use hecs::{
    Component, ComponentError, Entity, Query as HecsQuery, Ref, RefMut, World, Archetype, Access, Without, With, Fetch,
};
use std::marker::PhantomData;
use crate::ArchetypeAccess;

pub struct Query<'a, Q: HecsQuery> {
    pub(crate) world: &'a World,
    pub(crate) archetype_access: &'a ArchetypeAccess,
    _marker: PhantomData<Q>,
}

#[derive(Debug)]
pub enum QueryComponentError {
    CannotReadArchetype,
    CannotWriteArchetype,
    ComponentError(ComponentError),
}

impl<'a, Q: HecsQuery> Query<'a, Q> {
    #[inline]
    pub fn new(world: &'a World, archetype_access: &'a ArchetypeAccess) -> Self {
        Self {
            world,
            archetype_access,
            _marker: PhantomData::default()
        }
    }

    #[inline]
    pub fn iter(&mut self) -> QueryBorrow<'_, Q> {
        QueryBorrow::new(&self.world.archetypes)
    }

    /// Gets a reference to the entity's component of the given type. This will fail if the entity does not have
    /// the given component type or if the given component type does not match this query.
    pub fn get<T: Component>(&self, entity: Entity) -> Result<Ref<'_, T>, QueryComponentError> {
        if let Some(location) = self.world.get_entity_location(entity) {
            if self
                .archetype_access
                .immutable
                .contains(location.archetype as usize)
                || self
                    .archetype_access
                    .mutable
                    .contains(location.archetype as usize)
            {
                self.world
                    .get(entity)
                    .map_err(|err| QueryComponentError::ComponentError(err))
            } else {
                Err(QueryComponentError::CannotReadArchetype)
            }
        } else {
            Err(QueryComponentError::ComponentError(
                ComponentError::NoSuchEntity,
            ))
        }
    }

    /// Gets a mutable reference to the entity's component of the given type. This will fail if the entity does not have
    /// the given component type or if the given component type does not match this query.
    pub fn get_mut<T: Component>(
        &self,
        entity: Entity,
    ) -> Result<RefMut<'_, T>, QueryComponentError> {
        if let Some(location) = self.world.get_entity_location(entity) {
            if self
                .archetype_access
                .mutable
                .contains(location.archetype as usize)
            {
                self.world
                    .get_mut(entity)
                    .map_err(|err| QueryComponentError::ComponentError(err))
            } else {
                Err(QueryComponentError::CannotWriteArchetype)
            }
        } else {
            Err(QueryComponentError::ComponentError(
                ComponentError::NoSuchEntity,
            ))
        }
    }

    pub fn removed<C: Component>(&self) -> &[Entity] {
        self.world.removed::<C>()
    }

    /// Sets the entity's component to the given value. This will fail if the entity does not already have
    /// the given component type or if the given component type does not match this query.
    pub fn set<T: Component>(
        &self,
        entity: Entity,
        component: T,
    ) -> Result<(), QueryComponentError> {
        let mut current = self.get_mut::<T>(entity)?;
        *current = component;
        Ok(())
    }
}

/// A borrow of a `World` sufficient to execute the query `Q`
///
/// Note that borrows are not released until this object is dropped.
pub struct QueryBorrow<'w, Q: HecsQuery> {
    archetypes: &'w [Archetype],
    borrowed: bool,
    _marker: PhantomData<Q>,
}

impl<'w, Q: HecsQuery> QueryBorrow<'w, Q> {
    pub(crate) fn new(archetypes: &'w [Archetype]) -> Self {
        Self {
            archetypes,
            borrowed: false,
            _marker: PhantomData,
        }
    }

    /// Execute the query
    ///
    /// Must be called only once per query.
    #[inline]
    pub fn iter<'q>(&'q mut self) -> QueryIter<'q, 'w, Q> {
        self.borrow();
        QueryIter {
            borrow: self,
            archetype_index: 0,
            iter: None,
        }
    }

    fn borrow(&mut self) {
        if self.borrowed {
            panic!(
                "called QueryBorrow::iter twice on the same borrow; construct a new query instead"
            );
        }
        for x in self.archetypes {
            // TODO: Release prior borrows on failure?
            if Q::Fetch::access(x) >= Some(Access::Read) {
                Q::Fetch::borrow(x);
            }
        }
        self.borrowed = true;
    }

    /// Transform the query into one that requires a certain component without borrowing it
    ///
    /// This can be useful when the component needs to be borrowed elsewhere and it isn't necessary
    /// for the iterator to expose its data directly.
    ///
    /// Equivalent to using a query type wrapped in `With`.
    ///
    /// # Example
    /// ```
    /// # use hecs::*;
    /// let mut world = World::new();
    /// let a = world.spawn((123, true, "abc"));
    /// let b = world.spawn((456, false));
    /// let c = world.spawn((42, "def"));
    /// let entities = world.query::<(Entity, &i32)>()
    ///     .with::<bool>()
    ///     .iter()
    ///     .map(|(e, &i)| (e, i)) // Copy out of the world
    ///     .collect::<Vec<_>>();
    /// assert!(entities.contains(&(a, 123)));
    /// assert!(entities.contains(&(b, 456)));
    /// ```
    pub fn with<T: Component>(self) -> QueryBorrow<'w, With<T, Q>> {
        self.transform()
    }

    /// Transform the query into one that skips entities having a certain component
    ///
    /// Equivalent to using a query type wrapped in `Without`.
    ///
    /// # Example
    /// ```
    /// # use hecs::*;
    /// let mut world = World::new();
    /// let a = world.spawn((123, true, "abc"));
    /// let b = world.spawn((456, false));
    /// let c = world.spawn((42, "def"));
    /// let entities = world.query::<(Entity, &i32)>()
    ///     .without::<bool>()
    ///     .iter()
    ///     .map(|(e, &i)| (e, i)) // Copy out of the world
    ///     .collect::<Vec<_>>();
    /// assert_eq!(entities, &[(c, 42)]);
    /// ```
    pub fn without<T: Component>(self) -> QueryBorrow<'w, Without<T, Q>> {
        self.transform()
    }

    /// Helper to change the type of the query
    fn transform<R: HecsQuery>(mut self) -> QueryBorrow<'w, R> {
        let x = QueryBorrow {
            archetypes: self.archetypes,
            borrowed: self.borrowed,
            _marker: PhantomData,
        };
        // Ensure `Drop` won't fire redundantly
        self.borrowed = false;
        x
    }
}

unsafe impl<'w, Q: HecsQuery> Send for QueryBorrow<'w, Q> {}
unsafe impl<'w, Q: HecsQuery> Sync for QueryBorrow<'w, Q> {}

impl<'w, Q: HecsQuery> Drop for QueryBorrow<'w, Q> {
    fn drop(&mut self) {
        if self.borrowed {
            for x in self.archetypes {
                if Q::Fetch::access(x) >= Some(Access::Read) {
                    Q::Fetch::release(x);
                }
            }
        }
    }
}

impl<'q, 'w, Q: HecsQuery> IntoIterator for &'q mut QueryBorrow<'w, Q> {
    type IntoIter = QueryIter<'q, 'w, Q>;
    type Item = <Q::Fetch as Fetch<'q>>::Item;

    #[inline]
    fn into_iter(self) -> Self::IntoIter {
        self.iter()
    }
}

/// Iterator over the set of entities with the components in `Q`
pub struct QueryIter<'q, 'w, Q: HecsQuery> {
    borrow: &'q mut QueryBorrow<'w, Q>,
    archetype_index: u32,
    iter: Option<ChunkIter<Q>>,
}

unsafe impl<'q, 'w, Q: HecsQuery> Send for QueryIter<'q, 'w, Q> {}
unsafe impl<'q, 'w, Q: HecsQuery> Sync for QueryIter<'q, 'w, Q> {}

impl<'q, 'w, Q: HecsQuery> Iterator for QueryIter<'q, 'w, Q> {
    type Item = <Q::Fetch as Fetch<'q>>::Item;

    #[inline]
    fn next(&mut self) -> Option<Self::Item> {
        loop {
            match self.iter {
                None => {
                    let archetype = self.borrow.archetypes.get(self.archetype_index as usize)?;
                    self.archetype_index += 1;
                    unsafe {
                        self.iter = Q::Fetch::get(archetype, 0).map(|fetch| ChunkIter {
                            fetch,
                            len: archetype.len(),
                        });
                    }
                }
                Some(ref mut iter) => match unsafe { iter.next() } {
                    None => {
                        self.iter = None;
                        continue;
                    }
                    Some(components) => {
                        return Some(components);
                    }
                },
            }
        }
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        let n = self.len();
        (n, Some(n))
    }
}

impl<'q, 'w, Q: HecsQuery> ExactSizeIterator for QueryIter<'q, 'w, Q> {
    fn len(&self) -> usize {
        self.borrow
            .archetypes
            .iter()
            .filter(|&x| Q::Fetch::access(x).is_some())
            .map(|x| x.len() as usize)
            .sum()
    }
}

struct ChunkIter<Q: HecsQuery> {
    fetch: Q::Fetch,
    len: u32,
}

impl<Q: HecsQuery> ChunkIter<Q> {
    #[inline]
    unsafe fn next<'a, 'w>(&mut self) -> Option<<Q::Fetch as Fetch<'a>>::Item> {
        loop {
            if self.len == 0 {
                return None;
            }

            self.len -= 1;
            if self.fetch.should_skip() {
                // we still need to progress the iterator
                let _ = self.fetch.next();
                continue;
            }

            break Some(self.fetch.next());
        }
    }
}