eth_state_diff/

participation.rs

//! Compact delta encoding for Ethereum epoch participation flags.
//!
//! This module computes and applies deltas between validator participation
//! vectors.
//!
//! Participation changes are typically sparse: during an epoch transition,
//! most validators retain their existing participation flags while only a
//! subset of validators receive new values. The delta therefore stores only
//! modified indices and their replacement values rather than the complete
//! target vector.
//!
//! ## Encoding
//!
//! The delta has two representations:
//!
//! - [`ParticipationDiff::AllZeros`] represents a target vector containing
//!   only zero-valued participation flags.
//! - [`ParticipationDiff::Sparse`] stores only changed entries.
//!
//! For the sparse representation, modified indices are encoded as
//! delta-varint gaps between successive modified indices. The corresponding
//! replacement values are stored separately in `new_values`. This avoids
//! storing unchanged participation flags and makes sequences of nearby
//! changes particularly compact.
//!
//! Validators present in the target vector but not in the base vector are
//! stored separately in `extension` and appended during application.
//!
//! ## APIs
//!
//! [`diff_participation`] and [`apply_participation`] operate on contiguous
//! vectors and provide the specialized [`ParticipationDiff::AllZeros`] fast
//! path.
//!
//! [`diff_participation_iter`] and [`apply_participation_iter`] operate through
//! iterators and [`crate::ListMutTarget`], allowing consensus clients with
//! tree-backed or otherwise non-contiguous state representations to compute
//! and apply participation deltas without first materializing the complete
//! vector.
//!
//! The iterator-based diff API always produces the sparse representation.
//! The slice-based API can additionally detect an all-zero target and use the
//! more compact [`ParticipationDiff::AllZeros`] representation.
//!
//! ## Reconstruction
//!
//! Applying a sparse delta updates only the modified indices of the existing
//! collection and then appends any values in `extension`.
//!
//! Applying an [`ParticipationDiff::AllZeros`] delta replaces the destination
//! with a vector of the specified length containing only zero values.
//!
//! ## Complexity
//!
//! Diff generation is `O(n)` time and `O(m)` additional space, where `n` is
//! the number of participation flags examined and `m` is the number of
//! modified entries.
//!
//! Sparse delta application is `O(m + e)` time, where `m` is the number of
//! modified entries and `e` is the number of appended participation flags.
//!
//! The contiguous all-zero fast path performs `O(n)` work to initialize the
//! resulting vector.
//!
//! ## Serialization
//!
//! [`ParticipationDiff`] is designed to be serialized with `rkyv` and can be
//! subsequently compressed with a general-purpose compressor such as `zstd`.

use crate::{
    balances::{read_varint, write_varint},
    types::{ArchivedParticipationDiff, ParticipationDiff},
    Error,
};

/// Computes a compact delta between two participation flag slices.
///
/// The returned [`ParticipationDiff`] contains the information required to
/// reconstruct `target` from `base`.
///
/// If every flag in `target` is zero, the function uses the specialized
/// [`ParticipationDiff::AllZeros`] representation. Otherwise it produces a
/// sparse delta containing only modified flags.
///
/// This is the contiguous-slice convenience API. Clients whose participation
/// flags are stored in a non-contiguous representation can use
/// [`diff_participation_iter`] instead.
///
/// # Complexity
///
/// `O(n)` time and `O(m)` additional space, where `n` is the number of flags
/// examined and `m` is the number of modified flags.
pub fn diff_participation(base: &[u8], target: &[u8]) -> ParticipationDiff {
    if target.iter().all(|&value| value == 0) {
        return ParticipationDiff::AllZeros(target.len());
    }

    diff_participation_iter(base.iter().copied(), target.iter().copied())
}

/// Applies a participation delta to a contiguous vector in place.
///
/// This is the contiguous-vector convenience API. Sparse deltas are delegated
/// to [`apply_participation_iter`], while [`ParticipationDiff::AllZeros`] is
/// handled directly by replacing the destination with a zero-filled vector of
/// the encoded length.
///
/// After successful application, `base` contains the target participation
/// vector from which `delta` was produced.
///
/// # Errors
///
/// Returns [`Error::InvalidDelta`] if an encoded [`ParticipationDiff::AllZeros`]
/// length cannot be represented by the target vector or if a sparse delta is
/// internally inconsistent.
///
/// Returns [`Error::MalformedDelta`] if a sparse delta contains invalid
/// serialized payload data, such as a truncated or overflowing varint.
///
/// # Complexity
///
/// Sparse deltas require `O(m + e)` work, where `m` is the number of modified
/// entries and `e` is the number of appended flags.
///
/// An [`ParticipationDiff::AllZeros`] delta requires `O(n)` work to construct
/// the resulting zero-filled vector of length `n`.
pub fn apply_participation(
    base: &mut Vec<u8>,
    delta: &ArchivedParticipationDiff,
) -> Result<(), Error> {
    match delta {
        ArchivedParticipationDiff::AllZeros(len) => {
            let len = usize::try_from(len.to_native()).map_err(|_| {
                Error::InvalidDelta("all-zero participation length does not fit in usize".into())
            })?;

            base.clear();
            base.resize(len, 0);

            Ok(())
        }
        ArchivedParticipationDiff::Sparse { .. } => apply_participation_iter(base, delta),
    }
}

/// Computes a compact sparse delta between two participation flag iterators.
///
/// This API is intended for consensus clients whose participation flags are
/// stored in tree-backed or otherwise non-contiguous structures. The caller
/// can expose the values through [`ExactSizeIterator`]s without first
/// materializing the complete vectors as contiguous buffers.
///
/// The iterators are consumed during diff generation.
///
/// Unlike [`diff_participation`], this function always returns
/// [`ParticipationDiff::Sparse`]. It does not perform the all-zero
/// specialization because the iterator is consumed while determining the
/// changed entries.
///
/// Values remaining in `target` after the common portion are treated as
/// newly appended participation flags and are stored in the delta's
/// `extension` field.
///
/// # Complexity
///
/// `O(n)` time and `O(m + e)` additional space, where `n` is the size of the
/// common portion, `m` is the number of modified entries, and `e` is the
/// number of appended target entries.
pub fn diff_participation_iter<I1, I2>(mut base: I1, mut target: I2) -> ParticipationDiff
where
    I1: ExactSizeIterator<Item = u8>,
    I2: ExactSizeIterator<Item = u8>,
{
    let common_len = base.len().min(target.len());

    let mut sparse_indices = Vec::with_capacity(50_000);
    let mut new_values = Vec::with_capacity(50_000);
    let mut last_idx = 0u64;

    for i in 0..common_len {
        let Some(v1) = base.next() else {
            break;
        };

        let Some(v2) = target.next() else {
            break;
        };

        if v1 != v2 {
            let idx = i as u64;

            // `i >= last_idx` because changed indices are processed in
            // strictly increasing iterator order.
            let gap = idx - last_idx;
            write_varint(gap, &mut sparse_indices);

            new_values.push(v2);
            last_idx = idx;
        }
    }

    let extension = target.collect();

    ParticipationDiff::Sparse {
        sparse_indices,
        new_values,
        extension,
    }
}

/// Applies a sparse participation delta to a mutable collection in place.
///
/// This API is intended for consensus clients whose participation flags are
/// stored in tree-backed or otherwise non-contiguous structures.
///
/// The destination collection is updated according to the sparse entries in
/// `delta`. Each encoded index gap identifies the next modified entry, whose
/// value is replaced with the corresponding entry from `new_values`. Values
/// in `extension` are then appended to the destination.
///
/// [`ParticipationDiff::AllZeros`] is not supported by this generic API
/// because [`crate::ListMutTarget`] does not provide an operation for clearing
/// or resizing an existing collection. Callers should use
/// [`apply_participation`] when they need to support that representation.
///
/// # Errors
///
/// Returns [`Error::InvalidDelta`] if the supplied delta is not a sparse
/// representation, if the number of encoded indices does not match the number
/// of replacement values, if an encoded index cannot be represented as a
/// `usize`, if an index falls outside the destination collection, or if the
/// destination collection cannot provide the required element.
///
/// Returns [`Error::MalformedDelta`] if the sparse index payload contains an
/// invalid or truncated varint.
///
/// # Complexity
///
/// `O(m + e)` time and `O(1)` additional working space, excluding allocations
/// performed by the destination collection when it grows.
///
/// Here `m` is the number of modified entries and `e` is the number of
/// appended entries.
pub fn apply_participation_iter<T: crate::ListMutTarget<u8>>(
    target: &mut T,
    delta: &ArchivedParticipationDiff,
) -> Result<(), Error> {
    let ArchivedParticipationDiff::Sparse {
        sparse_indices,
        new_values,
        extension,
    } = delta
    else {
        return Err(Error::InvalidDelta(
            "AllZeros participation delta cannot be applied through the generic iterator API"
                .into(),
        ));
    };

    let indices_raw = sparse_indices.as_slice();
    let mut cursor = 0usize;
    let mut current_idx = 0usize;

    for value in new_values.iter() {
        let gap = read_varint(indices_raw, &mut cursor)?;

        let gap = usize::try_from(gap).map_err(|_| {
            Error::MalformedDelta("participation index gap does not fit in usize".into())
        })?;

        current_idx = current_idx.checked_add(gap).ok_or_else(|| {
            Error::MalformedDelta(
                "participation index overflow while decoding sparse indices".into(),
            )
        })?;

        let Some(target_value) = target.get_mut(current_idx) else {
            return Err(Error::InvalidDelta(format!(
                "participation index {current_idx} is outside target collection of length {}",
                target.len()
            )));
        };

        *target_value = *value;
    }

    if cursor != indices_raw.len() {
        return Err(Error::InvalidDelta(
            "participation sparse index payload contains unused bytes".into(),
        ));
    }

    for byte in extension.iter() {
        target.push(*byte);
    }

    Ok(())
}