differential-dataflow 0.24.0

//! Specialize differential dataflow operators.
//!
//! Differential dataflow introduces a small number of specialized operators on collections. These
//! operators have specialized implementations to make them work efficiently, and are in addition
//! to several operations defined directly on the `Collection` type (e.g. `map` and `filter`).

pub use self::iterate::Iterate;
pub use self::count::CountTotal;
pub use self::threshold::ThresholdTotal;

pub mod arrange;
pub mod reduce;
pub mod iterate;
pub mod join;
pub mod count;
pub mod threshold;

use crate::lattice::Lattice;
use crate::trace::Cursor;

/// An accumulation of (value, time, diff) updates.
pub struct EditList<V, T, D> {
    values: Vec<(V, usize)>,
    edits: Vec<(T, D)>,
}

impl<V: Copy, T: Ord + Lattice, D: crate::difference::Semigroup> EditList<V, T, D> {
    /// Creates an empty list of edits.
    #[inline]
    fn new() -> Self {
        EditList {
            values: Vec::new(),
            edits: Vec::new(),
        }
    }
    /// Walks the cursor's vals at the current key into `self`, advancing times by `meet` if supplied.
    ///
    /// The cursor is assumed to be positioned at a key already; callers that need
    /// to seek should use [`Cursor::populate_key`] (or [`ValueHistory::replay_key`])
    /// instead. This split avoids a redundant seek in the merge-join inner loop,
    /// where the cursor is positioned by the upstream merge step.
    fn load<'a, C>(&mut self, cursor: &mut C, storage: &'a C::Storage, meet: Option<&T>)
    where
        C: Cursor<Val<'a> = V, Time = T, Diff = D>,
    {
        self.clear();
        while let Some(val) = cursor.get_val(storage) {
            cursor.map_times(storage, |time, diff| {
                let mut t = C::owned_time(time);
                if let Some(m) = meet { t.join_assign(m); }
                self.push(t, C::owned_diff(diff));
            });
            self.seal(val);
            cursor.step_val(storage);
        }
    }
    /// Clears the list of edits.
    #[inline]
    pub fn clear(&mut self) {
        self.values.clear();
        self.edits.clear();
    }
    fn len(&self) -> usize { self.edits.len() }
    /// Inserts a new edit for an as-yet undetermined value.
    #[inline]
    pub fn push(&mut self, time: T, diff: D) {
        // TODO: Could attempt "insertion-sort" like behavior here, where we collapse if possible.
        self.edits.push((time, diff));
    }
    /// Associates all edits pushed since the previous `seal_value` call with `value`.
    #[inline]
    pub fn seal(&mut self, value: V) {
        let prev = self.values.last().map(|x| x.1).unwrap_or(0);
        crate::consolidation::consolidate_from(&mut self.edits, prev);
        if self.edits.len() > prev {
            self.values.push((value, self.edits.len()));
        }
    }
    fn map<F: FnMut(V, &T, &D)>(&self, mut logic: F) {
        for index in 0 .. self.values.len() {
            let lower = if index == 0 { 0 } else { self.values[index-1].1 };
            let upper = self.values[index].1;
            for edit in lower .. upper {
                logic(self.values[index].0, &self.edits[edit].0, &self.edits[edit].1);
            }
        }
    }
}

struct ValueHistory<V, T, D> {
    edits: EditList<V, T, D>,
    history: Vec<(T, T, usize, usize)>,     // (time, meet, value_index, edit_offset)
    buffer: Vec<((V, T), D)>,               // where we accumulate / collapse updates.
}

impl<V: Copy + Ord, T: Ord + Clone + Lattice, D: crate::difference::Semigroup> ValueHistory<V, T, D> {
    fn new() -> Self {
        ValueHistory {
            edits: EditList::new(),
            history: Vec::new(),
            buffer: Vec::new(),
        }
    }
    fn clear(&mut self) {
        self.edits.clear();
        self.history.clear();
        self.buffer.clear();
    }

    /// Loads and replays a specified key.
    ///
    /// If the key is absent, the replayed history will be empty.
    fn replay_key<'a, 'history, C>(
        &'history mut self,
        cursor: &mut C,
        storage: &'a C::Storage,
        key: C::Key<'a>,
        meet: Option<&T>,
    ) -> HistoryReplay<'history, V, T, D>
    where
        C: Cursor<Val<'a> = V, Time = T, Diff = D>,
    {
        self.clear();
        cursor.populate_key(storage, key, meet, &mut self.edits);
        self.replay()
    }

    /// Organizes history based on current contents of edits.
    fn replay<'history>(&'history mut self) -> HistoryReplay<'history, V, T, D> {

        self.buffer.clear();
        self.history.clear();
        for value_index in 0 .. self.edits.values.len() {
            let lower = if value_index > 0 { self.edits.values[value_index-1].1 } else { 0 };
            let upper = self.edits.values[value_index].1;
            for edit_index in lower .. upper {
                let time = self.edits.edits[edit_index].0.clone();
                self.history.push((time.clone(), time, value_index, edit_index));
            }
        }

        self.history.sort_by(|x,y| y.cmp(x));
        self.history.iter_mut().reduce(|prev, cur| { cur.1.meet_assign(&prev.1); cur });

        HistoryReplay { replay: self }
    }
}

struct HistoryReplay<'history, V, T, D> {
    replay: &'history mut ValueHistory<V, T, D>,
}

impl<'history, V: Copy + Ord, T: Ord + Clone + Lattice, D: Clone + crate::difference::Semigroup> HistoryReplay<'history, V, T, D> {
    fn time(&self) -> Option<&T> { self.replay.history.last().map(|x| &x.0) }
    fn meet(&self) -> Option<&T> { self.replay.history.last().map(|x| &x.1) }
    fn edit(&self) -> Option<(V, &T, &D)> {
        self.replay.history.last().map(|&(ref t, _, v, e)| (self.replay.edits.values[v].0, t, &self.replay.edits.edits[e].1))
    }

    fn buffer(&self) -> &[((V, T), D)] {
        &self.replay.buffer[..]
    }

    fn step(&mut self) {
        let (time, _, value_index, edit_offset) = self.replay.history.pop().unwrap();
        self.replay.buffer.push(((self.replay.edits.values[value_index].0, time), self.replay.edits.edits[edit_offset].1.clone()));
    }
    fn step_while_time_is(&mut self, time: &T) -> bool {
        let mut found = false;
        while self.time() == Some(time) {
            found = true;
            self.step();
        }
        found
    }
    fn advance_buffer_by(&mut self, meet: &T) {
        for element in self.replay.buffer.iter_mut() {
            (element.0).1.join_assign(meet);
        }
        crate::consolidation::consolidate(&mut self.replay.buffer);
    }
    fn is_done(&self) -> bool { self.replay.history.is_empty() }
}