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use core::cmp::{Ordering, PartialOrd};

use crate::{DeletionTs, Length, ReplicaId, ReplicaIdMap};

pub type DeletionMap = BaseMap<DeletionTs>;

pub type VersionMap = BaseMap<Length>;

/// A struct equivalent to a `HashMap<ReplicaId, T>`, but with the `ReplicaId`
/// of the local `Replica` (and the corresponding `T`) stored separately from
/// the `HashMap` itself.
#[derive(Clone, PartialEq, Eq)]
#[cfg_attr(
    any(feature = "encode", feature = "serde"),
    derive(serde::Serialize, serde::Deserialize)
)]
pub struct BaseMap<T> {
    /// The `ReplicaId` of the `Replica` that this map is used in.
    this_id: ReplicaId,

    /// The local value.
    this_value: T,

    /// The values of the remote `Replica`s.
    rest: ReplicaIdMap<T>,
}

impl<T: Copy> BaseMap<T> {
    #[inline]
    pub fn get(&self, replica_id: ReplicaId) -> T
    where
        T: Default,
    {
        if replica_id == self.this_id {
            self.this_value
        } else {
            self.rest.get(&replica_id).copied().unwrap_or_default()
        }
    }

    #[inline]
    pub fn get_mut(&mut self, replica_id: ReplicaId) -> &mut T
    where
        T: Default,
    {
        if replica_id == self.this_id {
            &mut self.this_value
        } else {
            self.rest.entry(replica_id).or_default()
        }
    }

    #[inline]
    pub fn fork(&self, new_id: ReplicaId, restart_at: T) -> Self {
        let mut forked = self.clone();
        forked.fork_in_place(new_id, restart_at);
        forked
    }

    #[inline]
    pub fn fork_in_place(&mut self, new_id: ReplicaId, restart_at: T) {
        self.insert(self.this_id, self.this_value);
        self.this_id = new_id;
        self.this_value = restart_at;
    }

    #[inline]
    pub fn insert(&mut self, replica_id: ReplicaId, value: T) {
        self.rest.insert(replica_id, value);
    }

    #[inline]
    fn iter(&self) -> impl Iterator<Item = (ReplicaId, T)> + '_ {
        let this_entry = core::iter::once((self.this_id, self.this_value));
        this_entry.chain(self.rest.iter().map(|(&id, &value)| (id, value)))
    }

    #[inline]
    pub fn new(this_id: ReplicaId, this_value: T) -> Self {
        Self { this_id, this_value, rest: ReplicaIdMap::default() }
    }

    #[inline]
    pub fn this(&self) -> T {
        self.this_value
    }

    #[inline]
    pub fn this_id(&self) -> ReplicaId {
        self.this_id
    }

    #[inline]
    pub fn this_mut(&mut self) -> &mut T {
        &mut self.this_value
    }
}

impl<T: core::fmt::Debug> core::fmt::Debug for BaseMap<T> {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        let this_entry = core::iter::once((&self.this_id, &self.this_value));
        f.debug_map().entries(this_entry.chain(self.rest.iter())).finish()
    }
}

impl PartialOrd for VersionMap {
    #[inline]
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        #[inline]
        fn update_order<I>(iter: I, mut order: Ordering) -> Option<Ordering>
        where
            I: Iterator<Item = (Length, Length)>,
        {
            for (this_value, other_value) in iter {
                match this_value.cmp(&other_value) {
                    Ordering::Greater => {
                        if order == Ordering::Less {
                            return None;
                        }
                        order = Ordering::Greater;
                    },

                    Ordering::Less => {
                        if order == Ordering::Greater {
                            return None;
                        }
                        order = Ordering::Less;
                    },

                    Ordering::Equal => {},
                }
            }

            Some(order)
        }

        let mut order = Ordering::Equal;

        let iter = self.iter().filter(|(_, value)| *value > 0).map(
            |(this_id, this_value)| {
                let other_value = other.get(this_id);
                (this_value, other_value)
            },
        );

        order = update_order(iter, order)?;

        let iter = other.iter().filter(|(_, value)| *value > 0).map(
            |(other_id, other_value)| {
                let this_value = self.get(other_id);
                (this_value, other_value)
            },
        );

        update_order(iter, order)
    }
}