const std = @import("std");

const c = @import("./c.zig");

const Id = @import("./id.zig").Id;
const Lookup = @import("./main.zig").Lookup;
const World = @import("./world.zig").World;

pub const EntityError = error{
    /// Id is `0`.
    IsNone,
    /// Id is not valid.
    IsInvalid,
    /// Id is not an `Entity`.
    IsNotEntity,
    /// Generation doesn't match.
    GenMismatch,
    /// Entity is not alive.
    IsNotAlive,
};

/// An `Entity` is an `Id` which represents a "thing" within the world. This
/// could be a traditional game object, but is also able to represent other
/// concepts such as component types, relationship types, systems, modules,
/// resources, and anything else you can make fit.
///
/// Each `Entity` can have a number of `Id`s added to it, which could be
/// components (such as `Position`), zero-size tags (such as `Disabled`) or
/// relationship `Pair`s (such as `(ChildOf, parent)`).
///
/// Entities can be created using `World.new()` and deleted with `delete()`.
/// When an entity is deleted it is no longer considered "alive". A world can
/// contain up to 4 billion alive entities.
///
/// An `Id` can be converted to an `Entity` using `Id.asEntity()` (fails if
/// the id isn't an entity), and back again with `asId()` (always succeeds).
pub fn Entity(comptime ctx: anytype) type {
    return struct {
        world: *World(ctx),
        raw: c.ecs_entity_t,

        const Self = @This();

        pub fn fromRaw(world: *World(ctx), raw: c.ecs_entity_t) Self {
            return .{ .world = world, .raw = raw };
        }

        /// Ensures this `Entity` is valid, returning an error otherwise.
        /// Entities that are valid can be used with API functions.
        ///
        /// An `Entity` is valid if ..
        /// - .. it is not `0`.
        /// - .. it does not have invalid bits set.
        /// - .. it does not have any `Id` flags set.
        /// - .. no entity with this id is alive, but this entity's generation is `0`.
        /// - .. an entity with this exact id and generation is alive in the world.
        pub fn ensureValid(self: Self) !Self {
            // `0` is not a valid entity id.
            if (self.raw == 0) return EntityError.IsNone;
            // Entity ids should not contain data in dead zone bits.
            const VALID_BITS_MASK: c.ecs_entity_t = 0xFF00FFFFFFFFFFFF;
            if ((self.raw & ~VALID_BITS_MASK) != 0) return EntityError.IsInvalid;
            // Entity ids should not contain `Id` flag bits.
            if ((self.raw & c.ECS_ID_FLAGS_MASK) != 0) return EntityError.IsNotEntity;

            // Get the currently alive entity entity with the same id.
            // If entity is alive, the generation has to match.
            // If entity is not alive, the generation has to be `0`.
            const entity = c.ecs_get_alive(self.world.raw, self.raw);
            if (c.ECS_GENERATION(entity) != c.ECS_GENERATION(self.raw))
                return EntityError.GenMismatch;

            return self;
        }

        /// Ensures this `Entity` is alive in this world, returning an error if not.
        pub fn ensureAlive(self: Self) !Self {
            return if ((try ensureValid(self)).isAlive()) self else EntityError.IsNotAlive;
        }

        pub fn isAlive(self: Self) bool {
            return c.ecs_is_alive(self.world.raw, self.raw);
        }

        pub fn asId(self: Self) Id(ctx) {
            return @bitCast(self);
        }

        pub fn getEntityId(self: Self) u32 {
            return @intCast(self.raw & c.ECS_ENTITY_MASK);
        }

        pub fn getGeneration(self: Self) u16 {
            return @intCast(c.ECS_GENERATION(self.raw));
        }

        /// Deletes this `Entity` and all of its components.
        pub fn delete(self: Self) void {
            c.ecs_delete(self.world.raw, self.raw);
        }

        pub fn get(self: Self, comptime T: type) ?*const T {
            const id = Lookup(ctx, T).id;
            return @alignCast(@ptrCast(c.ecs_get_id(self.world.raw, self.raw, id)));
        }

        pub fn set(self: Self, comptime T: type, value: T) Self {
            const id = Lookup(ctx, T).id;
            _ = c.ecs_set_id(self.world.raw, self.raw, id, @sizeOf(T), &value);
            return self;
        }
    };
}