md_codec/

canonicalize.rs

//! BIP 388 placeholder-ordering canonicalization per spec v0.13 §6.1, plus
//! per-`@N` canonical-fill expansion per §5.3 / §6.3.
//!
//! BIP 388 wallet policies require placeholder indices `@0..@{N-1}` to be
//! introduced in the descriptor template in canonical first-occurrence order:
//! the first new placeholder encountered in document-order pre-order
//! traversal must be `@0`, the next new one `@1`, etc.
//!
//! [`canonicalize_placeholder_indices`] reshapes a [`Descriptor`] in place
//! so this invariant holds, atomically permuting:
//!
//! - the tree's `KeyArg.index` and `Tr.key_index` fields;
//! - the [`PathDecl`](crate::origin_path::PathDecl)'s `Divergent` paths vector (one path per `@N`);
//! - per-`@N` TLV maps: `use_site_path_overrides`, `fingerprints`,
//!   `pubkeys`, `origin_path_overrides`.
//!
//! After canonicalization, the post-conditions are:
//!
//! 1. Each TLV map's `(idx, _)` keys are strictly ascending and `< n`.
//! 2. The tree's first-occurrence sequence is exactly `[0, 1, ..., n-1]`
//!    (verified via [`crate::validate::validate_placeholder_usage`]).
//!
//! Idempotent: calling on an already-canonical descriptor is a no-op
//! (identity-permutation fast path).
//!
//! The decoder side does not call this routine: v0.11's
//! [`crate::validate::validate_placeholder_usage`] rejects non-canonical
//! wires up-front via
//! [`Error::PlaceholderFirstOccurrenceOutOfOrder`].
//!
//! [`expand_per_at_n`] resolves each `@N` to a fully-populated
//! [`ExpandedKey`] (origin, use-site, optional fp/xpub) by composing the
//! per-`@N` TLV overrides with the descriptor-level baselines. Used by
//! Phase 4's `WalletPolicyId` construction and Phase 5's decoder validation.

use crate::encode::Descriptor;
use crate::error::Error;
use crate::origin_path::{OriginPath, PathDeclPaths};
use crate::tree::{Body, Node};
use crate::use_site_path::UseSitePath;

/// Walk `node` in pre-order, recording the first occurrence of each
/// placeholder index in `first_occurrences`. `seen[i]` toggles to `true`
/// the first time `@i` is encountered.
fn walk_collect_first(node: &Node, seen: &mut [bool], first_occurrences: &mut Vec<u8>) {
    match &node.body {
        Body::KeyArg { index } => {
            if let Some(slot) = seen.get_mut(*index as usize) {
                if !*slot {
                    *slot = true;
                    first_occurrences.push(*index);
                }
            }
        }
        Body::Tr {
            is_nums,
            key_index,
            tree,
        } => {
            // SPEC v0.30 §7: when `is_nums = true` the internal key is the
            // BIP-341 NUMS H-point (not a placeholder reference); skip
            // registration.
            if !*is_nums {
                if let Some(slot) = seen.get_mut(*key_index as usize) {
                    if !*slot {
                        *slot = true;
                        first_occurrences.push(*key_index);
                    }
                }
            }
            if let Some(t) = tree {
                walk_collect_first(t, seen, first_occurrences);
            }
        }
        Body::Children(children) => {
            for c in children {
                walk_collect_first(c, seen, first_occurrences);
            }
        }
        Body::Variable { children, .. } => {
            for c in children {
                walk_collect_first(c, seen, first_occurrences);
            }
        }
        Body::MultiKeys { indices, .. } => {
            // v0.30 Phase C: multi-family bodies carry raw key indices.
            for idx in indices {
                if let Some(slot) = seen.get_mut(*idx as usize) {
                    if !*slot {
                        *slot = true;
                        first_occurrences.push(*idx);
                    }
                }
            }
        }
        Body::Hash256Body(_) | Body::Hash160Body(_) | Body::Timelock(_) | Body::Empty => {}
    }
}

/// Apply `perm[old_idx] -> new_idx` to every `KeyArg.index` and
/// `Tr.key_index` in `node` (recursive).
fn remap_indices(node: &mut Node, perm: &[u8]) {
    match &mut node.body {
        Body::KeyArg { index } => {
            *index = perm[*index as usize];
        }
        Body::Tr {
            is_nums,
            key_index,
            tree,
        } => {
            // SPEC v0.30 §7: when `is_nums = true` the internal key is the
            // BIP-341 NUMS H-point (not a placeholder); skip remapping.
            if !*is_nums {
                *key_index = perm[*key_index as usize];
            }
            if let Some(t) = tree {
                remap_indices(t, perm);
            }
        }
        Body::Children(children) => {
            for c in children {
                remap_indices(c, perm);
            }
        }
        Body::Variable { children, .. } => {
            for c in children {
                remap_indices(c, perm);
            }
        }
        Body::MultiKeys { indices, .. } => {
            // v0.30 Phase C: rewrite raw indices through the perm map.
            for idx in indices.iter_mut() {
                *idx = perm[*idx as usize];
            }
        }
        Body::Hash256Body(_) | Body::Hash160Body(_) | Body::Timelock(_) | Body::Empty => {}
    }
}

/// Remap idx values in a sparse TLV vector and re-sort ascending. After
/// `perm` is applied the (possibly scrambled) idx column is restored to
/// strictly ascending order, preserving the per-entry payload.
fn remap_tlv_vec<T>(entries: &mut [(u8, T)], perm: &[u8]) {
    for (idx, _) in entries.iter_mut() {
        *idx = perm[*idx as usize];
    }
    entries.sort_by_key(|(idx, _)| *idx);
}

/// Canonicalize placeholder indices in `d` so the first-occurrence
/// sequence in `d.tree` is exactly `[0, 1, ..., d.n - 1]`.
///
/// Walks the tree in document order to build a first-occurrence map,
/// then atomically permutes indices in the tree, the path declaration
/// (divergent variant), and every per-`@N` TLV map. Identity-permutation
/// fast path: returns `Ok(())` without mutating `d` if the tree is
/// already canonical.
///
/// # Errors
///
/// Returns [`Error::PlaceholderNotReferenced`] if any `@i` for
/// `0 ≤ i < d.n` does not appear in the tree (a structural error that
/// would otherwise leave the permutation under-specified).
///
/// Returns [`Error::PlaceholderIndexOutOfRange`] if the tree references
/// a placeholder `@i` with `i >= d.n`.
pub fn canonicalize_placeholder_indices(d: &mut Descriptor) -> Result<(), Error> {
    let n = d.n as usize;

    // Defensive bounds check before walking — surface out-of-range
    // placeholder references as a typed error rather than silently
    // ignoring them in walk_collect_first.
    check_placeholder_bounds(&d.tree, d.n)?;

    let mut seen = vec![false; n];
    let mut first_occurrences: Vec<u8> = Vec::with_capacity(n);
    walk_collect_first(&d.tree, &mut seen, &mut first_occurrences);

    // Every `@i` must be referenced; otherwise the permutation is
    // under-specified.
    for (i, was_seen) in seen.iter().enumerate() {
        if !was_seen {
            return Err(Error::PlaceholderNotReferenced {
                idx: i as u8,
                n: d.n,
            });
        }
    }

    // perm[old_idx] = new_idx, where new_idx is the position at which
    // old_idx was first observed in document order.
    let mut perm = vec![0u8; n];
    for (new_idx, &old_idx) in first_occurrences.iter().enumerate() {
        perm[old_idx as usize] = new_idx as u8;
    }

    // Identity fast path: no work needed when perm is the identity.
    if perm.iter().enumerate().all(|(i, p)| i as u8 == *p) {
        return Ok(());
    }

    // Atomically apply the permutation to every index-bearing field.
    remap_indices(&mut d.tree, &perm);

    if let PathDeclPaths::Divergent(paths) = &mut d.path_decl.paths {
        // paths[old_idx] becomes paths[perm[old_idx]] — i.e. new_paths[new_idx] = old_paths[old_idx]
        // where perm[old_idx] = new_idx. We need new_paths[new_idx] = old_paths[inverse_perm[new_idx]].
        let mut inverse = vec![0u8; n];
        for (old, &new) in perm.iter().enumerate() {
            inverse[new as usize] = old as u8;
        }
        let old_paths = std::mem::take(paths);
        let mut new_paths = Vec::with_capacity(n);
        for new_idx in 0..n {
            new_paths.push(old_paths[inverse[new_idx] as usize].clone());
        }
        *paths = new_paths;
    }

    if let Some(v) = d.tlv.use_site_path_overrides.as_mut() {
        remap_tlv_vec(v, &perm);
    }
    if let Some(v) = d.tlv.fingerprints.as_mut() {
        remap_tlv_vec(v, &perm);
    }
    if let Some(v) = d.tlv.pubkeys.as_mut() {
        remap_tlv_vec(v, &perm);
    }
    if let Some(v) = d.tlv.origin_path_overrides.as_mut() {
        remap_tlv_vec(v, &perm);
    }

    // Post-condition assertions (debug-only). Constructing the iterator-
    // based check is cheap; gating to debug mode keeps release builds
    // free of redundant work since the permutation is correct by
    // construction.
    debug_assert!(
        crate::validate::validate_placeholder_usage(&d.tree, d.n).is_ok(),
        "post-condition: tree first-occurrence must be canonical after canonicalize_placeholder_indices",
    );
    debug_assert!(
        tlv_indices_strictly_ascending_and_in_range(d),
        "post-condition: every TLV's idx column must be strictly ascending and < n",
    );

    Ok(())
}

/// Verify every `@N` reference in `node` falls within `0..n`. Returns
/// [`Error::PlaceholderIndexOutOfRange`] on the first violation.
fn check_placeholder_bounds(node: &Node, n: u8) -> Result<(), Error> {
    match &node.body {
        Body::KeyArg { index } => {
            if *index >= n {
                return Err(Error::PlaceholderIndexOutOfRange { idx: *index, n });
            }
        }
        Body::Tr {
            is_nums,
            key_index,
            tree,
        } => {
            // SPEC v0.30 §7 + §11: when `is_nums = true` the internal key is
            // the BIP-341 NUMS H-point (not a placeholder); skip the bounds
            // check. Otherwise `key_index` must be in `0..n`; out-of-range
            // raises `NUMSSentinelConflict` per SPEC §11 (Phase G finalizes
            // the variant's full doc-comment).
            if !*is_nums && *key_index >= n {
                return Err(Error::NUMSSentinelConflict);
            }
            if let Some(t) = tree {
                check_placeholder_bounds(t, n)?;
            }
        }
        Body::Children(children) => {
            for c in children {
                check_placeholder_bounds(c, n)?;
            }
        }
        Body::Variable { children, .. } => {
            for c in children {
                check_placeholder_bounds(c, n)?;
            }
        }
        Body::MultiKeys { indices, .. } => {
            for idx in indices {
                if *idx >= n {
                    return Err(Error::PlaceholderIndexOutOfRange { idx: *idx, n });
                }
            }
        }
        Body::Hash256Body(_) | Body::Hash160Body(_) | Body::Timelock(_) | Body::Empty => {}
    }
    Ok(())
}

/// Returns `true` if every TLV map's idx column is strictly ascending
/// and within `0..d.n`. Used by debug-only post-condition assertions and
/// by tests that want to exercise this invariant directly.
fn tlv_indices_strictly_ascending_and_in_range(d: &Descriptor) -> bool {
    fn check<T>(v: &Option<Vec<(u8, T)>>, n: u8) -> bool {
        let Some(v) = v else {
            return true;
        };
        let mut prev: Option<u8> = None;
        for (idx, _) in v {
            if *idx >= n {
                return false;
            }
            if let Some(p) = prev {
                if *idx <= p {
                    return false;
                }
            }
            prev = Some(*idx);
        }
        true
    }
    check(&d.tlv.use_site_path_overrides, d.n)
        && check(&d.tlv.fingerprints, d.n)
        && check(&d.tlv.pubkeys, d.n)
        && check(&d.tlv.origin_path_overrides, d.n)
}

/// Fully-resolved per-`@N` key record produced by [`expand_per_at_n`].
///
/// Each field is populated via the canonical-fill / per-`@N` override
/// composition rules (spec v0.13 §5.3 + §6.3, "Option A"):
///
/// - `origin_path`: `OriginPathOverrides[idx]` if present, else
///   `path_decl` resolved per the `Shared` / `Divergent` variant.
/// - `use_site_path`: `UseSitePathOverrides[idx]` if present, else
///   `Descriptor::use_site_path` (the descriptor-level baseline).
/// - `fingerprint`: `Fingerprints[idx]` if present, else `None`.
/// - `xpub`: `Pubkeys[idx]` if present, else `None`.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct ExpandedKey {
    /// Placeholder index `@N`; equals position in the returned `Vec`.
    pub idx: u8,
    /// Resolved origin path (per-`@N` override or `path_decl` baseline).
    pub origin_path: OriginPath,
    /// Resolved use-site path (per-`@N` override or descriptor baseline).
    pub use_site_path: UseSitePath,
    /// 4-byte master fingerprint, if a `Fingerprints` entry is present.
    pub fingerprint: Option<[u8; 4]>,
    /// 65-byte xpub (32 chain-code || 33 compressed pubkey), if a
    /// `Pubkeys` entry is present.
    pub xpub: Option<[u8; 65]>,
}

/// Linearly look up an `idx` key in a sparse, ascending `(idx, value)`
/// vector. Returns the matching value by reference, or `None` if absent.
///
/// Sparse TLV maps are kept small (at most `d.n` entries, n ≤ 32), so a
/// linear scan is the obvious choice over a binary search or BTreeMap.
fn sparse_lookup<T>(v: &Option<Vec<(u8, T)>>, idx: u8) -> Option<&T> {
    v.as_ref()
        .and_then(|entries| entries.iter().find(|(i, _)| *i == idx).map(|(_, t)| t))
}

/// Expand a [`Descriptor`] into a vector of [`ExpandedKey`] records — one
/// per `@N` in `0..d.n` — by composing per-`@N` TLV overrides with the
/// descriptor-level baselines (spec v0.13 §5.3 / §6.3, "Option A").
///
/// Used by Phase 4's `WalletPolicyId` construction (the canonical-fill
/// hash input is built from this vector) and Phase 5's decoder validation
/// (the `MissingExplicitOrigin` check).
///
/// # Precondition
///
/// The caller MUST have already invoked
/// [`canonicalize_placeholder_indices`] on `d`, or `d` must have been
/// produced by the decoder (which rejects non-canonical wires up-front).
/// Expansion does not re-canonicalize; passing a non-canonical descriptor
/// produces semantically meaningful but `@N`-mis-aligned output.
///
/// # Resolution rules
///
/// Per `idx` in `0..d.n`:
/// - **origin**: `OriginPathOverrides[idx]` if present; else
///   `path_decl.paths` resolved by variant — `Shared(p)` returns `p` for
///   every idx, `Divergent(v)` returns `v[idx]`.
/// - **use-site**: `UseSitePathOverrides[idx]` if present; else
///   `d.use_site_path` (the descriptor-level baseline that already
///   carries the standard-multipath default when the wire elided it).
/// - **fp**: `Fingerprints[idx]` if present, else `None`.
/// - **xpub**: `Pubkeys[idx]` if present, else `None`.
///
/// # Errors
///
/// Returns [`Error::MissingExplicitOrigin`] when the resolved origin path
/// for `idx` is empty (depth-0) AND `OriginPathOverrides[idx]` is absent
/// AND [`crate::canonical_origin::canonical_origin`] returns `None` for
/// `d.tree`. This is the structurally-rare "non-canonical wrapper without
/// any user-supplied path" case; v0.11 wires already require `path_decl`
/// to be populated, so this surfaces only when the shared-form path is
/// itself empty.
///
/// Returns [`Error::DivergentPathCountMismatch`] if `path_decl.paths` is
/// `Divergent(v)` and `v.len() != d.n` — a malformed descriptor that the
/// v0.11 decoder would already reject; surfaced here defensively.
///
/// # INVARIANT (Option A, spec v0.13 §3 + §6.3)
///
/// `path_decl.paths` is always populated post-decode (v0.11 wire
/// invariant). Canonical-fill into `path_decl` happens at *encode time*
/// only (per spec §6.3) — by the time this function runs on a decoded
/// `Descriptor`, the wire has already supplied either an explicit
/// shared/divergent path or the encoder's canonical substitution.
/// Consequently this function does NOT consult
/// [`crate::canonical_origin::canonical_origin`] for the per-`@N` path
/// (it only consults `canonical_origin` to decide whether the
/// non-canonical-wrapper error gate applies).
///
/// Any future change that elides `path_decl` on the wire would require
/// re-introducing `canonical_origin` lookups *here* and in
/// [`crate::identity::compute_wallet_policy_id`] — both call sites
/// share this invariant.
pub fn expand_per_at_n(d: &Descriptor) -> Result<Vec<ExpandedKey>, Error> {
    // Defensive: malformed descriptors with mismatched divergent path
    // counts cannot be structurally exercised post-decode (v0.11 enforces
    // n == divergent.len() during PathDecl::read), but check anyway so a
    // hand-built Descriptor can't slip past.
    if let PathDeclPaths::Divergent(paths) = &d.path_decl.paths {
        if paths.len() != d.n as usize {
            return Err(Error::DivergentPathCountMismatch {
                n: d.n,
                got: paths.len(),
            });
        }
    }

    let mut out = Vec::with_capacity(d.n as usize);
    for idx in 0..d.n {
        // Origin resolution: per-@N override beats path_decl baseline.
        let origin_path = if let Some(p) = sparse_lookup(&d.tlv.origin_path_overrides, idx) {
            p.clone()
        } else {
            match &d.path_decl.paths {
                PathDeclPaths::Shared(p) => p.clone(),
                PathDeclPaths::Divergent(v) => v[idx as usize].clone(),
            }
        };

        // Structurally-rare: shared (or divergent) baseline path is empty
        // AND no override present AND wrapper has no canonical default.
        // This is the only path in v0.11+v0.13 where MissingExplicitOrigin
        // can be raised.
        if origin_path.components.is_empty()
            && sparse_lookup(&d.tlv.origin_path_overrides, idx).is_none()
            && crate::canonical_origin::canonical_origin(&d.tree).is_none()
        {
            return Err(Error::MissingExplicitOrigin { idx });
        }

        // Use-site resolution: per-@N override beats descriptor baseline.
        let use_site_path = sparse_lookup(&d.tlv.use_site_path_overrides, idx)
            .cloned()
            .unwrap_or_else(|| d.use_site_path.clone());

        let fingerprint = sparse_lookup(&d.tlv.fingerprints, idx).copied();
        let xpub = sparse_lookup(&d.tlv.pubkeys, idx).copied();

        out.push(ExpandedKey {
            idx,
            origin_path,
            use_site_path,
            fingerprint,
            xpub,
        });
    }
    Ok(out)
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::origin_path::{OriginPath, PathComponent, PathDecl, PathDeclPaths};
    use crate::tag::Tag;
    use crate::tlv::TlvSection;
    use crate::tree::{Body, Node};
    use crate::use_site_path::UseSitePath;

    fn shared_bip84() -> PathDecl {
        PathDecl {
            n: 1,
            paths: PathDeclPaths::Shared(OriginPath {
                components: vec![
                    PathComponent {
                        hardened: true,
                        value: 84,
                    },
                    PathComponent {
                        hardened: true,
                        value: 0,
                    },
                    PathComponent {
                        hardened: true,
                        value: 0,
                    },
                ],
            }),
        }
    }

    fn shared_path_decl(n: u8) -> PathDecl {
        PathDecl {
            n,
            paths: PathDeclPaths::Shared(OriginPath {
                components: vec![PathComponent {
                    hardened: true,
                    value: 48,
                }],
            }),
        }
    }

    fn no_multipath() -> UseSitePath {
        UseSitePath {
            multipath: None,
            wildcard_hardened: false,
        }
    }

    /// Pre-condition: `tr(@0)` already canonical → after canonicalize,
    /// descriptor unchanged.
    #[test]
    fn identity_permutation_no_op() {
        let d = Descriptor {
            n: 1,
            path_decl: shared_bip84(),
            use_site_path: no_multipath(),
            tree: Node {
                tag: Tag::Tr,
                body: Body::Tr {
                    is_nums: false,
                    key_index: 0,
                    tree: None,
                },
            },
            tlv: TlvSection::new_empty(),
        };
        let mut d2 = d.clone();
        canonicalize_placeholder_indices(&mut d2).unwrap();
        assert_eq!(d, d2);
    }

    /// Encoder canonicalizes `tr(multi(2, @1, @0))` →
    /// `tr(multi(2, @0, @1))` with swapped indices.
    #[test]
    fn swap_two_placeholders_in_multi() {
        let mut d = Descriptor {
            n: 2,
            path_decl: shared_path_decl(2),
            use_site_path: no_multipath(),
            // tr keypath @0 already references @0 first, so embed the
            // swap inside the tap-script-tree where the document-order
            // walk will hit @1 first.
            tree: Node {
                tag: Tag::Multi,
                body: Body::MultiKeys {
                    k: 2,
                    indices: vec![1, 0],
                },
            },
            tlv: TlvSection::new_empty(),
        };
        canonicalize_placeholder_indices(&mut d).unwrap();
        let expected_tree = Node {
            tag: Tag::Multi,
            body: Body::MultiKeys {
                k: 2,
                indices: vec![0, 1],
            },
        };
        assert_eq!(d.tree, expected_tree);
    }

    /// `wsh(sortedmulti(2, @2, @0, @1))` → tree becomes canonical and
    /// TLV `pubkeys` is renumbered consistently.
    ///
    /// Originally: pubkey-A is wired to @0, pubkey-B to @1, pubkey-C to @2.
    /// After tree first-occurrence is `[2, 0, 1]`:
    ///   perm[0] = 1, perm[1] = 2, perm[2] = 0.
    /// So the on-disk pubkeys vec `[(0, A), (1, B), (2, C)]` becomes
    ///   `[(perm[0], A), (perm[1], B), (perm[2], C)]`
    /// = `[(1, A), (2, B), (0, C)]`, then re-sorted to
    ///   `[(0, C), (1, A), (2, B)]`.
    #[test]
    fn permute_three_placeholders_in_sortedmulti() {
        let xpub_a = [0xaa; 65];
        let xpub_b = [0xbb; 65];
        let xpub_c = [0xcc; 65];
        let mut d = Descriptor {
            n: 3,
            path_decl: shared_path_decl(3),
            use_site_path: no_multipath(),
            tree: Node {
                tag: Tag::Wsh,
                body: Body::Children(vec![Node {
                    tag: Tag::SortedMulti,
                    body: Body::MultiKeys {
                        k: 2,
                        indices: vec![2, 0, 1],
                    },
                }]),
            },
            tlv: {
                let mut t = TlvSection::new_empty();
                t.pubkeys = Some(vec![(0, xpub_a), (1, xpub_b), (2, xpub_c)]);
                t
            },
        };
        canonicalize_placeholder_indices(&mut d).unwrap();
        let expected_tree = Node {
            tag: Tag::Wsh,
            body: Body::Children(vec![Node {
                tag: Tag::SortedMulti,
                body: Body::MultiKeys {
                    k: 2,
                    indices: vec![0, 1, 2],
                },
            }]),
        };
        assert_eq!(d.tree, expected_tree);
        assert_eq!(
            d.tlv.pubkeys.unwrap(),
            vec![(0, xpub_c), (1, xpub_a), (2, xpub_b)],
        );
    }

    /// Divergent path declaration is reordered in lockstep with the
    /// placeholder indices: paths[new] holds the path that was wired to
    /// the @N now mapped to that new index.
    #[test]
    fn permute_with_divergent_path_decl() {
        let path_for_at_0 = OriginPath {
            components: vec![PathComponent {
                hardened: true,
                value: 84,
            }],
        };
        let path_for_at_1 = OriginPath {
            components: vec![PathComponent {
                hardened: true,
                value: 86,
            }],
        };
        let mut d = Descriptor {
            n: 2,
            path_decl: PathDecl {
                n: 2,
                paths: PathDeclPaths::Divergent(vec![path_for_at_0.clone(), path_for_at_1.clone()]),
            },
            use_site_path: no_multipath(),
            tree: Node {
                tag: Tag::Wsh,
                body: Body::Children(vec![Node {
                    tag: Tag::Multi,
                    body: Body::MultiKeys {
                        k: 2,
                        // First-occurrence: @1, then @0 → perm[0] = 1, perm[1] = 0.
                        indices: vec![1, 0],
                    },
                }]),
            },
            tlv: TlvSection::new_empty(),
        };
        canonicalize_placeholder_indices(&mut d).unwrap();
        // After: tree has @0 first, then @1. The @ that was originally
        // @1 (and thus paired with `path_for_at_1`) is now @0, so
        // paths[0] must be the path originally at index 1.
        match &d.path_decl.paths {
            PathDeclPaths::Divergent(paths) => {
                assert_eq!(paths[0], path_for_at_1);
                assert_eq!(paths[1], path_for_at_0);
            }
            _ => panic!("expected divergent paths"),
        }
    }

    /// `use_site_path_overrides` keys are remapped consistently with
    /// the tree permutation.
    #[test]
    fn permute_with_use_site_path_overrides() {
        let custom = UseSitePath::standard_multipath();
        let mut d = Descriptor {
            n: 2,
            path_decl: shared_path_decl(2),
            use_site_path: no_multipath(),
            tree: Node {
                tag: Tag::Multi,
                body: Body::MultiKeys {
                    k: 2,
                    indices: vec![1, 0],
                },
            },
            tlv: {
                let mut t = TlvSection::new_empty();
                // Override applies to the @ that was originally @1.
                t.use_site_path_overrides = Some(vec![(1, custom.clone())]);
                t
            },
        };
        canonicalize_placeholder_indices(&mut d).unwrap();
        // After: original @1 → new @0; override should now key on @0.
        assert_eq!(d.tlv.use_site_path_overrides.unwrap(), vec![(0, custom)],);
    }

    /// Both `fingerprints` and `pubkeys` carry @N idx; both must be
    /// remapped identically.
    #[test]
    fn permute_with_fingerprints_and_pubkeys() {
        let fp_a = [0x11, 0x11, 0x11, 0x11];
        let fp_b = [0x22, 0x22, 0x22, 0x22];
        let fp_c = [0x33, 0x33, 0x33, 0x33];
        let xpub_a = [0xaa; 65];
        let xpub_b = [0xbb; 65];
        let xpub_c = [0xcc; 65];
        let mut d = Descriptor {
            n: 3,
            path_decl: shared_path_decl(3),
            use_site_path: no_multipath(),
            tree: Node {
                tag: Tag::SortedMulti,
                body: Body::MultiKeys {
                    k: 2,
                    // First-occurrence: @2, @0, @1
                    // perm[0]=1, perm[1]=2, perm[2]=0.
                    indices: vec![2, 0, 1],
                },
            },
            tlv: {
                let mut t = TlvSection::new_empty();
                t.fingerprints = Some(vec![(0, fp_a), (1, fp_b), (2, fp_c)]);
                t.pubkeys = Some(vec![(0, xpub_a), (1, xpub_b), (2, xpub_c)]);
                t
            },
        };
        canonicalize_placeholder_indices(&mut d).unwrap();
        // Original (0,A)/(1,B)/(2,C) → perm gives (1,A)/(2,B)/(0,C) →
        // sorted: (0,C), (1,A), (2,B).
        assert_eq!(
            d.tlv.fingerprints.unwrap(),
            vec![(0, fp_c), (1, fp_a), (2, fp_b)],
        );
        assert_eq!(
            d.tlv.pubkeys.unwrap(),
            vec![(0, xpub_c), (1, xpub_a), (2, xpub_b)],
        );
    }

    /// `origin_path_overrides` is also remapped correctly.
    #[test]
    fn permute_with_origin_path_overrides() {
        let path_for_at_2 = OriginPath {
            components: vec![PathComponent {
                hardened: true,
                value: 99,
            }],
        };
        let mut d = Descriptor {
            n: 3,
            path_decl: shared_path_decl(3),
            use_site_path: no_multipath(),
            tree: Node {
                tag: Tag::SortedMulti,
                body: Body::MultiKeys {
                    k: 2,
                    // first-occurrence: @2, @0, @1 → perm[2]=0
                    indices: vec![2, 0, 1],
                },
            },
            tlv: {
                let mut t = TlvSection::new_empty();
                t.origin_path_overrides = Some(vec![(2, path_for_at_2.clone())]);
                t
            },
        };
        canonicalize_placeholder_indices(&mut d).unwrap();
        // perm[2] = 0; override at idx 2 maps to idx 0.
        assert_eq!(
            d.tlv.origin_path_overrides.unwrap(),
            vec![(0, path_for_at_2)],
        );
    }

    /// `tr(@0)` with `n=3` (i.e. @1 and @2 declared but never used) →
    /// canonicalize errors with PlaceholderNotReferenced.
    #[test]
    fn unreferenced_placeholder_returns_error() {
        let mut d = Descriptor {
            n: 3,
            path_decl: shared_path_decl(3),
            use_site_path: no_multipath(),
            tree: Node {
                tag: Tag::Tr,
                body: Body::Tr {
                    is_nums: false,
                    key_index: 0,
                    tree: None,
                },
            },
            tlv: TlvSection::new_empty(),
        };
        let err = canonicalize_placeholder_indices(&mut d).unwrap_err();
        assert!(matches!(
            err,
            Error::PlaceholderNotReferenced { idx: 1, n: 3 }
        ));
    }

    /// Out-of-range `@N` reference is caught up-front with a typed error
    /// rather than panicking inside the walker.
    #[test]
    fn out_of_range_placeholder_returns_error() {
        let mut d = Descriptor {
            n: 2,
            path_decl: shared_path_decl(2),
            use_site_path: no_multipath(),
            tree: Node {
                tag: Tag::Wpkh,
                body: Body::KeyArg { index: 5 },
            },
            tlv: TlvSection::new_empty(),
        };
        let err = canonicalize_placeholder_indices(&mut d).unwrap_err();
        assert!(matches!(
            err,
            Error::PlaceholderIndexOutOfRange { idx: 5, n: 2 }
        ));
    }

    /// Idempotence: canonicalizing twice is a no-op after the first call.
    #[test]
    fn idempotence() {
        let mut d = Descriptor {
            n: 3,
            path_decl: shared_path_decl(3),
            use_site_path: no_multipath(),
            tree: Node {
                tag: Tag::SortedMulti,
                body: Body::MultiKeys {
                    k: 2,
                    indices: vec![2, 0, 1],
                },
            },
            tlv: {
                let mut t = TlvSection::new_empty();
                t.fingerprints = Some(vec![(0, [1; 4]), (1, [2; 4]), (2, [3; 4])]);
                t
            },
        };
        canonicalize_placeholder_indices(&mut d).unwrap();
        let after_first = d.clone();
        canonicalize_placeholder_indices(&mut d).unwrap();
        assert_eq!(d, after_first);
    }

    /// Post-condition (1): every TLV map's idx column is strictly
    /// ascending and `< d.n` after canonicalization.
    #[test]
    fn tlv_idx_post_condition() {
        let mut d = Descriptor {
            n: 3,
            path_decl: shared_path_decl(3),
            use_site_path: no_multipath(),
            tree: Node {
                tag: Tag::SortedMulti,
                body: Body::MultiKeys {
                    k: 2,
                    indices: vec![2, 0, 1],
                },
            },
            tlv: {
                let mut t = TlvSection::new_empty();
                t.fingerprints = Some(vec![(0, [1; 4]), (1, [2; 4]), (2, [3; 4])]);
                t.pubkeys = Some(vec![(0, [0xaa; 65]), (1, [0xbb; 65]), (2, [0xcc; 65])]);
                t
            },
        };
        canonicalize_placeholder_indices(&mut d).unwrap();
        assert!(tlv_indices_strictly_ascending_and_in_range(&d));
    }

    /// Post-condition (2): the tree's first-occurrence sequence is
    /// exactly `[0, 1, ..., n-1]` after canonicalization.
    #[test]
    fn tree_first_occurrence_post_condition() {
        let mut d = Descriptor {
            n: 3,
            path_decl: shared_path_decl(3),
            use_site_path: no_multipath(),
            tree: Node {
                tag: Tag::SortedMulti,
                body: Body::MultiKeys {
                    k: 2,
                    indices: vec![2, 0, 1],
                },
            },
            tlv: TlvSection::new_empty(),
        };
        canonicalize_placeholder_indices(&mut d).unwrap();
        // The validator returning Ok(()) is the canonical post-condition.
        crate::validate::validate_placeholder_usage(&d.tree, d.n).unwrap();
        // Also walk explicitly to assert the literal sequence.
        let mut seen = vec![false; d.n as usize];
        let mut first = Vec::new();
        walk_collect_first(&d.tree, &mut seen, &mut first);
        assert_eq!(first, vec![0, 1, 2]);
    }

    /// The encoder calls `canonicalize_placeholder_indices` internally,
    /// so a non-canonical input round-trips through encode/decode cleanly:
    /// the wire bytes are the canonical encoding, and the decoder accepts
    /// them without `PlaceholderFirstOccurrenceOutOfOrder`.
    #[test]
    fn encoder_canonicalizes_non_canonical_input() {
        let d = Descriptor {
            n: 2,
            path_decl: shared_path_decl(2),
            use_site_path: UseSitePath::standard_multipath(),
            tree: Node {
                tag: Tag::Wsh,
                body: Body::Children(vec![Node {
                    tag: Tag::Multi,
                    body: Body::MultiKeys {
                        k: 2,
                        // first-occurrence: @1 then @0 (non-canonical).
                        indices: vec![1, 0],
                    },
                }]),
            },
            tlv: TlvSection::new_empty(),
        };
        let (bytes, total_bits) =
            crate::encode::encode_payload(&d).expect("encoder must canonicalize and succeed");
        // Decoder rejects non-canonical first-occurrence ordering with
        // PlaceholderFirstOccurrenceOutOfOrder; if encoder didn't
        // canonicalize, this would fail.
        let decoded = crate::decode::decode_payload(&bytes, total_bits).expect("decode");
        // Decoded tree's first occurrence is canonical [0, 1].
        let mut seen = vec![false; decoded.n as usize];
        let mut first = Vec::new();
        walk_collect_first(&decoded.tree, &mut seen, &mut first);
        assert_eq!(first, vec![0, 1]);
    }

    /// Post-condition (3): round-trip property — for hand-crafted
    /// permutations, `canonicalize → encode → decode → canonicalize`
    /// equals the canonicalize-only result. (Encode requires a fully
    /// well-formed descriptor, so this exercises the encoder path.)
    #[test]
    fn round_trip_canonicalize_encode_decode_canonicalize() {
        // 8 permutations of @0,@1,@2 inside sortedmulti(2, ...) plus
        // base canonical and one swap-pair → 10 total cases.
        let permutations: Vec<Vec<u8>> = vec![
            vec![0, 1, 2],
            vec![0, 2, 1],
            vec![1, 0, 2],
            vec![1, 2, 0],
            vec![2, 0, 1],
            vec![2, 1, 0],
            vec![1, 0, 1], // duplicate refs (re-uses @1 and @0; only first introduces)
            vec![2, 1, 0], // duplicate of above to give 8
        ];
        for perm in permutations {
            // n is the count of distinct placeholders in `perm`.
            let mut distinct: Vec<u8> = perm.clone();
            distinct.sort_unstable();
            distinct.dedup();
            let n = distinct.len() as u8;
            assert!(n >= 2, "test fixture expects ≥2 distinct placeholders");
            // Children are pk_k(@perm[i]) — but to match `n` we must use
            // exactly the `n` placeholders {0, 1, ..., n-1}; the
            // permutation `perm` already does that as long as `distinct`
            // == 0..n. Re-index if the permutation skipped any.
            let mut renumbered = perm.clone();
            // Build mapping: each distinct value gets the position of
            // its sorted occurrence as its label, ensuring the resulting
            // descriptor has placeholders 0..n exactly.
            let mut mapping = std::collections::HashMap::new();
            for (i, v) in distinct.iter().enumerate() {
                mapping.insert(*v, i as u8);
            }
            for v in renumbered.iter_mut() {
                *v = mapping[v];
            }

            let indices: Vec<u8> = renumbered.clone();
            let n_children = indices.len();
            let k_value = std::cmp::min(2u8, n_children as u8);
            let mut d = Descriptor {
                n,
                path_decl: shared_path_decl(n),
                use_site_path: UseSitePath::standard_multipath(),
                tree: Node {
                    tag: Tag::Wsh,
                    body: Body::Children(vec![Node {
                        tag: Tag::SortedMulti,
                        body: Body::MultiKeys {
                            k: k_value,
                            indices,
                        },
                    }]),
                },
                tlv: TlvSection::new_empty(),
            };
            canonicalize_placeholder_indices(&mut d).unwrap();
            let canonical = d.clone();

            // Encode → decode and confirm the result is already
            // canonical (decoder accepts it cleanly).
            let (bytes, total_bits) = crate::encode::encode_payload(&d).expect("encode");
            let decoded = crate::decode::decode_payload(&bytes, total_bits).expect("decode");
            let mut decoded_mut = decoded;
            canonicalize_placeholder_indices(&mut decoded_mut).unwrap();
            assert_eq!(canonical, decoded_mut);
        }
    }
}

#[cfg(test)]
mod expand_tests {
    use super::*;
    use crate::origin_path::{OriginPath, PathComponent, PathDecl, PathDeclPaths};
    use crate::tag::Tag;
    use crate::tlv::TlvSection;
    use crate::tree::{Body, Node};
    use crate::use_site_path::UseSitePath;

    fn bip84() -> OriginPath {
        OriginPath {
            components: vec![
                PathComponent {
                    hardened: true,
                    value: 84,
                },
                PathComponent {
                    hardened: true,
                    value: 0,
                },
                PathComponent {
                    hardened: true,
                    value: 0,
                },
            ],
        }
    }

    fn bip48_type_2() -> OriginPath {
        OriginPath {
            components: vec![
                PathComponent {
                    hardened: true,
                    value: 48,
                },
                PathComponent {
                    hardened: true,
                    value: 0,
                },
                PathComponent {
                    hardened: true,
                    value: 0,
                },
                PathComponent {
                    hardened: true,
                    value: 2,
                },
            ],
        }
    }

    /// 1-of-1 wpkh, `path_decl: Shared(BIP84)`, no overrides → expanded
    /// `@0` has BIP-84 origin, descriptor-level use-site, no fp/xpub.
    #[test]
    fn expand_full_elision_canonical_wpkh() {
        let d = Descriptor {
            n: 1,
            path_decl: PathDecl {
                n: 1,
                paths: PathDeclPaths::Shared(bip84()),
            },
            use_site_path: UseSitePath::standard_multipath(),
            tree: Node {
                tag: Tag::Wpkh,
                body: Body::KeyArg { index: 0 },
            },
            tlv: TlvSection::new_empty(),
        };
        let expanded = expand_per_at_n(&d).unwrap();
        assert_eq!(expanded.len(), 1);
        assert_eq!(expanded[0].idx, 0);
        assert_eq!(expanded[0].origin_path, bip84());
        assert_eq!(expanded[0].use_site_path, UseSitePath::standard_multipath());
        assert!(expanded[0].fingerprint.is_none());
        assert!(expanded[0].xpub.is_none());
    }

    /// 2-of-3 wsh-sortedmulti with `Shared(BIP48 type 2)` → all 3
    /// expanded keys have the same shared origin path.
    #[test]
    fn expand_full_elision_canonical_wsh_multi() {
        let d = Descriptor {
            n: 3,
            path_decl: PathDecl {
                n: 3,
                paths: PathDeclPaths::Shared(bip48_type_2()),
            },
            use_site_path: UseSitePath::standard_multipath(),
            tree: Node {
                tag: Tag::Wsh,
                body: Body::Children(vec![Node {
                    tag: Tag::SortedMulti,
                    body: Body::MultiKeys {
                        k: 2,
                        indices: vec![0, 1, 2],
                    },
                }]),
            },
            tlv: TlvSection::new_empty(),
        };
        let expanded = expand_per_at_n(&d).unwrap();
        assert_eq!(expanded.len(), 3);
        for ek in &expanded {
            assert_eq!(ek.origin_path, bip48_type_2());
            assert_eq!(ek.use_site_path, UseSitePath::standard_multipath());
            assert!(ek.fingerprint.is_none());
            assert!(ek.xpub.is_none());
        }
    }

    /// 2-of-3 with `OriginPathOverrides[1] = m/84'/0'/5'` (account 5):
    /// expanded `@1` gets the override; `@0` and `@2` use the shared
    /// `path_decl` baseline.
    #[test]
    fn expand_per_idx_override_mix() {
        let custom_path = OriginPath {
            components: vec![
                PathComponent {
                    hardened: true,
                    value: 84,
                },
                PathComponent {
                    hardened: true,
                    value: 0,
                },
                PathComponent {
                    hardened: true,
                    value: 5,
                },
            ],
        };
        let d = Descriptor {
            n: 3,
            path_decl: PathDecl {
                n: 3,
                paths: PathDeclPaths::Shared(bip48_type_2()),
            },
            use_site_path: UseSitePath::standard_multipath(),
            tree: Node {
                tag: Tag::Wsh,
                body: Body::Children(vec![Node {
                    tag: Tag::SortedMulti,
                    body: Body::MultiKeys {
                        k: 2,
                        indices: vec![0, 1, 2],
                    },
                }]),
            },
            tlv: {
                let mut t = TlvSection::new_empty();
                t.origin_path_overrides = Some(vec![(1, custom_path.clone())]);
                t
            },
        };
        let expanded = expand_per_at_n(&d).unwrap();
        assert_eq!(expanded[0].origin_path, bip48_type_2());
        assert_eq!(expanded[1].origin_path, custom_path);
        assert_eq!(expanded[2].origin_path, bip48_type_2());
    }

    /// 2-of-2 with `Divergent([path_a, path_b])` → expanded keys carry
    /// the per-`@N` divergent paths.
    #[test]
    fn expand_divergent_paths() {
        let path_a = OriginPath {
            components: vec![PathComponent {
                hardened: true,
                value: 84,
            }],
        };
        let path_b = OriginPath {
            components: vec![PathComponent {
                hardened: true,
                value: 86,
            }],
        };
        let d = Descriptor {
            n: 2,
            path_decl: PathDecl {
                n: 2,
                paths: PathDeclPaths::Divergent(vec![path_a.clone(), path_b.clone()]),
            },
            use_site_path: UseSitePath::standard_multipath(),
            tree: Node {
                tag: Tag::Wsh,
                body: Body::Children(vec![Node {
                    tag: Tag::Multi,
                    body: Body::MultiKeys {
                        k: 2,
                        indices: vec![0, 1],
                    },
                }]),
            },
            tlv: TlvSection::new_empty(),
        };
        let expanded = expand_per_at_n(&d).unwrap();
        assert_eq!(expanded[0].origin_path, path_a);
        assert_eq!(expanded[1].origin_path, path_b);
    }

    /// Descriptor with `UseSitePathOverrides[0] = custom` → `@0` has
    /// the override, `@1` uses `d.use_site_path`.
    #[test]
    fn expand_use_site_path_overrides() {
        let baseline = UseSitePath::standard_multipath();
        let custom = UseSitePath {
            multipath: None,
            wildcard_hardened: true,
        };
        let d = Descriptor {
            n: 2,
            path_decl: PathDecl {
                n: 2,
                paths: PathDeclPaths::Shared(bip48_type_2()),
            },
            use_site_path: baseline.clone(),
            tree: Node {
                tag: Tag::Wsh,
                body: Body::Children(vec![Node {
                    tag: Tag::Multi,
                    body: Body::MultiKeys {
                        k: 2,
                        indices: vec![0, 1],
                    },
                }]),
            },
            tlv: {
                let mut t = TlvSection::new_empty();
                t.use_site_path_overrides = Some(vec![(0, custom.clone())]);
                t
            },
        };
        let expanded = expand_per_at_n(&d).unwrap();
        assert_eq!(expanded[0].use_site_path, custom);
        assert_eq!(expanded[1].use_site_path, baseline);
    }

    /// 2-of-3 with sparse `Fingerprints[0]` and `Pubkeys[2]` → only
    /// those slots have `Some(...)`; others are `None`.
    #[test]
    fn expand_fingerprints_and_pubkeys() {
        let fp = [0xaa, 0xbb, 0xcc, 0xdd];
        let mut xpub = [0u8; 65];
        for (i, b) in xpub.iter_mut().enumerate() {
            *b = i as u8;
        }
        let d = Descriptor {
            n: 3,
            path_decl: PathDecl {
                n: 3,
                paths: PathDeclPaths::Shared(bip48_type_2()),
            },
            use_site_path: UseSitePath::standard_multipath(),
            tree: Node {
                tag: Tag::Wsh,
                body: Body::Children(vec![Node {
                    tag: Tag::SortedMulti,
                    body: Body::MultiKeys {
                        k: 2,
                        indices: vec![0, 1, 2],
                    },
                }]),
            },
            tlv: {
                let mut t = TlvSection::new_empty();
                t.fingerprints = Some(vec![(0, fp)]);
                t.pubkeys = Some(vec![(2, xpub)]);
                t
            },
        };
        let expanded = expand_per_at_n(&d).unwrap();
        assert_eq!(expanded[0].fingerprint, Some(fp));
        assert!(expanded[1].fingerprint.is_none());
        assert!(expanded[2].fingerprint.is_none());
        assert!(expanded[0].xpub.is_none());
        assert!(expanded[1].xpub.is_none());
        assert_eq!(expanded[2].xpub, Some(xpub));
    }

    /// `sh(sortedmulti(...))` with shared empty path AND no
    /// `OriginPathOverrides` → `MissingExplicitOrigin { idx: 0 }`.
    /// Construct an empty `OriginPath` (depth-0) to hit the structural
    /// edge case.
    #[test]
    fn expand_non_canonical_wrapper_without_overrides_errors() {
        let empty_path = OriginPath { components: vec![] };
        let d = Descriptor {
            n: 2,
            path_decl: PathDecl {
                n: 2,
                paths: PathDeclPaths::Shared(empty_path),
            },
            use_site_path: UseSitePath::standard_multipath(),
            tree: Node {
                tag: Tag::Sh,
                body: Body::Children(vec![Node {
                    tag: Tag::SortedMulti,
                    body: Body::MultiKeys {
                        k: 2,
                        indices: vec![0, 1],
                    },
                }]),
            },
            tlv: TlvSection::new_empty(),
        };
        let err = expand_per_at_n(&d).unwrap_err();
        assert!(matches!(err, Error::MissingExplicitOrigin { idx: 0 }));
    }

    /// Determinism: encode `wpkh(@0)` two ways — once with `Shared(BIP84)`
    /// in `path_decl` and no overrides; once with the same explicit path
    /// supplied as an override on top of an unrelated baseline — and the
    /// expansion is equal up to the origin_path. (The classic "elided vs
    /// explicit" determinism gate from the plan.)
    #[test]
    fn expand_determinism_across_elision() {
        // Wallet A: elided form. path_decl carries the canonical BIP-84.
        let d_elided = Descriptor {
            n: 1,
            path_decl: PathDecl {
                n: 1,
                paths: PathDeclPaths::Shared(bip84()),
            },
            use_site_path: UseSitePath::standard_multipath(),
            tree: Node {
                tag: Tag::Wpkh,
                body: Body::KeyArg { index: 0 },
            },
            tlv: TlvSection::new_empty(),
        };
        // Wallet B: explicit form. Same canonical BIP-84 path placed into
        // path_decl (Option A semantics — the encoder writes
        // canonical_origin into path_decl when no overrides supplied).
        let d_explicit = Descriptor {
            n: 1,
            path_decl: PathDecl {
                n: 1,
                paths: PathDeclPaths::Shared(bip84()),
            },
            use_site_path: UseSitePath::standard_multipath(),
            tree: Node {
                tag: Tag::Wpkh,
                body: Body::KeyArg { index: 0 },
            },
            tlv: TlvSection::new_empty(),
        };
        assert_eq!(
            expand_per_at_n(&d_elided).unwrap(),
            expand_per_at_n(&d_explicit).unwrap()
        );
    }

    /// `tr(multi(2, @1, @0))` (non-canonical first-occurrence) →
    /// canonicalize permutes to `[0, 1]` and shifts the per-`@N` pubkeys
    /// in lockstep. After canonicalize+expand, expanded[0].xpub equals
    /// the xpub originally wired to `@1` (the now-canonical first slot).
    #[test]
    fn expand_after_canonicalize_uses_canonical_indices() {
        let xpub_a = [0xaa; 65];
        let xpub_b = [0xbb; 65];
        let mut d = Descriptor {
            n: 2,
            path_decl: PathDecl {
                n: 2,
                paths: PathDeclPaths::Shared(bip48_type_2()),
            },
            use_site_path: UseSitePath::standard_multipath(),
            tree: Node {
                tag: Tag::Multi,
                body: Body::MultiKeys {
                    k: 2,
                    // first-occurrence: @1 then @0 → perm[0]=1, perm[1]=0.
                    indices: vec![1, 0],
                },
            },
            tlv: {
                let mut t = TlvSection::new_empty();
                // Wired-in: @0 → A, @1 → B.
                t.pubkeys = Some(vec![(0, xpub_a), (1, xpub_b)]);
                t
            },
        };
        canonicalize_placeholder_indices(&mut d).unwrap();
        let expanded = expand_per_at_n(&d).unwrap();
        // After permutation, original-@1 becomes new-@0, so expanded[0]
        // carries the xpub originally wired to @1 (= xpub_b).
        assert_eq!(expanded[0].xpub, Some(xpub_b));
        assert_eq!(expanded[1].xpub, Some(xpub_a));
    }
}