khora_data/ecs/

query.rs

// Copyright 2025 eraflo
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

use crate::ecs::{
    page::{AnyVec, ComponentPage},
    Component, EntityId, World,
};
use std::{any::TypeId, marker::PhantomData};

// ------------------------- //
// ---- WorldQuery Part ---- //
// ------------------------- //

/// A trait implemented by types that can be used to query data from the `World`.
///
/// This "sealed" trait provides the necessary information for the query engine to
/// find the correct pages and safely access component data. It is implemented for
/// component references (`&T`, `&mut T`), `EntityId`, filters like `Without<T>`,
/// and tuples of other `WorldQuery` types.
pub trait WorldQuery {
    /// The type of item that the query iterator will yield (e.g., `(&'a Position, &'a mut Velocity)`).
    type Item<'a>;

    /// Returns the sorted list of `TypeId`s for the components to be INCLUDED in the query.
    /// This signature is used to find `ComponentPage`s that contain all these components.
    fn type_ids() -> Vec<TypeId>;

    /// Returns the sorted list of `TypeId`s for components to be EXCLUDED from the query.
    /// Used to filter out pages that contain these components.
    fn without_type_ids() -> Vec<TypeId> {
        Vec::new()
    }

    /// Fetches the query's item from a specific row in a `ComponentPage`.
    ///
    /// # Safety
    ///
    /// This function is unsafe because the caller (the `Query` iterator) must guarantee
    /// several invariants:
    /// 1. The page pointed to by `page_ptr` is valid and matches the query's signature.
    /// 2. `row_index` is a valid index within the page's columns.
    /// 3. Aliasing rules are not violated (e.g., no two `&mut T` to the same data).
    unsafe fn fetch<'a>(page_ptr: *const ComponentPage, row_index: usize) -> Self::Item<'a>;
}

// Implementation for a query of a single, immutable component reference.
impl<T: Component> WorldQuery for &T {
    type Item<'a> = &'a T;

    fn type_ids() -> Vec<TypeId> {
        vec![TypeId::of::<T>()]
    }

    /// Fetches a reference to the component `T` from the specified row.
    ///
    /// # Safety
    /// The caller MUST guarantee that the page contains a column for component `T`
    /// and that `row_index` is in bounds.
    unsafe fn fetch<'a>(page_ptr: *const ComponentPage, row_index: usize) -> Self::Item<'a> {
        // 1. Get a reference to the `ComponentPage`.
        let page = &*page_ptr;

        // 2. Get the type-erased column for the component `T`.
        // We can unwrap because the caller guarantees the column exists.
        let column: &dyn AnyVec = &**page.columns.get(&TypeId::of::<T>()).unwrap();

        // 3. Downcast the column to its concrete `Vec<T>` type.
        // First, cast to `&dyn Any`, then `downcast_ref`.
        let vec: &Vec<T> = column.as_any().downcast_ref::<Vec<T>>().unwrap();

        // 4. Get the component from the vector at the specified row.
        // We use `get_unchecked` for performance, as the caller guarantees the
        // index is in bounds. This avoids a bounds check.
        vec.get_unchecked(row_index)
    }
}

// Implementation for a query of a single, mutable component reference.
impl<T: Component> WorldQuery for &mut T {
    type Item<'a> = &'a mut T;

    fn type_ids() -> Vec<TypeId> {
        vec![TypeId::of::<T>()]
    }

    /// Fetches a mutable reference to the component `T` from the specified row.
    ///
    /// # Safety
    /// The caller MUST guarantee that:
    /// 1. The page contains a column for component `T`.
    /// 2. `row_index` is in bounds.
    /// 3. No other mutable reference to this specific component exists at the same time.
    ///    The query engine must enforce this.
    unsafe fn fetch<'a>(page_ptr: *const ComponentPage, row_index: usize) -> Self::Item<'a> {
        // UNSAFE: We cast the const pointer to a mutable one.
        // This is safe ONLY if the query engine guarantees no other access.
        let page = &mut *(page_ptr as *mut ComponentPage);
        let column = page.columns.get_mut(&TypeId::of::<T>()).unwrap();
        let vec = column.as_any_mut().downcast_mut::<Vec<T>>().unwrap();
        vec.get_unchecked_mut(row_index)
    }
}

// Implementation for a query of a 2-item tuple.
// This is now generic over any two types that implement `WorldQuery`.
impl<Q1: WorldQuery, Q2: WorldQuery> WorldQuery for (Q1, Q2) {
    type Item<'a> = (Q1::Item<'a>, Q2::Item<'a>);

    fn type_ids() -> Vec<TypeId> {
        let mut ids = Q1::type_ids();
        ids.extend(Q2::type_ids());
        // Sorting is crucial to maintain a canonical signature.
        ids.sort();
        // We also remove duplicates, in case of queries like `(&T, &T)`.
        ids.dedup();
        ids
    }

    fn without_type_ids() -> Vec<TypeId> {
        let mut ids = Q1::without_type_ids();
        ids.extend(Q2::without_type_ids());
        ids.sort();
        ids.dedup();
        ids
    }

    unsafe fn fetch<'a>(page_ptr: *const ComponentPage, row_index: usize) -> Self::Item<'a> {
        // Fetch data for each part of the tuple individually.
        let item1 = Q1::fetch(page_ptr, row_index);
        let item2 = Q2::fetch(page_ptr, row_index);
        (item1, item2)
    }
}

// Implementation for a query of a 3-item tuple.
impl<Q1: WorldQuery, Q2: WorldQuery, Q3: WorldQuery> WorldQuery for (Q1, Q2, Q3) {
    type Item<'a> = (Q1::Item<'a>, Q2::Item<'a>, Q3::Item<'a>);

    fn type_ids() -> Vec<TypeId> {
        let mut ids = Q1::type_ids();
        ids.extend(Q2::type_ids());
        ids.extend(Q3::type_ids());
        ids.sort();
        ids.dedup();
        ids
    }

    fn without_type_ids() -> Vec<TypeId> {
        let mut ids = Q1::without_type_ids();
        ids.extend(Q2::without_type_ids());
        ids.extend(Q3::without_type_ids());
        ids.sort();
        ids.dedup();
        ids
    }

    unsafe fn fetch<'a>(page_ptr: *const ComponentPage, row_index: usize) -> Self::Item<'a> {
        // Fetch data for each part of the tuple individually.
        let item1 = Q1::fetch(page_ptr, row_index);
        let item2 = Q2::fetch(page_ptr, row_index);
        let item3 = Q3::fetch(page_ptr, row_index);
        (item1, item2, item3)
    }
}

// Implementation for a query of a 4-item tuple.
impl<Q1: WorldQuery, Q2: WorldQuery, Q3: WorldQuery, Q4: WorldQuery> WorldQuery
    for (Q1, Q2, Q3, Q4)
{
    type Item<'a> = (Q1::Item<'a>, Q2::Item<'a>, Q3::Item<'a>, Q4::Item<'a>);

    fn type_ids() -> Vec<TypeId> {
        let mut ids = Q1::type_ids();
        ids.extend(Q2::type_ids());
        ids.extend(Q3::type_ids());
        ids.extend(Q4::type_ids());
        ids.sort();
        ids.dedup();
        ids
    }

    fn without_type_ids() -> Vec<TypeId> {
        let mut ids = Q1::without_type_ids();
        ids.extend(Q2::without_type_ids());
        ids.extend(Q3::without_type_ids());
        ids.extend(Q4::without_type_ids());
        ids.sort();
        ids.dedup();
        ids
    }

    unsafe fn fetch<'a>(page_ptr: *const ComponentPage, row_index: usize) -> Self::Item<'a> {
        (
            Q1::fetch(page_ptr, row_index),
            Q2::fetch(page_ptr, row_index),
            Q3::fetch(page_ptr, row_index),
            Q4::fetch(page_ptr, row_index),
        )
    }
}

// To fetch an entity's ID, we need to access the page's own entity list.
// We also need to query for the entity ID itself.
impl WorldQuery for EntityId {
    type Item<'a> = EntityId;

    // EntityId is not a component, it doesn't have a TypeId in the page signature.
    fn type_ids() -> Vec<TypeId> {
        Vec::new()
    }

    // It doesn't have a `without` filter either.
    // The default implementation is sufficient.

    unsafe fn fetch<'a>(page_ptr: *const ComponentPage, row_index: usize) -> Self::Item<'a> {
        let page = &*page_ptr;
        // The entity ID is fetched from the page's own list of entities.
        *page.entities.get_unchecked(row_index)
    }
}

// -------------------- //
// ---- Query Part ---- //
// -------------------- //

/// An iterator that yields the results of a `WorldQuery`.
///
/// This struct is created by the [`World::query()`] method. It holds a reference
/// to the world and iterates through all `ComponentPage`s that match the query's
/// signature, fetching the requested data for each entity in those pages.
pub struct Query<'a, Q: WorldQuery> {
    /// A raw pointer to the world. Using a pointer avoids lifetime variance issues
    /// and gives us the flexibility to provide either mutable or immutable access.
    world_ptr: *const World,

    /// The list of page indices that match this query.
    matching_page_indices: Vec<u32>,

    /// The index of the current page we are iterating through.
    current_page_index: usize,

    /// The index of the next row to fetch within the current page.
    current_row_index: usize,

    /// A marker to associate this iterator with the lifetime `'a` and the query type `Q`.
    /// It tells Rust that our iterator behaves as if it's borrowing `&'a Q` from the world.
    _phantom: PhantomData<(&'a (), Q)>,
}

impl<'a, Q: WorldQuery> Query<'a, Q> {
    /// (Internal) Creates a new `Query` iterator.
    ///
    /// This is intended to be called only by `World::query()`.
    /// It takes the world and the list of matching pages as arguments.
    pub(crate) fn new(world: &'a World, matching_page_indices: Vec<u32>) -> Self {
        Self {
            world_ptr: world as *const _,
            matching_page_indices,
            current_page_index: 0,
            current_row_index: 0,
            _phantom: PhantomData,
        }
    }
}

impl<'a, Q: WorldQuery> Iterator for Query<'a, Q> {
    /// The type of item yielded by this iterator, as defined by the `WorldQuery` trait.
    type Item = Q::Item<'a>;

    /// Advances the iterator and returns the next item.
    fn next(&mut self) -> Option<Self::Item> {
        loop {
            // 1. Check if there are any pages left to iterate through.
            if self.current_page_index >= self.matching_page_indices.len() {
                return None; // No more pages, iteration is finished.
            }

            // Unsafe block because we are dereferencing a raw pointer.
            // This is safe because the `Query` is only created from a valid `World` reference.
            let world = unsafe { &*self.world_ptr };

            // 2. Get the current page.
            let page_id = self.matching_page_indices[self.current_page_index];
            let page = &world.pages[page_id as usize];

            // 3. Check if there are rows left in the current page.
            if self.current_row_index < page.row_count() {
                // There are rows left, so we can fetch the data.
                let item = unsafe {
                    // This is safe because we've already matched the page signature
                    // and we are iterating within the page's bounds.
                    Q::fetch(page as *const _, self.current_row_index)
                };

                // Advance the row index for the next call.
                self.current_row_index += 1;
                return Some(item);
            } else {
                // 4. No more rows in this page. Move to the next page.
                self.current_page_index += 1;
                self.current_row_index = 0; // Reset the row index for the new page.
                                            // The `loop` will then re-evaluate with the new page index.
            }
        }
    }
}

/// A `WorldQuery` filter that matches entities that do NOT have component `T`.
///
/// This is used as a marker in a query tuple to exclude entities. For example,
/// `Query<(&Position, Without<Velocity>)>` will iterate over all entities
/// that have a `Position` but no `Velocity`.
pub struct Without<T: Component>(PhantomData<T>);

// `Without<T>` itself doesn't fetch any data, so its `WorldQuery` implementation
// is mostly empty. It acts as a signal to the query engine.
impl<T: Component> WorldQuery for Without<T> {
    /// This query item is a zero-sized unit type, as it fetches no data.
    type Item<'a> = ();

    /// A `Without` filter does not add any component types to the query's main
    /// signature for page matching. The filtering is handled separately.
    fn type_ids() -> Vec<TypeId> {
        Vec::new() // Returns an empty Vec.
    }

    /// This is the key part of the filter: it returns the TypeId of the component to filter out.
    fn without_type_ids() -> Vec<TypeId> {
        // This is the key change: it returns the TypeId of the component to filter out.
        vec![TypeId::of::<T>()]
    }

    /// Fetches nothing. Returns a unit type `()`.
    unsafe fn fetch<'a>(_page_ptr: *const ComponentPage, _row_index: usize) -> Self::Item<'a> {
        // This function will be called but its result is ignored.
    }
}