const std = @import("std");

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 {
            const result = lookupMut(T).*;
            if (result == 0) std.debug.panic("No lookup for {s}", .{@typeName(T)});
            return result;
        }

        /// 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"),
            };
        }

        /// Registers type lookups for the builtin Flecs entities.
        /// These are defined in the `./src/builtin/` directory.
        pub fn registerFlecsLookups(world: *World) !void {
            const error_writer = std.io.getStdErr().writer();
            const flecs = @import("./builtin/flecs.zig");
            const root = try lookupAndRegister(world, null, "flecs", flecs, error_writer);
            try lookupAndRegisterDeclarations(world, root, flecs, error_writer);
        }

        pub fn registerFlecsCoreLookups(world: *World) !void {
            const error_writer = std.io.getStdErr().writer();
            const flecs = @import("./builtin/flecs.zig");
            const flecs_core = @import("./builtin/flecs.core.zig");
            const root = try lookupAndRegister(world, null, "flecs", flecs, error_writer);
            const core = try lookupAndRegister(world, root, "core", flecs_core, error_writer);
            try lookupAndRegisterDeclarations(world, core, flecs_core, error_writer);
        }

        fn lookupAndRegister(
            world: *World,
            parent: ?Entity,
            comptime name: []const u8,
            comptime T: type,
            error_writer: anytype,
        ) !Entity {
            if (world.lookupChild(parent, name)) |entity| {
                const id_ptr = lookupMut(T);
                if (id_ptr.* != 0 and id_ptr.* != entity.raw) {
                    if (!meta.isNull(error_writer))
                        try std.fmt.format(error_writer, "Trying to register lookup for entity '{}' as {d}, but it's already registered as {d}\n", .{ entity, entity.raw, id_ptr.* });
                    return error.LookupIdMismatch;
                }
                id_ptr.* = entity.raw;
                return entity;
            } else {
                if (!meta.isNull(error_writer)) {
                    if (parent) |p|
                        try std.fmt.format(error_writer, "Could not find child '{s}' of parent '{}'\n", .{ name, p })
                    else
                        try std.fmt.format(error_writer, "Could not find root entity '{s}'\n", .{name});
                }
                return error.EntityNotFound;
            }
        }

        fn lookupAndRegisterDeclarations(
            world: *World,
            parent: Entity,
            comptime T: type,
            error_writer: anytype,
        ) !void {
            inline for (@typeInfo(T).Struct.decls) |decl| {
                const ChildType = @field(T, decl.name);
                if (@TypeOf(ChildType) != type) continue;

                comptime var child_name = decl.name;
                if (@hasDecl(T, "_" ++ decl.name ++ "_Name"))
                    child_name = @field(T, "_" ++ decl.name ++ "_Name");

                const child = try lookupAndRegister(world, parent, child_name, ChildType, error_writer);
                try lookupAndRegisterDeclarations(world, child, ChildType, error_writer);
            }
        }
    };
}