core/pmmr/backend.rs
+++ b/core/src/core/pmmr/backend.rs
@ -58,6 +58,10 @@ pub trait Backend<T: PMMRable> {
 	/// Number of leaves
 	fn n_unpruned_leaves(&self) -> u64;

+	/// Iterator over current (unpruned, unremoved) leaf insertion index.
+	/// Note: This differs from underlying MMR pos - [0, 1, 2, 3, 4] vs. [1, 2, 4, 5, 8].
+	fn leaf_idx_iter(&self, from_idx: u64) -> Box<dyn Iterator<Item = u64> + '_>;
+
 	/// Remove Hash by insertion position. An index is also provided so the
 	/// underlying backend can implement some rollback of positions up to a
 	/// given index (practically the index is the height of a block that
--- a/core/src/core/pmmr/pmmr.rs
+++ b/core/src/core/pmmr/pmmr.rs
@ -84,6 +84,11 @@ where
 		self.backend.n_unpruned_leaves()
 	}

+	/// Iterator over current (unpruned, unremoved) leaf insertion indices.
+	pub fn leaf_idx_iter(&self, from_idx: u64) -> impl Iterator<Item = u64> + '_ {
+		self.backend.leaf_idx_iter(from_idx)
+	}
+
 	/// Returns a vec of the peaks of this MMR.
 	pub fn peaks(&self) -> Vec<Hash> {
 		let peaks_pos = peaks(self.last_pos);
--- a/core/src/core/pmmr/readonly_pmmr.rs
+++ b/core/src/core/pmmr/readonly_pmmr.rs
@ -91,6 +91,11 @@ where
 		self.backend.leaf_pos_iter()
 	}

+	/// Iterator over current (unpruned, unremoved) leaf insertion indices.
+	pub fn leaf_idx_iter(&self, from_idx: u64) -> impl Iterator<Item = u64> + '_ {
+		self.backend.leaf_idx_iter(from_idx)
+	}
+
 	/// Is the MMR empty?
 	pub fn is_empty(&self) -> bool {
 		self.last_pos == 0
--- a/core/src/core/pmmr/vec_backend.rs
+++ b/core/src/core/pmmr/vec_backend.rs
@ -0,0 +1,153 @@
+// Copyright 2019 The Grin Developers
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use std::collections::HashSet;
+use std::convert::TryFrom;
+use std::fs::File;
+
+use croaring::Bitmap;
+
+use crate::core::hash::Hash;
+use crate::core::pmmr::{self, Backend};
+use crate::core::BlockHeader;
+use crate::ser::PMMRable;
+
+/// Simple/minimal/naive MMR backend implementation backed by Vec<T> and Vec<Hash>.
+/// Removed pos are maintained in a HashSet<u64>.
+#[derive(Clone, Debug)]
+pub struct VecBackend<T: PMMRable> {
+	/// Backend elements (optional, possible to just store hashes).
+	pub data: Option<Vec<T>>,
+	/// Vec of hashes for the PMMR (both leaves and parents).
+	pub hashes: Vec<Hash>,
+	/// Positions of removed elements (is this applicable if we do not store data?)
+	pub removed: HashSet<u64>,
+}
+
+impl<T: PMMRable> Backend<T> for VecBackend<T> {
+	fn append(&mut self, elmt: &T, hashes: Vec<Hash>) -> Result<(), String> {
+		if let Some(data) = &mut self.data {
+			data.push(elmt.clone());
+		}
+		self.hashes.append(&mut hashes.clone());
+		Ok(())
+	}
+
+	fn get_hash(&self, position: u64) -> Option<Hash> {
+		if self.removed.contains(&position) {
+			None
+		} else {
+			self.get_from_file(position)
+		}
+	}
+
+	fn get_data(&self, position: u64) -> Option<T::E> {
+		if self.removed.contains(&position) {
+			None
+		} else {
+			self.get_data_from_file(position)
+		}
+	}
+
+	fn get_from_file(&self, position: u64) -> Option<Hash> {
+		let idx = usize::try_from(position.saturating_sub(1)).expect("usize from u64");
+		self.hashes.get(idx).cloned()
+	}
+
+	fn get_data_from_file(&self, position: u64) -> Option<T::E> {
+		if let Some(data) = &self.data {
+			let idx = usize::try_from(pmmr::n_leaves(position).saturating_sub(1))
+				.expect("usize from u64");
+			data.get(idx).map(|x| x.as_elmt())
+		} else {
+			None
+		}
+	}
+
+	fn data_as_temp_file(&self) -> Result<File, String> {
+		unimplemented!()
+	}
+
+	/// Number of leaves in the MMR
+	fn n_unpruned_leaves(&self) -> u64 {
+		unimplemented!()
+	}
+
+	fn leaf_pos_iter(&self) -> Box<dyn Iterator<Item = u64> + '_> {
+		Box::new(
+			self.hashes
+				.iter()
+				.enumerate()
+				.map(|(x, _)| (x + 1) as u64)
+				.filter(move |x| pmmr::is_leaf(*x) && !self.removed.contains(x)),
+		)
+	}
+
+	fn leaf_idx_iter(&self, from_idx: u64) -> Box<dyn Iterator<Item = u64> + '_> {
+		let from_pos = pmmr::insertion_to_pmmr_index(from_idx + 1);
+		Box::new(
+			self.leaf_pos_iter()
+				.skip_while(move |x| *x < from_pos)
+				.map(|x| pmmr::n_leaves(x).saturating_sub(1)),
+		)
+	}
+
+	fn remove(&mut self, position: u64) -> Result<(), String> {
+		self.removed.insert(position);
+		Ok(())
+	}
+
+	fn rewind(&mut self, position: u64, _rewind_rm_pos: &Bitmap) -> Result<(), String> {
+		if let Some(data) = &mut self.data {
+			let idx = pmmr::n_leaves(position);
+			data.truncate(usize::try_from(idx).expect("usize from u64"));
+		}
+		self.hashes
+			.truncate(usize::try_from(position).expect("usize from u64"));
+		Ok(())
+	}
+
+	fn snapshot(&self, _header: &BlockHeader) -> Result<(), String> {
+		Ok(())
+	}
+
+	fn release_files(&mut self) {}
+
+	fn dump_stats(&self) {}
+}
+
+impl<T: PMMRable> VecBackend<T> {
+	/// Instantiates a new empty vec backend.
+	pub fn new() -> VecBackend<T> {
+		VecBackend {
+			data: Some(vec![]),
+			hashes: vec![],
+			removed: HashSet::new(),
+		}
+	}
+
+	/// Instantiate a new empty "hash only" vec backend.
+	pub fn new_hash_only() -> VecBackend<T> {
+		VecBackend {
+			data: None,
+			hashes: vec![],
+			removed: HashSet::new(),
+		}
+	}
+
+	/// Size of this vec backend in hashes.
+	pub fn size(&self) -> u64 {
+		self.hashes.len() as u64
+	}
+}
--- a/core/tests/block.rs
+++ b/core/tests/block.rs
@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.

-pub mod common;
+mod common;
 use crate::common::{new_block, tx1i2o, tx2i1o, txspend1i1o};
 use crate::core::consensus::BLOCK_OUTPUT_WEIGHT;
 use crate::core::core::block::Error;
--- a/core/tests/common.rs
+++ b/core/tests/common.rs
@ -15,15 +15,18 @@
 //! Common test functions

 use grin_core::core::{Block, BlockHeader, KernelFeatures, Transaction};
+use grin_core::core::hash::DefaultHashable;
 use grin_core::libtx::{
 	build::{self, input, output},
 	proof::{ProofBuild, ProofBuilder},
 	reward,
 };
 use grin_core::pow::Difficulty;
+use grin_core::ser::{self, FixedLength, PMMRable, Readable, Reader, Writeable, Writer};
 use keychain::{Identifier, Keychain};

 // utility producing a transaction with 2 inputs and a single outputs
+#[allow(dead_code)]
 pub fn tx2i1o() -> Transaction {
 	let keychain = keychain::ExtKeychain::from_random_seed(false).unwrap();
 	let builder = ProofBuilder::new(&keychain);
@ -41,6 +44,7 @@ pub fn tx2i1o() -> Transaction {
 }

 // utility producing a transaction with a single input and output
+#[allow(dead_code)]
 pub fn tx1i1o() -> Transaction {
 	let keychain = keychain::ExtKeychain::from_random_seed(false).unwrap();
 	let builder = ProofBuilder::new(&keychain);
@ -59,6 +63,7 @@ pub fn tx1i1o() -> Transaction {
 // utility producing a transaction with a single input
 // and two outputs (one change output)
 // Note: this tx has an "offset" kernel
+#[allow(dead_code)]
 pub fn tx1i2o() -> Transaction {
 	let keychain = keychain::ExtKeychain::from_random_seed(false).unwrap();
 	let builder = ProofBuilder::new(&keychain);
@ -77,6 +82,7 @@ pub fn tx1i2o() -> Transaction {

 // utility to create a block without worrying about the key or previous
 // header
+#[allow(dead_code)]
 pub fn new_block<K, B>(
 	txs: Vec<&Transaction>,
 	keychain: &K,
@ -101,6 +107,7 @@ where

 // utility producing a transaction that spends an output with the provided
 // value and blinding key
+#[allow(dead_code)]
 pub fn txspend1i1o<K, B>(
 	v: u64,
 	keychain: &K,
@ -120,3 +127,40 @@ where
 	)
 	.unwrap()
 }
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub struct TestElem(pub [u32; 4]);
+
+impl DefaultHashable for TestElem {}
+
+impl FixedLength for TestElem {
+	const LEN: usize = 16;
+}
+
+impl PMMRable for TestElem {
+	type E = Self;
+
+	fn as_elmt(&self) -> Self::E {
+		self.clone()
+	}
+}
+
+impl Writeable for TestElem {
+	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ser::Error> {
+		r#try!(writer.write_u32(self.0[0]));
+		r#try!(writer.write_u32(self.0[1]));
+		r#try!(writer.write_u32(self.0[2]));
+		writer.write_u32(self.0[3])
+	}
+}
+
+impl Readable for TestElem {
+	fn read(reader: &mut dyn Reader) -> Result<TestElem, ser::Error> {
+		Ok(TestElem([
+			reader.read_u32()?,
+			reader.read_u32()?,
+			reader.read_u32()?,
+			reader.read_u32()?,
+		]))
+	}
+}
--- a/core/tests/merkle_proof.rs
+++ b/core/tests/merkle_proof.rs
@ -12,12 +12,12 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.

-mod vec_backend;
+mod common;

 use self::core::core::merkle_proof::MerkleProof;
-use self::core::core::pmmr::PMMR;
+use self::core::core::pmmr::{VecBackend, PMMR};
 use self::core::ser::{self, PMMRIndexHashable};
-use crate::vec_backend::{TestElem, VecBackend};
+use crate::common::TestElem;
 use grin_core as core;

 #[test]
--- a/core/tests/pmmr.rs
+++ b/core/tests/pmmr.rs
@ -12,12 +12,12 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.

-mod vec_backend;
+mod common;

 use self::core::core::hash::Hash;
-use self::core::core::pmmr::{self, PMMR};
+use self::core::core::pmmr::{self, VecBackend, PMMR};
 use self::core::ser::PMMRIndexHashable;
-use crate::vec_backend::{TestElem, VecBackend};
+use crate::common::TestElem;
 use chrono::prelude::Utc;
 use grin_core as core;
 use std::u64;
@ -433,7 +433,7 @@ fn pmmr_prune() {

 	// First check the initial numbers of elements.
 	assert_eq!(ba.hashes.len(), 16);
-	assert_eq!(ba.remove_list.len(), 0);
+	assert_eq!(ba.removed.len(), 0);

 	// pruning a leaf with no parent should do nothing
 	{
@ -442,7 +442,7 @@ fn pmmr_prune() {
 		assert_eq!(orig_root, pmmr.root().unwrap());
 	}
 	assert_eq!(ba.hashes.len(), 16);
-	assert_eq!(ba.remove_list.len(), 1);
+	assert_eq!(ba.removed.len(), 1);

 	// pruning leaves with no shared parent just removes 1 element
 	{
@ -451,7 +451,7 @@ fn pmmr_prune() {
 		assert_eq!(orig_root, pmmr.root().unwrap());
 	}
 	assert_eq!(ba.hashes.len(), 16);
-	assert_eq!(ba.remove_list.len(), 2);
+	assert_eq!(ba.removed.len(), 2);

 	{
 		let mut pmmr: PMMR<'_, TestElem, _> = PMMR::at(&mut ba, sz);
@ -459,7 +459,7 @@ fn pmmr_prune() {
 		assert_eq!(orig_root, pmmr.root().unwrap());
 	}
 	assert_eq!(ba.hashes.len(), 16);
-	assert_eq!(ba.remove_list.len(), 3);
+	assert_eq!(ba.removed.len(), 3);

 	// pruning a non-leaf node has no effect
 	{
@ -468,7 +468,7 @@ fn pmmr_prune() {
 		assert_eq!(orig_root, pmmr.root().unwrap());
 	}
 	assert_eq!(ba.hashes.len(), 16);
-	assert_eq!(ba.remove_list.len(), 3);
+	assert_eq!(ba.removed.len(), 3);

 	// TODO - no longer true (leaves only now) - pruning sibling removes subtree
 	{
@ -477,7 +477,7 @@ fn pmmr_prune() {
 		assert_eq!(orig_root, pmmr.root().unwrap());
 	}
 	assert_eq!(ba.hashes.len(), 16);
-	assert_eq!(ba.remove_list.len(), 4);
+	assert_eq!(ba.removed.len(), 4);

 	// TODO - no longer true (leaves only now) - pruning all leaves under level >1
 	// removes all subtree
@ -487,7 +487,7 @@ fn pmmr_prune() {
 		assert_eq!(orig_root, pmmr.root().unwrap());
 	}
 	assert_eq!(ba.hashes.len(), 16);
-	assert_eq!(ba.remove_list.len(), 5);
+	assert_eq!(ba.removed.len(), 5);

 	// pruning everything should only leave us with a single peak
 	{
@ -498,7 +498,7 @@ fn pmmr_prune() {
 		assert_eq!(orig_root, pmmr.root().unwrap());
 	}
 	assert_eq!(ba.hashes.len(), 16);
-	assert_eq!(ba.remove_list.len(), 9);
+	assert_eq!(ba.removed.len(), 9);
 }

 #[test]
--- a/core/tests/vec_backend.rs
+++ b/core/tests/vec_backend.rs
@ -12,139 +12,56 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.

-use std::fs::File;
+mod common;

-use self::core::core::hash::{DefaultHashable, Hash};
-use self::core::core::pmmr::{self, Backend};
-use self::core::core::BlockHeader;
-use self::core::ser;
-use self::core::ser::{FixedLength, PMMRable, Readable, Reader, Writeable, Writer};
-use croaring;
-use croaring::Bitmap;
+use self::core::core::pmmr::{VecBackend, PMMR};
+use crate::common::TestElem;
 use grin_core as core;

-#[derive(Copy, Clone, Debug, PartialEq, Eq)]
-pub struct TestElem(pub [u32; 4]);
-
-impl DefaultHashable for TestElem {}
-
-impl FixedLength for TestElem {
-	const LEN: usize = 16;
+#[test]
+fn leaf_pos_and_idx_iter_test() {
+	let elems = [
+		TestElem([0, 0, 0, 1]),
+		TestElem([0, 0, 0, 2]),
+		TestElem([0, 0, 0, 3]),
+		TestElem([0, 0, 0, 4]),
+		TestElem([0, 0, 0, 5]),
+	];
+	let mut backend = VecBackend::new();
+	let mut pmmr = PMMR::new(&mut backend);
+	for x in &elems {
+		pmmr.push(x).unwrap();
+	}
+	assert_eq!(
+		vec![0, 1, 2, 3, 4],
+		pmmr.leaf_idx_iter(0).collect::<Vec<_>>()
+	);
+	assert_eq!(
+		vec![1, 2, 4, 5, 8],
+		pmmr.leaf_pos_iter().collect::<Vec<_>>()
+	);
 }

-impl PMMRable for TestElem {
-	type E = Self;
-
-	fn as_elmt(&self) -> Self::E {
-		self.clone()
-	}
-}
-
-impl Writeable for TestElem {
-	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ser::Error> {
-		writer.write_u32(self.0[0])?;
-		writer.write_u32(self.0[1])?;
-		writer.write_u32(self.0[2])?;
-		writer.write_u32(self.0[3])
-	}
-}
-
-impl Readable for TestElem {
-	fn read(reader: &mut dyn Reader) -> Result<TestElem, ser::Error> {
-		Ok(TestElem([
-			reader.read_u32()?,
-			reader.read_u32()?,
-			reader.read_u32()?,
-			reader.read_u32()?,
-		]))
-	}
-}
-
-/// Simple MMR backend implementation based on a Vector. Pruning does not
-/// compact the Vec itself.
-#[derive(Clone, Debug)]
-pub struct VecBackend<T: PMMRable> {
-	/// Backend elements
-	pub data: Vec<T>,
-	pub hashes: Vec<Hash>,
-	/// Positions of removed elements
-	pub remove_list: Vec<u64>,
-}
-
-impl<T: PMMRable> Backend<T> for VecBackend<T> {
-	fn append(&mut self, data: &T, hashes: Vec<Hash>) -> Result<(), String> {
-		self.data.push(data.clone());
-		self.hashes.append(&mut hashes.clone());
-		Ok(())
-	}
-
-	fn get_hash(&self, position: u64) -> Option<Hash> {
-		if self.remove_list.contains(&position) {
-			None
-		} else {
-			self.get_from_file(position)
-		}
-	}
-
-	fn get_data(&self, position: u64) -> Option<T::E> {
-		if self.remove_list.contains(&position) {
-			None
-		} else {
-			self.get_data_from_file(position)
-		}
-	}
-
-	fn get_from_file(&self, position: u64) -> Option<Hash> {
-		let hash = &self.hashes[(position - 1) as usize];
-		Some(hash.clone())
-	}
-
-	fn get_data_from_file(&self, position: u64) -> Option<T::E> {
-		let idx = pmmr::n_leaves(position);
-		let data = self.data[(idx - 1) as usize].clone();
-		Some(data.as_elmt())
-	}
-
-	fn data_as_temp_file(&self) -> Result<File, String> {
-		unimplemented!()
-	}
-
-	fn leaf_pos_iter(&self) -> Box<dyn Iterator<Item = u64> + '_> {
-		unimplemented!()
-	}
-
-	fn n_unpruned_leaves(&self) -> u64 {
-		unimplemented!()
-	}
-
-	fn remove(&mut self, position: u64) -> Result<(), String> {
-		self.remove_list.push(position);
-		Ok(())
-	}
-
-	fn rewind(&mut self, position: u64, _rewind_rm_pos: &Bitmap) -> Result<(), String> {
-		let idx = pmmr::n_leaves(position);
-		self.data = self.data[0..(idx as usize) + 1].to_vec();
-		self.hashes = self.hashes[0..(position as usize) + 1].to_vec();
-		Ok(())
-	}
-
-	fn snapshot(&self, _header: &BlockHeader) -> Result<(), String> {
-		Ok(())
-	}
-
-	fn release_files(&mut self) {}
-
-	fn dump_stats(&self) {}
-}
-
-impl<T: PMMRable> VecBackend<T> {
-	/// Instantiates a new VecBackend<T>
-	pub fn new() -> VecBackend<T> {
-		VecBackend {
-			data: vec![],
-			hashes: vec![],
-			remove_list: vec![],
-		}
+#[test]
+fn leaf_pos_and_idx_iter_hash_only_test() {
+	let elems = [
+		TestElem([0, 0, 0, 1]),
+		TestElem([0, 0, 0, 2]),
+		TestElem([0, 0, 0, 3]),
+		TestElem([0, 0, 0, 4]),
+		TestElem([0, 0, 0, 5]),
+	];
+	let mut backend = VecBackend::new_hash_only();
+	let mut pmmr = PMMR::new(&mut backend);
+	for x in &elems {
+		pmmr.push(x).unwrap();
 	}
+	assert_eq!(
+		vec![0, 1, 2, 3, 4],
+		pmmr.leaf_idx_iter(0).collect::<Vec<_>>()
+	);
+	assert_eq!(
+		vec![1, 2, 4, 5, 8],
+		pmmr.leaf_pos_iter().collect::<Vec<_>>()
+	);
 }
--- a/store/Cargo.toml
+++ b/store/Cargo.toml
@ -10,6 +10,7 @@ workspace = ".."
 edition = "2018"

 [dependencies]
+bit-vec = "0.6"
 byteorder = "1"
 croaring = "0.3.9"
 env_logger = "0.5"
--- a/store/src/pmmr.rs
+++ b/store/src/pmmr.rs
@ -148,6 +148,28 @@ impl<T: PMMRable> Backend<T> for PMMRBackend<T> {
 		}
 	}

+	/// Returns an iterator over all the leaf insertion indices (0-indexed).
+	/// If our pos are [1,2,4,5,8] (first 5 leaf pos) then our insertion indices are [0,1,2,3,4]
+	fn leaf_idx_iter(&self, from_idx: u64) -> Box<dyn Iterator<Item = u64> + '_> {
+		// pass from_idx in as param
+		// convert this to pos
+		// iterate, skipping everything prior to this
+		// pass in from_idx=0 then we want to convert to pos=1
+
+		let from_pos = pmmr::insertion_to_pmmr_index(from_idx + 1);
+
+		if self.prunable {
+			Box::new(
+				self.leaf_set
+					.iter()
+					.skip_while(move |x| *x < from_pos)
+					.map(|x| pmmr::n_leaves(x).saturating_sub(1)),
+			)
+		} else {
+			panic!("leaf_idx_iter not implemented for non-prunable PMMR")
+		}
+	}
+
 	fn data_as_temp_file(&self) -> Result<File, String> {
 		self.data_file
 			.as_temp_file()
--- a/store/tests/pmmr.rs
+++ b/store/tests/pmmr.rs
@ -28,6 +28,36 @@ use crate::core::ser::{
 	Writer,
 };

+#[test]
+fn pmmr_leaf_idx_iter() {
+	let (data_dir, elems) = setup("leaf_idx_iter");
+	{
+		let mut backend = store::pmmr::PMMRBackend::new(
+			data_dir.to_string(),
+			true,
+			false,
+			ProtocolVersion(1),
+			None,
+		)
+		.unwrap();
+
+		// adding first set of 4 elements and sync
+		let mmr_size = load(0, &elems[0..5], &mut backend);
+		backend.sync().unwrap();
+
+		{
+			let pmmr: PMMR<'_, TestElem, _> = PMMR::at(&mut backend, mmr_size);
+			let leaf_idx = pmmr.leaf_idx_iter(0).collect::<Vec<_>>();
+			let leaf_pos = pmmr.leaf_pos_iter().collect::<Vec<_>>();
+
+			// The first 5 leaves [0,1,2,3,4] are at pos [1,2,4,5,8] in the MMR.
+			assert_eq!(leaf_idx, vec![0, 1, 2, 3, 4]);
+			assert_eq!(leaf_pos, vec![1, 2, 4, 5, 8]);
+		}
+	}
+	teardown(data_dir);
+}
+
 #[test]
 fn pmmr_append() {
 	let (data_dir, elems) = setup("append");