tr.rs

// Tapscript

use crate::policy::semantic::Policy;
use crate::policy::Liftable;
use crate::util::{varint_len, witness_size};
use crate::{DescriptorTrait, ForEach, ForEachKey, Satisfier, ToPublicKey, TranslatePk};

use super::checksum::{desc_checksum, verify_checksum};
use crate::errstr;
use crate::expression::{self, FromTree};
use crate::miniscript::Miniscript;
use crate::Tap;
use crate::{Error, MiniscriptKey};
use bitcoin::blockdata::opcodes;
use bitcoin::util::taproot::{
    LeafVersion, TaprootBuilder, TaprootBuilderError, TaprootSpendInfo, TAPROOT_CONTROL_BASE_SIZE,
    TAPROOT_CONTROL_MAX_NODE_COUNT, TAPROOT_CONTROL_NODE_SIZE,
};
use bitcoin::{self, secp256k1, Script};
use std::cmp::{self, max};
use std::hash;
use std::sync::{Arc, Mutex};
use std::{fmt, str::FromStr};

/// A Taproot Tree representation.
// Hidden leaves are not yet supported in descriptor spec. Conceptually, it should
// be simple to integrate those here, but it is best to wait on core for the exact syntax.
#[derive(Clone, Ord, PartialOrd, Eq, PartialEq, Hash)]
pub enum TapTree<Pk: MiniscriptKey> {
    /// A taproot tree structure
    Tree(Arc<TapTree<Pk>>, Arc<TapTree<Pk>>),
    /// A taproot leaf denoting a spending condition
    // A new leaf version would require a new Context, therefore there is no point
    // in adding a LeafVersion with Leaf type here. All Miniscripts right now
    // are of Leafversion::default
    Leaf(Arc<Miniscript<Pk, Tap>>),
}

/// A taproot descriptor
pub struct Tr<Pk: MiniscriptKey> {
    /// A taproot internal key
    internal_key: Pk,
    /// Optional Taproot Tree with spending conditions
    tree: Option<TapTree<Pk>>,
    /// Optional spending information associated with the descriptor
    /// This will be [`None`] when the descriptor is not derived.
    /// This information will be cached automatically when it is required
    //
    // The inner `Arc` here is because Rust does not allow us to return a reference
    // to the contents of the `Option` from inside a `MutexGuard`. There is no outer
    // `Arc` because when this structure is cloned, we create a whole new mutex.
    spend_info: Mutex<Option<Arc<TaprootSpendInfo>>>,
}

impl<Pk: MiniscriptKey> Clone for Tr<Pk> {
    fn clone(&self) -> Self {
        // When cloning, construct a new Mutex so that distinct clones don't
        // cause blocking between each other. We clone only the internal `Arc`,
        // so the clone is always cheap (in both time and space)
        Self {
            internal_key: self.internal_key.clone(),
            tree: self.tree.clone(),
            spend_info: Mutex::new(
                self.spend_info
                    .lock()
                    .expect("Lock poisoned")
                    .as_ref()
                    .map(Arc::clone),
            ),
        }
    }
}

impl<Pk: MiniscriptKey> PartialEq for Tr<Pk> {
    fn eq(&self, other: &Self) -> bool {
        self.internal_key == other.internal_key && self.tree == other.tree
    }
}

impl<Pk: MiniscriptKey> Eq for Tr<Pk> {}

impl<Pk: MiniscriptKey> PartialOrd for Tr<Pk> {
    fn partial_cmp(&self, other: &Self) -> Option<cmp::Ordering> {
        match self.internal_key.partial_cmp(&other.internal_key) {
            Some(cmp::Ordering::Equal) => {}
            ord => return ord,
        }
        self.tree.partial_cmp(&other.tree)
    }
}

impl<Pk: MiniscriptKey> Ord for Tr<Pk> {
    fn cmp(&self, other: &Self) -> cmp::Ordering {
        match self.internal_key.cmp(&other.internal_key) {
            cmp::Ordering::Equal => {}
            ord => return ord,
        }
        self.tree.cmp(&other.tree)
    }
}

impl<Pk: MiniscriptKey> hash::Hash for Tr<Pk> {
    fn hash<H: hash::Hasher>(&self, state: &mut H) {
        self.internal_key.hash(state);
        self.tree.hash(state);
    }
}

impl<Pk: MiniscriptKey> TapTree<Pk> {
    // Helper function to compute height
    // TODO: Instead of computing this every time we add a new leaf, we should
    // add height as a separate field in taptree
    fn taptree_height(&self) -> usize {
        match *self {
            TapTree::Tree(ref left_tree, ref right_tree) => {
                1 + max(left_tree.taptree_height(), right_tree.taptree_height())
            }
            TapTree::Leaf(..) => 1,
        }
    }

    /// Iterate over all miniscripts
    pub fn iter(&self) -> TapTreeIter<Pk> {
        TapTreeIter {
            stack: vec![(0, self)],
        }
    }

    // Helper function to translate keys
    fn translate_helper<FPk, FPkh, Q, Error>(
        &self,
        translatefpk: &mut FPk,
        translatefpkh: &mut FPkh,
    ) -> Result<TapTree<Q>, Error>
    where
        FPk: FnMut(&Pk) -> Result<Q, Error>,
        FPkh: FnMut(&Pk::Hash) -> Result<Q::Hash, Error>,
        Q: MiniscriptKey,
    {
        let frag = match self {
            TapTree::Tree(l, r) => TapTree::Tree(
                Arc::new(l.translate_helper(translatefpk, translatefpkh)?),
                Arc::new(r.translate_helper(translatefpk, translatefpkh)?),
            ),
            TapTree::Leaf(ms) => {
                TapTree::Leaf(Arc::new(ms.translate_pk(translatefpk, translatefpkh)?))
            }
        };
        Ok(frag)
    }
}

impl<Pk: MiniscriptKey> fmt::Display for TapTree<Pk> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            TapTree::Tree(ref left, ref right) => write!(f, "{{{},{}}}", *left, *right),
            TapTree::Leaf(ref script) => write!(f, "{}", *script),
        }
    }
}

impl<Pk: MiniscriptKey> fmt::Debug for TapTree<Pk> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            TapTree::Tree(ref left, ref right) => write!(f, "{{{:?},{:?}}}", *left, *right),
            TapTree::Leaf(ref script) => write!(f, "{:?}", *script),
        }
    }
}

impl<Pk: MiniscriptKey> Tr<Pk> {
    /// Create a new [`Tr`] descriptor from internal key and [`TapTree`]
    pub fn new(internal_key: Pk, tree: Option<TapTree<Pk>>) -> Result<Self, Error> {
        let nodes = tree.as_ref().map(|t| t.taptree_height()).unwrap_or(0);

        if nodes <= TAPROOT_CONTROL_MAX_NODE_COUNT {
            Ok(Self {
                internal_key,
                tree,
                spend_info: Mutex::new(None),
            })
        } else {
            Err(Error::MaxRecursiveDepthExceeded)
        }
    }

    fn to_string_no_checksum(&self) -> String {
        let key = &self.internal_key;
        match self.tree {
            Some(ref s) => format!("tr({},{})", key, s),
            None => format!("tr({})", key),
        }
    }

    /// Obtain the internal key of [`Tr`] descriptor
    pub fn internal_key(&self) -> &Pk {
        &self.internal_key
    }

    /// Obtain the [`TapTree`] of the [`Tr`] descriptor
    pub fn taptree(&self) -> &Option<TapTree<Pk>> {
        &self.tree
    }

    /// Iterate over all scripts in merkle tree. If there is no script path, the iterator
    /// yields [`None`]
    pub fn iter_scripts(&self) -> TapTreeIter<Pk> {
        match self.tree {
            Some(ref t) => t.iter(),
            None => TapTreeIter { stack: vec![] },
        }
    }

    /// Compute the [`TaprootSpendInfo`] associated with this descriptor if spend data is `None`.
    ///
    /// If spend data is already computed (i.e it is not `None`), this does not recompute it.
    ///
    /// [`TaprootSpendInfo`] is only required for spending via the script paths.
    pub fn spend_info(&self) -> Arc<TaprootSpendInfo>
    where
        Pk: ToPublicKey,
    {
        // If the value is already cache, read it
        // read only panics if the lock is poisoned (meaning other thread having a lock panicked)
        let read_lock = self.spend_info.lock().expect("Lock poisoned");
        if let Some(ref spend_info) = *read_lock {
            return Arc::clone(spend_info);
        }
        drop(read_lock);

        // Get a new secp context
        // This would be cheap operation after static context support from upstream
        let secp = secp256k1::Secp256k1::verification_only();
        // Key spend path with no merkle root
        let data = if self.tree.is_none() {
            TaprootSpendInfo::new_key_spend(&secp, self.internal_key.to_x_only_pubkey(), None)
        } else {
            let mut builder = TaprootBuilder::new();
            for (depth, ms) in self.iter_scripts() {
                let script = ms.encode();
                builder = builder
                    .add_leaf(depth, script)
                    .expect("Computing spend data on a valid Tree should always succeed");
            }
            // Assert builder cannot error here because we have a well formed descriptor
            match builder.finalize(&secp, self.internal_key.to_x_only_pubkey()) {
                Ok(data) => data,
                Err(e) => match e {
                    TaprootBuilderError::InvalidMerkleTreeDepth(_) => {
                        unreachable!("Depth checked in struct construction")
                    }
                    TaprootBuilderError::NodeNotInDfsOrder => {
                        unreachable!("Insertion is called in DFS order")
                    }
                    TaprootBuilderError::OverCompleteTree => {
                        unreachable!("Taptree is a well formed tree")
                    }
                    TaprootBuilderError::InvalidInternalKey(_) => {
                        unreachable!("Internal key checked for validity")
                    }
                    TaprootBuilderError::IncompleteTree => {
                        unreachable!("Taptree is a well formed tree")
                    }
                    TaprootBuilderError::EmptyTree => {
                        unreachable!("Taptree is a well formed tree with atleast 1 element")
                    }
                },
            }
        };
        let spend_info = Arc::new(data);
        *self.spend_info.lock().expect("Lock poisoned") = Some(Arc::clone(&spend_info));
        spend_info
    }
}

impl<Pk: MiniscriptKey + ToPublicKey> Tr<Pk> {
    /// Obtain the corresponding script pubkey for this descriptor
    /// Same as[`DescriptorTrait::script_pubkey`] for this descriptor
    pub fn spk(&self) -> Script {
        let output_key = self.spend_info().output_key();
        let builder = bitcoin::blockdata::script::Builder::new();
        builder
            .push_opcode(opcodes::all::OP_PUSHNUM_1)
            .push_slice(&output_key.serialize())
            .into_script()
    }

    /// Obtain the corresponding script pubkey for this descriptor
    /// Same as[`DescriptorTrait::address`] for this descriptor
    pub fn addr(&self, network: bitcoin::Network) -> Result<bitcoin::Address, Error> {
        let spend_info = self.spend_info();
        Ok(bitcoin::Address::p2tr_tweaked(
            spend_info.output_key(),
            network,
        ))
    }
}

/// Iterator for Taproot structures
/// Yields a pair of (depth, miniscript) in a depth first walk
/// For example, this tree:
///                                     - N0 -
///                                    /     \\
///                                   N1      N2
///                                  /  \    /  \\
///                                 A    B  C   N3
///                                            /  \\
///                                           D    E
/// would yield (2, A), (2, B), (2,C), (3, D), (3, E).
///
#[derive(Debug, Clone)]
pub struct TapTreeIter<'a, Pk: MiniscriptKey> {
    stack: Vec<(u8, &'a TapTree<Pk>)>,
}

impl<'a, Pk> Iterator for TapTreeIter<'a, Pk>
where
    Pk: MiniscriptKey + 'a,
{
    type Item = (u8, &'a Miniscript<Pk, Tap>);

    fn next(&mut self) -> Option<Self::Item> {
        while !self.stack.is_empty() {
            let (depth, last) = self.stack.pop().expect("Size checked above");
            match &*last {
                TapTree::Tree(l, r) => {
                    self.stack.push((depth + 1, &r));
                    self.stack.push((depth + 1, &l));
                }
                TapTree::Leaf(ref ms) => return Some((depth, ms)),
            }
        }
        None
    }
}

impl<Pk: MiniscriptKey> FromTree for Tr<Pk>
where
    Pk: MiniscriptKey + FromStr,
    Pk::Hash: FromStr,
    <Pk as FromStr>::Err: ToString,
    <<Pk as MiniscriptKey>::Hash as FromStr>::Err: ToString,
{
    fn from_tree(top: &expression::Tree) -> Result<Self, Error> {
        // Helper function to parse taproot script path
        fn parse_tr_script_spend<Pk: MiniscriptKey>(
            tree: &expression::Tree,
        ) -> Result<TapTree<Pk>, Error>
        where
            Pk: MiniscriptKey + FromStr,
            Pk::Hash: FromStr,
            <Pk as FromStr>::Err: ToString,
            <<Pk as MiniscriptKey>::Hash as FromStr>::Err: ToString,
        {
            match tree {
                expression::Tree { name, args } if !name.is_empty() && args.is_empty() => {
                    let script = Miniscript::<Pk, Tap>::from_str(name)?;
                    Ok(TapTree::Leaf(Arc::new(script)))
                }
                expression::Tree { name, args } if name.is_empty() && args.len() == 2 => {
                    let left = parse_tr_script_spend(&args[0])?;
                    let right = parse_tr_script_spend(&args[1])?;
                    Ok(TapTree::Tree(Arc::new(left), Arc::new(right)))
                }
                _ => {
                    return Err(Error::Unexpected(
                        "unknown format for script spending paths while parsing taproot descriptor"
                            .to_string(),
                    ));
                }
            }
        }

        if top.name == "tr" {
            match top.args.len() {
                1 => {
                    let key = &top.args[0];
                    if !key.args.is_empty() {
                        return Err(Error::Unexpected(format!(
                            "#{} script associated with `key-path` while parsing taproot descriptor",
                            key.args.len()
                        )));
                    }
                    Ok(Tr {
                        internal_key: expression::terminal(key, Pk::from_str)?,
                        tree: None,
                        spend_info: Mutex::new(None),
                    })
                }
                2 => {
                    let ref key = top.args[0];
                    if !key.args.is_empty() {
                        return Err(Error::Unexpected(format!(
                            "#{} script associated with `key-path` while parsing taproot descriptor",
                            key.args.len()
                        )));
                    }
                    let ref tree = top.args[1];
                    let ret = parse_tr_script_spend(tree)?;
                    Ok(Tr {
                        internal_key: expression::terminal(key, Pk::from_str)?,
                        tree: Some(ret),
                        spend_info: Mutex::new(None),
                    })
                }
                _ => {
                    return Err(Error::Unexpected(format!(
                        "{}[#{} args] while parsing taproot descriptor",
                        top.name,
                        top.args.len()
                    )));
                }
            }
        } else {
            return Err(Error::Unexpected(format!(
                "{}[#{} args] while parsing taproot descriptor",
                top.name,
                top.args.len()
            )));
        }
    }
}

impl<Pk: MiniscriptKey> FromStr for Tr<Pk>
where
    Pk: MiniscriptKey + FromStr,
    Pk::Hash: FromStr,
    <Pk as FromStr>::Err: ToString,
    <<Pk as MiniscriptKey>::Hash as FromStr>::Err: ToString,
{
    type Err = Error;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        let desc_str = verify_checksum(s)?;
        let top = parse_tr_tree(desc_str)?;
        Self::from_tree(&top)
    }
}

impl<Pk: MiniscriptKey> fmt::Debug for Tr<Pk> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self.tree {
            Some(ref s) => write!(f, "tr({:?},{:?})", self.internal_key, s),
            None => write!(f, "tr({:?})", self.internal_key),
        }
    }
}

impl<Pk: MiniscriptKey> fmt::Display for Tr<Pk> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        let desc = self.to_string_no_checksum();
        let checksum = desc_checksum(&desc).map_err(|_| fmt::Error)?;
        write!(f, "{}#{}", &desc, &checksum)
    }
}

// Helper function to parse string into miniscript tree form
fn parse_tr_tree(s: &str) -> Result<expression::Tree, Error> {
    for ch in s.bytes() {
        if !ch.is_ascii() {
            return Err(Error::Unprintable(ch));
        }
    }

    let ret = if s.len() > 3 && &s[..3] == "tr(" && s.as_bytes()[s.len() - 1] == b')' {
        let rest = &s[3..s.len() - 1];
        if !rest.contains(',') {
            let internal_key = expression::Tree {
                name: rest,
                args: vec![],
            };
            return Ok(expression::Tree {
                name: "tr",
                args: vec![internal_key],
            });
        }
        // use str::split_once() method to refactor this when compiler version bumps up
        let (key, script) = split_once(rest, ',')
            .ok_or_else(|| Error::BadDescriptor("invalid taproot descriptor".to_string()))?;

        let internal_key = expression::Tree {
            name: key,
            args: vec![],
        };
        if script.is_empty() {
            return Ok(expression::Tree {
                name: "tr",
                args: vec![internal_key],
            });
        }
        let (tree, rest) = expression::Tree::from_slice_delim(script, 1, '{')?;
        if rest.is_empty() {
            Ok(expression::Tree {
                name: "tr",
                args: vec![internal_key, tree],
            })
        } else {
            Err(errstr(rest))
        }
    } else {
        Err(Error::Unexpected("invalid taproot descriptor".to_string()))
    };
    return ret;
}

fn split_once(inp: &str, delim: char) -> Option<(&str, &str)> {
    let ret = if inp.is_empty() {
        None
    } else {
        let mut found = inp.len();
        for (idx, ch) in inp.chars().enumerate() {
            if ch == delim {
                found = idx;
                break;
            }
        }
        // No comma or trailing comma found
        if found >= inp.len() - 1 {
            Some((&inp[..], ""))
        } else {
            Some((&inp[..found], &inp[found + 1..]))
        }
    };
    return ret;
}

impl<Pk: MiniscriptKey> Liftable<Pk> for TapTree<Pk> {
    fn lift(&self) -> Result<Policy<Pk>, Error> {
        fn lift_helper<Pk: MiniscriptKey>(s: &TapTree<Pk>) -> Result<Policy<Pk>, Error> {
            match s {
                TapTree::Tree(ref l, ref r) => {
                    Ok(Policy::Threshold(1, vec![lift_helper(l)?, lift_helper(r)?]))
                }
                TapTree::Leaf(ref leaf) => leaf.lift(),
            }
        }

        let pol = lift_helper(&self)?;
        Ok(pol.normalized())
    }
}

impl<Pk: MiniscriptKey> Liftable<Pk> for Tr<Pk> {
    fn lift(&self) -> Result<Policy<Pk>, Error> {
        match &self.tree {
            Some(root) => Ok(Policy::Threshold(
                1,
                vec![
                    Policy::KeyHash(self.internal_key.to_pubkeyhash()),
                    root.lift()?,
                ],
            )),
            None => Ok(Policy::KeyHash(self.internal_key.to_pubkeyhash())),
        }
    }
}

impl<Pk: MiniscriptKey> DescriptorTrait<Pk> for Tr<Pk> {
    fn sanity_check(&self) -> Result<(), Error> {
        for (_depth, ms) in self.iter_scripts() {
            ms.sanity_check()?;
        }
        Ok(())
    }

    fn address(&self, network: bitcoin::Network) -> Result<bitcoin::Address, Error>
    where
        Pk: ToPublicKey,
    {
        self.addr(network)
    }

    fn script_pubkey(&self) -> Script
    where
        Pk: ToPublicKey,
    {
        self.spk()
    }

    fn unsigned_script_sig(&self) -> Script
    where
        Pk: ToPublicKey,
    {
        Script::new()
    }

    fn explicit_script(&self) -> Result<Script, Error>
    where
        Pk: ToPublicKey,
    {
        Err(Error::TrNoScriptCode)
    }

    fn get_satisfaction<S>(&self, satisfier: S) -> Result<(Vec<Vec<u8>>, Script), Error>
    where
        Pk: ToPublicKey,
        S: Satisfier<Pk>,
    {
        best_tap_spend(&self, satisfier, false /* allow_mall */)
    }

    fn get_satisfaction_mall<S>(&self, satisfier: S) -> Result<(Vec<Vec<u8>>, Script), Error>
    where
        Pk: ToPublicKey,
        S: Satisfier<Pk>,
    {
        best_tap_spend(&self, satisfier, true /* allow_mall */)
    }

    fn max_satisfaction_weight(&self) -> Result<usize, Error> {
        let mut max_wieght = Some(65);
        for (depth, ms) in self.iter_scripts() {
            let script_size = ms.script_size();
            let max_sat_elems = match ms.max_satisfaction_witness_elements() {
                Ok(elem) => elem,
                Err(..) => continue,
            };
            let max_sat_size = match ms.max_satisfaction_size() {
                Ok(sz) => sz,
                Err(..) => continue,
            };
            let control_block_sz = control_block_len(depth);
            let wit_size = 4 + // scriptSig len byte
            control_block_sz + // first element control block
            varint_len(script_size) +
            script_size + // second element script len with prefix
            varint_len(max_sat_elems) +
            max_sat_size; // witness
            max_wieght = cmp::max(max_wieght, Some(wit_size));
        }
        max_wieght.ok_or(Error::ImpossibleSatisfaction)
    }

    fn script_code(&self) -> Result<Script, Error>
    where
        Pk: ToPublicKey,
    {
        Err(Error::TrNoScriptCode)
    }
}

impl<Pk: MiniscriptKey> ForEachKey<Pk> for Tr<Pk> {
    fn for_each_key<'a, F: FnMut(ForEach<'a, Pk>) -> bool>(&'a self, mut pred: F) -> bool
    where
        Pk: 'a,
        Pk::Hash: 'a,
    {
        let script_keys_res = self
            .iter_scripts()
            .all(|(_d, ms)| ms.for_each_key(&mut pred));
        script_keys_res && pred(ForEach::Key(&self.internal_key))
    }
}

impl<P: MiniscriptKey, Q: MiniscriptKey> TranslatePk<P, Q> for Tr<P> {
    type Output = Tr<Q>;

    fn translate_pk<Fpk, Fpkh, E>(
        &self,
        mut translatefpk: Fpk,
        mut translatefpkh: Fpkh,
    ) -> Result<Self::Output, E>
    where
        Fpk: FnMut(&P) -> Result<Q, E>,
        Fpkh: FnMut(&P::Hash) -> Result<Q::Hash, E>,
        Q: MiniscriptKey,
    {
        let translate_desc = Tr {
            internal_key: translatefpk(&self.internal_key)?,
            tree: match &self.tree {
                Some(tree) => Some(tree.translate_helper(&mut translatefpk, &mut translatefpkh)?),
                None => None,
            },
            spend_info: Mutex::new(None),
        };
        Ok(translate_desc)
    }
}

// Helper function to compute the len of control block at a given depth
fn control_block_len(depth: u8) -> usize {
    TAPROOT_CONTROL_BASE_SIZE + (depth as usize) * TAPROOT_CONTROL_NODE_SIZE
}

// Helper function to get a script spend satisfaction
// try script spend
fn best_tap_spend<Pk, S>(
    desc: &Tr<Pk>,
    satisfier: S,
    allow_mall: bool,
) -> Result<(Vec<Vec<u8>>, Script), Error>
where
    Pk: ToPublicKey,
    S: Satisfier<Pk>,
{
    let spend_info = desc.spend_info();
    // First try the key spend path
    if let Some(sig) = satisfier.lookup_tap_key_spend_sig() {
        Ok((vec![sig.to_vec()], Script::new()))
    } else {
        // Since we have the complete descriptor we can ignore the satisfier. We don't use the control block
        // map (lookup_control_block) from the satisfier here.
        let (mut min_wit, mut min_wit_len) = (None, None);
        for (depth, ms) in desc.iter_scripts() {
            let mut wit = if allow_mall {
                match ms.satisfy_malleable(&satisfier) {
                    Ok(wit) => wit,
                    Err(..) => continue, // No witness for this script in tr descriptor, look for next one
                }
            } else {
                match ms.satisfy(&satisfier) {
                    Ok(wit) => wit,
                    Err(..) => continue, // No witness for this script in tr descriptor, look for next one
                }
            };
            // Compute the final witness size
            // Control block len + script len + witnesssize + varint(wit.len + 2)
            // The extra +2 elements are control block and script itself
            let wit_size = witness_size(&wit)
                + control_block_len(depth)
                + ms.script_size()
                + varint_len(ms.script_size());
            if min_wit_len.is_some() && Some(wit_size) > min_wit_len {
                continue;
            } else {
                let leaf_script = (ms.encode(), LeafVersion::TapScript);
                let control_block = spend_info
                    .control_block(&leaf_script)
                    .expect("Control block must exist in script map for every known leaf");
                wit.push(leaf_script.0.into_bytes()); // Push the leaf script
                                                      // There can be multiple control blocks for a (script, ver) pair
                                                      // Find the smallest one amongst those
                wit.push(control_block.serialize());
                // Finally, save the minimum
                min_wit = Some(wit);
                min_wit_len = Some(wit_size);
            }
        }
        match min_wit {
            Some(wit) => Ok((wit, Script::new())),
            None => Err(Error::CouldNotSatisfy), // Could not satisfy all miniscripts inside Tr
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::ForEachKey;

    #[test]
    fn test_for_each() {
        let desc = "tr(acc0, {
            multi_a(3, acc10, acc11, acc12), {
              and_v(
                v:multi_a(2, acc10, acc11, acc12),
                after(10)
              ),
              and_v(
                v:multi_a(1, acc10, acc11, ac12),
                after(100)
              )
            }
         })";
        let desc = desc.replace(&[' ', '\n'][..], "");
        let tr = Tr::<String>::from_str(&desc).unwrap();
        // Note the last ac12 only has ac and fails the predicate
        assert!(!tr.for_each_key(|k| match k {
            ForEach::Key(k) => k.starts_with("acc"),
            ForEach::Hash(_h) => unreachable!(),
        }));
    }
}