zcash_client_backend: Generalize & extend account metadata query API

This generalizes the previous account metadata query API to be able to
represent more complex queries, and also to return note totals in
addition to note counts.

components/zcash_protocol/CHANGELOG.md

@@ -10,6 +10,8 @@ and this library adheres to Rust's notion of
 ### Added
 - `zcash_protocol::value::QuotRem`
 - `zcash_protocol::value::Zatoshis::div_with_remainder`
+- `impl Mul<u64> for zcash_protocol::value::Zatoshis`
+- `impl Div<NonZeroU64> for zcash_protocol::value::Zatoshis`
 
 ### Changed
 - MSRV is now 1.77.0.

components/zcash_protocol/src/value.rs

@@ -2,7 +2,7 @@ use std::convert::{Infallible, TryFrom};
 use std::error;
 use std::iter::Sum;
 use std::num::NonZeroU64;
-use std::ops::{Add, Mul, Neg, Sub};
+use std::ops::{Add, Div, Mul, Neg, Sub};
 
 use memuse::DynamicUsage;
 
@@ -321,6 +321,7 @@ impl Zatoshis {
     /// Divides this `Zatoshis` value by the given divisor and returns the quotient and remainder.
     pub fn div_with_remainder(&self, divisor: NonZeroU64) -> QuotRem<Zatoshis> {
         let divisor = u64::from(divisor);
+        // `self` is already bounds-checked, so we don't need to re-check it in division
         QuotRem {
             quotient: Zatoshis(self.0 / divisor),
             remainder: Zatoshis(self.0 % divisor),
@@ -394,11 +395,19 @@ impl Sub<Zatoshis> for Option<Zatoshis> {
     }
 }
 
+impl Mul<u64> for Zatoshis {
+    type Output = Option<Self>;
+
+    fn mul(self, rhs: u64) -> Option<Zatoshis> {
+        Zatoshis::from_u64(self.0.checked_mul(rhs)?).ok()
+    }
+}
+
 impl Mul<usize> for Zatoshis {
     type Output = Option<Self>;
 
     fn mul(self, rhs: usize) -> Option<Zatoshis> {
-        Zatoshis::from_u64(self.0.checked_mul(u64::try_from(rhs).ok()?)?).ok()
+        self * u64::try_from(rhs).ok()?
     }
 }
 
@@ -414,6 +423,15 @@ impl<'a> Sum<&'a Zatoshis> for Option<Zatoshis> {
     }
 }
 
+impl Div<NonZeroU64> for Zatoshis {
+    type Output = Zatoshis;
+
+    fn div(self, rhs: NonZeroU64) -> Zatoshis {
+        // `self` is already bounds-checked, so we don't need to re-check it
+        Zatoshis(self.0 / u64::from(rhs))
+    }
+}
+
 /// A type for balance violations in amount addition and subtraction
 /// (overflow and underflow of allowed ranges)
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]

zcash_client_backend/CHANGELOG.md

@@ -11,8 +11,11 @@ and this library adheres to Rust's notion of
 - `zcash_client_backend::data_api`:
   - `Progress`
   - `WalletSummary::progress`
-  - `WalletMeta`
+  - `PoolMeta`
+  - `AccountMeta`
   - `impl Default for wallet::input_selection::GreedyInputSelector`
+  - `BoundedU8`
+  - `NoteFilter`
 - `zcash_client_backend::fees`
   - `SplitPolicy`
   - `StandardFeeRule` has been moved here from `zcash_primitives::fees`. Relative
@@ -32,7 +35,7 @@ and this library adheres to Rust's notion of
 - MSRV is now 1.77.0.
 - Migrated to `arti-client 0.23`.
 - `zcash_client_backend::data_api`:
-  - `InputSource` has an added method `get_wallet_metadata`
+  - `InputSource` has an added method `get_account_metadata`
   - `error::Error` has additional variant `Error::Change`. This necessitates
     the addition of two type parameters to the `Error` type,
     `ChangeErrT` and `NoteRefT`.
@@ -65,7 +68,7 @@ and this library adheres to Rust's notion of
     changed.
 - `zcash_client_backend::fees`:
   - `ChangeStrategy` has changed. It has two new associated types, `MetaSource`
-    and `WalletMeta`, and its `FeeRule` associated type now has an additional
+    and `AccountMetaT`, and its `FeeRule` associated type now has an additional
     `Clone` bound. In addition, it defines a new `fetch_wallet_meta` method, and
     the arguments to `compute_balance` have changed.
   - `zip317::SingleOutputChangeStrategy` has been made polymorphic in the fee

zcash_client_backend/src/data_api.rs

@@ -794,6 +794,35 @@ impl<NoteRef> SpendableNotes<NoteRef> {
     }
 }
 
+/// Metadata about the structure of unspent outputs in a single pool within a wallet account.
+///
+/// This type is often used to represent a filtered view of outputs in the account that were
+/// selected according to the conditions imposed by a [`NoteFilter`].
+#[derive(Debug, Clone)]
+pub struct PoolMeta {
+    note_count: usize,
+    value: NonNegativeAmount,
+}
+
+impl PoolMeta {
+    /// Constructs a new [`PoolMeta`] value from its constituent parts.
+    pub fn new(note_count: usize, value: NonNegativeAmount) -> Self {
+        Self { note_count, value }
+    }
+
+    /// Returns the number of unspent outputs in the account, potentially selected in accordance
+    /// with some [`NoteFilter`].
+    pub fn note_count(&self) -> usize {
+        self.note_count
+    }
+
+    /// Returns the total value of unspent outputs in the account that are accounted for in
+    /// [`Self::note_count`].
+    pub fn value(&self) -> NonNegativeAmount {
+        self.value
+    }
+}
+
 /// Metadata about the structure of the wallet for a particular account.
 ///
 /// At present this just contains counts of unspent outputs in each pool, but it may be extended in
@@ -802,58 +831,185 @@ impl<NoteRef> SpendableNotes<NoteRef> {
 /// Values of this type are intended to be used in selection of change output values. A value of
 /// this type may represent filtered data, and may therefore not count all of the unspent notes in
 /// the wallet.
-pub struct WalletMeta {
-    sapling_note_count: usize,
-    #[cfg(feature = "orchard")]
-    orchard_note_count: usize,
+///
+/// A [`AccountMeta`] value is normally produced by querying the wallet database via passing a
+/// [`NoteFilter`] to [`InputSource::get_account_metadata`].
+#[derive(Debug, Clone)]
+pub struct AccountMeta {
+    sapling: Option<PoolMeta>,
+    orchard: Option<PoolMeta>,
 }
 
-impl WalletMeta {
-    /// Constructs a new [`WalletMeta`] value from its constituent parts.
-    pub fn new(
-        sapling_note_count: usize,
-        #[cfg(feature = "orchard")] orchard_note_count: usize,
-    ) -> Self {
-        Self {
-            sapling_note_count,
-            #[cfg(feature = "orchard")]
-            orchard_note_count,
-        }
+impl AccountMeta {
+    /// Constructs a new [`AccountMeta`] value from its constituent parts.
+    pub fn new(sapling: Option<PoolMeta>, orchard: Option<PoolMeta>) -> Self {
+        Self { sapling, orchard }
+    }
+
+    /// Returns metadata about Sapling notes belonging to the account for which this was generated.
+    ///
+    /// Returns [`None`] if no metadata is available or it was not possible to evaluate the query
+    /// described by a [`NoteFilter`] given the available wallet data.
+    pub fn sapling(&self) -> Option<&PoolMeta> {
+        self.sapling.as_ref()
+    }
+
+    /// Returns metadata about Orchard notes belonging to the account for which this was generated.
+    ///
+    /// Returns [`None`] if no metadata is available or it was not possible to evaluate the query
+    /// described by a [`NoteFilter`] given the available wallet data.
+    pub fn orchard(&self) -> Option<&PoolMeta> {
+        self.orchard.as_ref()
+    }
+
+    fn sapling_note_count(&self) -> Option<usize> {
+        self.sapling.as_ref().map(|m| m.note_count)
+    }
+
+    fn orchard_note_count(&self) -> Option<usize> {
+        self.orchard.as_ref().map(|m| m.note_count)
     }
 
     /// Returns the number of unspent notes in the wallet for the given shielded protocol.
-    pub fn note_count(&self, protocol: ShieldedProtocol) -> usize {
+    pub fn note_count(&self, protocol: ShieldedProtocol) -> Option<usize> {
         match protocol {
-            ShieldedProtocol::Sapling => self.sapling_note_count,
-            #[cfg(feature = "orchard")]
-            ShieldedProtocol::Orchard => self.orchard_note_count,
-            #[cfg(not(feature = "orchard"))]
-            ShieldedProtocol::Orchard => 0,
+            ShieldedProtocol::Sapling => self.sapling_note_count(),
+            ShieldedProtocol::Orchard => self.orchard_note_count(),
+        }
+    }
+
+    /// Returns the total number of unspent shielded notes belonging to the account for which this
+    /// was generated.
+    ///
+    /// Returns [`None`] if no metadata is available or it was not possible to evaluate the query
+    /// described by a [`NoteFilter`] given the available wallet data. If metadata is available
+    /// only for a single pool, the metadata for that pool will be returned.
+    pub fn total_note_count(&self) -> Option<usize> {
+        let s = self.sapling_note_count();
+        let o = self.orchard_note_count();
+        s.zip(o).map(|(s, o)| s + o).or(s).or(o)
+    }
+
+    fn sapling_value(&self) -> Option<NonNegativeAmount> {
+        self.sapling.as_ref().map(|m| m.value)
+    }
+
+    fn orchard_value(&self) -> Option<NonNegativeAmount> {
+        self.orchard.as_ref().map(|m| m.value)
+    }
+
+    /// Returns the total value of shielded notes represented by [`Self::total_note_count`]
+    ///
+    /// Returns [`None`] if no metadata is available or it was not possible to evaluate the query
+    /// described by a [`NoteFilter`] given the available wallet data. If metadata is available
+    /// only for a single pool, the metadata for that pool will be returned.
+    pub fn total_value(&self) -> Option<NonNegativeAmount> {
+        let s = self.sapling_value();
+        let o = self.orchard_value();
+        s.zip(o)
+            .map(|(s, o)| (s + o).expect("Does not overflow Zcash maximum value."))
+            .or(s)
+            .or(o)
+    }
+}
+
+/// A `u8` value in the range 0..=MAX
+#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
+pub struct BoundedU8<const MAX: u8>(u8);
+
+impl<const MAX: u8> BoundedU8<MAX> {
+    /// Creates a constant `BoundedU8` from a [`u8`] value.
+    ///
+    /// Panics: if the value is outside the range `0..=MAX`.
+    pub const fn new_const(value: u8) -> Self {
+        assert!(value <= MAX);
+        Self(value)
+    }
+
+    /// Creates a `BoundedU8` from a [`u8`] value.
+    ///
+    /// Returns `None` if the provided value is outside the range `0..=MAX`.
+    pub fn new(value: u8) -> Option<Self> {
+        if value <= MAX {
+            Some(Self(value))
+        } else {
+            None
         }
     }
 
-    /// Returns the number of unspent Sapling notes belonging to the account for which this was
-    /// generated.
-    pub fn sapling_note_count(&self) -> usize {
-        self.sapling_note_count
+    /// Returns the wrapped [`u8`] value.
+    pub fn value(&self) -> u8 {
+        self.0
     }
+}
 
-    /// Returns the number of unspent Orchard notes belonging to the account for which this was
-    /// generated.
-    #[cfg(feature = "orchard")]
-    pub fn orchard_note_count(&self) -> usize {
-        self.orchard_note_count
+impl<const MAX: u8> From<BoundedU8<MAX>> for u8 {
+    fn from(value: BoundedU8<MAX>) -> Self {
+        value.0
     }
+}
 
-    /// Returns the total number of unspent shielded notes belonging to the account for which this
-    /// was generated.
-    pub fn total_note_count(&self) -> usize {
-        self.sapling_note_count + self.note_count(ShieldedProtocol::Orchard)
+impl<const MAX: u8> From<BoundedU8<MAX>> for usize {
+    fn from(value: BoundedU8<MAX>) -> Self {
+        usize::from(value.0)
+    }
+}
+
+/// A small query language for filtering notes belonging to an account.
+///
+/// A filter described using this language is applied to notes individually. It is primarily
+/// intended for retrieval of account metadata in service of making determinations for how to
+/// allocate change notes, and is not currently intended for use in broader note selection
+/// contexts.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub enum NoteFilter {
+    /// Selects notes having value greater than or equal to the provided value.
+    ExceedsMinValue(NonNegativeAmount),
+    /// Selects notes having value greater than or equal to approximately the n'th percentile of
+    /// previously sent notes in the account, irrespective of pool. The wrapped value must be in
+    /// the range `1..=99`. The value `n` is respected in a best-effort fashion; results are likely
+    /// to be inaccurate if the account has not yet completed scanning or if insufficient send data
+    /// is available to establish a distribution.
+    // TODO: it might be worthwhile to add an optional parameter here that can be used to ignore
+    // low-valued (test/memo-only) sends when constructing the distribution to be drawn from.
+    ExceedsPriorSendPercentile(BoundedU8<99>),
+    /// Selects notes having value greater than or equal to the specified percentage of the account
+    /// balance across all shielded pools. The wrapped value must be in the range `1..=99`
+    ExceedsBalancePercentage(BoundedU8<99>),
+    /// A note will be selected if it satisfies both of the specified conditions.
+    ///
+    /// If it is not possible to evaluate one of the conditions (for example,
+    /// [`NoteFilter::ExceedsPriorSendPercentile`] cannot be evaluated if no sends have been
+    /// performed) then that condition will be ignored. If neither condition can be evaluated,
+    /// then the entire condition cannot be evaluated.
+    Combine(Box<NoteFilter>, Box<NoteFilter>),
+    /// A note will be selected if it satisfies the first condition; if it is not possible to
+    /// evaluate that condition (for example, [`NoteFilter::ExceedsPriorSendPercentile`] cannot
+    /// be evaluated if no sends have been performed) then the second condition will be used for
+    /// evaluation.
+    Attempt {
+        condition: Box<NoteFilter>,
+        fallback: Box<NoteFilter>,
+    },
+}
+
+impl NoteFilter {
+    /// Constructs a [`NoteFilter::Combine`] query node.
+    pub fn combine(l: NoteFilter, r: NoteFilter) -> Self {
+        Self::Combine(Box::new(l), Box::new(r))
+    }
+
+    /// Constructs a [`NoteFilter::Attempt`] query node.
+    pub fn attempt(condition: NoteFilter, fallback: NoteFilter) -> Self {
+        Self::Attempt {
+            condition: Box::new(condition),
+            fallback: Box::new(fallback),
+        }
     }
 }
 
 /// A trait representing the capability to query a data store for unspent transaction outputs
-/// belonging to a wallet.
+/// belonging to a account.
 #[cfg_attr(feature = "test-dependencies", delegatable_trait)]
 pub trait InputSource {
     /// The type of errors produced by a wallet backend.
@@ -900,14 +1056,17 @@ pub trait InputSource {
     ///
     /// The returned metadata value must exclude:
     /// - spent notes;
-    /// - unspent notes having value less than the specified minimum value;
+    /// - unspent notes excluded by the provided selector;
     /// - unspent notes identified in the given `exclude` list.
-    fn get_wallet_metadata(
+    ///
+    /// Implementations of this method may limit the complexity of supported queries. Such
+    /// limitations should be clearly documented for the implementing type.
+    fn get_account_metadata(
         &self,
         account: Self::AccountId,
-        min_value: NonNegativeAmount,
+        selector: &NoteFilter,
         exclude: &[Self::NoteRef],
-    ) -> Result<WalletMeta, Self::Error>;
+    ) -> Result<AccountMeta, Self::Error>;
 
     /// Fetches the transparent output corresponding to the provided `outpoint`.
     ///