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

pub fn Context(comptime ctx: anytype) type {
    return struct {
        pub const Entity = @import("./entity.zig").Entity(ctx);
        pub const Id = @import("./id.zig").Id(ctx);
        pub const Iter = @import("./iter.zig").Iter(ctx);
        pub const Pair = @import("./pair.zig").Pair(ctx);
        pub const World = @import("./world.zig").World(ctx);

        /// Looks up an entity ID unique to this `Context` for the provided
        /// type that has been registered previously, typically done by
        /// calling functions such as `World.component(...)`.
        pub fn lookup(comptime T: type) c.ecs_entity_t {
            return lookupMut(T).*;
        }

        /// Returns a pointer unique to this `Context` and the provided type
        /// that holds an entity ID. Useful for associating types to entities.
        pub fn lookupMut(comptime T: type) *c.ecs_entity_t {
            _ = T; // Only necessary to create a unique type.
            const EntityHolder = struct {
                pub var id: c.ecs_entity_t = 0;
            };
            return &EntityHolder.id;
        }

        /// Converts the specified value to an `ecs_entity_t`.
        ///
        /// If `null` is passed to this function, either directly or through an
        /// optional type of one of the supported types, `0` is returned. It's up to
        /// the caller to do any necessary testing for `0` values, and whether the
        /// returned entity is valid or alive.
        ///
        /// The following can be converted:
        /// - `Entity` and `ecs_entity_t` - Self-explanatory.
        /// - `type` - Looks up the entity associated with that type.
        pub fn anyToEntity(value: anytype) c.ecs_entity_t {
            return switch (@TypeOf(value)) {
                @TypeOf(null) => 0,
                c.ecs_entity_t => value,
                Entity => value.raw,
                ?Entity => if (value) |v| v.raw else 0,
                type => lookup(value),
                else => @compileError("Value of type " ++ @typeName(@TypeOf(value)) ++ " can't be converted to Entity"),
            };
        }

        /// Converts the specified value to an `ecs_id_t`.
        ///
        /// If `null` is passed to this function, either directly or through an
        /// optional type of one of the supported types, `0` is returned. It's up to
        /// the caller to do any necessary testing for `0` values, and whether the
        /// returned id is valid.
        ///
        /// The following can be converted:
        /// - `Id` and `ecs_id_it` - Self-explanatory.
        /// - `Entity` and `ecs_entity_t` - Self-explanatory.
        /// - `Pair` - Converts to the equivalent `Id`.
        /// - `.{ relation, target }` - A `Pair`, converted using `anyToEntity()`.
        /// - `type` - Looks up the entity associated with that type.
        pub fn anyToId(value: anytype) c.ecs_id_t {
            const T = @TypeOf(value);
            if (comptime meta.isTuple(T)) {
                if (@typeInfo(T).Struct.fields.len != 2)
                    @compileError("Value of type " ++ @typeName(T) ++ " must be a tuple with 2 elements, to be a Pair");
                const relation = anyToEntity(value[0]);
                const target = anyToEntity(value[1]);
                return c.ecs_make_pair(relation, target);
            }
            return switch (T) {
                @TypeOf(null) => 0,
                c.ecs_id_t => value,
                Id => value.raw,
                ?Id => if (value) |v| v.raw else 0,
                Pair => value.raw,
                ?Pair => if (value) |v| v.raw else 0,
                // Technically same type as `ecs_id_it`.
                // c.ecs_entity_t => value,
                Entity => value.raw,
                ?Entity => if (value) |v| v.raw else 0,
                type => lookup(value),
                else => @compileError("Value of type " ++ @typeName(T) ++ " can't be converted to Id"),
            };
        }
    };
}