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#[cfg(feature = "persistent")]
use dogged::DVec;
use snapshot_vec as sv;
use std::ops;
use std::marker::PhantomData;

use super::{VarValue, UnifyKey, UnifyValue};

#[allow(dead_code)] // rustc BUG
type Key<S> = <S as UnificationStore>::Key;

/// Largely internal trait implemented by the unification table
/// backing store types. The most common such type is `InPlace`,
/// which indicates a standard, mutable unification table.
pub trait UnificationStore: ops::Index<usize, Output = VarValue<Key<Self>>> + Clone {
    type Key: UnifyKey<Value = Self::Value>;
    type Value: UnifyValue;
    type Snapshot;

    fn new() -> Self;

    fn start_snapshot(&mut self) -> Self::Snapshot;

    fn rollback_to(&mut self, snapshot: Self::Snapshot);

    fn commit(&mut self, snapshot: Self::Snapshot);

    fn len(&self) -> usize;

    fn push(&mut self, value: VarValue<Self::Key>);

    fn update<F>(&mut self, index: usize, op: F)
        where F: FnOnce(&mut VarValue<Self::Key>);

    fn tag() -> &'static str {
        Self::Key::tag()
    }
}

/// Backing store for an in-place unification table.
/// Not typically used directly.
#[derive(Clone, Debug)]
pub struct InPlace<K: UnifyKey> {
    values: sv::SnapshotVec<Delegate<K>>
}

impl<K: UnifyKey> UnificationStore for InPlace<K> {
    type Key = K;
    type Value = K::Value;
    type Snapshot = sv::Snapshot;

    #[inline]
    fn new() -> Self {
        InPlace { values: sv::SnapshotVec::new() }
    }

    #[inline]
    fn start_snapshot(&mut self) -> Self::Snapshot {
        self.values.start_snapshot()
    }

    #[inline]
    fn rollback_to(&mut self, snapshot: Self::Snapshot) {
        self.values.rollback_to(snapshot);
    }

    #[inline]
    fn commit(&mut self, snapshot: Self::Snapshot) {
        self.values.commit(snapshot);
    }

    #[inline]
    fn len(&self) -> usize {
        self.values.len()
    }

    #[inline]
    fn push(&mut self, value: VarValue<Self::Key>) {
        self.values.push(value);
    }

    #[inline]
    fn update<F>(&mut self, index: usize, op: F)
        where F: FnOnce(&mut VarValue<Self::Key>)
    {
        self.values.update(index, op)
    }
}

impl<K> ops::Index<usize> for InPlace<K>
    where K: UnifyKey
{
    type Output = VarValue<K>;
    fn index(&self, index: usize) -> &VarValue<K> {
        &self.values[index]
    }
}

#[derive(Copy, Clone, Debug)]
struct Delegate<K>(PhantomData<K>);

impl<K: UnifyKey> sv::SnapshotVecDelegate for Delegate<K> {
    type Value = VarValue<K>;
    type Undo = ();

    fn reverse(_: &mut Vec<VarValue<K>>, _: ()) {}
}

#[cfg(feature = "persistent")]
#[derive(Clone, Debug)]
pub struct Persistent<K: UnifyKey> {
    values: DVec<VarValue<K>>
}

#[cfg(feature = "persistent")]
impl<K: UnifyKey> UnificationStore for Persistent<K> {
    type Key = K;
    type Value = K::Value;
    type Snapshot = Self;

    #[inline]
    fn new() -> Self {
        Persistent { values: DVec::new() }
    }

    #[inline]
    fn start_snapshot(&mut self) -> Self::Snapshot {
        self.clone()
    }

    #[inline]
    fn rollback_to(&mut self, snapshot: Self::Snapshot) {
        *self = snapshot;
    }

    #[inline]
    fn commit(&mut self, _snapshot: Self::Snapshot) {
    }

    #[inline]
    fn len(&self) -> usize {
        self.values.len()
    }

    #[inline]
    fn push(&mut self, value: VarValue<Self::Key>) {
        self.values.push(value);
    }

    #[inline]
    fn update<F>(&mut self, index: usize, op: F)
        where F: FnOnce(&mut VarValue<Self::Key>)
    {
        let p = &mut self.values[index];
        op(p);
    }
}

#[cfg(feature = "persistent")]
impl<K> ops::Index<usize> for Persistent<K>
    where K: UnifyKey
{
    type Output = VarValue<K>;
    fn index(&self, index: usize) -> &VarValue<K> {
        &self.values[index]
    }
}