Utility structs to handle the 3 hashtrees (output, range proof, kernel) more conveniently and transactionally.

以“树”的形式，保存了整个区块链简要、关键的几个数据。

以“优雅”的方式进行存储，以达到“高效”的读取数据。

主要是 4 个哈希的集合

• header_pmmr_h BlockHeader

• output_pmmr_h OutputIdentifier

• rproof_pmmr_h RangeProof

• kernel_pmmr_h TxKernel

外加关联对象（此处只关联存储对象）

• commit_index ChainStore

方便某些情况下管理“交易”。

可用于快速检索、校验、验证。

TxHashSet

主要是关联 PMMRHandle 和 TxKernel 对象。

是“链”对象的子集。

pub struct TxHashSet {
    /// Header MMR to support the header_head chain.
    /// This is rewound and applied transactionally with the
    /// output, rangeproof and kernel MMRs during an extension or a
    /// readonly_extension.
    /// It can also be rewound and applied separately via a header_extension.
    header_pmmr_h: PMMRHandle<BlockHeader>,

    /// Header MMR to support exploratory sync_head.
    /// The header_head and sync_head chains can diverge so we need to maintain
    /// multiple header MMRs during the sync process.
    ///
    /// Note: this is rewound and applied separately to the other MMRs
    /// via a "sync_extension".
    sync_pmmr_h: PMMRHandle<BlockHeader>,

    output_pmmr_h: PMMRHandle<Output>,
    rproof_pmmr_h: PMMRHandle<RangeProof>,
    kernel_pmmr_h: PMMRHandle<TxKernel>,

    // chain store used as index of commitments to MMR positions
    commit_index: Arc<ChainStore>,
}

和下面的 Extension 差不多，只不过它更加具体一点，因为它所使用的 PMMRHandle 本身已经包含 PMMRBackend，所以这里不用指定。

PMMRBackend

xxx

croaring Bitmap

xxx

TxHashSetRoots

/// A helper to hold the roots of the txhashset in order to keep them
/// readable.
#[derive(Debug)]
pub struct TxHashSetRoots {
    /// Header root
    pub header_root: Hash,
    /// Output root
    pub output_root: Hash,
    /// Range Proof root
    pub rproof_root: Hash,
    /// Kernel root
    pub kernel_root: Hash,
}

Extension

扩展 TxHashSet，大大增强了其功能。

/// Allows the application of new blocks on top of the sum trees in a
/// reversible manner within a unit of work provided by the `extending`
/// function.
pub struct Extension<'a> {
    output_pmmr: PMMR<'a, OutputIdentifier, PMMRBackend<OutputIdentifier>>,
    rproof_pmmr: PMMR<'a, RangeProof, PMMRBackend<RangeProof>>,
    kernel_pmmr: PMMR<'a, TxKernel, PMMRBackend<TxKernel>>,

    commit_index: Arc<ChainStore>,
    new_output_commits: HashMap<Commitment, u64>,
    new_block_markers: HashMap<Hash, BlockMarker>,
    rollback: bool,
}

BlockMarker

/// The output and kernel positions that define the size of the MMRs for a
/// particular block.
#[derive(Debug, Clone)]
pub struct BlockMarker {
    /// The output (and rangeproof) MMR position of the final output in the
    /// block
    pub output_pos: u64,
    /// The kernel position of the final kernel in the block
    pub kernel_pos: u64,
}