tx_graph.rs

//! Module for structures that store and traverse transactions.
//!
//! [`TxGraph`] contains transactions and indexes them so you can easily traverse the graph of
//! those transactions. `TxGraph` is *monotone* in that you can always insert a transaction -- it
//! does not care whether that transaction is in the current best chain or whether it conflicts with
//! any of the existing transactions or what order you insert the transactions. This means that you
//! can always combine two [`TxGraph`]s together, without resulting in inconsistencies. Furthermore,
//! there is currently no way to delete a transaction.
//!
//! Transactions can be either whole or partial (i.e., transactions for which we only know some
//! outputs, which we usually call "floating outputs"; these are usually inserted using the
//! [`insert_txout`] method.).
//!
//! The graph contains transactions in the form of [`TxNode`]s. Each node contains the txid, the
//! transaction (whole or partial), the blocks that it is anchored to (see the [`Anchor`]
//! documentation for more details), and the timestamp of the last time we saw the transaction as
//! unconfirmed.
//!
//! # Canonicalization
//!
//! Conflicting transactions are allowed to coexist within a [`TxGraph`]. A process called
//! canonicalization is required to get a conflict-free view of transactions.
//!
//! * [`canonical_iter`](TxGraph::canonical_iter) returns a [`CanonicalIter`] which performs
//!   incremental canonicalization. This is useful when you only need to check specific transactions
//!   (e.g., verifying whether a few unconfirmed transactions are canonical) without computing the
//!   entire canonical view.
//! * [`canonical_view`](TxGraph::canonical_view) returns a [`CanonicalView`] which provides a
//!   complete canonical view of the graph. This is required for typical wallet operations like
//!   querying balances, listing outputs, transactions, and UTXOs. You must construct this first
//!   before performing these operations.
//!
//! All these methods require a `chain` and `chain_tip` argument. The `chain` must be a
//! [`ChainOracle`] implementation (such as [`LocalChain`](crate::local_chain::LocalChain)) which
//! identifies which blocks exist under a given `chain_tip`.
//!
//! The canonicalization algorithm uses the following associated data to determine which
//! transactions have precedence over others:
//!
//! * [`Anchor`] - This bit of data represents that a transaction is anchored in a given block. If
//!   the transaction is anchored in chain of `chain_tip`, or is an ancestor of a transaction
//!   anchored in chain of `chain_tip`, then the transaction must be canonical.
//! * `last_seen` - This is the timestamp of when a transaction is last-seen in the mempool. This
//!   value is updated by [`insert_seen_at`](TxGraph::insert_seen_at) and
//!   [`apply_update`](TxGraph::apply_update). Transactions that are seen later have higher priority
//!   than those that are seen earlier. `last_seen` values are transitive. This means that the
//!   actual `last_seen` value of a transaction is the max of all the `last_seen` values from it's
//!   descendants.
//! * `last_evicted` - This is the timestamp of when a transaction last went missing from the
//!   mempool. If this value is equal to or higher than the transaction's `last_seen` value, then it
//!   will not be considered canonical.
//!
//! # Graph traversal
//!
//! You can use [`TxAncestors`]/[`TxDescendants`] to traverse ancestors and descendants of a given
//! transaction, respectively.
//!
//! # Applying changes
//!
//! The [`ChangeSet`] reports changes made to a [`TxGraph`]; it can be used to either save to
//! persistent storage, or to be applied to another [`TxGraph`].
//!
//! Methods that change the state of [`TxGraph`] will return [`ChangeSet`]s.
//!
//! # Generics
//!
//! Anchors are represented as generics within `TxGraph<A>`. To make use of all functionality of the
//! `TxGraph`, anchors (`A`) should implement [`Anchor`].
//!
//! Anchors are made generic so that different types of data can be stored with how a transaction is
//! *anchored* to a given block. An example of this is storing a merkle proof of the transaction to
//! the confirmation block - this can be done with a custom [`Anchor`] type. The minimal [`Anchor`]
//! type would just be a [`BlockId`] which just represents the height and hash of the block which
//! the transaction is contained in. Note that a transaction can be contained in multiple
//! conflicting blocks (by nature of the Bitcoin network).
//!
//! ```
//! # use bdk_chain::BlockId;
//! # use bdk_chain::tx_graph::TxGraph;
//! # use bdk_chain::example_utils::*;
//! # use bitcoin::Transaction;
//! # let tx_a = tx_from_hex(RAW_TX_1);
//! let mut tx_graph: TxGraph = TxGraph::default();
//!
//! // insert a transaction
//! let changeset = tx_graph.insert_tx(tx_a);
//!
//! // We can restore the state of the `tx_graph` by applying all
//! // the changesets obtained by mutating the original (the order doesn't matter).
//! let mut restored_tx_graph: TxGraph = TxGraph::default();
//! restored_tx_graph.apply_changeset(changeset);
//!
//! assert_eq!(tx_graph, restored_tx_graph);
//! ```
//!
//! A [`TxGraph`] can also be updated with another [`TxGraph`] which merges them together.
//!
//! ```
//! # use bdk_chain::{Merge, BlockId};
//! # use bdk_chain::tx_graph::{self, TxGraph};
//! # use bdk_chain::example_utils::*;
//! # use bitcoin::Transaction;
//! # use std::sync::Arc;
//! # let tx_a = tx_from_hex(RAW_TX_1);
//! # let tx_b = tx_from_hex(RAW_TX_2);
//! let mut graph: TxGraph = TxGraph::default();
//!
//! let mut update = tx_graph::TxUpdate::default();
//! update.txs.push(Arc::new(tx_a));
//! update.txs.push(Arc::new(tx_b));
//!
//! // apply the update graph
//! let changeset = graph.apply_update(update.clone());
//!
//! // if we apply it again, the resulting changeset will be empty
//! let changeset = graph.apply_update(update);
//! assert!(changeset.is_empty());
//! ```
//! [`insert_txout`]: TxGraph::insert_txout

use crate::collections::*;
use crate::BlockId;
use crate::CanonicalIter;
use crate::CanonicalView;
use crate::CanonicalizationParams;
use crate::{Anchor, ChainOracle, Merge};
use alloc::collections::vec_deque::VecDeque;
use alloc::sync::Arc;
use alloc::vec::Vec;
use bdk_core::ConfirmationBlockTime;
pub use bdk_core::TxUpdate;
use bitcoin::{Amount, OutPoint, SignedAmount, Transaction, TxOut, Txid};
use core::fmt::{self, Formatter};
use core::{
    convert::Infallible,
    ops::Deref,
};

impl<A: Ord> From<TxGraph<A>> for TxUpdate<A> {
    fn from(graph: TxGraph<A>) -> Self {
        let mut tx_update = TxUpdate::default();
        tx_update.txs = graph.full_txs().map(|tx_node| tx_node.tx).collect();
        tx_update.txouts = graph
            .floating_txouts()
            .map(|(op, txo)| (op, txo.clone()))
            .collect();
        tx_update.anchors = graph
            .anchors
            .into_iter()
            .flat_map(|(txid, anchors)| anchors.into_iter().map(move |a| (a, txid)))
            .collect();
        tx_update.seen_ats = graph.last_seen.into_iter().collect();
        tx_update.evicted_ats = graph.last_evicted.into_iter().collect();
        tx_update
    }
}

impl<A: Anchor> From<TxUpdate<A>> for TxGraph<A> {
    fn from(update: TxUpdate<A>) -> Self {
        let mut graph = TxGraph::<A>::default();
        let _ = graph.apply_update(update);
        graph
    }
}

/// A graph of transactions and spends.
///
/// See the [module-level documentation] for more.
///
/// [module-level documentation]: crate::tx_graph
#[derive(Clone, Debug, PartialEq)]
pub struct TxGraph<A = ConfirmationBlockTime> {
    txs: HashMap<Txid, TxNodeInternal>,
    spends: HashMap<OutPoint, HashSet<Txid>>,
    spend_vouts_by_txid: HashMap<Txid, BTreeSet<u32>>,
    anchors: HashMap<Txid, BTreeSet<A>>,
    first_seen: HashMap<Txid, u64>,
    last_seen: HashMap<Txid, u64>,
    last_evicted: HashMap<Txid, u64>,

    txs_by_highest_conf_heights: BTreeSet<(u32, Txid)>,
    txs_by_last_seen: BTreeSet<(u64, Txid)>,

    // The following fields exist so that methods can return references to empty sets.
    // FIXME: This can be removed once `HashSet::new` and `BTreeSet::new` are const fns.
    empty_outspends: HashSet<Txid>,
    empty_anchors: BTreeSet<A>,
}

impl<A> Default for TxGraph<A> {
    fn default() -> Self {
        Self {
            txs: Default::default(),
            spends: Default::default(),
            spend_vouts_by_txid: Default::default(),
            anchors: Default::default(),
            first_seen: Default::default(),
            last_seen: Default::default(),
            last_evicted: Default::default(),
            txs_by_highest_conf_heights: Default::default(),
            txs_by_last_seen: Default::default(),
            empty_outspends: Default::default(),
            empty_anchors: Default::default(),
        }
    }
}

/// A transaction node in the [`TxGraph`].
#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
pub struct TxNode<'a, T, A> {
    /// Txid of the transaction.
    pub txid: Txid,
    /// A partial or full representation of the transaction.
    pub tx: T,
    /// The blocks that the transaction is "anchored" in.
    pub anchors: &'a BTreeSet<A>,
    /// The first-seen unix timestamp of the transaction as unconfirmed.
    pub first_seen: Option<u64>,
    /// The last-seen unix timestamp of the transaction as unconfirmed.
    pub last_seen: Option<u64>,
    /// The last-evicted-from-mempool unix timestamp of the transaction.
    pub last_evicted: Option<u64>,
}

impl<T, A> Deref for TxNode<'_, T, A> {
    type Target = T;

    fn deref(&self) -> &Self::Target {
        &self.tx
    }
}

/// Internal representation of a transaction node of a [`TxGraph`].
///
/// This can either be a whole transaction, or a partial transaction (where we only have select
/// outputs).
#[derive(Clone, Debug, PartialEq)]
enum TxNodeInternal {
    Whole(Arc<Transaction>),
    Partial(BTreeMap<u32, TxOut>),
}

impl Default for TxNodeInternal {
    fn default() -> Self {
        Self::Partial(BTreeMap::new())
    }
}

/// Errors returned by `TxGraph::calculate_fee`.
#[derive(Debug, PartialEq, Eq)]
pub enum CalculateFeeError {
    /// Missing `TxOut` for one or more of the inputs of the tx
    MissingTxOut(Vec<OutPoint>),
    /// When the transaction is invalid according to the graph it has a negative fee
    NegativeFee(SignedAmount),
}

impl fmt::Display for CalculateFeeError {
    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
        match self {
            CalculateFeeError::MissingTxOut(outpoints) => {
                let max_show = 3;
                let shown: Vec<_> = outpoints.iter().take(max_show).collect();
                let remaining = outpoints.len().saturating_sub(max_show);

                write!(f, "cannot calculate fee, missing previous output(s): ")?;
                if outpoints.is_empty() {
                    write!(f, "<none>")
                } else {
                    write!(f, "{}", shown[0])?;
                    for op in &shown[1..] {
                        write!(f, ", {}", op)?;
                    }
                    if remaining > 0 {
                        write!(f, " (+{} more)", remaining)?;
                    }
                    Ok(())
                }
            }
            CalculateFeeError::NegativeFee(fee) => {
                write!(
                    f,
                    "invalid transaction: negative fee {}",
                    fee.display_dynamic()
                )?;
                Ok(())
            }
        }
    }
}

impl core::error::Error for CalculateFeeError {}

impl<A> TxGraph<A> {
    /// Iterate over all tx outputs known by [`TxGraph`].
    ///
    /// This includes txouts of both full transactions as well as floating transactions.
    pub fn all_txouts(&self) -> impl Iterator<Item = (OutPoint, &TxOut)> {
        self.txs.iter().flat_map(|(txid, tx)| match tx {
            TxNodeInternal::Whole(tx) => tx
                .as_ref()
                .output
                .iter()
                .enumerate()
                .map(|(vout, txout)| (OutPoint::new(*txid, vout as _), txout))
                .collect::<Vec<_>>(),
            TxNodeInternal::Partial(txouts) => txouts
                .iter()
                .map(|(vout, txout)| (OutPoint::new(*txid, *vout as _), txout))
                .collect::<Vec<_>>(),
        })
    }

    /// Iterate over floating txouts known by [`TxGraph`].
    ///
    /// Floating txouts are txouts that do not have the residing full transaction contained in the
    /// graph.
    pub fn floating_txouts(&self) -> impl Iterator<Item = (OutPoint, &TxOut)> {
        self.txs
            .iter()
            .filter_map(|(txid, tx_node)| match tx_node {
                TxNodeInternal::Whole(_) => None,
                TxNodeInternal::Partial(txouts) => Some(
                    txouts
                        .iter()
                        .map(|(&vout, txout)| (OutPoint::new(*txid, vout), txout)),
                ),
            })
            .flatten()
    }

    /// Iterate over all full transactions in the graph.
    pub fn full_txs(&self) -> impl Iterator<Item = TxNode<'_, Arc<Transaction>, A>> {
        self.txs.iter().filter_map(|(&txid, tx)| match tx {
            TxNodeInternal::Whole(tx) => Some(TxNode {
                txid,
                tx: tx.clone(),
                anchors: self.anchors.get(&txid).unwrap_or(&self.empty_anchors),
                first_seen: self.first_seen.get(&txid).copied(),
                last_seen: self.last_seen.get(&txid).copied(),
                last_evicted: self.last_evicted.get(&txid).copied(),
            }),
            TxNodeInternal::Partial(_) => None,
        })
    }

    /// Iterate over graph transactions with no anchors or last-seen.
    pub fn txs_with_no_anchor_or_last_seen(
        &self,
    ) -> impl Iterator<Item = TxNode<'_, Arc<Transaction>, A>> {
        self.full_txs().filter_map(|tx| {
            if tx.anchors.is_empty() && tx.last_seen.is_none() {
                Some(tx)
            } else {
                None
            }
        })
    }

    /// Get a transaction by txid. This only returns `Some` for full transactions.
    ///
    /// Refer to [`get_txout`] for getting a specific [`TxOut`].
    ///
    /// [`get_txout`]: Self::get_txout
    pub fn get_tx(&self, txid: Txid) -> Option<Arc<Transaction>> {
        self.get_tx_node(txid).map(|n| n.tx)
    }

    /// Get a transaction node by txid. This only returns `Some` for full transactions.
    pub fn get_tx_node(&self, txid: Txid) -> Option<TxNode<'_, Arc<Transaction>, A>> {
        match &self.txs.get(&txid)? {
            TxNodeInternal::Whole(tx) => Some(TxNode {
                txid,
                tx: tx.clone(),
                anchors: self.anchors.get(&txid).unwrap_or(&self.empty_anchors),
                first_seen: self.first_seen.get(&txid).copied(),
                last_seen: self.last_seen.get(&txid).copied(),
                last_evicted: self.last_evicted.get(&txid).copied(),
            }),
            _ => None,
        }
    }

    /// Obtains a single tx output (if any) at the specified outpoint.
    pub fn get_txout(&self, outpoint: OutPoint) -> Option<&TxOut> {
        match &self.txs.get(&outpoint.txid)? {
            TxNodeInternal::Whole(tx) => tx.as_ref().output.get(outpoint.vout as usize),
            TxNodeInternal::Partial(txouts) => txouts.get(&outpoint.vout),
        }
    }

    /// Returns known outputs of a given `txid`.
    ///
    /// Returns a [`BTreeMap`] of vout to output of the provided `txid`.
    pub fn tx_outputs(&self, txid: Txid) -> Option<BTreeMap<u32, &TxOut>> {
        Some(match &self.txs.get(&txid)? {
            TxNodeInternal::Whole(tx) => tx
                .as_ref()
                .output
                .iter()
                .enumerate()
                .map(|(vout, txout)| (vout as u32, txout))
                .collect::<BTreeMap<_, _>>(),
            TxNodeInternal::Partial(txouts) => txouts
                .iter()
                .map(|(vout, txout)| (*vout, txout))
                .collect::<BTreeMap<_, _>>(),
        })
    }

    /// Calculates the fee of a given transaction. Returns [`Amount::ZERO`] if `tx` is a coinbase
    /// transaction. Returns `OK(_)` if we have all the [`TxOut`]s being spent by `tx` in the
    /// graph (either as the full transactions or individual txouts).
    ///
    /// To calculate the fee for a [`Transaction`] that depends on foreign [`TxOut`] values you must
    /// first manually insert the foreign TxOuts into the tx graph using the [`insert_txout`]
    /// function. Only insert TxOuts you trust the values for!
    ///
    /// Note `tx` does not have to be in the graph for this to work.
    ///
    /// [`insert_txout`]: Self::insert_txout
    pub fn calculate_fee(&self, tx: &Transaction) -> Result<Amount, CalculateFeeError> {
        if tx.is_coinbase() {
            return Ok(Amount::ZERO);
        }

        let (inputs_sum, missing_outputs) = tx.input.iter().fold(
            (SignedAmount::ZERO, Vec::new()),
            |(mut sum, mut missing_outpoints), txin| match self.get_txout(txin.previous_output) {
                None => {
                    missing_outpoints.push(txin.previous_output);
                    (sum, missing_outpoints)
                }
                Some(txout) => {
                    sum += txout.value.to_signed().expect("valid `SignedAmount`");
                    (sum, missing_outpoints)
                }
            },
        );
        if !missing_outputs.is_empty() {
            return Err(CalculateFeeError::MissingTxOut(missing_outputs));
        }

        let outputs_sum = tx
            .output
            .iter()
            .map(|txout| txout.value.to_signed().expect("valid `SignedAmount`"))
            .sum::<SignedAmount>();

        let fee = inputs_sum - outputs_sum;
        fee.to_unsigned()
            .map_err(|_| CalculateFeeError::NegativeFee(fee))
    }

    /// The transactions spending from this output.
    ///
    /// [`TxGraph`] allows conflicting transactions within the graph. Obviously the transactions in
    /// the returned set will never be in the same active-chain.
    pub fn outspends(&self, outpoint: OutPoint) -> &HashSet<Txid> {
        self.spends.get(&outpoint).unwrap_or(&self.empty_outspends)
    }

    /// Iterates over the transactions spending from `txid`.
    ///
    /// The iterator item is a union of `(vout, txid-set)` where:
    ///
    /// - `vout` is the provided `txid`'s outpoint that is being spent
    /// - `txid-set` is the set of txids spending the `vout`.
    pub fn tx_spends(
        &self,
        txid: Txid,
    ) -> impl DoubleEndedIterator<Item = (u32, &HashSet<Txid>)> + '_ {
        self.spend_vouts_by_txid
            .get(&txid)
            .into_iter()
            .flat_map(move |vouts| {
                vouts.iter().filter_map(move |vout| {
                    self.spends
                        .get(&OutPoint::new(txid, *vout))
                        .map(|spends| (*vout, spends))
                })
            })
    }
}

impl<A: Clone + Ord> TxGraph<A> {
    /// Creates an iterator that filters and maps ancestor transactions.
    ///
    /// The iterator starts with the ancestors of the supplied `tx` (ancestor transactions of `tx`
    /// are transactions spent by `tx`). The supplied transaction is excluded from the iterator.
    ///
    /// The supplied closure takes in two inputs `(depth, ancestor_tx)`:
    ///
    /// * `depth` is the distance between the starting `Transaction` and the `ancestor_tx`. I.e., if
    ///   the `Transaction` is spending an output of the `ancestor_tx` then `depth` will be 1.
    /// * `ancestor_tx` is the `Transaction`'s ancestor which we are considering to walk.
    ///
    /// The supplied closure returns an `Option<T>`, allowing the caller to map each `Transaction`
    /// it visits and decide whether to visit ancestors.
    pub fn walk_ancestors<'g, T, F, O>(&'g self, tx: T, walk_map: F) -> TxAncestors<'g, A, F, O>
    where
        T: Into<Arc<Transaction>>,
        F: FnMut(usize, Arc<Transaction>) -> Option<O> + 'g,
    {
        TxAncestors::new_exclude_root(self, tx, walk_map)
    }

    /// Creates an iterator that filters and maps descendants from the starting `txid`.
    ///
    /// The supplied closure takes in two inputs `(depth, descendant_txid)`:
    ///
    /// * `depth` is the distance between the starting `txid` and the `descendant_txid`. I.e., if
    ///   the descendant is spending an output of the starting `txid` then `depth` will be 1.
    /// * `descendant_txid` is the descendant's txid which we are considering to walk.
    ///
    /// The supplied closure returns an `Option<T>`, allowing the caller to map each node it visits
    /// and decide whether to visit descendants.
    pub fn walk_descendants<'g, F, O>(
        &'g self,
        txid: Txid,
        walk_map: F,
    ) -> TxDescendants<'g, A, F, O>
    where
        F: FnMut(usize, Txid) -> Option<O> + 'g,
    {
        TxDescendants::new_exclude_root(self, txid, walk_map)
    }
}

impl<A> TxGraph<A> {
    /// Creates an iterator that both filters and maps conflicting transactions (this includes
    /// descendants of directly-conflicting transactions, which are also considered conflicts).
    ///
    /// Refer to [`Self::walk_descendants`] for `walk_map` usage.
    pub fn walk_conflicts<'g, F, O>(
        &'g self,
        tx: &'g Transaction,
        walk_map: F,
    ) -> TxDescendants<'g, A, F, O>
    where
        F: FnMut(usize, Txid) -> Option<O> + 'g,
    {
        let txids = self.direct_conflicts(tx).map(|(_, txid)| txid);
        TxDescendants::from_multiple_include_root(self, txids, walk_map)
    }

    /// Given a transaction, return an iterator of txids that directly conflict with the given
    /// transaction's inputs (spends). The conflicting txids are returned with the given
    /// transaction's vin (in which it conflicts).
    ///
    /// Note that this only returns directly conflicting txids and won't include:
    /// - descendants of conflicting transactions (which are technically also conflicting)
    /// - transactions conflicting with the given transaction's ancestors
    pub fn direct_conflicts<'g>(
        &'g self,
        tx: &'g Transaction,
    ) -> impl Iterator<Item = (usize, Txid)> + 'g {
        let txid = tx.compute_txid();
        tx.input
            .iter()
            .enumerate()
            .filter_map(move |(vin, txin)| self.spends.get(&txin.previous_output).zip(Some(vin)))
            .flat_map(|(spends, vin)| core::iter::repeat(vin).zip(spends.iter().cloned()))
            .filter(move |(_, conflicting_txid)| *conflicting_txid != txid)
    }

    /// Get all transaction anchors known by [`TxGraph`].
    pub fn all_anchors(&self) -> &HashMap<Txid, BTreeSet<A>> {
        &self.anchors
    }

    /// Whether the graph has any transactions or outputs in it.
    pub fn is_empty(&self) -> bool {
        self.txs.is_empty()
    }
}

impl<A: Anchor> TxGraph<A> {
    /// Transform the [`TxGraph`] to have [`Anchor`]s of another type.
    ///
    /// This takes in a closure of signature `FnMut(A) -> A2` which is called for each [`Anchor`] to
    /// transform it.
    pub fn map_anchors<A2: Anchor, F>(self, f: F) -> TxGraph<A2>
    where
        F: FnMut(A) -> A2,
    {
        let mut new_graph = TxGraph::<A2>::default();
        new_graph.apply_changeset(self.initial_changeset().map_anchors(f));
        new_graph
    }

    /// Construct a new [`TxGraph`] from a list of transactions.
    pub fn new(txs: impl IntoIterator<Item = Transaction>) -> Self {
        let mut new = Self::default();
        for tx in txs.into_iter() {
            let _ = new.insert_tx(tx);
        }
        new
    }

    /// Inserts the given [`TxOut`] at [`OutPoint`].
    ///
    /// Inserting floating txouts are useful for determining fee/feerate of transactions we care
    /// about.
    ///
    /// The [`ChangeSet`] result will be empty if the `outpoint` (or a full transaction containing
    /// the `outpoint`) already existed in `self`.
    ///
    /// [`apply_changeset`]: Self::apply_changeset
    pub fn insert_txout(&mut self, outpoint: OutPoint, txout: TxOut) -> ChangeSet<A> {
        let mut changeset = ChangeSet::<A>::default();
        let tx_node = self.txs.entry(outpoint.txid).or_default();
        match tx_node {
            TxNodeInternal::Whole(_) => {
                // ignore this txout we have the full one already.
                // NOTE: You might think putting a debug_assert! here to check the output being
                // replaced was actually correct is a good idea but the tests have already been
                // written assuming this never panics.
            }
            TxNodeInternal::Partial(partial_tx) => {
                match partial_tx.insert(outpoint.vout, txout.clone()) {
                    Some(old_txout) => {
                        debug_assert_eq!(
                            txout, old_txout,
                            "txout of the same outpoint should never change"
                        );
                    }
                    None => {
                        changeset.txouts.insert(outpoint, txout);
                    }
                }
            }
        }
        changeset
    }

    /// Insert the given transaction into [`TxGraph`].
    ///
    /// The [`ChangeSet`] returned will be empty if no changes are made to the graph.
    ///
    /// # Updating Existing Transactions
    ///
    /// An unsigned transaction can be inserted first and have it's witness fields updated with
    /// further transaction insertions (given that the newly introduced transaction shares the same
    /// txid as the original transaction).
    ///
    /// The witnesses of the newly introduced transaction will be merged with the witnesses of the
    /// original transaction in a way where:
    ///
    /// * A non-empty witness has precedence over an empty witness.
    /// * A smaller witness has precedence over a larger witness.
    /// * If the witness sizes are the same, we prioritize the two witnesses with lexicographical
    ///   order.
    pub fn insert_tx<T: Into<Arc<Transaction>>>(&mut self, tx: T) -> ChangeSet<A> {
        // This returns `Some` only if the merged tx is different to the `original_tx`.
        fn _merge_tx_witnesses(
            original_tx: &Arc<Transaction>,
            other_tx: &Arc<Transaction>,
        ) -> Option<Arc<Transaction>> {
            debug_assert_eq!(
                original_tx.input.len(),
                other_tx.input.len(),
                "tx input count must be the same"
            );
            let merged_input = Iterator::zip(original_tx.input.iter(), other_tx.input.iter())
                .map(|(original_txin, other_txin)| {
                    let original_key = core::cmp::Reverse((
                        original_txin.witness.is_empty(),
                        original_txin.witness.size(),
                        &original_txin.witness,
                    ));
                    let other_key = core::cmp::Reverse((
                        other_txin.witness.is_empty(),
                        other_txin.witness.size(),
                        &other_txin.witness,
                    ));
                    if original_key > other_key {
                        original_txin.clone()
                    } else {
                        other_txin.clone()
                    }
                })
                .collect::<Vec<_>>();
            if merged_input == original_tx.input {
                return None;
            }
            if merged_input == other_tx.input {
                return Some(other_tx.clone());
            }
            Some(Arc::new(Transaction {
                input: merged_input,
                ..(**original_tx).clone()
            }))
        }

        let tx: Arc<Transaction> = tx.into();
        let txid = tx.compute_txid();
        let mut changeset = ChangeSet::<A>::default();

        let tx_node = self.txs.entry(txid).or_default();
        match tx_node {
            TxNodeInternal::Whole(existing_tx) => {
                if existing_tx.as_ref() != tx.as_ref() {
                    // Allowing updating witnesses of txs.
                    if let Some(merged_tx) = _merge_tx_witnesses(existing_tx, &tx) {
                        *existing_tx = merged_tx.clone();
                        changeset.txs.insert(merged_tx);
                    }
                }
            }
            partial_tx => {
                for txin in &tx.input {
                    // this means the tx is coinbase so there is no previous output
                    if txin.previous_output.is_null() {
                        continue;
                    }
                    self.spend_vouts_by_txid
                        .entry(txin.previous_output.txid)
                        .or_default()
                        .insert(txin.previous_output.vout);
                    self.spends
                        .entry(txin.previous_output)
                        .or_default()
                        .insert(txid);
                }
                *partial_tx = TxNodeInternal::Whole(tx.clone());
                changeset.txs.insert(tx);
            }
        }

        changeset
    }

    /// Batch insert unconfirmed transactions.
    ///
    /// Items of `txs` are tuples containing the transaction and a *last seen* timestamp. The
    /// *last seen* communicates when the transaction is last seen in mempool which is used for
    /// conflict-resolution (refer to [`TxGraph::insert_seen_at`] for details).
    pub fn batch_insert_unconfirmed<T: Into<Arc<Transaction>>>(
        &mut self,
        txs: impl IntoIterator<Item = (T, u64)>,
    ) -> ChangeSet<A> {
        let mut changeset = ChangeSet::<A>::default();
        for (tx, seen_at) in txs {
            let tx: Arc<Transaction> = tx.into();
            changeset.merge(self.insert_seen_at(tx.compute_txid(), seen_at));
            changeset.merge(self.insert_tx(tx));
        }
        changeset
    }

    /// Inserts the given `anchor` into [`TxGraph`].
    ///
    /// The [`ChangeSet`] returned will be empty if graph already knows that `txid` exists in
    /// `anchor`.
    pub fn insert_anchor(&mut self, txid: Txid, anchor: A) -> ChangeSet<A> {
        // These two variables are used to determine how to modify the `txid`'s entry in
        // `txs_by_highest_conf_heights`.
        // We want to remove `(old_top_h?, txid)` and insert `(new_top_h?, txid)`.
        let mut old_top_h = None;
        let mut new_top_h = anchor.confirmation_height_upper_bound();

        let is_changed = match self.anchors.entry(txid) {
            hash_map::Entry::Occupied(mut e) => {
                old_top_h = e
                    .get()
                    .iter()
                    .last()
                    .map(Anchor::confirmation_height_upper_bound);
                if let Some(old_top_h) = old_top_h {
                    if old_top_h > new_top_h {
                        new_top_h = old_top_h;
                    }
                }
                let is_changed = e.get_mut().insert(anchor.clone());
                is_changed
            }
            hash_map::Entry::Vacant(e) => {
                e.insert(core::iter::once(anchor.clone()).collect());
                true
            }
        };

        let mut changeset = ChangeSet::<A>::default();
        if is_changed {
            let new_top_is_changed = match old_top_h {
                None => true,
                Some(old_top_h) if old_top_h != new_top_h => true,
                _ => false,
            };
            if new_top_is_changed {
                if let Some(prev_top_h) = old_top_h {
                    self.txs_by_highest_conf_heights.remove(&(prev_top_h, txid));
                }
                self.txs_by_highest_conf_heights.insert((new_top_h, txid));
            }
            changeset.anchors.insert((anchor, txid));
        }
        changeset
    }

    /// Updates the first-seen and last-seen timestamps for a given `txid` in the [`TxGraph`].
    ///
    /// This method records the time a transaction was observed by updating both:
    /// - the **first-seen** timestamp, which only changes if `seen_at` is earlier than the current
    ///   value, and
    /// - the **last-seen** timestamp, which only changes if `seen_at` is later than the current
    ///   value.
    ///
    /// `seen_at` is a UNIX timestamp in seconds.
    ///
    /// Returns a [`ChangeSet`] representing any changes applied.
    pub fn insert_seen_at(&mut self, txid: Txid, seen_at: u64) -> ChangeSet<A> {
        let mut changeset_first_seen = self.update_first_seen(txid, seen_at);
        let changeset_last_seen = self.update_last_seen(txid, seen_at);
        changeset_first_seen.merge(changeset_last_seen);
        changeset_first_seen
    }

    /// Updates `first_seen` given a new `seen_at`.
    fn update_first_seen(&mut self, txid: Txid, seen_at: u64) -> ChangeSet<A> {
        let is_changed = match self.first_seen.entry(txid) {
            hash_map::Entry::Occupied(mut e) => {
                let first_seen = e.get_mut();
                let change = *first_seen > seen_at;
                if change {
                    *first_seen = seen_at;
                }
                change
            }
            hash_map::Entry::Vacant(e) => {
                e.insert(seen_at);
                true
            }
        };

        let mut changeset = ChangeSet::<A>::default();
        if is_changed {
            changeset.first_seen.insert(txid, seen_at);
        }
        changeset
    }

    /// Updates `last_seen` given a new `seen_at`.
    fn update_last_seen(&mut self, txid: Txid, seen_at: u64) -> ChangeSet<A> {
        let mut old_last_seen = None;
        let is_changed = match self.last_seen.entry(txid) {
            hash_map::Entry::Occupied(mut e) => {
                let last_seen = e.get_mut();
                old_last_seen = Some(*last_seen);
                let change = *last_seen < seen_at;
                if change {
                    *last_seen = seen_at;
                }
                change
            }
            hash_map::Entry::Vacant(e) => {
                e.insert(seen_at);
                true
            }
        };

        let mut changeset = ChangeSet::<A>::default();
        if is_changed {
            if let Some(old_last_seen) = old_last_seen {
                self.txs_by_last_seen.remove(&(old_last_seen, txid));
            }
            self.txs_by_last_seen.insert((seen_at, txid));
            changeset.last_seen.insert(txid, seen_at);
        }
        changeset
    }

    /// Inserts the given `evicted_at` for `txid` into [`TxGraph`].
    ///
    /// The `evicted_at` timestamp represents the last known time when the transaction was observed
    /// to be missing from the mempool. If `txid` was previously recorded with an earlier
    /// `evicted_at` value, it is updated only if the new value is greater.
    pub fn insert_evicted_at(&mut self, txid: Txid, evicted_at: u64) -> ChangeSet<A> {
        let is_changed = match self.last_evicted.entry(txid) {
            hash_map::Entry::Occupied(mut e) => {
                let last_evicted = e.get_mut();
                let change = *last_evicted < evicted_at;
                if change {
                    *last_evicted = evicted_at;
                }
                change
            }
            hash_map::Entry::Vacant(e) => {
                e.insert(evicted_at);
                true
            }
        };

        let mut changeset = ChangeSet::<A>::default();
        if is_changed {
            changeset.last_evicted.insert(txid, evicted_at);
        }
        changeset
    }

    /// Batch inserts `(txid, evicted_at)` pairs into [`TxGraph`] for `txid`s that the graph is
    /// tracking.
    ///
    /// The `evicted_at` timestamp represents the last known time when the transaction was observed
    /// to be missing from the mempool. If `txid` was previously recorded with an earlier
    /// `evicted_at` value, it is updated only if the new value is greater.
    pub fn batch_insert_relevant_evicted_at(
        &mut self,
        evicted_ats: impl IntoIterator<Item = (Txid, u64)>,
    ) -> ChangeSet<A> {
        let mut changeset = ChangeSet::default();
        for (txid, evicted_at) in evicted_ats {
            // Only record evictions for transactions the graph is tracking.
            if self.txs.contains_key(&txid) {
                changeset.merge(self.insert_evicted_at(txid, evicted_at));
            }
        }
        changeset
    }

    /// Extends this graph with the given `update`.
    ///
    /// The returned [`ChangeSet`] is the set difference between `update` and `self` (transactions
    /// that exist in `update` but not in `self`).
    pub fn apply_update(&mut self, update: TxUpdate<A>) -> ChangeSet<A> {
        let mut changeset = ChangeSet::<A>::default();
        for tx in update.txs {
            changeset.merge(self.insert_tx(tx));
        }
        for (outpoint, txout) in update.txouts {
            changeset.merge(self.insert_txout(outpoint, txout));
        }
        for (anchor, txid) in update.anchors {
            changeset.merge(self.insert_anchor(txid, anchor));
        }
        for (txid, seen_at) in update.seen_ats {
            changeset.merge(self.insert_seen_at(txid, seen_at));
        }
        for (txid, evicted_at) in update.evicted_ats {
            changeset.merge(self.insert_evicted_at(txid, evicted_at));
        }
        changeset
    }

    /// Determines the [`ChangeSet`] between `self` and an empty [`TxGraph`].
    pub fn initial_changeset(&self) -> ChangeSet<A> {
        ChangeSet {
            txs: self.full_txs().map(|tx_node| tx_node.tx).collect(),
            txouts: self
                .floating_txouts()
                .map(|(op, txout)| (op, txout.clone()))
                .collect(),
            anchors: self
                .anchors
                .iter()
                .flat_map(|(txid, anchors)| anchors.iter().map(|a| (a.clone(), *txid)))
                .collect(),
            first_seen: self.first_seen.iter().map(|(&k, &v)| (k, v)).collect(),
            last_seen: self.last_seen.iter().map(|(&k, &v)| (k, v)).collect(),
            last_evicted: self.last_evicted.iter().map(|(&k, &v)| (k, v)).collect(),
        }
    }

    /// Applies [`ChangeSet`] to [`TxGraph`].
    pub fn apply_changeset(&mut self, changeset: ChangeSet<A>) {
        for tx in changeset.txs {
            let _ = self.insert_tx(tx);
        }
        for (outpoint, txout) in changeset.txouts {
            let _ = self.insert_txout(outpoint, txout);
        }
        for (anchor, txid) in changeset.anchors {
            let _ = self.insert_anchor(txid, anchor);
        }
        for (txid, seen_at) in changeset.last_seen {
            let _ = self.insert_seen_at(txid, seen_at);
        }
        for (txid, evicted_at) in changeset.last_evicted {
            let _ = self.insert_evicted_at(txid, evicted_at);
        }
    }
}

impl<A: Anchor> TxGraph<A> {
    /// List txids by descending anchor height order.
    ///
    /// If multiple anchors exist for a txid, the highest anchor height will be used. Transactions
    /// without anchors are excluded.
    pub fn txids_by_descending_anchor_height(
        &self,
    ) -> impl ExactSizeIterator<Item = (u32, Txid)> + '_ {
        self.txs_by_highest_conf_heights.iter().copied().rev()
    }

    /// List txids by descending last-seen order.
    ///
    /// Transactions without last-seens are excluded. Transactions with a last-evicted timestamp
    /// equal or higher than it's last-seen timestamp are excluded.
    pub fn txids_by_descending_last_seen(&self) -> impl Iterator<Item = (u64, Txid)> + '_ {
        self.txs_by_last_seen
            .iter()
            .copied()
            .rev()
            .filter(|(last_seen, txid)| match self.last_evicted.get(txid) {
                Some(last_evicted) => last_evicted < last_seen,
                None => true,
            })
    }

    /// Returns a [`CanonicalIter`].
    pub fn canonical_iter<'a, C: ChainOracle>(
        &'a self,
        chain: &'a C,
        chain_tip: BlockId,
        params: CanonicalizationParams,
    ) -> CanonicalIter<'a, A, C> {
        CanonicalIter::new(self, chain, chain_tip, params)
    }

    /// Returns a [`CanonicalView`].
    pub fn try_canonical_view<'a, C: ChainOracle>(
        &'a self,
        chain: &'a C,
        chain_tip: BlockId,
        params: CanonicalizationParams,
    ) -> Result<CanonicalView<A>, C::Error> {
        CanonicalView::new(self, chain, chain_tip, params)
    }

    /// Returns a [`CanonicalView`].
    pub fn canonical_view<'a, C: ChainOracle<Error = Infallible>>(
        &'a self,
        chain: &'a C,
        chain_tip: BlockId,
        params: CanonicalizationParams,
    ) -> CanonicalView<A> {
        CanonicalView::new(self, chain, chain_tip, params).expect("infallible")
    }

    /// Construct a `TxGraph` from a `changeset`.
    pub fn from_changeset(changeset: ChangeSet<A>) -> Self {
        let mut graph = Self::default();
        graph.apply_changeset(changeset);
        graph
    }
}

/// The [`ChangeSet`] represents changes to a [`TxGraph`].
///
/// Since [`TxGraph`] is monotone, the "changeset" can only contain transactions to be added and
/// not removed.
///
/// Refer to [module-level documentation] for more.
///
/// [module-level documentation]: crate::tx_graph
#[derive(Debug, Clone, PartialEq)]
#[cfg_attr(
    feature = "serde",
    derive(serde::Deserialize, serde::Serialize),
    serde(bound(
        deserialize = "A: Ord + serde::Deserialize<'de>",
        serialize = "A: Ord + serde::Serialize",
    ))
)]
#[must_use]
pub struct ChangeSet<A = ()> {
    /// Added transactions.
    pub txs: BTreeSet<Arc<Transaction>>,
    /// Added txouts.
    pub txouts: BTreeMap<OutPoint, TxOut>,
    /// Added anchors.
    pub anchors: BTreeSet<(A, Txid)>,
    /// Added last-seen unix timestamps of transactions.
    pub last_seen: BTreeMap<Txid, u64>,
    /// Added timestamps of when a transaction is last evicted from the mempool.
    #[cfg_attr(feature = "serde", serde(default))]
    pub last_evicted: BTreeMap<Txid, u64>,
    /// Added first-seen unix timestamps of transactions.
    #[cfg_attr(feature = "serde", serde(default))]
    pub first_seen: BTreeMap<Txid, u64>,
}

impl<A> Default for ChangeSet<A> {
    fn default() -> Self {
        Self {
            txs: Default::default(),
            txouts: Default::default(),
            anchors: Default::default(),
            first_seen: Default::default(),
            last_seen: Default::default(),
            last_evicted: Default::default(),
        }
    }
}

impl<A> ChangeSet<A> {
    /// Iterates over all outpoints contained within [`ChangeSet`].
    pub fn txouts(&self) -> impl Iterator<Item = (OutPoint, &TxOut)> {
        self.txs
            .iter()
            .flat_map(|tx| {
                tx.output
                    .iter()
                    .enumerate()
                    .map(move |(vout, txout)| (OutPoint::new(tx.compute_txid(), vout as _), txout))
            })
            .chain(self.txouts.iter().map(|(op, txout)| (*op, txout)))
    }

    /// Iterates over the heights of that the new transaction anchors in this changeset.
    ///
    /// This is useful if you want to find which heights you need to fetch data about in order to
    /// confirm or exclude these anchors.
    pub fn anchor_heights(&self) -> impl Iterator<Item = u32> + '_
    where
        A: Anchor,
    {
        let mut dedup = None;
        self.anchors
            .iter()
            .map(|(a, _)| a.anchor_block().height)
            .filter(move |height| {
                let duplicate = dedup == Some(*height);
                dedup = Some(*height);
                !duplicate
            })
    }
}

impl<A: Ord> Merge for ChangeSet<A> {
    fn merge(&mut self, other: Self) {
        // We use `extend` instead of `BTreeMap::append` due to performance issues with `append`.
        // Refer to https://github.com/rust-lang/rust/issues/34666#issuecomment-675658420
        self.txs.extend(other.txs);
        self.txouts.extend(other.txouts);
        self.anchors.extend(other.anchors);

        // first_seen timestamps should only decrease
        self.first_seen.extend(
            other
                .first_seen
                .into_iter()
                .filter(|(txid, update_fs)| match self.first_seen.get(txid) {
                    Some(existing) => update_fs < existing,
                    None => true,
                })
                .collect::<Vec<_>>(),
        );

        // last_seen timestamps should only increase
        self.last_seen.extend(
            other
                .last_seen
                .into_iter()
                .filter(|(txid, update_ls)| self.last_seen.get(txid) < Some(update_ls))
                .collect::<Vec<_>>(),
        );
        // last_evicted timestamps should only increase
        self.last_evicted.extend(
            other
                .last_evicted
                .into_iter()
                .filter(|(txid, update_lm)| self.last_evicted.get(txid) < Some(update_lm))
                .collect::<Vec<_>>(),
        );
    }

    fn is_empty(&self) -> bool {
        self.txs.is_empty()
            && self.txouts.is_empty()
            && self.anchors.is_empty()
            && self.first_seen.is_empty()
            && self.last_seen.is_empty()
            && self.last_evicted.is_empty()
    }
}

impl<A: Ord> ChangeSet<A> {
    /// Transform the [`ChangeSet`] to have [`Anchor`]s of another type.
    ///
    /// This takes in a closure of signature `FnMut(A) -> A2` which is called for each [`Anchor`] to
    /// transform it.
    pub fn map_anchors<A2: Ord, F>(self, mut f: F) -> ChangeSet<A2>
    where
        F: FnMut(A) -> A2,
    {
        ChangeSet {
            txs: self.txs,
            txouts: self.txouts,
            anchors: BTreeSet::<(A2, Txid)>::from_iter(
                self.anchors.into_iter().map(|(a, txid)| (f(a), txid)),
            ),
            first_seen: self.first_seen,
            last_seen: self.last_seen,
            last_evicted: self.last_evicted,
        }
    }
}

impl<A> AsRef<TxGraph<A>> for TxGraph<A> {
    fn as_ref(&self) -> &TxGraph<A> {
        self
    }
}

/// An iterator that traverses ancestors of a given root transaction.
///
/// The iterator excludes partial transactions.
///
/// Returned by the [`walk_ancestors`] method of [`TxGraph`].
///
/// [`walk_ancestors`]: TxGraph::walk_ancestors
pub struct TxAncestors<'g, A, F, O>
where
    F: FnMut(usize, Arc<Transaction>) -> Option<O>,
{
    graph: &'g TxGraph<A>,
    visited: HashSet<Txid>,
    queue: VecDeque<(usize, Arc<Transaction>)>,
    filter_map: F,
}

impl<'g, A, F, O> TxAncestors<'g, A, F, O>
where
    F: FnMut(usize, Arc<Transaction>) -> Option<O>,
{
    /// Creates a `TxAncestors` that includes the starting `Transaction` when iterating.
    pub(crate) fn new_include_root(
        graph: &'g TxGraph<A>,
        tx: impl Into<Arc<Transaction>>,
        filter_map: F,
    ) -> Self {
        Self {
            graph,
            visited: Default::default(),
            queue: [(0, tx.into())].into(),
            filter_map,
        }
    }

    /// Creates a `TxAncestors` that excludes the starting `Transaction` when iterating.
    pub(crate) fn new_exclude_root(
        graph: &'g TxGraph<A>,
        tx: impl Into<Arc<Transaction>>,
        filter_map: F,
    ) -> Self {
        let mut ancestors = Self {
            graph,
            visited: Default::default(),
            queue: Default::default(),
            filter_map,
        };
        ancestors.populate_queue(1, tx.into());
        ancestors
    }

    /// Creates a `TxAncestors` from multiple starting `Transaction`s that includes the starting
    /// `Transaction`s when iterating.
    #[allow(unused)]
    pub(crate) fn from_multiple_include_root<I>(
        graph: &'g TxGraph<A>,
        txs: I,
        filter_map: F,
    ) -> Self
    where
        I: IntoIterator,
        I::Item: Into<Arc<Transaction>>,
    {
        Self {
            graph,
            visited: Default::default(),
            queue: txs.into_iter().map(|tx| (0, tx.into())).collect(),
            filter_map,
        }
    }

    /// Creates a `TxAncestors` from multiple starting `Transaction`s that excludes the starting
    /// `Transaction`s when iterating.
    #[allow(unused)]
    pub(crate) fn from_multiple_exclude_root<I>(
        graph: &'g TxGraph<A>,
        txs: I,
        filter_map: F,
    ) -> Self
    where
        I: IntoIterator,
        I::Item: Into<Arc<Transaction>>,
    {
        let mut ancestors = Self {
            graph,
            visited: Default::default(),
            queue: Default::default(),
            filter_map,
        };
        for tx in txs {
            ancestors.populate_queue(1, tx.into());
        }
        ancestors
    }

    /// Traverse all ancestors that are not filtered out by the provided closure.
    pub fn run_until_finished(self) {
        self.for_each(|_| {})
    }

    fn populate_queue(&mut self, depth: usize, tx: Arc<Transaction>) {
        let ancestors = tx
            .input
            .iter()
            .map(|txin| txin.previous_output.txid)
            .filter(|&prev_txid| self.visited.insert(prev_txid))
            .filter_map(|prev_txid| self.graph.get_tx(prev_txid))
            .map(|tx| (depth, tx));
        self.queue.extend(ancestors);
    }
}

impl<A, F, O> Iterator for TxAncestors<'_, A, F, O>
where
    F: FnMut(usize, Arc<Transaction>) -> Option<O>,
{
    type Item = O;

    fn next(&mut self) -> Option<Self::Item> {
        loop {
            // we have exhausted all paths when queue is empty
            let (ancestor_depth, tx) = self.queue.pop_front()?;
            // ignore paths when user filters them out
            let item = match (self.filter_map)(ancestor_depth, tx.clone()) {
                Some(item) => item,
                None => continue,
            };
            self.populate_queue(ancestor_depth + 1, tx);
            return Some(item);
        }
    }
}

/// An iterator that traverses transaction descendants.
///
/// Returned by the [`walk_descendants`] method of [`TxGraph`].
///
/// [`walk_descendants`]: TxGraph::walk_descendants
pub struct TxDescendants<'g, A, F, O>
where
    F: FnMut(usize, Txid) -> Option<O>,
{
    graph: &'g TxGraph<A>,
    visited: HashSet<Txid>,
    queue: VecDeque<(usize, Txid)>,
    filter_map: F,
}

impl<'g, A, F, O> TxDescendants<'g, A, F, O>
where
    F: FnMut(usize, Txid) -> Option<O>,
{
    /// Creates a `TxDescendants` that includes the starting `txid` when iterating.
    #[allow(unused)]
    pub(crate) fn new_include_root(graph: &'g TxGraph<A>, txid: Txid, filter_map: F) -> Self {
        Self {
            graph,
            visited: Default::default(),
            queue: [(0, txid)].into(),
            filter_map,
        }
    }

    /// Creates a `TxDescendants` that excludes the starting `txid` when iterating.
    pub(crate) fn new_exclude_root(graph: &'g TxGraph<A>, txid: Txid, filter_map: F) -> Self {
        let mut descendants = Self {
            graph,
            visited: Default::default(),
            queue: Default::default(),
            filter_map,
        };
        descendants.populate_queue(1, txid);
        descendants
    }

    /// Creates a `TxDescendants` from multiple starting transactions that includes the starting
    /// `txid`s when iterating.
    pub(crate) fn from_multiple_include_root<I>(
        graph: &'g TxGraph<A>,
        txids: I,
        filter_map: F,
    ) -> Self
    where
        I: IntoIterator<Item = Txid>,
    {
        Self {
            graph,
            visited: Default::default(),
            queue: txids.into_iter().map(|txid| (0, txid)).collect(),
            filter_map,
        }
    }

    /// Creates a `TxDescendants` from multiple starting transactions that excludes the starting
    /// `txid`s when iterating.
    #[allow(unused)]
    pub(crate) fn from_multiple_exclude_root<I>(
        graph: &'g TxGraph<A>,
        txids: I,
        filter_map: F,
    ) -> Self
    where
        I: IntoIterator<Item = Txid>,
    {
        let mut descendants = Self {
            graph,
            visited: Default::default(),
            queue: Default::default(),
            filter_map,
        };
        for txid in txids {
            descendants.populate_queue(1, txid);
        }
        descendants
    }

    /// Traverse all descendants that are not filtered out by the provided closure.
    pub fn run_until_finished(self) {
        self.for_each(|_| {})
    }

    fn populate_queue(&mut self, depth: usize, txid: Txid) {
        let spend_paths = self
            .graph
            .tx_spends(txid)
            .flat_map(|(_, spends)| spends)
            .map(|&txid| (depth, txid));
        self.queue.extend(spend_paths);
    }
}

impl<A, F, O> Iterator for TxDescendants<'_, A, F, O>
where
    F: FnMut(usize, Txid) -> Option<O>,
{
    type Item = O;

    fn next(&mut self) -> Option<Self::Item> {
        let (op_spends, txid, item) = loop {
            // we have exhausted all paths when queue is empty
            let (op_spends, txid) = self.queue.pop_front()?;
            // we do not want to visit the same transaction twice
            if self.visited.insert(txid) {
                // ignore paths when user filters them out
                if let Some(item) = (self.filter_map)(op_spends, txid) {
                    break (op_spends, txid, item);
                }
            }
        };

        self.populate_queue(op_spends + 1, txid);
        Some(item)
    }
}