pipecircuit 0.0.19

Correct. Robust. Fast. Portable. Pick four.
Documentation 
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use super::bucket::{next_bucket, probe_distance, reduce};
use super::{DenseKeyIndex, EMPTY_ROW, FindOutcome, IndexInconsistency};
use crate::storage::warp_table::IndexKey;

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub(in crate::storage::warp_table) struct PreparedRemoval<Key> {
    bucket: usize,
    pub(in crate::storage::warp_table) row: usize,
    pub(in crate::storage::warp_table) relocation: RowRelocation<Key>,
}

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub(in crate::storage::warp_table) enum RowRelocation<Key> {
    Stable,
    Relocated {
        key: Key,
        bucket: usize,
        from: usize,
        to: usize,
        stored_to: u32,
    },
}

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
#[must_use]
pub(in crate::storage::warp_table) enum PrepareRemoveOutcome<Key> {
    Prepared(PreparedRemoval<Key>),
    Absent,
}

impl DenseKeyIndex {
    pub(in crate::storage::warp_table) fn prepare_remove<Key: IndexKey>(
        &self,
        key: Key,
        keys: &[Key],
        last_row: usize,
    ) -> Result<PrepareRemoveOutcome<Key>, IndexInconsistency> {
        let (bucket, stored_row, row) = match self.find(key, keys)? {
            FindOutcome::Present {
                bucket,
                stored_row,
                row,
            } => (bucket, stored_row, row),
            FindOutcome::Absent => return Ok(PrepareRemoveOutcome::Absent),
        };
        let relocation = if row == last_row {
            RowRelocation::Stable
        } else {
            let Some(moved_key) = keys.get(last_row).copied() else {
                return Err(IndexInconsistency::KeyUnavailable { row: last_row });
            };
            match self.find(moved_key, keys)? {
                FindOutcome::Present {
                    bucket: moved_bucket,
                    row: moved_row,
                    ..
                } if moved_row == last_row => RowRelocation::Relocated {
                    key: moved_key,
                    bucket: moved_bucket,
                    from: last_row,
                    to: row,
                    stored_to: stored_row,
                },
                FindOutcome::Present { row: actual, .. } => {
                    return Err(IndexInconsistency::StoredRowMismatch {
                        expected: last_row,
                        actual,
                    });
                }
                FindOutcome::Absent => {
                    return Err(IndexInconsistency::KeyUnavailable { row: last_row });
                }
            }
        };
        self.validate_cluster_after(bucket, keys)?;
        Ok(PrepareRemoveOutcome::Prepared(PreparedRemoval {
            bucket,
            row,
            relocation,
        }))
    }

    fn validate_cluster_after<Key: IndexKey>(
        &self,
        removed_bucket: usize,
        keys: &[Key],
    ) -> Result<(), IndexInconsistency> {
        let mut bucket = next_bucket(removed_bucket, self.buckets.len());
        for _ in 0..self.buckets.len() {
            let Some(stored_row) = self.buckets.get(bucket).copied() else {
                return Err(IndexInconsistency::BucketUnavailable { bucket });
            };
            if stored_row == EMPTY_ROW {
                return Ok(());
            }
            let row = usize::try_from(stored_row)
                .map_err(|_| IndexInconsistency::StoredRowUnrepresentable { stored_row })?;
            if keys.get(row).is_none() {
                return Err(IndexInconsistency::KeyUnavailable { row });
            }
            bucket = next_bucket(bucket, self.buckets.len());
        }
        Err(IndexInconsistency::ProbeLimitReached)
    }

    #[allow(
        clippy::indexing_slicing,
        reason = "prepared removal proves every modulo bucket and stored row before mutation"
    )]
    pub(in crate::storage::warp_table) fn commit_remove<Key: IndexKey>(
        &mut self,
        removal: PreparedRemoval<Key>,
        keys: &[Key],
    ) {
        let relocated = match removal.relocation {
            RowRelocation::Stable => None,
            RowRelocation::Relocated {
                key,
                bucket,
                from: _,
                to,
                stored_to,
            } => {
                self.buckets[bucket] = stored_to;
                Some((key, to))
            }
        };
        let mut vacancy = removal.bucket;
        let mut bucket = next_bucket(vacancy, self.buckets.len());
        loop {
            let stored_row = self.buckets[bucket];
            if stored_row == EMPTY_ROW {
                self.buckets[vacancy] = EMPTY_ROW;
                return;
            }
            let row = stored_row as usize;
            let word = match relocated {
                Some((key, relocated_row)) if row == relocated_row => key.index_word(),
                _ => keys[row].index_word(),
            };
            let ideal = reduce(word, self.buckets.len());
            if probe_distance(ideal, bucket, self.buckets.len())
                > probe_distance(ideal, vacancy, self.buckets.len())
            {
                self.buckets[vacancy] = stored_row;
                vacancy = bucket;
            }
            bucket = next_bucket(bucket, self.buckets.len());
        }
    }
}